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Mat foundations
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Mat foundations
1
Description STAAD has the ability to generate supports for structures like slabs on grade, which also go by the name mat foundations. A mat foundation is a large concrete slab sitting on soil. The support for the structure is the soil itself. The resistance of the soil is represented through a term called Modulus of Subgrade Reaction, the definition of which may be found in many textbooks on foundation analysis. The general approach to solving such problems is to sub-divide the slab into several plate elements. Each node of the meshed slab will then have an influence area or a contributory area, which is to say that soil within the area surrounding that node acts like a spring. The influence area is then multiplied by the subgrade modulus to arrive at the spring constant. Subgrade modulus has units of force per length^3. So, the spring will have units of force/length.
The problem with using this method is that, for irregularly-shaped or large slabs with many nodes, computing the influence area for each node can become quite tedious and time-consuming. The model below exemplifies the problem.
STAAD.Pro 2004 – Training Manual – Advanced Topics
2
This is where the Foundation type of support can be useful. STAAD will calculate the influence areas of all the nodes by itself and derive the spring constants for you. In STAAD, we refer to facility as SPRING SUPPORT GENERATION. STAAD has two options for such supports:
a) The ELASTIC MAT option b) The PLATE MAT option
The ELASTIC MAT option : When the spring support generation facility was first introduced in STAAD, it was based on this method. In fact, this was the only method available until and including STAAD.Pro 2002 Build 1004. This method calculates the influence area of the various nodes using the Delaunay triangle method.
STAAD.Pro 2004 – Training Manual – Advanced Topics 3
The distinguishing aspect of this method is that it uses the joint-list that accompanies the ELASTIC MAT command to form a closed surface. The area within this closed surface is then determined and the share of this area for each node in the list is then calculated. Hence, while specifying the joint-list, one should make sure that these joints make up a closed surface. Without a proper closed surface, the area calculated for the region may be indeterminate and the spring constant values may be erroneous. Consequently, the list should have at a minimum, 3 nodes. While forming the closed surface, namely, a polygon, the sides of the polygon have to be assembled by lining up points along the edges. The edge detection aspects of this method are very sensitive to out-of-straightness, which may occur if the coordinates of the nodes aren't precise to a significant number of digits. Also, the internal angle formed by 2 adjacent lines connecting 3 consecutive nodes in the list should be less than 180 degrees, which is to say that, the region should have the shape of a convex polygon. Failure to form straight edges and convex polygons can lead to erroneous influence area values and consequently, erroneous spring constants. This is the limitation of this feature. The example below explains the method that may be used to get around a situation where a convex polygon is not available. For the model comprised of plate elements 100 to 102 in the figure below, one wishes to generate the spring supports at nodes 1 to 8. However, a single ELASTIC MAT command will not suffice because the internal angle between the edges 1-8 and 8-7 at node 8 is 270 degrees, which violates the requirements of a convex polygon.
STAAD.Pro 2004 – Training Manual – Advanced Topics
4 So, one should break it up into 2 commands:
1 2 3 8 ELASTIC MAT DIREC Y SUBG 200. 3 4 5 6 7 8 ELASTIC MAT DIREC Y SUBG 200.
Joints 3 and 8 will hence get the contribution from both of the above commands. Because this method uses nodes to generate contours, it may be used whether the mat is defined using plates, or solids. This is the advantage of this method.
STAAD.Pro 2004 – Training Manual – Advanced Topics 5
The PLATE MAT option : If the foundation slab is modeled using plate elements, the influence area can be calculated using the principles used in determining the tributary area of the nodes from the finite element modeling standpoint. In other words, the rules used by the program in converting a uniform pressure load on an element into fixed end actions at the nodes are used in calculating the influence area of the node, which is then multiplied by the subgrade modulus to obtain the spring constant. This feature has been available since STAAD.Pro 2002 Build 1005. The advantage of this method is that it overcomes one of the major limitations of the Delaunay triangle method, which is that the contour formed by the nodes of the mat must form a convex hull. Example SUPPORTS 17054 TO 17081 PLATE MAT DIR YONLY SUBGRADE 5000.0 PRINT YR -.01 0.01 PLATE MAT DIR YONLY SUBGRADE 5000.0 The first of the above 2 commands instructs STAAD to internally generate supports for the nodes at the corners of plate elements 17054 TO 17081. The second example instructs STAAD to internally generate supports for the nodes at the corners of plate elements which lie in the global XZ plane bound by the YRANGE value of -0.01 and +0.01 length units. Another advantage of the PLATE MAT method is that it enables us to view soil pressure contours beneath the base of the slab. After the analysis, go to the post-processing mode, and click on the Plates page. In the selection box for choosing the type of result to plot, choose base pressures. This is not currently available with the ELASTIC MAT method.
STAAD.Pro 2004 – Training Manual – Advanced Topics
6 Question : How do I tell STAAD that my soil spring is effective only in
COMPRESSION, and should not be considered when it goes into tension?
Answer : This may be done by using the ELASTIC MAT or PLATE MAT command in conjunction with the SPRING COMPRESSION command. The program iteratively solves the problem so that the final answer reflects the condition corresponding to actual contact between slab & soil. Example problem 27 illustrates this.
Question : Is it possible to get a report which shows the influence area generated by STAAD for each support node?
Answer : Yes. Use the PRINT option available with the ELASTIC MAT or PLATE MAT commands. This will produce a report of the influence areas. An example of such a report is shown below.
To get a report of the spring constants themselves, use the command
PRINT SUPPORT INFORMATION
STAAD.Pro 2004 – Training Manual – Advanced Topics 7 Question : Is it possible to find out the base pressure at each node for each
load case?
Answer : Yes. In the post-processing mode, go to the Node – Base pressure page. A table will appear along the right side of the screen showing these values. The Summary tab will show the maximum and minimum pressure along with the associated node for each of the 3 global directions.
Question : How does subgrade modulus differ from soil bearing capacity?
Answer : A soil must be capable of carrying the loads it is subjected to, without undergoing a shear failure, or excessive settlements. This capacity is referred to as the soil bearing capacity. The modulus of subgrade reaction is a measure of the stiffness of soil if it were to behave like a spring. It is the relationship between bearing pressure and soil deflection. The modulus of subgrade reaction is the quantity by which the influence area of a support node is multiplied by to get the equivalent spring constant which can be used at the analysis stage. One would provide this as an input item when one wishes STAAD to generate spring supports using the ELASTIC MAT command, as explained in section 5.27.3 of the STAAD.Pro Technical Reference manual. At the end of the mat foundation analysis, the maximum soil pressure you get from STAAD’s soil pressure diagram should be within the limits of the soil’s bearing capacity. If the actual pressure exceeds the capacity, it is an indication of failure.
STAAD.Pro 2004 – Training Manual – Advanced Topics
8 Question : If you have the value for soil bearing pressure, how do you use
that to come up with the subgrade modulus that STAAD uses for elastic mat definitions?
Answer : One doesn't use the bearing capacity of soil to determine the subgrade modulus. Instead, it is a separate attribute of soil. If you have a look at the text book "Foundation Analysis and Design" by Joseph Bowles, you will find a few sections devoted to that topic, with specific values listed for specific types of soil. The basic difference between these 2 attributes is that, bearing capacity (or bearing pressure) is the pressure at which the soil fails, either in shear or compression. It hence has units of force per unit area. Subgrade Modulus on the other hand is a measure of the "spring constant" of soil. It is the distance that a unit area of soil would deflect under a unit load.
Correlation between soil bearing capacity and modulus of subgrade reaction
Apurba Tribedi Senior Product Manager
Bentley Systems Inc.
Yorba Linda, CA, US
The author is a Senior Product Manager at Bentley. He has been involved in architecting and coding structural software for more than 18 years. He is one of the core developers of the STAAD.Pro program and currently manages the STAAD Foundation product. After graduating from Calcutta University, he joined Research Engineers as a software developer and has since worked in different areas including graphics, user interface, database, analysis and design engine.
Abstract: Engineers increasingly using software to design mat as flexible foundation to save concrete. Instead of soil bearing capacity these software programs often ask for a property called “modulus of subgrade reaction”. Why this soil property is needed? Is there any relationship between these two parameters? Can one parameter be estimated from the other? This paper digs dip to explain the significance of these parameters and how one parameter relates to the other.
Introduction Probably the most widely used value in a soil report is soil bearing capacity. The obvious reason is the basic examples given in most text books almost always use bearing capacity to calculate the plan dimension of a footing. Because of simplicity and ease of use, that method is still the fundamental soil parameter for foundation design. However, that simplicity assumes that the footing will behave as a rigid body. That assumption works well in practice for small and single column footings. But for large and multi column foundations, most engineers prefer flexible analysis. Manual computation of flexible analysis could be challenging and in almost all cases software programs such as STAAD, SAFE, GT STRUDL etc. are used. However, these computer programs often ask for an input called “modulus of subgrade reaction”. Many engineers are not familiar with this term and often try to compare it with bearing capacity. As more and more engineers will use software to design foundations, it is more essential now than ever for engineers to have a fundamental understanding of this soil parameter. Is there any relationship between bearing capacity and modulus of subgrade reaction? Here we will discuss the concepts and possible relationship.
Modulus of subgrade reaction (Ks) This term is measured and expressed as load intensity per unit of displacement. For the English unit system it is often expressed in kip/in2/in and in SI system in kN/m2/m. Some often expresses this term in kip/in3 (or kN/m3) which could be misleading. Numerically kip/in3 is correct but does not properly represent the physical significance of the measured value and it could be mistaken as density unit or a volumetric measurement. Mathematically, the coefficient of subgrade reaction is expressed as: (1)
… . 1
where p = contact pressure intensity and s = soil settlement As Terzaghi mentioned,(2) proper estimation of contact pressure for a flexible foundation could be very cumbersome, so it is assumed that Ks remains constant for the entire footing. In other words, the ratio between pressure and settlement at all locations of a footing will remain constant. So the displacement diagram of a footing with a load at center will have a dishing effect. A point at the center of the footing will experience the highest displacement. Displacement reduces as it moves away from the center. Figure 1‐a, shows a simple slab on grade foundation. It was modeled and analyzed in STAAD Foundation as “Mat”, which is a flexible foundation, and the soil was defined using coefficient of subgrade reaction. For this exercise, the software default value for the modulus of subgrade reaction was used. The displacement diagram shows a dishing effect as discussed earlier. Figure 1‐b shows the soil pressure contour. It is also obvious that the pressure intensity at the center is maximum and reduces as the elements (or node coordinates) moves away from the center. So, it is to assume that the ratio of pressure intensity and settlement is constant.
Figure 1 –Deflection diagram and Soil pressure contour Let us investigate some of the numbers from the same example. Soil pressure, corresponding displacement and the ratio is listed in Table 1 below. The points are represented on a diagonal to illustrate the variation of pressure and displacement as the points move away from the center to the most distant point in the corner of the rectangular footing. Figure 2 shows the points on the mat slab.
Figure 2: Selected points to compare base pressure, deflection and ratio
Node number Soil pressure (p) Node displacement () Ratio (p/)
(kN/m2) (mm) (kN/m2/m)
1 (top‐left corner) 58.38282 5.377 10858
41 61.94684 5.70524 10858
51 65.56358 6.03834 10858
61 69.19262 6.37257 10858
71 72.64874 6.69087 10858
81 (middle) 75.31719 6.93664 10858
Table 1: soil pressure, node displacement and their ratio
Now this is hardly a surprise as, by definition, modulus of subgrade reaction (Ks) is a constant for the entire footing and the program used Ks as its soil property. It is also important to note that the software default Ks value (10858 kN/m2/m) was exactly the same as the constant ratio calculated in table 1. Base pressure was calculated from the support reaction. So, one might think that the ratio of support reaction and corresponding displacement will also be a constant. Let us examine some of the numbers as listed in table 2. Obviously the ratios are not constant for all but for most. This brings us to our next topic on how Ks value is used inside the program and the base pressure is calculated.
Node number Support Reaction(P) Node displacement () Ratio (P/)
(kN) (mm) (kN/m)
1 (top‐left corner) 1.313609 5.377 244.3
41 5.575193 5.70524 977.2
51 5.900749 6.03834 977.2
61 6.227366 6.37257 977.2
71 6.538362 6.69087 977.2
81 (middle) 6.778522 6.93664 977.2
Table 2: Support reaction and displacement
Tributary area/influence surface area Often an assumption is made to calculate how much area of a plate can be attributed to a node or, in other words, the influence of each node on the surface area of a plate. It depends on the shape of the plate. For a perfect square or rectangular plate, each node will influence exactly 1/4th of the plate surface area (Figure 3‐a). But for a generalized quadrilateral, the best practice would be to calculate the center of the mass of the plate and then draw lines from that center point to the middle points of each side. The shaded area represents the influence surface area of the corresponding node (Figure 3‐b).
Figure 3: Node tributary area
Calculation of spring support constant The above described tributary area calculation is the key procedure used internally by the program to calculate the linear spring constant. The program first calculates the tributary area for each node of the footing and then multiplies the modulus of subgrade reaction by the corresponding tributary area for each node to get the linear spring constant at each node.
… . 2 where
is the spring constant at ith node is the influence area of ith node
Ks is the modulus of subgrade reaction For a concrete foundation analysis, those springs have to be defined as compression‐only as concrete is assumed not to carry any tensile force. The base pressure is calculated at each support node by dividing the support reaction with the corresponding node tributary area. If we look at the above example, node 1 has a much smaller tributary area than the rest of the nodes. It can also be noted that all other nodes have same tributary area which explains Table 2 as it shows ratio for node 1 is different than other nodes. Figure 4 shows the tributary area for different nodes. Node 1 has a tributary area which is 25% of Node 81. Table 3 is an extension of Table 1 and Table 2 which shows how constant ratio is achieved for all nodes.
Figure 4: Influence area of selected nodes
Node number
Support Reaction(P)
Influence area
Base Pressure (p)
Displacement () Ratio (p/)
(kN) (m2) (kN/m2) (mm) (kN/m2/m)
1 (top‐left corner)
1.313609 .0225 58.38282 5.377 10858
41 5.575193 .09 61.94684 5.70524 10858
51 5.900749 .09 65.56358 6.03834 10858
61 6.227366 .09 69.19262 6.37257 10858
71 6.538362 .09 72.64874 6.69087 10858
81 (middle) 6.778522 .09 75.31719 6.93664 10858
Table 3: Reaction, base pressure, displacement, Ks constant
Bearing Capacity dependency on allowable settlement Bearing capacity is the measurement of the soil pressure which soil can safely bear. In other words, bearing capacity is the pressure which soil can withstand before it fails. The two most important soil failure criteria are:
1) Shear failure 2) Maximum allowable settlement
Among many factors, foundation width (B) can influence failure criteria. Normally, shear failure governs for smaller foundations and settlement failure governs bigger foundations. The following table is a typical example which shows the relationship among different foundation sizes and failure criteria.
Shape B m
L m
qa (kPa) Governing Criteria
Square 1 1 113 Shear 2 2 117 Shear 3 3 111 Settlement 4 4 92 Settlement 6 6 75 Settlement 10 10 64 Settlement Table 4: Final allowable bearing capacity for allowable settlement = 25 mm. and a given embedment depth To estimate settlement failure, an allowable settlement value is assumed (normally 25 mm or 1 inch). When soil settles more than that allowable value, the soil fails. So, even for a bearing capacity calculation, an allowable soil settlement is used and structural engineers should be
aware of that value while designing a footing. The allowable soil settlement value is typically an integral part of any soil report.
Why use the modulus of subgrade reaction It was previously stated that to design a flexible mat foundation, the modulus of subgrade reaction is used instead of bearing capacity of soil. But why is it so? The answer lies in the underlying assumptions of how a foundation might behave. Foundations can be rigid or flexible. Bearing capacity is used to design rigid foundations but subgrade reaction is used for flexible foundations. The very assumption of a rigid foundation is “that the distribution of the subgrade reaction p over the base of the foundation must be planar, because a rigid foundation remains plane when it settles” (3). Let us consider a simply supported beam loaded at center as shown in the figure 5‐a. By statics, we can obtain R1 = P/2 and R2 = P/2. If the same beam is loaded eccentrically, reaction can be calculated as shown in 5‐b.
Figure 5: Reactions for a simply supported beam
The same concept is extended for rigid foundation design. But instead of the end supports, the whole foundation is supported. It is also assumed that the relative stiffness of the concrete slab is much higher than the soil stiffness. So, the slab is assumed to remain planar even after the application of load. Figure 6‐a shows a footing loaded at the center. From a rigid wide beam analogy, P = R x L. Similarly for an eccentrically loaded footing the reaction will vary linearly from one end to the
P
R1 R2
P
R1 R2
La
R2 = P x a / L P = R1 + R2 R2 = P – R1
(a)
(b)
other as shown in figure 6‐c. Equations 3 and 4 can be solved to find end reactions. But none of the equations contain modulus of subgrade reaction (Ks). So, the “distribution of subgrade reaction on the base of a rigid footing is independent of the degree of compressibility of the subgrade”(4) it is resting on. As many authors concluded, a rigid foundation can be safely designed using bearing capacity as in most cases this method yields more conservative results.
12
… . 3
16
13
… . 4
Figure 6: Sub grade reactions for an isolated footing
But a mat foundation is often designed as a flexible foundation as it can be large in size and there may be many load application points and other complexities, such as holes and grade beams. Widespread availability of FEA software contributes to this trend. But a flexible foundation cannot have linear subgrade reaction unlike rigid foundations. Rather, it depends on the compressibility of the foundation as well as the structural rigidity. A flexible foundation will be subjected to internal bending and relative displacements between two slab points. The greater the structural rigidity is, the less the relative displacement. The author tested the case with very high elasticity of the slab elements and it resulted in a nearly planar surface after the application of the load. Similarly, the greater the modulus of subgrade reaction is, the less the pressure distribution. In other words higher Ks value will absorb more pressure at the load application point. Hence, the modulus of subgrade reaction —which is the function of soil settlement and the external pressure— is used for flexible foundation.
