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INTRODUCTION
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Design ObjectivesFor reinforced concrete structures, the
design objectives of the structural
engineer typically consist of the
following:1. To configure a workable and
economical structural system. This
involves the selection of the
appropriate structural types and laying
out the locations and arrangement of
structural elements such as columnsand beams.
2. To select structural dimensions,
depth and width, of individual
members, and the concrete cover.
3. To determine the required
reinforcement, both longitudinal andtransverse.
4. Detailing of reinforcement such as
development lengths, hooks, and
bends.
5. To satisfy serviceability requirements
such as deflections and crack widths
Loads
Analysis is performedon an idealized
structure .
1.Determinate
Structures.
2. Indeterminate
Structures.
3. Matrix Structural
Analysis
4. Finite Element
Analysis
Materials
Mechanics of materialsand structures
1. Steel Structures.
2. Wood Structures.
3. Bricks and Stones.
4. Concrete Structures.
5. Composite Structures
6. Reinforced ConcreteStructures.
7. Prestressed Concrete
Structures
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Design
The task of the structural engineer is to design a structure which satisfies
the needs of the client and the user. Specifically the structure should be
safe, economical to build and maintain, and aesthetically pleasing. But what
does the design process involve?Design is a word that means different things to different people. In
dictionaries the word is described as a mental plan, preliminary sketch,
pattern, construction, plot or invention. Even among those closely involved
with the built environment there are considerable differences in
interpretation. Architects, for example, may interpret design as being the
production of drawings and models to show what a new building willactually look like.
To civil and structural engineers, however, design is
taken to mean the entire planning process
for a new building structure, bridge, tunnel, road, etc., from outline
concepts and feasibility studies through mathematical calculations toworking drawings which could show every last nut and bolt in the project.
Together with the drawings there will be bills of quantities, a specification
and a contract, which will form the necessary legal and organizational
framework within which a contractor, under the supervision of engineers
and architects, can construct the scheme.
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There are many inputs into the
engineering design process as
illustrated by Fig.
1. client brief
2. experience3. imagination
4. a site investigation
5. model and laboratory tests
6. economic factors
7. environmental factors
Structural Design
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Allowable Stress Design
The design concept is based on the elastic
theory assuming a straight line stress
distribution along the depth of the
concrete section under service loads. The
members are proportioned on the basis
of certain allowable stresses in concrete
and steel. The allowable stresses are
fractions of the crushing strength of
concrete and yield strength of steel. This
method has been deleted from the ACI
Code. The application of this approach is
still used in the design of prestressed
concrete members under service load
conditions
Ultimate/Unified Strength
DesignThe unified design method (UDM) is
based on the strength of structural
members assuming a failure condition,
whether due to the crushing of the
concrete or to the yield of the reinforcing
steel bars. Although there is some
additional strength in the bars after
yielding (due to strain hardening), this
additional strength is not considered in
the analysis of reinforced concrete
members. In this approach, the actual
loads, or working loads, are multiplied by
load factors to obtain the factored design
loads. The load factors represent a high
percentage of the factor for safety
required in the design
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Alternative Design Method
A second approach for the design ofreinforced and prestressed concrete
flexural and compression members is called
the strength design method, or the
alternative provisions (ADM), as introduced
in the ACI Code, Appendix B. When this
method is used in the design, the designer
must adhere to all sections of Appendixes
Band C and substitute accordingly for the
corresponding sections of the Code.
Reinforcement limits, strength reduction
factors, load factors, and moment
redistribution are affected. The provisions
of this method satisfy the Code and are
equally acceptable
Strut and Tie MethodAn other approach for the design of
concrete members is called the strut and
tie method (STM). The provisions of this
method are introduced in the ACI Code,
Appendix A. It applies effectively in
regions of discontinuity such as supportand load applications on beams.
Consequently, the structural element is
divided into segments and then analyzed
using the truss analogy approach, where
the concrete resists compression forces as
a strut, while the steel reinforcement
resists tensile forces as a tie.