P
R
P
R2 R1
(a)
(b)
(c)
Correlation between bearing capacity and modulus of subgrade reaction The most common —and probably the safest— answer is that there is no correlation. But there should be one, as both are the measurements of soil capacities and any of these two parameters can be used to design a regular foundation. Let us look at the definition of Ks again, which is the pressure per unit settlement. So, in other words, soil capacity to withstand pressure for a given displacement. From earlier discussions, it is also clear that even bearing capacity has an allowable settlement. So, it is tempting to conclude that modulus of subgrade reaction is the bearing capacity per unit settlement. This conclusion is very similar to the equation presented by Bowles.(5)
: 40 /
: 12 / where SF = Safety factor and qa is the allowable bearing capacity. In the above equations, the allowable bearing capacity is first converted to ultimate bearing capacity by multiplying with a safety factor. The author assumed one inch or 25 mm settlement. The final equation is then formulated dividing the ultimate bearing capacity by the assumed settlement. The more generic form of the equation can be written as:
/
where I = Safety factor qa is the allowable bearing capacity is the allowable soil settlement
From above equations, it is evident that the appropriate safety factor must be used and the Ks value can be better compared with ultimate bearing capacity rather than the allowable bearing capacity. The safety factor can vary depending on projects and geotechnical engineers. The other important factor is the assumed allowable settlement for the calculated bearing capacity. .
However the above mentioned equations have its limitations. It can be applied to the footings where settlement failure governs but cannot be related to the footings where shear failure occurs before reaching allowable settlement limit. So, Engineers must exercise caution before using these equations.
Conclusion The correlation between bearing capacity and modulus of subgrade reaction is at best estimation. It can be used for estimation but Ks value determined by a plate load test should always be used if available or should be requested whenever possible. However, the above discussion gives insight into these values and helps engineers to understand the physical significance of modulus of subgrade reaction. References: (1), (2), (3), (4) Soil Mechanics in Engineering Practice (Third Edition) – Terzaghi, Peck, Mesri (5) Foundation Analysis and Design (Fifth Edition) – Joseph E. Bowles
By: Jignesh V ChokshiL&T Sargent & Lundy Limited, Vadodara
Procedure to Calculate Tributary Area and Vertical Spring Constants for
foundation modeled with soil as elastic supports in FE based programs
IntroductionIn Foundations, many times to estimate true behavior of mat; elastic property of soil is taken into considerationin FEM models. The base slab is divided into finite number of 2D plate elements representing the mat and thesupport condition is elastic based on the modulus of subgrade reaction of soil. Some popular FE programs donot include the facility of surface support and hence to model soil as elastic support, calculation of verticalspring stiffness at each node of the mesh becomes necessary.
The spring value shall be in Force/Unit displacement. The unit of Modulus of subgrade reaction is force/unitarea/unit deflection. Hence, when vertical spring constant is to be calculated for each joint, the tributary area ofmat at each node shall be calculated and then multiplied with the modulus of subgrade reaction. This procedureis simple for regularly divided meshes. However for most practical problems, it is a cumbersome process tocalculate the tributary area at each node and then calculate the spring constants. Also, after the analysis iscompleted, the output of such programs provides only vertical spring force in force unit. For determination ofactual base pressure, it becomes necessary to divide the spring force by the tributary area at each node.
ProcedureTo simplify the calculation of tributary area and vertical spring constant, a simple, accurate and quicker methodis suggested.
STEP 1 – Prepare the model with or without superstructure and the base modeled as 2D plate elements.STEP 2 – Copy the model and extract the mesh at base by deleting all members/elements above base level.STEP 3 – Provide PIN support at each node of the base and create two loading cases.STEP 4 – In Load case 1, apply surface load equal to the magnitude of modulus of subgrade reaction on thebase mat and in Load case 2, apply surface load equal to Unity on the mat.STEP 5 – Run the model for static analysis and print the support reactions.It is evident that, the vertical support reaction due to uniform surface load will be tributary area multiplied bythe surface load. This is nothing but the spring constant when the surface loading is equal to modulus ofsubgrade reaction and tributary area when surface loading is unity. Thus, we achieved the spring constant andtributary area at each node. In the original model export these values as spring supports by providingappropriate command. Retain the tributary area for future use.Alternatively, to calculate tributary areas from load case 1 only,STEP 5 – Extract the output of support reaction for load case 1 and import it in the Excel. Divide each reactionby modulus of subgrade reaction. This gives the tributary area at each node.STEP 6 – After the analysis of entire original model is completed; the support reaction will be spring force ateach node of base mesh. To obtain base pressure magnitude, divide each spring force by correspondingtributary area obtained in the STEP – 5.
Specifying/Defining Supports
We all know that without specifying/defining supports, STAAD.Pro will
not analyze a structure. So defining a support system is essential and
proper definition of support is utmost important to have actual
response (moments, shear, axial loads and torsions etc.) to provide
proper resistance against this response in designing or proportioning
members sizes.
STAAD.Pro provides facilities to assign support both parallel and inclined
orientation to global axes. The supports that can be specified are:
a. PINNED
b. FIXED
c. FIXED BUT
d. SPRING
From the definition we know that a pinned support can resist
translational movement but provides no resistance against rotation. A
fixed support can resist all type of movement in all directions i.e. a fixed
support have reactions for all forces and also for moments.In support
condition FIXED BUT is self definitive. There have facilities to make
release supports in all desired directions but against forces and
moments i.e. Fx, Fy,Fz,Mx,My,Mz and othersThe others contain a
translational springs and rotational springs. These springs can be
defined as spring constant.
Translational spring: (FX FY FZ )
The input data is translational spring constant. This can be defined as
the force required to displace a joint (supported by this spring) one unit
length in the direction specified by the user. The direction must be in
global axes.
Rotational spring (MX MY MZ)
The input data is rotational spring constant. This can be defined as the
force required to rotate a joint (supported by this spring) one degree
around user specified direction. The rotation must be referenced global
axes systemsFor specifying joints & directions, requiring enforced
support displacements this command is also used. Dear reader we will
publish springs in relation to mat foundation analysis in this blog very
soon.
03 SUPPORT OPTIONS
Please note the unit for fixed but is KN/m or Ton/m etc .wherein u
have to multiply sub grade reaction with influence area and if u go for
elastic/plate mat option than u have to give only the sub grade reaction
value which may be Kn/sq2/m et. Ton/sq2/m .so u can go for
either of this three which suits u.This is from STAAD help
menu. Whenever u have any doubt go to STAAD help menu. U have a
search option wherein u can get your doubts clari
The FOOTING option :
If you want to specify the influence area of a joint yourself and have
STAAD simply multiply the area you specified by the sub-grade
modulus, use the FOOTING option. Situations where this may be
appropriate are such as when a spread footing is located beneath a joint
where you want to specify a spring support.
The Elastic option :
If you want to have STAAD calculate the influence area for the joint
(instead of you specifying an area yourself) and use that area along with
the sub-grade modulus to determine the spring stiffness value, use the
Elastic otipon. Situations where this may be appropriate are such as
when a slab is on soil and carries the weight of the structure above. You
may have modeled the entire slab as finite elements and wish to generate
spring supports at the nodes of the elements.
The PLATE Option :
Similar to the Elastic optin except for the method used to compute the
influence area for the joints. If your consists of plate elements and all of
the influence areas are incorporated in the plate areas, then this option is
preferable
AUTOMATIC SPRING SUPPORT GENERATION FOR FOOTINGS
AND SLAB ON GRADE
INTRODUCTION:
STAAD.Pro foundation support generator
STAAD.Pro has a foundation support generator with the following
three options:
1. Footing
2. Elastic MAT
3. Plate MAT
In this blog, only the Footing and Plate MAT options will be
discussed.
Option 1: Footing
Suppose a 10' tall (12" x 12") square column placed on a (8' x 8') square
footing is to be modeled in the STAAD.Pro environment. The footing is
resting on soil with sub-grade modulus of 144 kip/ft3. The engineer would
like to model the soil as spring supports.
Figure 1: Physical and analytical models of a simple column on a footing.
This model can be easily created using the STAAD.Pro V8i
interface. To assign the supports:
1. Click on the General -> Supports control tab on the left.
2. Click on the Create button on the right. The Create Support
dialog box shown in Figure 2 will appear.
3. Select the foundation tab in the Create Support dialog box.
Figure 2: Create support dialog box.
Note that in this dialog box there are three options for generating
the foundation support. In this case, the Footing option will be
selected.
4. Populate the dialog box as shown in Figure 3.
Figure 3: Foundation Support Generator - Footing Option.
This footing has to be assigned to the stick model.
Calculations:
Area of the footing (A) = 10' x 10' = 100 sq.ft
Sub-grade Modulus (E) = 144 kip/ft3
Spring Constant (K) = (A) X (E) = 100 sq.ft X 144 kip/ft3 = 14,400
kip/ft For a column reaction load of P=10 kips, Support .in.
Figure 4 shows the displacement of node 3 in the STAAD.Pro
model. Please note that the displacement is same as what we have
calculated above.
Figure 4: Displacement at node 3.
The STAAD.Pro model is attached in Appendix A of this document.
Option 2: Plate MAT
Suppose a (80' x 80') square MAT foundation is to be modeled in the
STAAD.Pro environment. The MAT foundation is resting on soil with a sub-
grade modulus of 144 kip/ft3. The engineer would like to model the soil as
spring supports.
(Physical Model)
(Analytical Model)
Figure 5: Physical and analytical model of a simple MAT foundation.
This model can be easily created using the STAAD.Pro V8i
interface. To assign the supports:
1. Click on the General -> Supports control tab on the left.
2. Click on the Create button on the right. The Create Support
dialog box shown in Figure 6 will appear.
3. Select the foundation tab in the Create Support dialog box.
4. Populate the dialog box as shown in Figure 6.
Figure 6: Foundation Support Generator - Plate MAT Option.
This support has to be assigned to the all the plates that represent
the MAT foundation.
Figure 7: Foundation Support Generator - Plate MAT Option.
Figure 7 shows the influence area for node 342 which is
connected to four plates (4'x 4') square plates (shown with green
outlines).
Calculations:
Influence Area for node 342 (A) = 4' x 4' = 16 sq.ft
Sub-grade Modulus (E) = 144 kip/ft3
Spring Constant for node 342 (K) = (A) X (E) =16 sq.ft X 144
kip/ft3 = 2304 kip/ft
For uniform distributed MAT load of Q=0.2 kip/ft2,
Total force on Influence Area = Q X A = 0.2 kip/ft2 X 16 sq.ft =
0.32 kips Approximate Support displacement (d) = P/K = 0.32
kips/ 2304kip/ft X 12 in/ft = 0.017 in.
Figure 8: Displacement at node 342.
Figure 8 shows the displacement of node 342 in the STAAD.Pro
model. Please note that the displacement is same as what we have
calculated above. Figure 9 shows the influence area of node 342
in the STAAD.Pro output file. Please note that the influence area
of 16 sq.ft is same as what we have calculated above.
Figure 9: Influence Area of node 342.
The STAAD.Pro model is attached in Appendix B of this document.
Appendix A
STAAD PLANE
START JOB INFORMATION
ENGINEER DATE 19-Mar-09
END JOB INFORMATION
INPUT WIDTH 79
UNIT FEET KIP
JOINT COORDINATES
1 0 0 0; 2 0 10 0; 3 10 0 0; 4 10 10 0;
MEMBER INCIDENCES
1 1 2; 2 2 4; 3 3 4;
DEFINE MATERIAL START
ISOTROPIC CONCRETE
E 453600
POISSON 0.17
DENSITY 0.14999
ALPHA 5.5e-006
DAMP 0.05
END DEFINE MATERIAL
MEMBER PROPERTY AMERICAN
1 TO 3 PRIS YD 1 ZD 1
CONSTANTS
MATERIAL CONCRETE ALL
SUPPORTS
1 3 ELASTIC FOOTING 10 10 DIRECT Y SUBGRADE 144
LOAD 1 LOADTYPE None TITLE LOAD CASE 1
JOINT LOAD
2 4 FY -10
PERFORM ANALYSIS PRINT ALL
FINISH
STAAD PRO V8i
Syllabus:
Chapter- 1:
1. Introduction To Structural Engineering
2. What is a Structure?
3. About STAAD.Pro V8i
4. Getting Started
Chapter- 2:
1. Starting STAAD.Pro V8i
2. Methods Of Model Generation
3. Translational Repeat
4. Circular Repeat
5. Insert Node
6. Add Beam
Chapter- 3:
1. Run Structure Wizard
2. Generation Structure Models
3. Merging the Generated Model in STAAD.Pro
4. Importing CAD Models
Chapter- 4:
1. Support Specification
2. Support Page
3. Member Property
4. Member Offset
Chapter- 5:
1. Loading – 1
2. Loading – 2
3. Wind Load Generation
4. Assigning Wind Loads
Chapter- 5:
1. Analysis
2. Concrete Design
3. Time History Analysis
Chapter- 6:
1. Introduction to FEM
2. Plate
3. Surface
4. Meshing
Chapter- 7: (Slabs)
1. Desgin Of Slab
2. Design Of One Way Slab
3. Design Of Two Way Slab
4. Design Of Staircase
5. Design of Bridge using STAAD .Beava
Chapter- 8: (Bridge Deck Preprocessor Worked Example)
1. Bridge Deck Preprocessing Using STAAD.Beava
Chapter- 9: (Steel)
1. Design Of Steel Structures
2. Member Specification
Table Member Property
Chapter- 10: (Seismic Loads Worked Examples)
1. Calculate Natural Frequency of a Building By Response Spectrum Analysis
2. Calculate Natural Frequency of a Building By Rayleigh Method
3. Calculate Natural Frequency of a Building By Modal Shape
Chapter- 11: (Wind Load Intensity Worked Examples)
Calculate Wind Load Intensity In a Building
Chapter- 1:
1. Introduction To Structural Engineering
2. What is a Structure?
3. About STAAD.Pro V8i
4. Getting Started
1. Introduction to Structural Engineering
Structural Engineering is a field of civil engineering dealing with analysis and design of structures that support or resist loads. Structural engineering is usually considered a speciality within civil engineering, but it can also be studied inits own right. Structural engineering are most commonly involved in the design of buildings and large non-building structures but they can also be involved in the design of buildings and large non-building structures but they can also be involved in the design of machinery, medical equipment, vehicles or any item where structural integrity affects the item’s function or safety. Structural engineers must ensure their design satisfy given design criteria, predicated on safety or serviceability and performance. Buildings are made to endure massive loads as well as changing climate and natural disasters.
Structural engineers are responsible for engineering design and analysis. Entry-level structural engineers may design the individual structural elements of a structure, for example the beams, columns and floor of the building. More experienced engineers would be responsible for the structural design and integrity of an entire system, such as building.
Structural engineering depends upon a detailed knowledge of loads. To apply the knowledge successfully a structural engineer will need a detailed knowledge of mathematics and of relevant empirical and theoretical design codes.
2.What is a Structure?
A Structure can be defined as an interrelated or independent parts forming a more complex, unified whole and serving a common purpose. For instance, a building can be defined as a structural system designed and constructed to support and transmit applied lateral and gravity loads safely to the ground without exceeding the allowable stresses in its members. The super structure is the vertical extension of the building above the foundation. Columns, beams and load bearing wall support floors and roof structures. The substructure is the underlying structure forming the foundation of the building.
Types of Structures:
1. Space Structure:
Suitable for any arrangement of model geometry and loading. Allows three dimensional Structures. Allows loading in any direction. Allows deformations in all three global axes. Coordinate system tracks right hand rule.
2. Plane Structure:
Suitable only for two dimensional modes in x y plane with no loading or distortions upright to this plane.
All loads and distortions are in the plane of the structure.
3. Truss Structure:
Allows loading in any direction, but members only deliver axial resistance. Members dismiss resist bending or shear loads.
Allows three dimensional structures. Allows distortions in all three global directions. Coordinate system tracks right hand rule.
4. Floor Structure:
Suitable for two dimensional models in x z plane with loading and distortions perpendicular to this plane.
All loads and distortions are corresponding to the global y axis.
3. About STAAD.Pro V8i
STAAD.Pro V8i is the leading Structural Analysis and Design Software from Bentley. The
Letter “i” stands for intutive, interactive, incredible and interoperable. STAAD.Pro is the
professional’s choice for steel, concrete, timber, aluminium and cold formed steel
design of virtually any structures including culverts, pertrochemical plants, tunnels,
bridges, piles and much more. Bentley sounds V8i is the most complete and noteworthy
release in its history, which took a total investment of over a billion dollars and extents
across the vast array disciplines with fundamental subject and assignment endures to
be Sustaining Infrastructure.
STAAD.Pro is a overall resolution program for execution of analysis and design of a extensive variation of types of structures. The simple three activities which are to be carried out to accomplish that goal – a) model generation,
b) the calculations to obtain the analytical results and
c) result verification – are all simplified by tools enclosed in the program’s graphical environment.
This guidebook comprises three sample tutorials which guide the user to execute those 3 activities.
1. Graphical model generation services as well as text editor based commands for generating the calculated model. Beam and column members are characterized using lines. Walls, slabs and panel type objects are characterized by means of triangular and quadrilateral finite elements. Solid blocks are characterized by means of brick elements. These functions allow the user to generate the geometry, assign properties, orient cross sections as wanted, assign materials like steel, concrete, timber, aluminium, specify supports, apply loads obviously as well as have the program produce loads, design parameters etc.
2. Analysis engines for executing linear elastic and p delta analysis, finite element analysis, regularity in extraction, and response spectrum analysis & time history analysis.
3. Design engines for code inspection and optimization of steel, aluminium and timber members. Reinforcement designs for concrete beams, columns, slabs and shear walls. Design of shear and moment acquaintances for steel members.
4. Result inspecting, result confirmation and report preparation tools for Inspecting displacement diagrams, bending moment and shear force diagrams, beam, plate and solid stress contours, etc.
5. Exterior tools for actions like import and export of data from and to other broadly recognized formats, links with other general softwares for Place areas like reinforced and pre stressed concrete slab design, footing design, steel connection design, etc.
6. A library of visible utilities called Open STAAD which permits users to Right of entry STAAD.Pro’s internal tasks and practises as well as its graphical instructions to tap into STAAD’s catalogue and link input and output data to third-party software inscribed using languages like C, C++, VBA, FORTRAN, Java, etc. Thus, Open STAAD allows users to relation in-house or third-party presentations with STAAD.Pro.
4.0 Getting Started
STAAD.Pro V8i Application Window:
STAAD.Pro V8i screen is shown below. The screen has five major elements as shown below.
1. Menu bar.
2. Tool bar.
3. Page control.
4. Main Window.
5. Data Window.
In STAAD.Pro V8i:
Geometry is the “Elements of your Structure”. The Elements are given below:
Nodes Members (beams and columns) Plates (Slab, Walls and Raft Foundations) Surfaces (Slab, Walls and Raft Foundations)
Nodes:
Stiffed Joint with 6 reactions. It is located at each end of the Beam and each corner of the Plate Nodes considered the essence of the geometry of any structure in STAAD.Pro. Each node holds the following informations:
Node Number. Node Coordinates in XYZ space.