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Limit State DesignLimit state design is a further step in the
strength design method. It indicates the state
of the member in which it ceases to meet the
service requirements such as losing its ability
to withstand external loads, or suffering
excessive deformation, cracking, or local
damage. According to the limit state design,
reinforced concrete members have to be
analyzed with regard to three limiting states:
1. Load carrying capacity (safety, stability, and
durability)
2. Deformation (deflections, vibrations, and
impact)3. The formation of cracks.
The aim of this analysis is to ensure that no
limiting state will appear in the structural
member during its service life
Loads are those forces for which a given
structure should be proportioned. In
general, loads may be classified as deador live
Dead loads include the weight of the
structure (its self-weight) and any
permanent material placed on the
structure, such as tiles, roofing materials,and walls. Dead loads can be determined
with a high degree of accuracy from the
dimensions of the elements and the unit
weight of materials.
Live loads are all other loads that are notdead loads. They may be steady or
unsteady or movable or moving; they
may be applied slowly, suddenly,
vertically, or laterally, and their
magnitudes may fluctuate with time.
Loads
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Type of Live LoadsOccupancy loads caused by
the weight of the people,
furniture, and goods
Forces resulting from wind
action and temperature
changes
The weight of snow if
accumulation is probable
The pressure of liquids or
earth on retaining structures
The weight of traffic on a
bridge
Dynamic forces resulting
from moving loads (impact),
earthquakes, or blast loading
Live loads for highway bridges are specified by the
American Association of State Highway and
Transportation Officials (AASHTO) in its LRFD Bridge
Design Specifications
For railway bridges, the American Railway Engineering
and Maintenance-of-Way Association (AREMA) has
published the Manual of Railway Engineering , which
specifies traffic load
Environmental loads consist mainly of snow loads, wind
pressure and suction, earthquake loads (i.e., inertia
forces caused by earthquake motions), soil pressures
on subsurface portions of structures, loads from
possible ponding of rainwater on flat surfaces,and forces caused by temperature differentials. Like
live loads, environmental loads at any given time are
uncertain in both magnitude and distribution. ASCE
manual contains much information on environmental
loads, which is often modified locally depending, for
instance, on local climatic or seismic conditions.
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Occupancy LoadsThe minimum live loads for which the floors and roof of a building should be
designed are usually specified in the building code that governs at the site of
construction. Representative values of minimum live loads to be used in a wide variety
of buildings are found in Minimum Design Loads for Buildings and Other Structures ofAmerican Society of Civil Engineers (ASCE).
The table gives uniformly distributed live loads for various types of
occupancies; these include impact provisions where necessary. These loads are
expected maxima and considerably exceed average values.
In addition to these uniformly distributed loads, it is recommended that as an
alternative to the uniform load floors be designed to support safely certainconcentrated loads if these produce a greater stress.
Certain reductions are often permitted in live loads for members supporting
large areas, on the premise that it is not likely that the entire area would be fully loaded
at one time.
Tabulated live loads cannot always be used. The type of occupancy should be
considered and the probable loads computed as accurately as possible. Warehouses forheavy storage may be designed for loads as high as 500 psf or more; unusually heavy
operations in manufacturing buildings may require an increase in the 250 psf value
specified in Table ; special provisions must be made for all definitely located heavy
concentrated loads.
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For occupancies or uses not
designated in this book's chapter4,
the live load shall be determined in
accordance with a method
approved by the authority having
jurisdiction.
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Simple HousesFor structural members in
one and two-family dwellings
supporting more than one
floor load, the following floorlive load reduction shall be
permitted as an alternative to
Eq. 4.7-1:
L = 0.7 (Lo1 + Lo2 + )
Lo1, Lo2,
are theunreduced floor live loads
applicable to each of multiple
supported story levels
regardless of tributary area.
The reduced floor live load
effect, L, shall not be lessthan that produced by the
effect of the largest
unreduced floor live load on a
given story level acting alone.
Heavy Live Loads
Live loads that exceed 100 lb/ft2 shall not be reduced.
EXCEPTION:
Live loads for members supporting two or more floors
shall be permitted to be reduced by 20 percentLimitations on One-Way Slabs
The tributary area,At for one-way slabs shall not exceed
an area defined by the slab span times a width normal to
the span of 1.5 times the slab span.
Seismic forces may be found for a particular structure byelastic or inelastic dynamic analysis, considering expected
ground accelerations and the mass, stiffness, and damping
characteristics of the construction. However, often the
design is based on equivalent static forces . The base shear
is found by considering such factors as location, type of
structure and its occupancy, total dead load, and theparticular soil condition. The total lateral force is
distributed to floors over the entire height of the structure
in such a way as to approximate the distribution of forces
obtained from a dynamic analysis.
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Design Criteria
In achieving the design objectives, there are four general design criteria of SAFE that must be
satisfied:
1. Safety, strength, and stability. Structural systems and member must be designed with
sufficient margin of safety against failure.
2. Aesthetics. Aesthetics include such considerations as shape, geometrical proportions,
symmetry, surface texture, and articulation. These are especially important for structures of
high visibility such as signature buildings and bridges. The structural engineer must work in
close coordination with planners, architects, other design professionals, and the affected
community in guiding them on the structural and construction consequences of decisions
derived from aesthetical considerations.
3. Functional requirements. A structure must always be designed to serve its intended
function as specified by the project requirements. Constructability is a major part of the
functional requirement. A structural design must be practical and economical to build.
4. Economy. Structures must be designed and built within the target budget of the project.For reinforced concrete structures, economical design is usually not achieved by minimizing
the amount of concrete and reinforcement quantities. A large part of the construction cost
are the costs of labor, formwork, and false work. Therefore, designs that replicate member
sizes and simplify reinforcement placement to result in easier and faster construction will
usually result in being more economical than a design that achieves minimum material
Quantities.
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Design LoadsTo serve its purpose, a
structure must be safe against
collapse and serviceable in use.
Serviceability requiresthat deflections be adequately
small; that cracks, if any, be kept
to tolerable limits; that vibrations
be minimized; etc.
Safety requires that the
strength of the structure beadequate for all loads that may
foresee ably act on it.
If the strength of a
structure, built as designed, could
be predicted accurately, and if the
loads and their internal effects(moments, shears, axial forces)
were known accurately, safety
could be ensured by providing a
carrying capacity just barely in
excess of the known loads.
Uncertainty in Design1. Actual loads may differ from those assumed.
2. Actual loads may be distributed in a manner
different from that assumed.
3. The assumptions and simplifications inherent in anyanalysis may result in
calculated load effects-moments, shears, etc.-different
from those that, in fact, act in the structure.
4. The actual structural behavior may differ from that
assumed, owing to imperfect knowledge.
5. Actual member dimensions may differ from thosespecified.
6. Reinforcement may not be in its proper position.
7. Actual material strength may be different from that
specified.
Real Load = Design load x (1 + A )Real Strength = Design Strength x (1 B )
For safety real load= real strength
Design strength/design load = FOS=( 1+A)/(1-B)
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Advantages of Strength Design
1. The derivation of the strength design expressions takes into account the nonlinear shape of
the stressstrain diagram. When the resulting equations are applied, decidedly better estimates
of load-carrying ability are obtained.
2. With strength design, a more consistent theory is used throughout the designs of reinforced
concrete structures. For instance, with working-stress design the transformed-area or straight-line method was used for beam design, and a strength design procedure was used for columns.
3. A more realistic factor of safety is used in strength design. The designer can certainly estimate
the magnitudes of the dead loads that a structure will have to support more accurately than he
or she can estimate the live and environmental loads. With working stress design, the same
safety factor was used for dead, live, and environmental loads. This is not the case for strength
design. For this reason, use of different load or safety factors in strength design for the differenttypes of loads is a definite improvement.
4. A structure designed by the strength method will have a more uniform safety factor against
collapse throughout. The strength method takes considerable advantage of higher strength
steels, whereas working-stress design did so only partly. The result is better economy for
strength design.
5. The strength method permits more flexible designs than did the working-stress method. Forinstance, the percentage of steel may be varied quite a bit. As a result, large sections may be
used with small percentages of steel, or small sections may be used with large percentages of
steel. Such variations were not the case in the relatively fixed working stress method. If the
same amount of steel is used in strength design for a particular beam as would have been used
with WSD, a smaller section will result. If the same size section is used as required by WSD, a
smaller amount of steel will be required.