Beam:
Any member in the structure, that can be beam, column, bracing member or truss member. Beams are actually defined based on the Nodes at their ends. Each beam holds the following information:
Beam Number. The Node numbers at its ends.
Plates:
A thin shell with 4 node shaped element. It can be slab or wall element. Each plate will holds the following information:
Plate Number. Node Number at each corner of it.
Surface:
A thin shell in green color with mutli-nodded shape starting from 3 nodes and more. It can be anything of slabs, walls and raft foundations. It holds the following information:
Surface Number. Node Numbers at each corner of it.
Hardware Requirements:
The following requirements are suggested minimums. Systems with increased capacity provide enhanced performance.
PC with Intel-Pentium or equivalent. Graphics card and monitor with 1024×768 resolution, 256 color display (16 bit
high color recommended). 128 MB RAM or higher.
Windows NT 4.0 or higher operating system. Running it on Windows 95 & Windows 98 systems is not recommended as performance may be degraded. The program works best on Windows 2000 and XP operating systems.
Sufficient free space on the hard disk to hold the program and data files. The disk space requirement will vary depending on the modules you are installing. A typical minimum is 500MB free space.
A multi-media ready system with sound card and speakers is needed to run the tutorial movies and slide shows.
Chapter- 2:
1. Starting STAAD.Pro V8i
2. Methods Of Model Generation
3. Translational Repeat
4. Circular Repeat
5. Insert Node
6. Add Beam
1. Starting STAAD.Pro V8i
Creating a Project:
Once you stared the STAAD.Pro application follow the instructions:
1. In the Project Tasks box, click New Project.
2. A New Project dialog box appears is shown below:
3. Before starting a project, you must be aware of the type of structure. The structure
type can be defined as Space, Plane, Floor, or Truss.
Space: A SPACE structure, which is a three-dimensional framed structure with loads applied in any plane, is the most general. The loading causes the structure to deform in all 3 global axes.
Plane: The type of geometry, loading and deformation are restricted to the global X-Y plane only.
Floor: The geometry of structure is kept at the X-Z plane. Truss: The structure transmits loading by pure axial action. Truss members are
considered to be in capable of carrying shear, bending and torsion.
4. Set the length units and loading units and click Next button.
Note: The units can be altered later if needed, at any point of the model creation.
5. Now Where do you want to go? dialog box appears. You have specify the method for building
Add Beam: Sets the program in the Snap Node/Beam dialog and snap grid to construct your model by creating new joints and beams using the construction grid, drawing tools and spreadsheets.
Add Plate: Sets the program up with the Snap Node/Plate dialog to construct your model by creating new joints and 3-noded and 4-noded plate elements using the construction grid, drawing tools and spreadsheets.
Add Solid: Sets the program up with the Snap Node/Plate dialog to construct your model by creating new joints and 8-noded solid/brick elements using the construction grid, drawing tools and spreadsheets.
Open Structure Wizard: Opens the library of readymade structure templates which can be extracted and modified parametric model standard, parametric structural templates for trusses, surfaces, bay frames and much more.
Open STAAD.Editor: Allows you to build your model using STAAD syntax commands (non-graphical interface) through the STAAD editor.
Edit Job Information: Automatically opens the Job Information dialog box which provide information about the job (i.e. client’s name, job title, engineers involved, etc.) before building your model.
2. Methods Of Model Generation
STAAD.Pro V8i consists of three parts:
Pre Processor: Generates the model with all the data needed for the analysis. Analysis Engine: Calculates displacements, member forces, reactions, stresses,
etc.
Post Processing: Displays the results of the analysis and design.
Creating Nodes:
When you select the Nodes command in geometry menu, it shows a dialog box where you can enter the joint coordinates.
After creating the joint i.e. entering the coordinates, you can able to see the joint in the modelling area.
JOINT COORDINATES
i1, x1, y1, z1, (i2, x2, y2, z2, i3)
REPEAT n, xi,yi1, zi1, (xi1, yi2, . . . . xin, yin, zin)
REPEAT ALL n,xi1, yi1, zi1, (xi2, yi2, zi2, . . . . xin, yin, zin)
Enhanced Grid Tool:
The options in Snap/Grid Node tools in the geometry menu have been improved to
1. Allow multiple grids to be created.
2. Import a DXF file and use it as be created.
3. Import grid files created in different STAAD.Pro model.
Beams, plates and 8 nodes solid element can be created using the suitable Snap/Grid tool. When this function is propelled, the following dialog is opened which includes a Default Grid. This grid will be of type ‘linear’, there are also options to create Radial, and Irregular grids.
As new grids are added or modified, the information is stored in the STAAD.Pro data folder with a GRD allowance that permits other STAAD.Pro file to re-use these defined grids. To alter the starting of this grid, click on the Edit button to show the existing grid properties.
The current plane of the grid is set by selecting the required option. This can rotated about one of the global planes by selecting the axis of rotation and setting the angle.
The origin of the grid is marked on the graphics, with a small circle. The location of the origin, specified in global coordinates, can either be defined explicitly in the given X.Y and Z coordinates, or it can be set to the coordinates of an existing node by clicking on the icon and then on the node itself in the graphical window. Note that at this point the origin coordinate is updated.
The construction lines are used to specify how many gridlines are created either side of the origin, the spacing between the gridlines and if there should be a skew in degrees along either axis.
Click on the OK button to accept these settings.
Additional grids can be defined by clicking in the Create button. Three different types of standard grid can be created:
Linear Radial Irregular
The type of the grid required can be selected from the drop down list available at the top of the property sheet.
Each new grid should be identified with a unique name for future reference. The functionality for each type of grid is given below:
Linear:
Two dimensional system of regularity spaced linear construction lines creating a plane of snap points.
Plane is defined as being coincident with the global XY, XZ or YZ planes or at an angle skewed with respect to the global planes.
Location of the origin can be defined with respect to global X, Y and Z coordinates systems.
Radial:
Two dimensional system of regularly spaced radial and circumferential construction lines creating a plane of snap points.
Plane is defined as being coincident with the global XY, XZ and YZ planes or at angle skewed with respect to the global planes.
Location of the origin can be defined with respect to global X, Y and Z coordinates systems.
Well suited for drawing circular models using piece-wise linear techniques.
The settings for a Radial grid are defined in the following window:
The Plane, Angle of Plane and Grid origin option are as for the linear.
Irregular:
Two dimensional system of regularity or irregularly spaced linear construction lines creating a plane of snap points.
Plane is defined as being coincident with the global XY, XZ or YZ planes or at an angle skewed respect to the global planes or at an arbitrary plane.
The settings for an irregular grid are defined in the following window:
3. Translational Repeat
Translational Option allows to copy the entire structure or a portion of the structure in a linear direction. We may generate one or more several copies of the selected components. Select the structural elements to repeat. Select Geometry→ Translational
Repeat option from the geometry menu or Click Translational Repeat Icon . The Translational Repeat dialog box appears as shown below:
Global Direction:
Choose any one of the three possible global direction along which the selected structural elements should be copies.
No of Steps:
Specify the numbers of steps to repeated you need.
Default Step Spacing:
Type the default spacing between steps in the edit box in current length units. For each step, the default value of the spacing will be what we provide in the Default step spacing box. We can change the spacing of individual steps if we choose to do so.
Step Spacing Table:
This table consists of two columns: Step and Spacing. We can change the spacing of any type in the table.
Renumber Bay:
This is the way of instructing the program to use a user-specified starting number for the members generated in each step of the translational repeat activity.
Geometry Only:
The Translational Repeat allows the copying of the elements without having their loads properties, steel design parameters, etc. being copied with it. By default (when the Geometry Only option is not checked) all loads, properties, design parameters, members releases, etc. on the selected elements will automatically be copied along with the elements. By checking the option labelled Geometry Only, the translational repeating will be per formed using only geometry data.
Link Steps and Open Base:
If you want to automatically connect the steps or copies by new members, along the specified global directions, check the Link Steps check box. In other words, the Link Steps option is applicable when the newly created units are physically removed from the existing units and when one wishes to connect those using members. To avoid joining the base of the copied structures, check the Open Base box.
Here you can see the Frame model copied using the Translational Repeat option:
4. Circular Repeat
Circular Repeat allows to copy of the entire structure on an portion of if in a circular direction. Select the structural elements to repeat and select the Circular Repeat option from the geometry menu. The 3D Circular dialog box appears as shown in the figure.
Axis of Rotation:
Click the radio button to choose the axis of rotation for repeating the selected components.
Through:
The new highlight node button selects the Node on Plane. Click on this icon to be able to select the node from the main model. Once the cursor changes the shape, simply select a node from the model. The Node and Point boxes will automatically fill up with the correct information. Otherwise, type an existing Node number or location Point coordinates to define the axis of rotation.
Use this as Reference Point for Beta angle generation. In previous versions of STAAD.Pro, one limitations of the Circular Repeat feature was that the member orientation was not taken into consideration during the circular generation. This limitation has been addressed now.
If the Use this as Reference Point for Beta angle generation switch is turned on, the point through which the axis for circular repeat operation passes will be used as the member reference point for all the generated members. This point along with the local X axis of the generated member will define the local X-Y plane of the member and hence the member orientation gets automatically set.
Total Angle:
Provide the total sweep angle of rotation between the original structure and the last copied structure.
No of Steps:
Provide the number of steps we want over the specified Total Angle.
Link Steps and open Base:
If you want to automatically connect the steps by new members, check the Link Steps check box. To avoid joining the base of the copied structure, check the Open Base box.
The Circular Repeat. Rotate and Mirror dialog boxes have been enhanced to remain open so that the selection beams, nodes, etc. can be accomplished even while the box is open. Also, selection of critical points such as the node, point or plane where the axis of rotation crosses can now be selected graphically while the box remains open. This eliminates the inconvenience in the past where if this location was known before selecting one of the geometry options, the box had to be closed down to determine the location first.
1. Select the objects to be copied.
2. Click Geometry→ Circulation Repeat.
3. In 3D Circular dialog box, select the Axis of Rotation and Point or Coordinate of Axis.
4. Type the Total angle and No of Steps.
5. Click OK.
5. Insert Node
This facility allows the user to insert node on an existing member. The member is split into the corresponding number of segments with automatic generation of node and member numbers, member properties and loads.
If you choose this option, the Insert Node cursor appears. By using that cursor, you can select the member to split. The Insert Node dialog appears, as shown below:
Beam Length:
This lists the distance from node A to node B along the beam to be split.
New Insertion Point:
Provide the Distance from the start node of the member in current length units. Alternatively, provide Proportion of the total length of the member to position the new node. Click Add New Point to add the node.
Add New Point:
After providing the Distance or the Proportion, click on the Add New Point to add the node.
Add Mid Point:
To split the member into two segments, click on this button.
Add n Points:
To divide the beam in a number of equal segments, provide the number of intermediate points in the n = edit box and click on Add n Points. Note that this value should be an integer.
Insertion Points:
The locations of the newly created points are listed in this list box, shown as the distance from the start node of the member, To accept the new nodes that appear in the Insertion Point list box, click the OK button.
Remove:
To remove a node from the list of inserted nodes, highlight the desired node and click on this button.
Enhancement of Insert Node Operation:
Users can now select multiple members and split the members at a given fractional position or a specified distances from the starting node positions. The new feature will enable the users to perform the operation in one sight command which will reduce the modeling time.
New point by distance:
Specify the distance in current length units at which the beam is to split. The value for the distance is entered in the Distance edit box and is measured from the start node of the beam.
New point by proportion:
This option allows the users to specify the distance in terms of a ratio. For example, to split a beam at the midpoint, enter 0.5 as the proportion .To split the beam at quarter points, use a proportion value of 0.25.
Add mid point:
The beam are split at their midpoints.
Add ‘n’ points:
To split a beam by inserting ‘n’ number of points, use this option. The beams are split up into n+1 segments.
6.Add Beam
This option in geometry menu allows you to add members by connecting existing nodes. Choosing this option brings up the following sub-menu.
Add Beam from Point to Point:
In prior versions to STAAD.Pro, the Add Beam option was a facility for adding a beam between two existing nodes. This has now been extended to be able to create beams from nodes that have not been previously defined. The nodes can now be dynamically generated at the time of creating the beam similar to the way beams are created using the Snap/Grid Beam command.
To create a beam dynamically without the start and end nodes defined, go to Geometry| Add Beam |Add Beam from Point to Point from the main menu. The Add Beams cursor appears. Click on any point on the existing beam where the starting node of the new beam will lie. if an existing node is not present at that point, a dialog box will prompt for a new node to be created.
Click on Yes to create a new node. The Insert Nodes dialog box will prompt for the exact location where the nodes is to be created. once the desired node or nodes have been input that box, click on the OK button to generate the new nodes on the selected beam. If the new node input is not within a close proximity of the point clicked on the screen, no “draggable”line will be shown. Click on the new node to start the creation of the beam. Then, drag the mouse to another existing node location or repeat the same steps again to dynamically create another new node.
Chapter- 3:
1. Run Structure Wizard
2. Generation Structure Models
3. Merging the Generated Model in STAAD.Pro
4. Importing CAD Models
1. Run Structure Wizard
The Run Structure Wizard option offers a library of ore-defined structure prototypes, such as Pratt truss, North light Truss, cylindrical Frame, etc. We may parametrically generate a structural model and then transfer and superimpose it on the current structure.
When we select the Run Structure Wizard option from the Geometry menu, the Structure Wizard window appears as shown below.
The Protype Models and Saved User Models options on the top of the left side of the screen. If the Prototype Models option is selected, the Model Type will list the types of prototype structure available as shown below. If the Saved User Models option is selected, the Model Type will display the list previously done and saved models by the user.
Adding and Deleting items to the library:
Items can be deleted or added with certain settings from and to the list. The modified item list can be saved in different files and called when requires. In brief , the item list is customizable.
To insert any customized item under any Model type, select that Model Type and click the mouse at the bottom of the same pane. Right-click the mouse and from the context menu, select Add Plug-in and you can load the corresponding “.dll” file. We can also delete a particular structural item by selecting that particular item and by clicking the Delete Model Plug-in from the context menu. A structural item under any Model Type may be renamed by using Rename Model Generator from the context menu.
The customized list of the Prototype can be saved in different files. By default, STAAD.Pro/Structure Wizard uses the default .STP file. We can save any changes in this file. Also changes can be saved in any file other than default .STP. To save the changes, select Save As…. from the File menu in the Structure Wizard window. Provide the path and name of the .STP file and press OK.
To open any .STP file to use the customized Structure Libraries, select the File| Open menu option from Structure Wizard main menu. Specify the path and name of the .STP file and press OK.
Use the View, Zoom, Pan and Rotate icons to change the orientation of the model.
2.Generation of Structure from Models
In this section, the process of generating a structural model and combining it with the existing STAAD.Pro structure will be explained using a Howe Roof Truss. Follow these steps to create the other truss types also.
Selection of Unit:
The unit of the length should be specified before the generation of a model. From the File menu, click Select Unit and the Select Unit dialog box will appear as shown below. We can select any unit of length from Imperial or SI/Metric system of units.
Model Type: Truss
Select the Howe Roof structure type under model type Trusses. Drag the item into the right side window and release the button. The Select Parameters dialog box will appears to specify the Truss parameter as shown below:
After defining the parameters click Apply and the prototype truss will appears with the X, Y and Z axes on the screen.
Right click in the right side window containing the generated model. The context-menu will display the options Change Property, Scale and Delete. We can edit the value of the parameters by clicking the Change Property, which will pop-up the select Parameters dialog box. Enter the length, height and width of the truss and the number of bays along those directions. To modify the spacing of individual bays, click the browse button and in the dialog box that appears, type new spacing and click OK. Click the Apply button to parametrically generated the truss model. Click Close to finish.
We can re-scale the model in X, Y and Z directions separately using Scale from the context menu. You can also delete the particular model by clicking Delete from the context menu.
3.Merging the Generated Model to STAAD.Pro
Select the Merge Model with STAAD.Pro sub menu from the File menu to combine the generated model to the current STAAD.Pro structure.
The structure Wizard window will now close. In the STAAD.Pro window, the Paste Prototype Model dialog box will appears., in which we can type the shift of the origin of the Structure Wizard model from the origin of the STAAD.Pro axis system or we can type coordinate of the node of the STAAD.Pro structure with which we can want to connect the Structure Wizard model or click on the Reference Pt button to connect the node of the existing structure in STAAD.Pro with the Structure Wizard model by clicking on the joints where they will be connected. Click OK to finish.
In the Frame Models Continuous Beam, Bay Frame, Grid Frame and Floor Grid have similar parameters in the Select Parameter dialog box. Type values for Length, Height & Width and number of bays for each. To modify the spacing of the bays, click the browse button and in the dialog box that appears, type new spacing and click OK. Click the Apply button to the parametrically generated model.
The Cylindrical Frame, Reverse Cylindrical Frame and Circular Beam have similar Parameter in the Select Parameter dialog box. Type values for Length, Radius, Angle and number of bays along length and periphery. To modify the spacing of bars, click the browse button and in dialog box that appears, type new spacing and click OK, Click the Apply button to parametrically generate the model.
4.Importing CAD Models
This feature can import CAD models, has two separate utilities, Scan DXF and STAAD Models.
Scan DXF:
If the geometry of the model is created using the drawing program like AutoCAD and saved in a DXF file format, it can be imported using this option. After dragging the Scan DXF icon into the right side window, a Open dialog box appears and noe=w locate the “DXF” file, which we want to open, select that file and press OK. This feature supports the limited number of CAD entities like Line, 3D-Polyline and 3D-Face.
STAAD Models:
This allows the geometry of the previously created model to be imported and altered. After dragging the STAAD Models icon into the right side window, an Open dialog box will appear. Now locate the “STD” file which you want to open, select that “STD” file and press OK. The geometry from that STD file will be imported. That model can be scaled up or down along the global X, Y and Z directions by clicking the right mouse button, choosing the Scale option and provide the desired values.
Chapter- 4:
1. Support Specification
2. Support Page
3. Member Property
4. Member Offset
1.Support Specification
This allows the user to define the support conditions of the structure by providing fixed, pinned, roller, inclined, spring supports, etc. Supports can defined and assigned from the General| Support page also. This menu option is used to specify the supports on the structures. The Support Specification menu offers several sub-menu options, as follow.
Click Commands→ Support Specifications.
Pinned:
This allows user to create the pinned support tag and assigned it to the selected nodes. A pinned support is restrained in all three translational degree of freedom and free in the 3 rotational degrees of freedom.
Fixed:
This allows the user to create a fixed support tag and assign that to the selected nodes. A fixed support is restrained in all 6 degree of freedom.
Fixed But / Spring:
This allows the user to create various types of roller, hinge and spring support with specified restrained degrees of freedom and to assign them to selected nodes.
Enforced:
The Enforced support is the same the fixed support except that the restrained degrees of freedom are defined in terms of being stiff springs. Enforced supports are identical to the ‘FIXED’ type of supports in most respects. The real advantage of using the ENFORCED type lies in the fact that is enables STAAD to accept loads such as support displacements loads in case of plates and solids. Support displacement loads are not permitted for plates and solids if the FIXED support type is used. So, for structures without these characteristics, the FIXED type of support offers the same level of functionality as the ENFORCED support type.
Enforced But:
Enforced But support type is the same as the “Enforced” support except that we have the choice on the degrees of freedom we wish to restrain. For example, we can select Enforced But and restrain just the FX, FY and FZ degree of freedom and let the remaining 3 free to deformation.
Inclined:
This allows the user to create supports that restraints in an axis system that is inclined with respect to the global axis system. There are two aspects defining the inclined supports:
The reference point which inclined axis system. The restraints, releases and springs.
Foundation:
To define a spring support for an isolated footing, click the Footing radio button. Provide the dimension of the footing in current units settings and choose the Direction of the spring action. Provide the soil Sub-grade value in the edit box. Click the Add button to add the foundation support tag to the structure or click Assign to assign this support to selected nodes.
Elastic Mat: In this method, the area is calculated using a Delaunay triangle principle. Hence the candidates for this options are nodes which define the mat. To achieve best results, one needs to ensure that the contour formed by the nodes form a convex hull.
Plate Mat: If the foundation slab is modeled using plate elements, the spring supports can be generated using an influence area calculated using the principles used in determining the tributary area of nodes from the finite element modelling standpoint. Hence the candidates for this option are the plates which define the mat. When the mat is modeled using plates. this produces superior results than the ELASTIC MAT type.
2.Support Page
When the General | Support Page is opened, a Supported Nodes tables and a Supports dialog box appears in the data area. We may specify supports in two ways. We must first create Support Specification and then select the nodes to which this support is to be attached to. Alternatively, we may first select the nodes and then specify a support to be assigned to the selected nodes. In second case, a new Support Specification is created along with a support reference number. Also note that the Assign button become active if we have already selected the nodes to which the support is to be applied.
Supported Nodes Table list all nodes for which supports have been defined. The type of support is also displayed. The Supports dialog box allows us to define supports and assign them to nodes. All supports that have been defined for the model are listed in the Supports dialog box.
Create:
The Create button is for creating the supports to be applied on the structure. When you click this button Create Support dialog box appears.
Edit:
For certain types of supports, the parameters of the support can be modified after the support is created. The Edit button is available for that purpose. To do this, first select that support type from the list. Click on Edit and dialog box corresponding to that support will be re-displayed, allowing for changes to be made.
Delete:
Use this button to delete a previously assigned support.
Assignment Method:
The options under the Assignment Method offer different choices for assigning supports to the structure.
Assign To Selected Nodes:
To assign a support to selected nodes, first select the support from the supports dialog box. The support selected is highlighted. Then select the nodes to which this support is to be assigned. When all the desired nodes are selected, click the Assign To Selected Nodes radio button, then click the Assign button.
Assign To View:
To assign a support to all free nodes in a view, first select the support from the Supports dialog box. The selected supported is highlighted. Select the Assign To View radio button, then click the Assign button. All free nodes in the structure are assigned this support after getting the confirmation.
Use Cursor To Assign:
To assign a support to nodes using the cursor, first select the support from the Supports dialog box. The selected support is highlighted. Select the Use Cursor To Assign radio button, then click the Assign button. The button will appear depressed and label will change to Assigning. Make sure that the Nodes Cursor is selected so that we can select the nodes. Using the cursor, click on the nodes to which this support is to be assigned. Click on the Assign button again to finish.
Assign To Edit List:
To assign a support using a typed list of node numbers, first select the support from the Supports dialog box. The selected support is highlighted. Select the Assign To Edit List radio button, then type the list of node numbers and click the Assign button.
3.Member Property
This allows the user to provide the cross sectional properties of members with or without the material specification. The same options can be gained access from the General | Property page. The Member Property menu option is used to create the property tag and then assign the specified property tag to select members through the Property Page. Alternatively, we may first select members and then define the member property to be assigned to these members.
The Member Property menu offers several sub-menu options as shown below:
Prismatic:
This allows the user to assign Circular, Rectangular, Tee, Trapezoidal, General, etc. Cross sections to the frame members.
When we select the Prismatic option, the Property dialog box appears as shown below. Also note that the Properties dialog box also opens simultaneously letting us utilize some of the other operations available from that dialog box.
Material: Check this box and select the material from the drop down list if the new member property tag should include the materials constants.
Circle: To define a circular section, click on the Circle tab as shown in the previous figure. Enter the section diameter YD and select the material.
Rectangle: To define the rectangle section, Click on the Rectangle tab. Enter the height YD and width ZD of the section and select the material.
Tee: To define a tee section, click on the tee tab. Enter the height YD, width ZD stem height YB and stem width ZB and select the material.
Trapezoidal: To define a trapezoidal section, click on the Trapezoidal tab. Enter the height YD, top width ZD, bottom width ZB and select the material.
Tapered Tube: This allows the user to specify a I-section having a varying depth over the length of the member by using 7 parameters as shown below:
4.Member Offset
The beams and columns of structure are characterized by lines in the computer model. In the actual structure, a beam spans distance which in the clear span between the faces of columns. But in the computer model, the line for the beam spans among the centerlines of the column. The half depth portion of either column is significantly stiffer than the beam itself from the stand point of bending. To take benefit of this extra stiffness, we may affirm that the start and end faces of the beam are offset from the node by a distance identical to the half-column-depths.
Member offsets can be specified in other situations too. Examples are
When a bracing member does not meet the node which is defined in its incidence list.
A girder and top slab in the bridge where the centerline of the girder is several inches below the centerline of the slab.
This facility becomes very useful when the user wants to have the structural parameters of a member viz. shear force, bending moment by considering the clear distance of the member between the supports. This facility can accessed from the General | Specification also. When you select the offset menu option in the command menu, the Member Specification dialog box appears as shown below.
Location:
Location defines the offset end of the member. Start is the starting point of the member and End is the Ending point of the member. Start and End depends on the Member Incidence of the member. Selecting one of these options defines the member offset to be at the start point or at the end point of the member.
Direction:
Choose the Local for assigning the offsets in the local axis system. Otherwise, choose the global axis system.
Offsets:
Type the offset distance from the joint in the three global directions. Click the Add button to add this specification to the structure or click Assign to assign the specification to selected member as well as add this specification to the structure.
Chapter- 5:
1. Loading – 1
2. Loading – 2
3. Wind Load Generation
4. Assigning Wind Loads
Loading – 1
In STAAD.Pro V8i, loads in a structure can be detailed as Dead load, Live load, Wind load, Snow load, Seismic load, temperature load and fixed-end member load. STAAD.Pro V8i can also calculate the self-weight of the structure and make it as uniformly distributed loads (UDL) in analysis. Self-weight of the members can be applied in any desired direction.
Click Commands→ Loading→ Primary Loading.
Now the Create New Definitions / Load Cases / Load Items dialog box appears. Now you have to define the loads, then click Add button.
Dead Load or Self-weight:
Self-weight of all active members of the structure are calculated and applied as a uniformly distributed load. Please note that the property of the member must be defined before this command used.
Direction- Specify the direction in the self-weight load is to be applied by clicking on the X, Y or Z buttons.
Factor- Specify the factor with which the calculated self-weight are to be multiplied. A negative value indicates that the load is applies along the negative direction of the selected axis.
Nodal Load:
Nodal loads is the combination of forces and moments, it may be applied to any free node of a structure. These loads act in the global coordinate system of the structure. Two options are available under Nodal Load: Node and Support Displacement. Positive value forces acts in the positive coordinate directions of the axis.
Member Load:
The Member Load tab allows the user to apply loads on the span of frame members.
Concentrated Load:
To specify a concentrated force or moment, click the Concentrated Force or Concentrated Moment tab. The data items are explained below.
Linear Varying Load:
The load is applied over the entire length of the member, varies with respect to the distance.
Loading – 2
Area Load: This allows the user to apply load over area, which will be distributed on surrounding beams based on the one way distribution. This load is a one-way distributed pressure load on members that circumstances a panel. Enter the value of area load in current units. This load always acts along the positive local y direction on the two longest member on each panel.
Note: Area load should not specified on members declared as Member Cable, Member Truss or Member Tension.
Floor Load: User can apply the load over the panel, which will be distributed on surrounding beams based on a two-way distribution. This load is two-way distributed pressure load on members that circumscribe a panel. The data items are explained below:
Load – Floor load value in the current units. This load will act parallel to the global vertical axis.
Direction – The floor may be considered as acting perpendicular to plane of the panel on which it is defined. This is normal load static condition.
Range – Define X Range/ Y Range/ Z Range. Specify the location of the floor using the Define X Range option. The load will be calculated for all members lying between this range.
One Way Distribution – Check the box for one way distribution to get a one way type distribution of the pressure. In such cases, the program find out the shorter side of the panel. It then divides the load in between the long direction beams. No load is generated by this option if the panel is square in shape.
Plate Load: The Plate Load tab allows the user to apply elements loads. The Plate Load tab offers several sub-menu options as shown below.
Pressure On Full Plate:
Load – W1 is the variable using which the pressure value is defined, in pressure units.
Direction – The load may be applied along the local Z – axis, or along one of the global X, Y or Z – axis (GX, GY, GZ)
Concentrated Load:
Use this option to define a concentrated load that acts on specific point within the boundary of the element. If a load acts at a node point of an element, it is advisable to apply it using the Nodal Load option described in earlier pages.
Load – The magnitude of load is specified in the box alongside Force. X and Y define the location of the load, in terms of the distance from the origin of local X and Y axes of the element.
Direction – The load may be applied along the local Z-axis, or along one of the global X, Y or Z – axis (GX, GY, GZ).
Partial Plate Pressure:
To Specify a uniform pressure on the entire element or a non user specified portion of the element, use this facility. The data items are explained below:
Load – The element pressure (force per unit area) or Concentrated load (force unit). For concentrated load the values of X2 and Y2 must be omitted, while X1 and Y1 must be specified.
X1, Y1, X2, Y2 – For element pressure (force per unit area), these values represent the coordinates of the rectangular bpundary on which the pressure is applied. If X1, Y1, X2 and Y2 are all zero; the pressure is applied over the entire element. If X1 and Y1 are specified but X2 and Y2 are omitted, then W1 is treated as concentrated load.
Direction – GX, GY, GZ represent the global X, Y and Z direction along which the pressure may be applied Local Z indicates that the pressure is applied normal to the element in the local Z direction.
Trapezoidal: To specify a trapezoidal varying pressure load on a plate, select the Trapezoidal tab. The load is applied over the entire element in the local Z direction, varying along the positive local X or Y direction. The data items are explained below.
Direction of Pressure – GX, GY and GZ represent the global X, Y and Z direction along which the pressure may be applied. Local Z indicates that the pressure is applied normal to the element in the local Z direction. Enter the pressure intensity F1 at the lowest local coordinate location (start) and the intensity F2 at the highest local coordinate location (end), Start and End are defined basd on the positive direction of the local X-axis or local Y-axis.
Variation along element – Define the direction in which the pressure varies as either the local X ot Y direction or Choose the joint option, which is discussed next.
Joint – Check the joint option to apply different value of pressure at different nodes of the plate element. When checked, the dialog box will change as shown below. Apply different values of pressure in the edit boxes for the different nodes.
Hydrostatic: To model loads due to hydrostatic pressure on one or more adjacent elements, select the Hydrostatic tab. The hydorstatic load is converted to Trapezoidal
loads on the elements. The load is applied over the entire area of the element. The data items are explained below:
Force – Enter the value of the load at the minimum and maximum global axis in current units. For example, to model a retaining wall with soil pressure, W1 is the force at the bottom of the wall and W2 is the force at the top of the wall.
Interpolate along Global Axis – Specify the global axis (X, Y or Z) along which the load vary from W1 to W2. For example, the load would vary along the Y – axis on a vertical retaining wall.
Select Plate(s) – Unlike the load definition options, we must select plate(s) for this option to became active. Click on this button to select plate(s). Click on the Select Plates button. A dialog box will appear. Select all the plates of a wall on which we wish to apply hydrostatic load. Click on Done. The hydrostatic dialog box will re-appear.
Direction of pressure – Specify the direction of design pressure as Local Z axis or global axes (GX, GY or GZ) and click on Add. This will assign a linearly varying hydrostatic load on all the selected elements.
Element Joint Load: To specify a varying pressure at each joint on a plate, select the Element Joint Load option. The data items are explained below.
Joint Load Data – Choose Three Noded Facet / Four Noded Facet depending on whether the plate element is 3 noded 4 noded.
Direction – The load may be applied along the local Z – axis or along one of the global X, Y or Z – axis (GX, GY, GZ)
Add – After defining a load, click the Add button to add this under current load case in the Loads dialog box.
3.Wind Load Generation
The wind load generation is a utility, which takes place as an input wind pressure and height ranges over which these pressures act and generates nodal point and member loads.
This facility is available for two types of structures:
1. Panel type or closed structures. 2. Open structures.
Closest structures are ones like where non-structural entities like glass facade, aluminium sheets, timber panels or non-load bearing walls act as an obstruction to the wind. If these entities are n and of included in the structural mode, the load generated because of wind blowing against them needs to be computed. Therefore, the steps involved in load generation for such structure are
1. Identify the panels – regions circumscribed by members so that a polygonal closed area is formed. The area may also be formed between the ground level along one edge and members along other.
2. Calculate the panel area and multiple it by wind pressure. 3. Convert the resulting force into nodal point loads.
Plates and solids are not considered in the calculation of the panel area. Openings within the panels may be modeled with the help of exposure factors. An exposure factors is associated with each joint of the panel can be reduced or increased.
Open structures are those like transmission towers, in which the region between members is “Open” allowing the wind to blow through it. The procedure for load generation for open structures is
1. Calculate the exposed area of the individual members of the model. 2. Multiply that exposed area by the wind pressure to arrive at the force and apply
the force on the individual members as a uniformly distributed load. It is assumed that all members of the structures within the specified Rangers are
subjected to the pressure and hence, they will all receive the load. The concept of members on the Windward side shielding the members in the inside regions of their structures does not exist for open structures.
At a large structure may consist of hundreds of panel and members, the user with the help of this facility can avoid a considerable amount of work in calculating the loads.
The wind load menu option allows the user to define the parameters for automatic generation of wind loads on the structure.
STAAD.Pro V8i is now capable of generating the wind pressure profile for a structure in accordance with the ASCE-7-95 as well as the ASCE-7-02 codes. The pressure profile is the table of values of wind intensity versus height above ground.
The calculated pressure may then be applied on the structure to compute loads on the member using the in-built program’s wind load generation algorithm for the closed as well as open-lattice type structures.
When the wind load B&B of my menu option is selected, the new wind type dialog box appears, as shown below.
Enter the ‘Type No.’ which denotes the number by which the wind load type will be identified. Multiple wind types can be created in the same model. Click on the Add button within this dialogue box and then click on close.
The newly created TYPE 1 wind definition will appear underneath wind in the Load dialogue.
Select the TYPE 1 name in the tree control and click on the Add button. The dialogue box shown below will prompt for the pressure profile for this wind definition.
As we said earlier, the pressure profile is the table of wind intensity versus height above ground. If we know that, that information can be typed into the box above.
To calculate the wind intensity, use the following formula from IS 875-Part 3.
Vz = Vb k1 k2 k3 and pz = 0.6 Vz2
where, Vz = Design wind speed at any height
Vb = Base wind speed.
k1 = probability factor.
k = terrain, height and structure size factor.
k3 and = topography factor.
pz = design wind pressure.
Exposure:
The exposure tab is used to modify the influence area of wind load associated with particular joints in the structure. By default, the exposure factor is 1.0, thus the wind force is applied on the full influence area associated the joints. Click on Add to add this load under the current load case in the load dialogue box.
4.Assigning Wind Load
This tab allows the user to apply previously created wind load type on the structures through the means of a load case. If the model already contains previously defined wind load cases, a dialogue box resembling the one shown will appear.
Select type:
Choose a previously defined wind load type from the drop down list.
Direction:
Specify the global direction in which the wind load is to be generated by clicking the X, Z, -X or –Z radio button. When wind is generated in X direction, the wind load is applied on the near side and when –X is chosen the load is applied on the far side. Generation in Z or –Z also works the same way.
Factor:
Specify the factor to multiply the calculated wind loads.
Open structure:
By default, the load generation is based on the assumption that the region between members is covered by panels. To generate loads on open structures like highway signs or transmission towers, switch on this box. The members are selected and X is used and the factor is positive, then the exposed surface facing in the –X direction will be loaded in the positive X direction. If X and a negative factor, then the exposed surface facing in the X direction will be loaded in the negative x direction. If –X is entered and a positive factor, then the exposed surfaces facing in the +X direction will be loaded the positive X direction.
Chapter- 5:
1. Analysis
2. Concrete Design
3. Time History Analysis
Analysis
STAAD.Pro V8i offers STAAD engine for general purposes structural analysis and design. The modelling mode of STAAD environment is used to prepare structural input data. After the analysis is performed, used the menu option File→ View→ Output File→ STAAD Output to view the output files.
The STAAD Analysis engine perform analysis and design simultaneously. However, to carry out the design, the design parameters too must be specified along with the geometry, properties, etc. Before you perform the analysis. Also, note that you can
change the design code to be followed for design and the code check before performing the analysis/design.
Perform analysis:
To do the analysis must be need to add the command from Commands→ Analysis→
Perform analysis…
This allowed the user to specify the instructions for the type of analysis to be performed using STAAD engine. In addition, this command may be used to print various analysis-related data such as load information, statics check information, mode shapes etc.
The analysis menu offers several sub menu options. When you select one of the analysis commands, you may specify the analysis-related data to be printed in the STAAD output (.ANL) file by selecting the print option radio buttons, explained below:
Load data: print all the load data.
Statics check: provides summation of the applied load and support reaction as well as summation of moment of load and reactions taken around the origin.
Statics load: print everything that statics check does and summation of all internal and external forces at each joint.
Mode shapes: print mode shapes values at the joints or are calculated mode shapes.
Both: this option is equivalent to the load data plus statics check option.
All: this option is equivalent to load data plus statics data.
Run Analysis:
The Analysis is performed under the commands under the analyse menu in the Modelling Mode. Select the Run Analysis option to perform Analysis/Design.
The Analysis Status dialog box appears:
This dialog box displays the status of the analysis process. If an error occurs during the analysis, the above dialog box displays the error message.
View Output File: it will invoke the STAAD viewer with the analysis results presented in a textual format.
Go to Post Processing Mode: it will take you to the STAAD.Pro Post processor where you can graphically.
Stay in Modeling Mode: it will keep you in Modeling environment.
During the analysis, an output file is generated. This file may contain selected input data items, results and error messages. To include a report of the input data items in the output file, use the menu options under Commands | Pre Analysis Print. The generated output file may be viewed using the menu option File | View | Output File | STAAD Output.
Any errors that occur during the analysis process may be viewed using the menu option File→ View→ Output File.
Concrete Design
STAAD has the capabilities of performing concrete design on limit state method of IS 456 (2000).
Beam Design:
Beams are designed for flexure, shear and torsion. If required the effect the axial force may be considered. For all these forces, all active beam loadings are pre-scanned to identify the critical load cases at different section of the beams.
Column Design:
Columns are designed for axial forces and biaxial moments at the ends. All active load cases are tested to calculate reinforcement. The loading which yield maximum reinforcement is called the critical load. Column design is done for square, rectangular and circular sections. By default, square and rectangular column and designed with reinforcement distributed on each side equally for the sections under uni-axial moment.
Design Parameters:
The program contains several parameters which are needed to perform design as per IS 456 (2000). Default parameter values have been selected that they are frequently used numbers for conventional design parameters. These values may be changes to suit the particular design performed.
Performing Concrete Design:
1. Click Commands→ Design→ Concrete Design.
2. Now the user can specify the design parameters for the structure.
3. Concrete Design dialog box appears.
4. Click the Select Parameters button. Now you can select the desired parameters for the concrete design.
5. Click Ok. Then Click Define Parameters button, now you can define the parameter
Important Parameters To Be defined Are Given Below:
Parameter Name Default Value Description
FC 25000 KN/mm2 Concrete yield strength.
FYMAIN 415000 KN/mm2 Yield stress for main reinforcing steel.
FYSEC 415000 KN/mm2 Yield stress for secondary reinforcing steel.
MAXMAIN 60 mm Maximum main reinforcement bar size.
MAXSEC 12 mm Maximum secondary reinforcement bar size.
MINMAIN 10mm Minimum main reinforcement bar size.
MINSEC 8 mm Minimum secondary reinforcement bar size.
6. Now Click Commands button, Design Commands dialog box appears.
7. Click Add button to add the parameters, then assign the commands to the respective members.
8. Assign the DESIGN BEAM to the members parallel to X and Z direction.
Click Select→ Beam Parallel To X.
Click Select→ Beam Parallel To Z.
9. Assign the DESIGN COLUMN to the members parallel to Y direction.
Click Select→ Beam Parallel To Y.
10. Then Run Analysis, the result provide the suitable concrete design for the structure.
NOTE: After the analysis, double – click the member of the structure, it show the concrete design, if the concrete design of the element is missing, then it is said to unsafe.
Time History Analysis
Time history analysis is an advanced method of dynamic analysis. It has an ability to incorporate harmonic forcing functions that can be described by sinusoidal curves with a specified arrival time, frequency, amplitude and duration.
Define Time History Dialog:
Used to define the Forcing Function of a time varying load.
Click Commands→ Loading→ Definitions→ Time History→ Forcing Functions is
selected or The Add… button is clicked in the Load & Definition dialog found on the General | Load & Definition page.
Integration Time Step:
Solution time step used in the step-by-step integration of the uncoupled equations.
Type:
This refers to the number of the type of functions.
Loading type:
Select the Acceleration, Force or Moment option to define the type of functions being input.
Save:
Select this option to create an external file containing the history of displacements of every node of the structure at every time step.
Function Options:
Define Time VS <loading type>
Used to specify a time history forcing function, where the loading type is that selected above. Specify the values Time and corresponding Force or Acceleration. The time history function is plotted on the bottom of the dialog as data pairs are entered.
Harmonic:
Curve Shape:
Specify if the harmonic function is a SINE or COSINE curve.
Frequency or RPM:
Choose Frequency and enter circular frequency in cycles per second or RPM and enter revolutions per minute.
Amplitude:
Max. Amplitude forcing function in current units.
Phase:
Phase angle in degrees.
Cycles:
No.of cycles of loading.
Step of Sub Div:
Choose the step option to time step of loading SubDiv to sub divide a 1/4 cycle into this many integer time steps.
Spectrum:
Select this Function Option to provide spectrum parameters for your time history loading.
Time History Parameters Dialog:
Time Step:
Specify a solution time step to be used in the step-by-step integration of the uncoupled equations.
Damping:
The following options are available for specifying damping:
Damping-this is to be used for specifying a single model damping ratio which will be applied to all mode. The default value is 0.05.
CDAMP – if a damping ratio has already been specified under CONSTANTS based on the type of material in the structure, the value may be used directly in time history analysis. Check this option for that purpose.
MDAMP – we wish to utilise individual damping ratios for individual modes, that is achieved through the means of the MDAMP option. The first step to doing this is the specification of those individual damping ratios, as explained under section 5.26 .3 of the STAAD technical reference manual and is done graphically from the command-define damping menu. If this first step has been completed, the instruction to utilise MDAMP done by selecting this option shown above.
Arrival time:
specify values of possible arrival times of the various dynamic load types. The arrival time is the time at which the load type begins to act at a joint or at the base of the structure. The same load may have different arrival times for different joint and hence all these values must be specified here. The arrival time and time force pairs for the load
types are used to create the load vector needed for each time step of the analysis.
Chapter- 6:
1. Introduction to FEM
2. Plate
3. Surface
4. Meshing
Introduction to FEM
The Finite Element Method (FEM) is a numerical technique for finding approximate solution of partial differentially equation (PDE) as well as integral equation.
The finite element method is a good choice for solving partial differential equations more complicated the domains, when the domains changes, when the desired precision varies over the entire domains, or when the solution lacks smoothness.
The final element method originated from the need for solving complex elasticity and structural analysis in civil and aeronautical engineering. Its development can be traced back to the work by Alexander Hrennikoff and Richard Courant. While the approaches used by the pioneers are dramatically different, they share one essential characteristic: mesh discretization of continuous domains into a set of the sub-domains, usually called elements.
The development of final element that began in the earnest in the middle to late 1950s for airframe and structural analysis and gathered momentum at the University of stuttgart through the work of Jhon Argyris and at Berkeley through the work of Ray W. Clough in the 1960s for use in civil engineering. By late 1950s, the key concept of stiffness matrix and element software NASTRAN in 1965. The method was provided with rigourous mathematical foundation in 1970 with the publication of strang and Fix’s’An analysis of the finite element method has since been generalised into a branch applied mathematics for numerical modelling of physical system in a wide variety of engineering disciplines.
Plate
Add Plate:
This option allows you to Triangular or Quadrilateral plate elements by connecting existing nodes. To add quadrilateral plate, select Quad from the sub-menu. For triangular plates, select Triangle from the sub-menu . The cursor changes to Quad plate or Triangular Plate shapes. To create new elements, simply click on the existing nodes in the right sequence. A rubber banded area shows the boundary of the plate being generated.
Set New Plate Attribute:
Similar to the “ Set New Member Attribute” command in which the user is in can define the property, material and releases to each new plate element as it is created, has been introduced.
In order to define the attributes for plate element before they are created, go to Geometry→ Add Plate → Set New Plate Attributes from the main menu.
A dialogue box will prompt for various attributes of the plate to be pre-defined. A summary of a specific attributes are defined in the table below.
Button Function
Create New Property Prompts the plate thickness dialogue box that the thickness of the plate at each of the common node can be defined.
Create New Material Defined the various material properties of the plate including poison ratio, modulus of elasticity, shear modulus, etc.
Create New Release Define the degree of freedom to be released at each node of the plate to the plane stress no in plane rotation or no stiffness.
Multiple properties, releases and materials can be created and saved for future use. To choose from various pre-defined types, simply select the appropriate definition using the “Select Property”, “Select Material” or the “Plate Release” drop-down boxes.
For the program to recognize the pre-defined attributes, the “Assign these attributes while creating a new plates” check box must be checked. Any new plate element created from here on will now possess these attributes.
How to Sketch Plates:
1. Click the plate icon .
2. Now automatically beam cursor will change into plate cursor and nodes in the structure are visible.
3. Plate can drawn only clicking the four node points.
4. After placing the plate, click Commands menu→ Member Property→ Plate
Thickness. Now the Properties Whole Structure dialog box appears.
5. Click Thickness button, now the Plate Element/ Surface Property dialog box appears, where you add different types of member properties of plate and surface element.
6. Type respective value of thickness for the plate element. Click Add button.
7. Now click Select menu→ Plate cursor.
8. Now select the plates and select the radio button Assign to selected plates and click Assign button.
9. Now open the 3D rendering page. You can see the Plate with defined thickness.
Surface
Add Surface:
Adding surface is similar to the adding plates where the plates can be placed by clicking only 4 node points while the surface can be placed by clicking more than 4 node points. Finally you have to click the node point where you start placing the surface.
How to Add surface in the structure:
1. Click Add Surface icon or Click Geometry menu→ Add Surface.
2. Now the beam cursor changes into surface cursor.
3. Place the surface by clicking the node points and finally click the node point where begin.
4. As usual define the property for surface. Type the respective thickness value for surface member.
5. Now Assign the member property to the surface by selecting the surfaces using the surface cursor.
6. Now Open 3d Rendering View.
Meshing
Meshing is the process of creating a finite element mesh over the respective member. The nodes that form the corners of the polygon representing the super – element must already exit on the drawing before the facility can be availed. They can be selected in a sequence and the process launched.
Meshing can be done over the plate and surface they can be classified into two are
1. Plate mesh. 2. Surface mesh.
Plate Meshing:
This is an utility meant for taking an existing plate element and subdividing into a set of smaller elements. Consequently, a plate element must already exist on the drawing in order for this facility to be enabled. Using the Plate Cursor, Click the right mouse button on the element and select Generate Mesh. Alternatively select the Geometry menu→
Generate Plate Mesh.
If the element being meshed is triangular, the polygonal mesh feature described in the previous section will automatically become activated. If the element is quadrilateral, the user have to choose between polygonal and quadrilateral meshing.
1. Select the plate element, right click in the selected element.
2. Click Generate Plate Meshing. Now the Meshing type dialog box appears.
3. Choose the type of meshing, Click Polygonal Meshing and click OK
4. Now the Define Mesh Region dialog box appears, user have to define the boundary of the meshing surface. Click OK button.
5. Now the meshing surface is visible, then apply the plate load and proceed analysis.
Polygonal Meshing.
Quadrilateral Meshing.
Surface Meshing:
Surface meshing is similar to the process of plate meshing. Same procedure is followed for the surface meshing. Click Geometry menu→ Generate Plate Mesh. Now select
the surface element and provide respective boundary condition. Click OK button. Now you get the surface mesh.
Chapter- 7: (Slabs)
1. Desgin Of Slab
2. Design Of One Way Slab
3. Design Of Two Way Slab
4. DesignOf Staircase
5. Design of Bridge using STAAD.Beava
Slab Design
Slabs are the important structural component where the pre-stressing is applied. With increase in the demand for fast track, economical and efficient construction, pre-stressed slabs are becoming popular. The slabs are presented in two groups are
One way slabs Two way slabs
A slab is pre-stressed for the following benefits. 1. Increased span-to-depth ratio Typical values of span-to-depth ratios in slabs are given below.
Non-pre-stressed slab 28:1 Prestressed slab 45:1
2. Reduction in self weight. 3. Section remains uncracked under service loads which increases durability. 4. Quick release of formwork which help for fast construction. 5. Reduction in fabrication of reinforcement. 6. More flexibility in accommodating late design changes.
Design of One – Way Slab
One – Way Slab:
Rectangular slabs can be divided into two groups based on the support condition and length-to-breadth ratios. The one-way are identified as follows:
1. When a rectangular slab is supported on all the four edges and length-to-breadth (L/B) ratio equal to or greater than two, the slab is considered to be a one-way-slab. The slab spans predominantly in the direction parallel to the shorter edge.
2. when a rectangular slab is supported only on two opposite edges, it is a one-way slab spanning in the direction perpendicular to the edges. Precast planks fall in this group.
A slab in a framed building can be a one-way slab depending upon its length-to-breadth ratio. A one-way is designed for spanning direction only. For the transverse direction, a minimum amount of reinforcement is provided. A slab under flexural behavior like a beam. One-way slabs are analysed and designed for spanning direction similar to the rectangular beams. A slab of uniform thickness subjected to a bending moment uniformly distributed over its width. Although a one meter wide strip of the slab is considered as a beam for the analysis and design for flexural strength, there is a difference between the beam and slab as follows.
When a beam bends, the portion of the section above the neutral axis is under compression and hence subjected to a lateral condition. Hence after bending, the cross-section, will strictly not be a rectangular, but nearly a trapezoidal.
In the case of a one-way slab, for a design strip, such lateral displacements and strains are prevented by the remainder of the slab on either side i.e it retains the rectangular shape after even after bending. The final design involves the checking of the stresses in concrete at transfer and under service loads with respect to the allowable stresses. The allowable stresses depend on the type of slab. During the design, the reinforced bars are usually spaced uniformly over the width of the slab.
Design Steps In STAAD.Pro V8i:
1. Create a member to represent the slab.
2. Assign the suitable support to both ends.
3. Assign the Cross section properties.
4. Assign the load as follows
Dead Load.
1. Selfweight 2. Uniformly Distributed Load to represent the floor finish
Live Load
1. Uniform Distributed Load
Combination Load as per IS 456.
Design Of Two – Way Slab
If a concrete slab is supported by a beams along all four edges and reinforced with steel bars arranged perpendicularly, it is known as two-way slab. In other words, slab panels that deform with significant curvature in two orthogonal directions must be designed as two-way slabs, with the principle reinforcement placed in the two directions.
In general, twisting moments develop in addition to bending moments in a two-way slab element, except when the element is oriented along the principal curvatures. These twisting moments can become significant at points along the slab diagonals
Wall Supported VS Beam Supported Slabs:
The distributed load on the typical tw0-way slab is transmitted partly along the short to the long edge supports and partly along the long span to the short span supports. In wall-supported panels, these portions of the load are transmitted by the respective wall supports directly to their foundations vertically below. The design considerations of deflection control criteria.
In beam-supported panels, the portion of the load transmitted by the slab in any one direction is in turn transmitted by the beam in the perpendicular direction to the two supporting columns. Slabs supported by beams behave differently, when compared to slabs supported on walls, because of the influence of the following factors.
Deflections in the supporting beams. Torsion in the supporting beams. Displacements in the supporting beams.
Design Steps in STAAD.Pro:
1. Create the frame model of the structure.
2. Use the Parametric Modelling to find the optimisation size of the elements.
3. Assign the supports.
4. Assign the Primary and Combination Loads.
5. Do the analysis and design.
Design Of Staircase
Staircase is a vital element of a building providing entree to different floors and roof of the building. It comprises of a flight of steps and one or more midway landing slabs in the middle of the floor levels. Architectural thoughts including aesthetics, structural feasibility and functional desires are major characteristics to select a specific type of the staircase. Other persuading parameters for the selection of lighting, ventilation, comfort, accessibility, space etc.
The common terminologies used in staircase are:
Tread: The horizontal top portion of a step where foot rests is called as tread. The dimension varies from 270 mm for residential buildings and factories to 300 mm for public buildings where large number of persons use the staircase.
Riser: The vertical distance between two successive steps is called as riser. The dimension of the riser varies from 150 mm for public buildings to 190 mm for residential buildings and factories.
Waist: The thickness of the waist-slab on which steps are made is called as waist. The thickness of the waist is the minimum thickness perpendicular to the soffit of the staircase. The steps of the staircase resting on waist-slab can be made of bricks or concrete.
Design Procedure In STAAD.Pro:
Design the waist-slab type of the staircase
Finish Load = 1Kn/m2 Live Load = 5Kn/m2 Riser = 160mm Tread = 270mm Use M25 grade concrete and Fe 415 steel.
STAAD.Beava
The general philosophy governing the design of bridges is that, subject to set of loading rules and constraints, the worst effects due to load application should be established and designed. The process of load application can be complex as governing rules can impose inter-dependent parameter such as loaded length on a lane, lane factors and load intensity. To obtain the maximum design effects, engineers have to try many loading situations on a trial and error basis.
This leads to the generation of many live load application instances and a large volume of output data that has to be combined with dead load effects as well. In view of the above, a computer program has been developed to minimize the load application process while complying with national code requirements.
Users can avoid trial and error approach and eliminate any possible errors arising from inaccuracies associated with it. This program is based on the use of influence surface for a given effect on a bridge deck relates its value to movement of a unit load over the area of interest. The influence surface is a three dimensional form of an influence line for a single member.
STAAD.Pro V8i will automatically generates influence surfaces for effects such as bending moments for elements, deflection in all degree of freedom of nodes and support reaction. The engineer will then instruct the program to utilise the relevant influence surfaces and with due regards to code requirements, optimise load positions to obtain the maximum desired effects.
Once the influence surfaces have been generated, they are saved and can be used for any further investigation that may be requires. This will remain valid as long as the user has not altered the structural model. Changes to the structural model can alter the pattern of the influence surfaces and the user must ensure that a further run takes place before any further processing.
The Engineer’s knowledge and judgement is critical in deciding which effects are required and at which position to obtain them. This is where users can save lot of processing time and can ensure critical positions are not missed. The current versions of Bridge Engineering Automated Vehicle Application (B.E.A.V.A) supports the UK BS5400 part2, American AASHTO and Indian IRC 6:2000 standards.
All the relevant code instructions for loading definitions and traffic lane calculations are incoporated in BEAVA and in case where vehicle axle arrangements are not standard, it is possible to define a vehicle and save it in library for use it in analysis. BEAVA is fully integrated in STAAD.Pro and utilises the GUI for all input data.
The user defines the width of the carriageway as straight or curved parallel lines, BEAVA then automatically calculates the following in accordance with selected code:
Number of Notional Lanes Influence lines along the center line of notional lanes. Loaded length along the lanes. Critical location of uniformly distributed load. Critical location of knife edge load. Maximum effect value. Associates effects values.
Loading arrangements for the effects requested can be displayed on the model and for every loading arrangements are produced, the user can instruct the program to generate a STAAD.Pro load case. The added live load cases can be combined with dead loads i in the normal way of STAAD.Pro load combination generation. The final model can then analsed in STAAD.Pro and then post-processed.
To avoid inefficient use of the program, it is recommended that the following steps be taken in the order suggested.
Create the structural model including member properties and support conditions.
From the Mode menu select Bridge Deck Pre-processor; note that if your security device is not programmed for this module you will not be able to proceed.
The menu bar has been modified to show Deck and Vehicle. Select the elements/members that define the deck area of model. From Deck menu, select Create Deck to define the deck. From Deck menu, select Influence Surface generator. This will start analysis
procedure to create the influence surfaces. From Deck menu, select Define Carriageway and define either a straight or a
curved carriageway. From Deck menu select Load Generator. Proceed to select the required input, on
completion, select OK. The loading program is now engaged and will calculate all the requires loading arrangements that lead to max/min effects you have request. On completion, a text file will be displayed on the screen containing the loading arrangements, which you can now display graphically.
For each effect requested display the loading arrangements and examine the correctness.
For each effect requested, select Create Loading in STAAD Model from Deck menu.
After all load cases have created, from Mode menu select Modelling and return to carry on with other load generates and combinations.
Proceed with analysis and post processing in the usual way.
How To Design A Bridge By Using Bridge Deck Preprocessor
Bridge Deck Preprocessor By Using STAAD.Beava
Design Procedure For Bridge Deck Preprocessor:
1. Create the bridge model which comprises of Columns, Beams and Girders.
2. Assign the Pinned support to the columns.
3. Use the Geometry menu→ Generate Surface Meshing to create the bridge deck as
follows.
Mesh type = Quadrilateral
Number of division along length = 75 Number of division along width = 16
4. Now type the values and click Apply. Model appears like as shown below.
5. Assign the Member properties and Specifications for the elements
Column = Circular Cross Section with 1 meter dia in concrete. Beam parallel the X and Z direction = Rectangular Cross Section with 0.5 m x 0.5
m in concrete. Plate Elements = 0.3 m Thickness in concrete. Member Offset = Decide the suitable member offsets as per the cross section
dimensions.
6. Add a primary load case with Dead Load type and assign the Selfweight for all the elements.
7. Perform Analysis and Run Analysis. Note the node id, which has the maximum displacement due to load.
8. Click Mode menu→ Bridge Deck Preprocessor.
9. Select all the plates.
10. Click Deck menu→ Create Deck, name the deck.
11. Click the Loading menu→ Run Influence Generator. It takes sometimes to complete the analysis over the plates elements.
11. Click the Loading menu→ Influence Diagram… Check the diagram for all the
elements.
12. Click Deck menu→ Define Roadway.
13. Click New button. Define Roadway dialog box appears.
Click Add Lane Left Orientation = 90 Length = 75 Origin = 2 Width = 4 Add the Curb on the lane side by side
14. Click Ok. Now you can get the model with the corresponding lanes.
15. Click Vehicle menu→ Vehicle Database. Now Vehicle Database dialog box
appears.
16. Select the suitable IRC Vehicle for your design. Click Ok.
17. Click Loading menu→ Run Load Generator…
Limit = Ultimate Design Code = IRC Chapter 3 Loading Type = Class 70R Loading Enter the Node Displacement Values
18. Click Ok. Now automatically it will generate result values in notepad.
19. Verify and close the notepad.
20. Right click in the window, select the Structures Diagrams… Select the Results values and click OK.
21. Click Loading→ Create Loading in STAAD Model. Now the following dialog box
appears
22. Now Run Analysis and then move to post processing. In Post Processing window, click the Beam tab→ Forces.
3. Now
you can provide the concrete design over the model.
Chapter- 9: (Steel)
1. Design Of Steel Structures
2. Member Specification
Table Member Property
Design Of Steel Structures
STAAD.Pro V8i comprises an extensive set of accommodations for designing steel structural members as individual components of an analyzed structure. The member design services provide the user with the skill to carry out a number of different design procedures. These services may be used selectively in accord with the necessities of the design problem. The procedures to perform a design are:
Identify the members and the load cases to be considered in the design.
Identify whether to perform code checking or member selection.
Identify design parameter values, if different from the default values.
Presently, STAAD.Pro V8i supports steel design of wide flange, S shape, M shape, HP shapes, T shape, I shape, angle, double angle, channel, double channel, pipes, tubes, beams with cover plate and composite beams.
Design Process follows the following design checks
1. Slenderness
2. Section Classification
3. Tension
4. Compression
5. Shear
6. Bending
7. Combined Interaction Check
when a design is performed, the output file reports the maximum ratio from all the above mentioned checks.
Indian Steel Design IS 800:2007 Parameters
Parameter Name
Default Value
Description
FYLD 250 MPA Yield strength of steel.
KY 1.0 K value in local y-axis. Usually, this is minor axis.
KZ 1.0 K value in local z-axis. Usually, this is major axis.
LY Member Length Length in local y-axis to calculate slenderness ratio.
LZ Member Length Same as above except in local z-axis (major).
MAIN 180 Allowable Kl/r for slenderness calculations for compression members.
NSF 1.0 Net section factor for tension members.
RATIO 1.0 Permissible ratio of the actual to allowable stresses
TMAIN 400 Allowable Kl/r for slenderness calculations for tension members.
TRACK 0 0=Minimum detail1=intermediate detail level2=maximum detail
CMYCMZ 0.85 for sidesway and calculated for no sidesway
Cm value in local y & z axes
DFF None (Mandatory for deflection check)
“Deflection Length” / Max. allowable local deflection
DJ1 Start Joint of member
Joint No. denoting starting point for calculation of “Deflection Length”
DJ2 End Joint of member Joint No. denoting end point for calculation of “Deflection Length”
Member Specification
Cable:
This command may be used to model a specified set of members as CABLE members. The CABLE members, in addition to elastic axial deformation, are also capable of accommodating the stiffness of initial tension due to static loads. The Cable menu option under the commands menu→ Member Specification allows the user to define
cable members. When you select the Cable menu option, the Member Specification dialog box appears, as shown below:
Provide either the Initial TENSION in the cable as o force, or the Unstressed LENGTH of the cable of the cable member. Click the Add button to add this specification to the structure or click Assign to assign the specification to selected members as well as add this specification to the structure. The TENSION specified in the CABLE member is applied on the structure as an external load as well as is used to modify the stiffness of the member. The tension value must be positive to be treated as cable; otherwise, it is a truss. If TENSION or the value is omitted a minimum tension will be used.
This is truss member but not a tension only member unless you also include this member in a MEMBER TENSION input. Note also that Member releases are not allowed. The tension is a preload and will not be the final tension in the cable after the deformation due to this preload.
Tension/Compression:
This command may be used to designate certain member as Tension only or compression only members. The Tension Only / Compression Only menu option in the Member Specifications menu allows the user to define tension only or compression only members. These members are capable of carrying tensile forces only.
Click the Add button to add this specification to the structure or click Assign to assign the specification to selected members as well as add this specification to the structure.
MEMBER TENSION 0
This command switches off ALL tension/compression only specification for load cases, which are specified subsequent to this command, usually entered after a CHANGE command. There is no list associated with this command. Hence, for any further primary load cases, the tension/compression only attributed is disabled for ALL members.
Tension only member are truss/cable members that are capable of carrying tensile forces only. Thus they are automatically inactivated for load cases that create compression in them. Compression only members that are capable of carrying compressive forces only. Thus, they are automatically inactivated for load cases that
create tension in them. Member Releases are not allowed on members with this attribute.
The Procedure for analysis of Tension only or Compression only members requires iteration for every load cases and therefore may be moderately involved. The user may also consider using the INACTIVE specification if the solution time becomes unacceptably high. If a CHANGE command is used, then the SET NL command must be used to convey to STAAD that multiple analyses and multiple structural conditions are involved.
Table Member Property
Click Commands→ Member Property→ Steel Table
This allows the user to choose steel sections from the available in-built steel tables for different countries. The Steel Table option provides a sub-menu that includes available countries steel table types. Select the country from this sub-menu. Please note that the Properties dialog box also opens simultaneously letting us utilize some of the other options available from the dialog box. After choosing the country, the Steel Table dialog box appears as shown below:
Select the type of section by clicking on the appropriate tab and then select the specific section from the list box. Please note that the type of steel section available for selection will vary depending on the selected country. In addiction, depending on the type of section selected, additional properties may be specified. Click Add button to add this property to the structure or click Assign to assign the property to the selected members as well as add this property to the structure.
Click on the View Table button to display all the available dimensions in the active database for the sectional cross-section. This option displays all member properties for the current country steel table in a dialog box and provides the feature to customize steel section database.
User is provided with the following two options in the dialog box:
Select Single Section – Where user can select the section for the structural member
Selection Sections to Project Database – Where user can select/deselect sections from the Steel Table for the specified project.
Chapter- 10: (Seismic Loads Worked Examples)
1. Calculate Natural Frequency of a Buiding By Response Spectrum Analysis
2. Calculate Natural Frequency of a Building By Rayleigh Method
3. Calculate Natural Frequency of a Building By Modal Shape
Calculate Natural Frequency of a Building By Response Spectrum Analysis
Design Procedure For Response Spectrum Analysis:
1. Open STAAD.Pro V8i.
2. Click New Project and set the units as Kilo Newton & Meter.
3. In STAAD.Pro, open Run structure wizard in Geometry menu→ Run Structure
Wizard.
4. Change the Model Type into Frame Model and select Bay Frame, now the Select Parameter dialog appears.
5.Set the parameter of the structure as shown below.
6. Click File menu→ Merge Model with STAAD.Pro Model and place the model at origin.
7. Now assign Fixed support to the structure.
8. Assign the Member Property for column as YD=0.6 m & ZD= 0.6 m and for beam YD= 0.75 m & ZD= 0.6 m.
9. Now you can see the model in 3D Rendering.
10. Next Loading process, Click Commands→ Loading→ Definitions.
11. Select Seismic Definitions and click Add button.
12. Now Add New : Seismic Definitions dialog box appears, in Type drop down box select the respective codes for design. i.e IS 1893 – 2002/2005
13. Click Generate button, now the IS 1893 Seismic Parameter dialog box appears.
Choose the zone and it factors, Response Reduction factor, Importance Factor, Type of Structure and Type of soil.
14. Click Generate button. In IS 1893 Seismic Parameter dialog box type the Damping Value as 0.05. Click Add button.
15. Now your seismic definition is added, then add other factor of seismic definition. First you have to add the basic factors.
Self Weight Factor = 1, click Add button. Floor Weights must assigned as given below and click Add button.
16. Now you are going to add the Load Case Details.
17. Load Case Type 1 ( Here you assign floor loads only in GY direction and values must be negative.)
LOAD CASE 1 YRANGE 0 42 FLOAD 3.5 YRANGE 43 45 FLOAD 2.5 YRANGE 0 45 FLOAD 1.5
18. Load Case Type 2 Response Spectrum (Here you add the self weight of the structure in positive X and Z direction and negative Y direction. Different floor load in all three global directions.)
LOAD CASE 2
SELFWEIGHT X 1 LIST ALL
SELFWEIGHT Y -1 LIST ALL
SELFWEIGHT Z 1 LIST ALL
FLOOR LOAD
YRANGE 0 42 FLOAD 3.5 GX
YRANGE 0 42 FLOAD 3.5 GY
YRANGE 0 42 FLOAD 3.5 GZ
YRANGE 43 45 FLOAD 3.5 GX
YRANGE 43 45 FLOAD 3.5 GY
YRANGE 43 45 FLOAD 3.5 GZ
YRANGE 0 45 FLOAD 3.5 GX
YRANGE 0 45 FLOAD 3.5 GX
YRANGE 0 45 FLOAD 3.5 GX
19. Now you have to assign the self weight to structure by Assign to view.
20. Then add another load item Response Spectra as shown below.
21. Then Click Commands→ Miscellaneous→ Cut Off Mode Shape…. Mode
Shapes Value is 10.
22. Click Commands→ Analysis→ Perform Analysis Print All.
23. Then Run Analysis or press CTRL + F5.
24. Result Values
Eigen Values: Calculated frequencies for load case.
Mode Shape Values:
Mass Participation Factor is important for analysis.
25. Deflection:
26. Click Ok button.
How To Calculate Natural Frequency By Rayleigh Method
Design Procedure For Rayleigh Method:
1. Open STAAD.Pro V8i.
2. Click New Project and set the units as Kilo Newton & Meter.
3. In STAAD.Pro, open Run structure wizard in Geometry menu→ Run Structure
Wizard.
4. Change the Model Type into Frame Model and select Bay Frame, now the Select Parameter dialog appears.
5.Set the parameter of the structure as shown below.
6. Click File menu→ Merge Model with STAAD.Pro Model and place the model at
origin.
7. Now assign Fixed support to the structure.
8. Assign the Member Property for column as YD=0.6 m & ZD= 0.6 m and for beam YD= 0.75 m & ZD= 0.6 m, Plate Thicness = 0.15m.
9. Now you can see the model in 3D Rendering.
10. Next Loading process, Click Commands→ Loading.
11. Now the Load & Definitions dialog box opens, add the following loads:
12. Then Perform Analysis, Select All.
13. Now Run Analysis. Result you will get the Rayleigh Frequency for load case 1.
Note: Please ignore if you get any warnings.
14. Now add Load case 2: Assign the following loads
15. Again Run Analysis. Now you get the Rayleigh Frequency for load case 1 and load case 2.
16. Then check the deflection using Animation command. Provide suitable concrete design the control the deflection.
How To Calculate Natural Frequency of a Building By Modal Shape
Design Procedure For Modal Shape:
1. Open STAAD.Pro V8i.
2. Click New Project and set the units as Kilo Newton & Meter.
3. In STAAD.Pro, open Run structure wizard in Geometry menu→ Run Structure Wizard.
4. Change the Model Type into Frame Model and select Bay Frame, now the Select Parameter dialog appears.
5.Set the parameter of the structure as shown below.
6. Click File menu→ Merge Model with STAAD.Pro Model and place the model at
origin.
7. Now assign Fixed support to the structure.
8. Assign the Member Property for column as YD=0.6 m & ZD= 0.6 m and for beam YD= 0.75 m & ZD= 0.6 m, Plate Thickness = 0.15m.
9. Now you can see the model in 3D Rendering.
10. Click Commands→ Miscellaneous→ Cut Off Mode Shape...
11. Now the Cut Off Mode Shape dialog box appears. Enter the desired number of modes you want. Click Ok
12. Then add the following Load Cases:
13. Finally you have add the Modal Calculation command.
14. Now assign the respective loads to the elements.
15. Then Perform Analysis and Run Analysis.
16. In Result you get the Calculated Frequency of Load Case.
17. According to IS 1893, Mass Participation Factors must be atleast greater than 90.
18. Then Check the deflection using Animation command.
19. Now you have to provide the suitable concrete design to control the deflection.
Note: For more info and further details watch the video.
Chapter- 11: (Wind Load Intensity Worked Examples)
1. Calculate Wind Load Intensity In A Building
Wind Load Intensity Calculation
How to calculate wind load as per ASCE – 7 ?
Design Procedure To Calculate Wind Load Intensity:
1. Open STAAD.Pro V8i.
2. Click New Project and set the units as Kilo Newton & Meter.
3. Open the Grid and Form the following grid.
4. By using the Snap/Node Beam Add the members and select the members.
5. Now use the Translational Repeat option to build the structure.
6. Assign the respective support for the structure.
7. Assign the suitable member property for the model.
For Column = 0.75 x 0.75 m
For Beam: YD = 0.60; ZD = 0.40
8. Now open the structure in 3D rendering.
10. Now click Calculate as per ASCE-7 button. Now the select respective code and type of building. Click OK
11. Wind load in calculated according the varying height of the structure. Click Close button.
12. Add the suitable factor of exposure.
13. Assign the Exposure against the structure.
14. Assign the following Load Cases.
15. Perform Analysis and Run Analysis.
16. According to the result values provide the suitable concrete design for the structure.
17. Again Run the Analysis.
18. Now you get the concrete design of the elements.
19. In Post Processing, You can get the Bending Moment and Shear Force values.
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STAAD.Pro
If you open STAAD.Pro, you will be greeted by this window.
If we need a existing project we can select them. Otherwise click New Project. Or by clicking
File -> new
Before getting into the new project check whether the license configuration is set as Indian
Design Codes. Unless click the Indian standard design codes.
When we enter into the new project another window will be opened. In this window the 4
options will be there.
Space – if we select this, a 3D space will be created for designing framed structure.
Plane – if we get into this 2D plane
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Floor – this creates 2D to design with moments
Truss – it consists of 3D pre-constructed steel trusses
Now, we may get into Space.
If the File Name and check the Length Units and Load Units.
As per SI units click Meter and Kilo Newton respectively.
Then click Next.
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TRAINING AND PLACEMENT WING-UCEN, http://ucentp.in
In this window Add what we go to design or what we going to edit. We may add beam.
Then click Finish.
Now, a page is opened with grid in three axes
In this grid we have 3 base axes. It should be noted in the all stages of design from marking the
node to the applying loads.
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TRAINING AND PLACEMENT WING-UCEN, http://ucentp.in
Commands:
Nodes cursor
To select the Nodes
Beams Cursor
To select the Beam
Plates cursor
To select the Plates.
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Add beam
To add beams
Add 4 node plates
To form plates by selecting 4 nodes
Add surface
To form surface by selecting several nodes.
Snap node/ Beam
To make visible or invisible the grid.
Insert Node
If we require more nodes during operations this command will help.
To view the objects from Front, Back, Left Side, Right Side, Top, Bottom &
Isometric view respectively.
3D Rendered View
To view the view the full structure in three dimension.
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If we press the View command, it can be seen in front view ( in 2D).
The grid in the opening page is having ordinates in both axes. For example, if we want to
design beam with ordinates of 3, then we want to change the ordinate as 3. It can be done by
following steps.
When this page opens at the right side there will be a window named as Snap Node/ Beam.
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TRAINING AND PLACEMENT WING-UCEN, http://ucentp.in
Unless by clicking Snap node/ Beam the window can be opened.
Click Edit. A new window will be opened.
In this page under spacing we can change the ordinate in x as well as y axis. Further if we need
more ordinates in left or right side we can also change them.
Click in the grid at 3 ordinates. Now the node will be created in red color at the clicked points.
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TRAINING AND PLACEMENT WING-UCEN, http://ucentp.in
Now press the Snap node/ Beam command to release the cursor or press Esc and followed
by Geometry .
You can see the beam as below.
Now select the beams by Beam cursor
Shift + B – shows the number of beams
If you don’t want to display beam number same command should be used.
Now select the nodes by Nodes cursor
Shift + N – shows the number of nodes
If you don’t want to display node number same command should be used.
Setup
If we get into set up the necessary data about the project the can be enter into this as follows.
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Geometry
Under this we have the following commands.
Now we may enter Beam.
General
Under this we have the following commands.
Now click Property. The below window will be opened.
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In this click the Define. Then the next box may be opened.
In this window the properties of the beam may be assigned. Where we may assign beam as
Rectangular and assign the dimensions. Then enter Add. Close the window.
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Now click in Section as shown.
Assign To View » Assign
To check the beam shape click 3D rendered view
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General » Support
The following window will be opened.
Click Create support
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Then press the Add command. Now you may see the below window.
Use Cursor To Assign » Assign
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Now point cursor in the support nodes. Supports will be created.
General » Load & Definitions
The load & definition window will be created.
Click New in this.
Now the next window will be generated.
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In this box, click the Load Case
In this click Primary.
Give any reference number and the type of load. Press Enter.
Then press Load Items in the same page.
Click the Self-weight. Verify the direction of self-weight is vertical (and the ‘–ve‘ sign indicates
the downward direction)
Then click the Nodal load.
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The below window will be opened.
Then give the load in the respective direction (i.e., vertical). In shown case it is y direction. If we
would like to add moment, can also do it now.
Press Enter.
Load & Definition will be opened again.
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We can see the assigned load case in the window.
Use Cursor To Assign » Assign
Now click the cursor in the where the load is acting. Then you can see the load on the beam.
(The various loading condition will be discussed throughly in the upcoming modules)
General » Material
A new window will be opened.
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Click any of the material in which the beam is going to design.
Assign to view » Assign
Analysis/Print
Under this 3 things are available. The analysis should be done for all the structures designed to
check whether the design is safe and having any error.
If you press the Analysis the new window will be opened.
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If no printing facility is available click No Print and Add it.
At the right hand side of the screen the new tab will be opened.
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Where click the PERFORM ANALYSIS.
Then in the tool bar click Analyze and click the Run Analysis
The Analysis Output File will be generated as below.
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TRAINING AND PLACEMENT WING-UCEN, http://ucentp.in
Concrete slab Design:
Yes, you can design the concrete slab using STAAD, with plate elements and meshing it appropriately. But it is best practice to take the analysis results from the STAAD and do the manual design. While meshing see to that your aspect ratio of the elements are close 1.0 While designing it is adviced to consider the torsional moment in addition with the major moments. Mx = Mx + Mxy and similarily My = My + Mxy for Rebar calculations. Shear stress shall be directly taken from the STAAD and can be checked with the allowable shear stresses based on the Pt provided.
PLATE/SHELL ELEMENT:
When a user chooses to model the surface component using plate elements, he/she is taking on the responsibility of meshing.
With the new Surface type of entity, the burden of meshing is shifted from the user to the program to some degree.
ELEMENT stress and moment output is available at the following locations:
A. Center point of the element.
B. All corner nodes of the element.
C. At any user specified point within the element.
Following are the items included in the ELEMENT STRESS output.
SQX, SQY Shear stresses (Force/ unit len./ thk.)
SX, SY, SXY Membrane stresses (Force/unit len./ thk)
MX, MY, MXY Moments per unit width (Force x Length/length)
(For Mx, the unit width is a unit distance parallel to the local Y axis.
For My, the unit width is a unit distance parallel to the local X axis.
Mx and My cause bending, while Mxy causes the element to twist out-of-plane.)
SMAX, SMIN Principal stresses in the plane of the element (Force/unit area).
The 3rd principal stress is 0.0.
TMAX Maximum 2D shear stress in the plane of the element (Force/unit area)
ANGLE Orientation of the 2D principal plane (Degrees)
VONT, VONB 3D Von Mises stress, where
VM =0.707√ (SMAX-SMIN)2 +SMAX2+SMIN2
TRESCAT, TRESCAB Tresca stress, where
TRESCA = MAX[ |(Smax-Smin)| , |(Smax)| , |(Smin)| ]
ELEMENT FORCE outputs are available at the centre node of the element, all corner nodes of the element, and at any user-specified point within the element. The items included in the ELEMENT FORCE output are: QX, QY Transverse shear forces stated as force per unit length per unit element thickness. FX, FY, FXY Membrane forces stated as force per unit length per unit element thickness. MX, MY, MXY Bending moments stated as moment per unit length. SMAX, SMIN Principal stresses stated as force per unit area. TMAX Maximum in-plane shear stress stated as force per unit area. ANGLE The orientation of the principal plane stated in degrees measured anti-clockwise from the local x-axis. The top and bottom surfaces are identified on the basis of the direction of the local z-axis.
Units Kn met
When I perform concrete design on an element, the output contains expressions such as "LONG. REINF.", "TRANS. REINF.", "TOP", "BOTT.", etc. Can you explain what these terms mean?
The design of an element involves determination of the reinforcement for moments Mx and My at the centroid of the element. The reinforcement calculated to resist Mx is called longitudinal reinforcement, and is denoted in the output by the expression "LONG. REINF.".
The reinforcement calculated to resist My is called transverse reinforcement, and is denoted in the output by the expression "TRANS. REINF.".
The sign of Mx and My will determine which face of the element the steel has to be provided on. Every element has a "top" face, and a "bottom" face, as defined by the direction of the local Z axis of the elements. Mx will cause tension on one of those faces, and compression on the other. A similar effect will be caused by My. The output report of reinforcement provided on those faces contains the terms "TOP" for top face, and "BOTT" for the bottom face.
The procedure used by the program to arrive at these quantities is as follows :
For each element, the program first scans through all the active load cases, to find the following maxima :
Maximum positive Mx Maximum negative Mx Maximum positive My Maximum negative My
The element is then designed for all those four quantities. If any of these moments happen to be zero, or if the reinforcement required to resist that moment is less than the capacity of the element with minimum reinforcement, only minimum reinforcement is provided. For the ACI code, the rules governing provision of reinforcement for shrinkage and temperature are used in calculating minimum reinforcement.
The rules applicable for design of a beam for flexure are used in calculating the steel areas. The width used in this calculation is a unit width of the element. For determination of the effective depth, the steel for longitudinal moment is assumed to be the outer layer, and the steel for transverse moment is the inner layer.
The output will consist of the steel area required for all of four maximas. As described earlier, they will be reported using the terms LONG, TRANSVERSE, TOP and BOTT.
When I perform concrete design on an element, the output reports reinforcement in terms of "SQ.MM/MM". Can you please explain why?
When you ask for an element design or a slab design using the commands
DESIGN ELEMENT ..
or
DESIGN SLAB ..
STAAD designs the element for the moments MX and MY at the centroid of the element. By definition, MX and MY are termed as Moments per Unit width, since that is what they are. They have units of Force-length/length, as in 43.5 KN-mm/mm, or 43.5 KN-m/m. In other words, if you take a one metre width of the slab at the centroid of the element in question, the moment over that one metre width on that element is equal to 43.5 KN-m.
The design of that element hence has to be done on the basis of a unit width. Thus, in order to design an element for a 43.5 KN-m/m moment, one needs to use a one metre width of slab. The reinforcement required for that element is thus reported in terms of unit width of the element. The results are hence in the form Area of steel/unit-width of element, as in, "SQ.MM/MM".
A floor slab has been modeled using 4-noded plate elements. The elements are subjected to pressure loading in the vertically downward direction. A concrete design has been performed on the elements. (See below for the reinforcement report for many of those elements.) Why is it that the moments as well as reinforcement are appearing on the top and not on the bottom of the plates?
The reinforcement report for many of those elements looks like the following:
ELEMENT LONG. REINF (SQ.IN/FT)
MOM-X /LOAD (K-FT/FT)
TRANS. REINF (SQ.IN/FT)
MOM-Y /LOAD (K-FT/FT)
134 TOP : 5.944 1474.13 / 12 6.914 1679.58 / 12 BOTT: 1.296 0.00 / 0 1.296 0.00 / 0
Solution:
In the above output, the word TOP and BOTTOM refer to the "local" top and bottom surfaces of the individual elements, and not in the global axis sense. The local top and bottom surfaces depend on the way an element is defined in its incidence statement. TOP is defined as the surface which coincides with the positive side of the local Z axis. BOTTOM is defined as the surface which coincides with the negative side of the local Z axis. Shown below are two examples in which the element incidence is numbered in two contrasting ways. In the first figure, the local Z axis of the element points in the vertically upward direction. Consequently, the local top and bottom surfaces have the same sense as the global top and bottom.
In the next figure, the local Z axis of the element points in the vertically downward direction. Consequently, the local top and bottom surfaces have the opposite sense as the global top and bottom.
You can verify the direction of the local axes of the elements in your model by doing the following. Click the right mouse button and select Labels. Under the Plate category, switch on Plate Orientation. The local axes will be displayed as shown in these figures above.
PLATE AND SURFACE:
What is difference between plate element and Surface?
I am use to prepare model without including slab and assigning load on each beams by manual calculation of loads coming on each beams due to slab as per one way and two slab distribution rules. Now I want to create model with slabs by using floor area facility. Please advise what I have to do ?
1.use add surface button and surface cursor.
2.OR use Generate surface meshing and palate cursor.
Secondly, if some of my RC members have failed then what I have to do other than change size of failed member and re-analyzed and design whole structure.
3. If my all RC beams and columns has been passed but slab has been failed then what I have to do ?
4. In design of slab, is it necessary to convert 1- region into two or more regions. What is requirement?
If you want to consider the in plane rigidity offered by the slabs in the model then it is best to model it as plates.
Surfaces are special entities that need to be used for the modeling of shear walls and thereafter these shear walls need to be designed as per the concrete codes (ACI if you are using American code). So if you are modeling plates then the load can be provided as a pressure load on plates. But make sure that the plates are meshed properly so that there is proper load transfer from the plates to the adjacent beam members. If you don’t want to consider the in plane rigidity offered by the slab against lateral displacements then the loading can be done using floor load options.
You can either model the walls using plates or you may manually calculate the load and provide them as a uniformly distributed member load.
A good rule of thumb for starting mesh sizes is the lesser of span/10 or 1000mm.
ELEMENT FORCE outputs are available at the centre node of the element, all corner nodes of the element, and at any user-specified point within the element. The items included in the ELEMENT FORCE output are: QX, QY Transverse shear forces stated as force per unit length per unit element thickness. FX, FY, FXY Membrane forces stated as force per unit length per unit element thickness. MX, MY, MXY Bending moments stated as moment per unit length. SMAX, SMIN Principal stresses stated as force per unit area. TMAX Maximum in-plane shear stress stated as force per unit area. ANGLE The orientation of the principal plane stated in degrees measured anti-clockwise from the local x-axis. The top and bottom surfaces are identified on the basis of the direction of the local z-axis.
Ref:Modern structural Analysis by Iain MacLeod.
Section 5 5-169
5.27.3 Automatic Spring Support Generator for
Foundations
STAAD has a facility for automatic generation of spring supports
to model footings and foundation mats. This command is specified
under the SUPPORT command.
General Format:
SUPPORT
X joint-list ELASTIC FOOting f1 (f2) XOnly joint-list ELASTIC MAT DIR Y SUBgrade f3 plate-list PLATE MAT YOnly Z ZOnly
(PRINT) ( COMP )
MULTI
plate-list PLATE MAT DIR ALL SUBgrade f3 (f4 f5)
(PRINT) ( COMP ) MULTI
where
f1, f2 = Length and width of the footing. If f2 is not given, the
footing is assumed to be a square with sides f1
X,Y,Z = Global direction in which soil springs are to be
generated
f3 = Soil sub-grade modulus in force/area/length units.
STAAD Commands and Input Instructions
Section 5 5-170
ALL option
f3, f4, f5 = Soil sub-grade modulus in force/area/length units in
Y, X, Z directions respectively. f4, f5 default to f3 if
omitted.
Do not use this command with SET Z UP.
The ELASTIC FOOTING option : If you want to specify the
influence area of a joint yourself and have STAAD simply
multiply the area you specified by the sub-grade modulus, use the
FOOTING option. Situations where this may be appropriate are
such as when a spread footing is located beneath a joint where you
want to specify a spring support. Please note that it is absolutely
imperative that you provide f1 (and f2 if its a non -square footing)
if you choose the FOOTING option.
The ELASTIC MAT option : If you want to have STAAD
calculate the influence area for the joint (instead of you specifying
an area yourself) and use that area along with the sub-grade
modulus to determine the spring stiffness value, use the MAT
option. Situations where this may be appropriate are such as when
a slab is on soil and carries the weight of the structure above. You
may have modeled the entire slab as finite elements and wish to
generate spring supports at the nodes of the elements.
Section 5 5-171
The PLATE MAT option : Similar to the Elastic Mat except
for the method used to compute the influence area for the joints.
If your mat consists of plate elements and all of the influence
areas are incorporated in the plate areas, then this option is
preferable. Enter a list of plates or YRANGE f1 f2 at the
beginning of the command, the joint influence areas are then
calculated using the same principles as joint forces would be from
uniform pressure on these plates. This method overcomes a major
limitation of the Delaunay triangle method used in the ELASTIC
MAT option, which is that the contour formed by the nodes of the
mat must form a convex hull.
The PLATE MAT DIR ALL option : Similar to the Plate Mat
except that the spring supports are generated in all 3 directions.
If the compression only option is also specified, then the
compression direction will be assumed to be in the Y direction. If
the Y spring at a joint goes slack (lift off), then the X and Z
spring stiffnesses for that joint will also be set to zero. Otherwise
the X and Z springs act in both directions. The influence area for
the X and Z springs is the same as used for the Y spring. Three
values of subgrade reaction may be entered, the first is for the Y
direction, the second for X and the third for Z.
The DIRection option : The keyword DIR is followed by one of
the alphabets X, Y or Z (or XONLY, YONLY, or ZONLY) which
indicate the direction of resistance of the spring supports. If X or
Y or Z is selected then a spring support is generated in that
direction plus 3 other directions receive a fixed support, e.g. if Y
is selected, then FY is supported by a spring; FX and FZ and MY
are fixed supports; and MX and MZ are free. If XONLY,
YONLY, or ZONLY are selected then only a spring support in
that direction is generated.
The SUBGRADE option : The keyword SUBGRADE is followed
by the value of the subgrade reaction. The value should be
provided in the current unit system signified by the most recent
UNIT statement prior to the SUPPORT command.
STAAD Commands and Input Instructions
Section 5 5-172
The PRINT option : Prints the influence area of each joint.
The COMP option : The springs generated will be compression
only.
The MULTI option : The springs generated will be multilinear.
Add the associated multilinear curve input after each MAT
command (with the multi option) to describe the displacement -
spring constant curve. See section 5.27.4 for additional
information on this input format. The actual spring constant used
will be the subgrade modulus (f3 entered above) times the
influence area (computed by STAAD) times the si values entered
in the curve (so the curve stiffness values will likely be between
0.0 and 1.0).
SPRINGS d1 s1 d2 s2 …… dn sn
Example
SUPPORTS
1 TO 126 ELASTIC MAT DIREC Y SUBG 200.
1 TO 100 PLATE MAT DIREC Y SUBG 200.
YR -.01 .01 PLA MAT DIR Y SUBG 200 MUL
SPRINGS -0.51 40.0 -0.50 50.0 0.5 65.0
The first command above instructs STAAD to internally generate
supports for all nodes 1 through 126 with elastic springs. STAAD
first calculates the influence area perpendicular to the global Y
axis of each node and then multiplies the corresponding influence
area by the soil subgrade modulus of 200.0 to calculate the spring
constant to be applied to the node. In the 2nd example, the nodes
of plates 1 to 100 are assigned spring supports, generated using a
subgrade modulus of 200 units.
Section 5 5-173
Notes:
A closed surface is generated by the program based on the joint -list that
accompanies the ELASTIC MAT command. The area within this closed
surface is determined and the share of this area for each node in the list
is then calculated.
Hence, while specifying the joint-list, one should make sure that these
joints make up a closed surface. Without a proper closed surface, the
area calculated for the region may be indeterminate and the spring
constant values may be erroneous. Consequently, the list should have at
a minimum, 3 nodes.
The internal angle formed by 2 adjacent segments connecting 3
consecutive nodes in the list should be less than 180 degrees. In other
words, the region should have the shape of a convex polygon. The
example below explains the method that may be used to get around a
situation where a convex polygon is not available.
For the model comprised of plate elements 100 to 102 in the figure
below, one wishes to generate the spring supports at nodes 1 to 8.
However, a single ELASTIC MAT command will not suffice because the
internal angle between the edges 1-8 and 8-7 at node 8 is 270 degrees,
which violates the requirements of a convex polygon.
STAAD Commands and Input Instructions
Section 5 5-174
So, one should break it up into 2 commands:
1 2 3 8 ELASTIC MAT DIREC Y SUBG 200.
3 4 5 6 7 8 ELASTIC MAT DIREC Y SUBG 200.
Figure 5.19
Joints 3 and 8 will hence get the contribution from both of the above
commands.
The command works only when the plane of the closed region is parallel
to one of the global planes X-Y, Y-Z or X-Z. For regions that are
inclined to one of the global planes, the spring constant will have to be
evaluated manually and specified using the FIXED BUT type of spring
support.
1
6 7
8 5
2 3
4
100
102
101
Example Problem 27 265
Example Problem No. 27
This example illustrates the usage of commands necessary to apply the compression only attribute to spring supports for a slab on grade. The spring supports themselves are generated utilizing the built-in support generation facility. The slab is subjected to pressure and overturning loading. A tension/compression only analysis of the structure is performed. The numbers shown in the diagram below are the element numbers.
Part I - Application Examples Example Problem 27 266
STAAD SPACE SLAB ON GRADE * SPRING COMPRESSION EXAMPLE
Every STAAD input file has to begin with the word STAAD. The word SPACE signifies that the structure is a space frame and the geometry is defined through X, Y and Z axes. An optional title to identify this project is provided in the second line.
SET NL 3
This structure has to be analysed for 3 primary load cases. Consequently, the modeling of our problem requires us to define 3 sets of data, with each set containing a load case and an associated analysis command. Also, the supports which get switched off in the analysis for any load case have to be restored for the analysis for the subsequent load case. To accommodate these requirements, it is necessary to have 2 commands, one called “SET NL” and the other called “CHANGE”. The SET NL command is used above to indicate the total number of primary load cases that the file contains. The CHANGE command will come in later (after the PERFORM ANALYSIS command).
UNIT FEET KIP JOINT COORDINATES 1 0.0 0.0 40.0 2 0.0 0.0 36.0 3 0.0 0.0 28.167 4 0.0 0.0 20.333 5 0.0 0.0 12.5 6 0.0 0.0 6.5 7 0.0 0.0 0.0 REPEAT ALL 3 8.5 0.0 0.0 REPEAT 3 8.0 0.0 0.0 REPEAT 5 6.0 0.0 0.0 REPEAT 3 8.0 0.0 0.0 REPEAT 3 8.5 0.0 0.0
For joints 1 through 7, the joint number followed by the X, Y and Z coordinates are specified above. The coordinates of these joints is used as a basis for generating 21 more joints by incrementing the X coordinate of each of these 7 joints by 8.5 feet, 3 times.
Example Problem 27 267
REPEAT commands are used to generate the remaining joints of the structure. The results of the generation may be visually verified using the STAAD graphical viewing facilities.
ELEMENT INCIDENCES 1 1 8 9 2 TO 6 REPEAT 16 6 7
The incidences of element number 1 is defined and the data is used as the basis for generating the 2nd through the 6th element. The incidence pattern of the first 6 elements is then used to generate the incidences of 96 more elements using the REPEAT command.
UNIT INCH ELEMENT PROPERTIES 1 TO 102 TH 8.0
The thickness of elements 1 to 102 is specified as 8.0 inches following the command ELEMENT PROPERTIES.
CONSTANTS E 4000.0 ALL POISSON 0.12 ALL
The modulus of elasticity (E) and Poisson’s Ratio are specified following the command CONSTANTS.
SPRING COMPRESSION 1 TO 126 KFY
The above two lines declare the spring supports at nodes 1 to 126 as having the compression-only attribute. The supports themselves are being generated later (see the ELASTIC MAT command which appears later).
UNIT FEET SUPPORTS 1 TO 126 ELASTIC MAT DIRECTION Y SUBGRADE 12.0
Part I - Application Examples Example Problem 27 268
The above command is used to instruct STAAD to generate support springs which are effective in the global Y direction. These springs are located at nodes 1 to 126. The subgrade modulus of the soil is specified as 12 kip/cu.ft. The program will determine the area under the influence of each joint and multiply the influence area by the subgrade modulus to arrive at the spring stiffness for the "FY" degree of freedom at the joint. Units for length are changed to FEET to facilitate the input of subgrade modulus of soil. Additional information on this feature may be found in the STAAD Technical Reference Manual.
LOAD 1 'WEIGHT OF MAT & EARTH' ELEMENT LOAD 1 TO 102 PR GY -1.50
The above data describe a static load case. A pressure load of 1.50 kip/sq.ft acting in the negative global Y direction is applied on all the elements.
PERFORM ANALYSIS PRINT STATICS CHECK CHANGE
Tension/compression cases must each be followed by PERFORM ANALYSIS and CHANGE commands. The CHANGE command restores the original structure to prepare it for the analysis for the next primary load case.
LOAD 2 'COLUMN LOAD-DL+LL' JOINT LOADS 1 2 FY -217. 8 9 FY -109. 5 FY -308.7 6 FY -617.4 22 23 FY -410. 29 30 FY -205. 26 FY -542.7 27 FY -1085.4 43 44 50 51 71 72 78 79 FY -307.5 47 54 82 FY -264.2
Example Problem 27 269
48 55 76 83 FY -528.3 92 93 FY -205.0 99 100 FY -410.0 103 FY -487.0 104 FY -974.0 113 114 FY -109.0 120 121 FY -217.0 124 FY -273.3 125 FY -546.6 PERFORM ANALYSIS PRINT STATICS CHECK CHANGE
Load case 2 consists of several joint loads acting in the negative global Y direction. This is followed by another ANALYSIS command. The CHANGE command restores the original structure once again for the forthcoming load case.
LOAD 3 'COLUMN OVERTURNING LOAD' ELEMENT LOAD 1 TO 102 PR GY -1.50 JOINT LOADS 1 2 FY -100. 8 9 FY -50. 5 FY -150.7 6 FY -310.4 22 23 FY -205. 29 30 FY -102. 26 FY -271.7 27 FY -542.4 43 44 50 51 71 72 78 79 FY -153.5 47 54 82 FY -132.2 48 55 76 83 FY -264.3 92 93 FY 102.0 99 100 FY 205.0 103 FY 243.0 104 FY 487.0 113 114 FY 54.0 120 121 FY 108.0
Part I - Application Examples Example Problem 27 270
124 FY 136.3 125 FY 273.6 PERFORM ANALYSIS PRINT STATICS CHECK
Load case 3 consists of several joint loads acting in the upward direction at one end and downward on the other end to apply an overturning moment that will lift off one end. The CHANGE command is not needed after the last analysis.
LOAD LIST 3 PRINT JOINT DISPLACEMENTS LIST 113 114 120 121 PRINT ELEMENT STRESSES LIST 34 67 PRINT SUPPORT REACTIONS LIST 5 6 12 13
A list of joint displacements, element stresses for elements 34 and 67, and support reactions at a list of joints, are obtained for load case 3, with the help of the above commands.
FINISH
The STAAD run is terminated.
Example Problem 27 271
**************************************************** * * * STAAD.Pro * * Version Bld * * Proprietary Program of * * Research Engineers, Intl. * * Date= * * Time= * * * * USER ID: * **************************************************** 1. STAAD SPACE SLAB ON GRADE 2. * SPRING COMPRESSION EXAMPLE 3. SET NL 3 4. UNIT FEET KIP 6. JOINT COORDINATES 7. 1 0.0 0.0 40.0 8. 2 0.0 0.0 36.0 9. 3 0.0 0.0 28.167 10. 4 0.0 0.0 20.333 11. 5 0.0 0.0 12.5 12. 6 0.0 0.0 6.5 13. 7 0.0 0.0 0.0 14. REPEAT ALL 3 8.5 0.0 0.0 15. REPEAT 3 8.0 0.0 0.0 16. REPEAT 5 6.0 0.0 0.0 17. REPEAT 3 8.0 0.0 0.0 18. REPEAT 3 8.5 0.0 0.0 20. ELEMENT INCIDENCES 21. 1 1 8 9 2 TO 6 22. REPEAT 16 6 7 24. UNIT INCH 25. ELEMENT PROPERTIES 26. 1 TO 102 TH 8.0 28. CONSTANTS 29. E 4000.0 ALL 30. POISSON 0.12 ALL 32. SPRING COMPRESSION 33. 1 TO 126 KFY 35. UNIT FEET 36. SUPPORTS 37. 1 TO 126 ELASTIC MAT DIRECTION Y SUBGRADE 12.0 39. LOAD 1 'WEIGHT OF MAT & EARTH' 40. ELEMENT LOAD 41. 1 TO 102 PR GY -1.50 43. PERFORM ANALYSIS PRINT STATICS CHECK P R O B L E M S T A T I S T I C S ----------------------------------- NUMBER OF JOINTS/MEMBER+ELEMENTS/SUPPORTS = 126/ 102/ 126 ORIGINAL/FINAL BAND-WIDTH= 8/ 8/ 54 DOF TOTAL PRIMARY LOAD CASES = 1, TOTAL DEGREES OF FREEDOM = 756 SIZE OF STIFFNESS MATRIX = 41 DOUBLE KILO-WORDS REQRD/AVAIL. DISK SPACE = 13.0/ 40260.7 MB **NOTE-Tension/Compression converged after 1 iterations, Case= 1 STATIC LOAD/REACTION/EQUILIBRIUM SUMMARY FOR CASE NO. 1 'WEIGHT OF MAT & EARTH' ***TOTAL APPLIED LOAD ( KIP FEET ) SUMMARY (LOADING 1 ) SUMMATION FORCE-X = 0.00 SUMMATION FORCE-Y = -7740.00 SUMMATION FORCE-Z = 0.00 SUMMATION OF MOMENTS AROUND THE ORIGIN- MX= 154800.01 MY= 0.00 MZ= -499230.03 ***TOTAL REACTION LOAD( KIP FEET ) SUMMARY (LOADING 1 ) SUMMATION FORCE-X = 0.00 SUMMATION FORCE-Y = 7740.00 SUMMATION FORCE-Z = 0.00 SUMMATION OF MOMENTS AROUND THE ORIGIN-
Part I - Application Examples Example Problem 27 272
MX= -154800.01 MY= 0.00 MZ= 499230.02 MAXIMUM DISPLACEMENTS ( INCH /RADIANS) (LOADING 1) MAXIMUMS AT NODE X = 0.00000E+00 0 Y = -1.50000E+00 1 Z = 0.00000E+00 0 RX= 9.51342E-10 121 RY= 0.00000E+00 0 RZ= -4.19726E-10 80 ************ END OF DATA FROM INTERNAL STORAGE ************ 44. CHANGE 46. LOAD 2 'COLUMN LOAD-DL+LL' 47. JOINT LOADS 48. 1 2 FY -217. 49. 8 9 FY -109. 50. 5 FY -308.7 51. 6 FY -617.4 52. 22 23 FY -410. 53. 29 30 FY -205. 54. 26 FY -542.7 55. 27 FY -1085.4 56. 43 44 50 51 71 72 78 79 FY -307.5 57. 47 54 82 FY -264.2 58. 48 55 76 83 FY -528.3 59. 92 93 FY -205.0 60. 99 100 FY -410.0 61. 103 FY -487.0 62. 104 FY -974.0 63. 113 114 FY -109.0 64. 120 121 FY -217.0 65. 124 FY -273.3 66. 125 FY -546.6 68. PERFORM ANALYSIS PRINT STATICS CHECK **NOTE-Tension/Compression converged after 1 iterations, Case= 2 STATIC LOAD/REACTION/EQUILIBRIUM SUMMARY FOR CASE NO. 2 'COLUMN LOAD-DL+LL' ***TOTAL APPLIED LOAD ( KIP FEET ) SUMMARY (LOADING 2 ) SUMMATION FORCE-X = 0.00 SUMMATION FORCE-Y = -13964.90 SUMMATION FORCE-Z = 0.00 SUMMATION OF MOMENTS AROUND THE ORIGIN- MX= 301253.66 MY= 0.00 MZ= -884991.47 ***TOTAL REACTION LOAD( KIP FEET ) SUMMARY (LOADING 2 ) SUMMATION FORCE-X = 0.00 SUMMATION FORCE-Y = 13964.90 SUMMATION FORCE-Z = 0.00 SUMMATION OF MOMENTS AROUND THE ORIGIN- MX= -301253.66 MY= 0.00 MZ= 884991.47 MAXIMUM DISPLACEMENTS ( INCH /RADIANS) (LOADING 2) MAXIMUMS AT NODE X = 0.00000E+00 0 Y = -1.09725E+01 120 Z = 0.00000E+00 0 RX= 7.89606E-02 99 RY= 0.00000E+00 0 RZ= 9.69957E-02 6 ************ END OF DATA FROM INTERNAL STORAGE ************ 69. CHANGE 71. LOAD 3 'COLUMN OVERTURNING LOAD'
Example Problem 27 273
72. ELEMENT LOAD 73. 1 TO 102 PR GY -1.50 74. JOINT LOADS 75. 1 2 FY -100. 76. 8 9 FY -50. 77. 5 FY -150.7 78. 6 FY -310.4 79. 22 23 FY -205. 80. 29 30 FY -102. 81. 26 FY -271.7 82. 27 FY -542.4 83. 43 44 50 51 71 72 78 79 FY -153.5 84. 47 54 82 FY -132.2 85. 48 55 76 83 FY -264.3 86. 92 93 FY 102.0 87. 99 100 FY 205.0 88. 103 FY 243.0 89. 104 FY 487.0 90. 113 114 FY 54.0 91. 120 121 FY 108.0 92. 124 FY 136.3 93. 125 FY 273.6 95. PERFORM ANALYSIS PRINT STATICS CHECK **START ITERATION NO. 2 **START ITERATION NO. 3 **START ITERATION NO. 4 **START ITERATION NO. 5 **NOTE-Tension/Compression converged after 5 iterations, Case= 3 STATIC LOAD/REACTION/EQUILIBRIUM SUMMARY FOR CASE NO. 3 'COLUMN OVERTURNING LOAD' ***TOTAL APPLIED LOAD ( KIP FEET ) SUMMARY (LOADING 3 ) SUMMATION FORCE-X = 0.00 SUMMATION FORCE-Y = -10533.10 SUMMATION FORCE-Z = 0.00 SUMMATION OF MOMENTS AROUND THE ORIGIN- MX= 213519.36 MY= 0.00 MZ= -478687.78 ***TOTAL REACTION LOAD( KIP FEET ) SUMMARY (LOADING 3 ) SUMMATION FORCE-X = 0.00 SUMMATION FORCE-Y = 10533.10 SUMMATION FORCE-Z = 0.00 SUMMATION OF MOMENTS AROUND THE ORIGIN- MX= -213519.36 MY= 0.00 MZ= 478687.78 MAXIMUM DISPLACEMENTS ( INCH /RADIANS) (LOADING 3) MAXIMUMS AT NODE X = 0.00000E+00 0 Y = 2.83669E+01 120 Z = 0.00000E+00 0 RX= -1.22268E-01 120 RY= 0.00000E+00 0 RZ= 1.09786E-01 125 ************ END OF DATA FROM INTERNAL STORAGE ************ 97. LOAD LIST 3 98. PRINT JOINT DISPLACEMENTS LIST 113 114 120 121
Part I - Application Examples Example Problem 27 274
JOINT DISPLACEMENT (INCH RADIANS) STRUCTURE TYPE = SPACE ------------------ JOINT LOAD X-TRANS Y-TRANS Z-TRANS X-ROTAN Y-ROTAN Z-ROTAN 113 3 0.00000 19.17264 0.00000 -0.09579 0.00000 0.06945 114 3 0.00000 14.53915 0.00000 -0.09437 0.00000 0.06506 120 3 0.00000 28.36691 0.00000 -0.12227 0.00000 0.10056 121 3 0.00000 22.49737 0.00000 -0.11615 0.00000 0.08912 ************** END OF LATEST ANALYSIS RESULT ************** 99. PRINT ELEMENT STRESSES LIST 34 67 ELEMENT STRESSES FORCE,LENGTH UNITS= KIP FEET ---------------- STRESS = FORCE/UNIT WIDTH/THICK, MOMENT = FORCE-LENGTH/UNIT WIDTH ELEMENT LOAD SQX SQY MX MY MXY VONT VONB SX SY SXY TRESCAT TRESCAB 34 3 -4.50 -6.74 2.45 7.99 6.96 188.81 188.81 0.00 0.00 0.00 202.25 202.25 TOP : SMAX= 171.62 SMIN= -30.64 TMAX= 101.13 ANGLE= -34.2 BOTT: SMAX= 30.64 SMIN= -171.62 TMAX= 101.13 ANGLE= -34.2 67 3 37.83 6.21 -57.38 5.58 43.51 1303.44 1303.44 0.00 0.00 0.00 1449.91 1449.91 TOP : SMAX= 375.29 SMIN= -1074.62 TMAX= 724.96 ANGLE= -27.1 BOTT: SMAX= 1074.62 SMIN= -375.29 TMAX= 724.96 ANGLE= -27.1 **** MAXIMUM STRESSES AMONG SELECTED PLATES AND CASES **** MAXIMUM MINIMUM MAXIMUM MAXIMUM MAXIMUM PRINCIPAL PRINCIPAL SHEAR VONMISES TRESCA STRESS STRESS STRESS STRESS STRESS 1.074621E+03 -1.074621E+03 7.249564E+02 1.303438E+03 1.449913E+03 PLATE NO. 67 67 67 67 67 CASE NO. 3 3 3 3 3 ********************END OF ELEMENT FORCES******************** 100. PRINT SUPPORT REACTIONS LIST 5 6 12 13 SUPPORT REACTIONS -UNIT KIP FEET STRUCTURE TYPE = SPACE ----------------- JOINT LOAD FORCE-X FORCE-Y FORCE-Z MOM-X MOM-Y MOM Z 5 3 0.00 148.06 0.00 0.00 0.00 0.00 6 3 0.00 168.10 0.00 0.00 0.00 0.00 12 3 0.00 149.08 0.00 0.00 0.00 0.00 13 3 0.00 153.60 0.00 0.00 0.00 0.00 ************** END OF LATEST ANALYSIS RESULT **************
Question : How do I remove a load from a member without removing it from the load case?
Answer :
To demonstrate this, let us open EXAMP_01 located in the UK examples folder: X:\SPRO2005\STAAD\EXAMPLES\UK where
"X:" is the drive, and "SPRO2005" is the name of the installation folder.
The following picture will appear on the screen. We will explore two different ways of removing a load from a specific member. The load will continue to be present on the other members on which it was originally applied..
1
Say that we want to remove the load from member 10. To identify the member, let us first switch the beam numbers on. To do this, we can either press Shift + B on the key board or go to View | Structure Diagrams from the main menu and then switch on the beam numbers on from the Labels tab.
2
Next, go to the Load Page from the left side of the screen.
3
On the right side of the screen, there is a dialog box titled Load. Here, expand Load Cases Details.
4
Under load 1, highlight the expression UNI Y -13.5 kN/m. We will notice that this load is currently assigned to members 8, 9, 10, 11, 12, and 13.
Our goal is to remove the load from member 10. Here are the two methods: Method 1 � using the Toggle Load button The Toggle Load button is a switch setting which turns on what is called the toggle mode.
5
In this mode, when an attribute is selected and assigned using the "Use Cursor to Assign" method, the following happens.
• Click on the entity once - the attribute gets assigned. • Click on the same entity a second time - the attribute gets de-assigned. • Click on the same entity again, - the attribute gets re-assigned.
Thus each click will result in an assign if the attribute was un-assigned, and a de-assign if the attribute was assigned.
Let us use this Toggle Load option to remove the load from member 10. First, switch the Toggle Load box on. Then, after making sure that the �Use Cursor To Assign� method is selected, click on the Assign button.
The cursor will change as shown below.
6
Using this cursor, click on member 10. You will see that the load applied on member 10 has been removed.
To stop the process of removing loads, either hit the �Esc� key or go back and click on the Assign button again.
7
Method 2 � using the Edit button In this method, we will use the Edit button in the Load dialog box.
First, make sure that the load item is selected. Then, click on the Edit button. (You may also double-click on the expression UNI Y -13.5 kN/m. This will also bring up the Edit dialog box shown in the next page).
8
The following Edit dialog box will appear. Here, notice that the members on which the uniform force has been applied are listed.
Let us uncheck the box next to Member 10. Then, click on the Change button.
Once that is done, close the Edit dialog box.
9
You will see that the load has been removed from member 10.
10
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