Etabs engtips

Bigbeam (Structural) 22 Jun 08 10:49

I am trying to import my column locations from a dxf file but can't seem to get it to work. There was mention about creating the columns as blocks in autocad but I can't get that to work. I cant even get the pull down box in the import dxf to pick the layer that I have the columns on Ii would have thought columns would be points).

To bad there isn't an option to just get the entities into etabs and let the user do the actual assignments.

What am I doing wrong?

K slickdeals (Structural) 22 Jun 08

20:59 There are 2 ways to do it. One is the fancy way using blocks, the other is plain and simple way.

1. If you are using blocks, you need to draw a closed shape, I usually use a circle and define that as a new block. Add a new layer and then define all the columns defined as blocks in that layer. Save it as a dxf and when you import it, select that particular layer in the columns drop down box. It usually works 95% of the time.

2. The less fancy and error-free way to do it is to draw an "X" or a "+". Draw lines and not polylines. Once this is done for all columns, save and export as a dxf. Import the columns as beams. Once you do this, you get an "X" or "+" at each column location. Then all you have to do is to click at the intersections and you have your column layout. You can later delete the "X" and "+".

HTH Bigbeam (Structural) 23 Jun 08

20:09 I couldn't get the block method to work (I have no idead why they wouldn't do it with point objects as this is the way safe does it.

Next question is how do you import walls? I can't seem to find any way to easily convert a beam to a wall in etabs.

-k- slickdeals (Structural) 23 Jun 08

22:20 No way to automatically do it. You have to bring them in as beams and then draw wall elements connecting the ends in plan. drile007 (Structural) 24 Jun 08

2:49 You can import walls through dxf with 3D polyline or 3Dface. I prefer 3D polyline. Bigbeam (Structural) 24 Jun 08

15:36 drile007I can get a beam in but not a wall. How did you convert a beam to a wall? As slickdeals

said, you have to do that manually.-k- drile007 (Structural) 25 Jun 08

9:42 I didn't convert beam to a wall, I've drawn the walls in Acad with entities like 3dFace or polyline. Then I import dxf file into Etabs. drp181 (Structural) 29 Jul 08

3:44 You can import lines as beams, then convert the beams to walls using the "Extrude Lines to Areas" command. Set the delta z to the story height. This can be done for all walls at once, and would seem to be simpler than drawing 3D shapes in CAD.

zstone (Structural) 19 Jun 08 12:44

which is best way?In some training manuals of CSI,and video tutorials, shear walls are not meshed.Complet shear wall in one story is included as one finite element, this reason is for design of shear wall procedure or fore anyother reason, i dont knowi thing that to meshing shear waall is best way, because results will bee more logic and more realistic than in other case(unmeshed shear wall),sorry for my englishbest regards

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jugurtha12 (Civil/Environme) 28 Jun 08 4:35

I THINK THAT WE CAN NOT MESH A SHEAR WALL BECAUSE IT IS A STRUCTURAL ELEMENT BUT WE CAN MESH IT ONLY WHEN THERE IS AN OPENING IN THE WALL SORY FOR MY INGLISH slickdeals (Structural) 28 Jun 08

8:53 Any unmeshed shell element has an artificially high stiffness. It is recommended that you mesh your walls especially if you are designing tall buildings because there will be more flexibility.

You can internally mesh a wall element. sreichwein (Structural) 25 Jul 08

20:22 Meshing a shear wall is much more accurate to the actual behavior. However, complex shear walls in ETABS are extremely tricky to mesh. I usually assign automesh to each wall pier and assign a maximum mesh size equal to 24". The tricky part is getting the mesh to work. You must make sure that all meshed shell element boundaries are attached to an adjacent shell element. If not, you will get errors in the analysis or the model will not run at all. Practice makes perfect, so I recommend playing around with it until you get it down. If you need more information, let me know.

alexbzeta (Structural) 30 Jun 08

10:26 I am trying to track story shear for individual frames for a steel braced frame system. I tried using pier labels, but ETABS only allows labels to be assigned to vertical frame members or area elements (works great for moment frames and concrete shear walls). The only other option I see is to define section cuts. This is a pain because I have to define all of the groups and then all of the section cuts - all with unique names.

Anyone have any better suggestions? Find A Job or Post a Job Opening Click Here.

stressed (Civil/Environme) 4 Jul 08 15:29

I think you're right about the section cuts. That's the only way I can think of either. sreichwein (Structural) 25 Jul 08

20:16 The shear and axial diagrams are also useful. I've done it both ways and would also love to know if there is an easier way.

Skicat (Structural) 7 May 08 12:39

I am a new ETABS user and I recently attended an ADAPT seminar talking about the integration of ETABS and ADAPT floor pro. I know ETABS doesn't do PT. We use ADAPT floor & PT8 for our slab & beam design. We don't use ETABS to design concrete floors or beams. We only use it for gravity and lateral analysis of the concrete frame. We do very little steel design compared to concrete, so I guess it is more suited for steel design than concrete. I was just curious what others are using it for and if I can get more out of ETABS than I currently am. Thank you.

Check Out Our Whitepaper Library. Click Here.TtheD (Structural) 8 May 08

15:47 We use ETABS primarily for concrete shearwall design. It gives the boundary zone lengths in the output. masomenos (Civil/Environme) 28 Jun 08

19:43 steel and concrete blgs, parking garages and small stuff too like trusses and single column and beam designs. If you already have ETABS and are using it for concrete, you can use ETABS column and shear wall design even if you're using Adapt for the slabs. Just keep in mind that ETABS is a general purpose structural analysis and design package with frame and shell/membrane finite elements, albeit with features oriented toward design of bldg structures (story concept). I heard that ETABS was used on the Boston tunnel big dig design, so it's versatile if you know the modeling techniques. sreichwein (Structural) 25 Jul 08

20:14 Generally, we just use ETABS for the analysis of concrete shear walls and concrete moment frames. Once we are able to limit the drift, we can output the forces from ETABS into a spreadsheet for design. We design the piers of the shears walls for flexure and axial loads using PCA Column. We design the moment frames using PCA Slab. For the gravity design of the concrete floor system, we use RAM Advanse and SAFE. ETABS is also very useful in designing complex steel braced frame and moment frame lateral systems. It is especially useful in seismic applications. However, RAM is more suitable for the gravity design of

steel floor systems, in my opinion. Often we have two models; a RAM gravity model and an ETABS lateral model. zstone (Structural) 4 Jul 08

17:29 I like do perform an Response Spectrum Analysis on Etabs, I have define a Design Spectrum By EC8 - import from file case, and i have set a scale factor value equal to 1.0,im i correct, because the Spectrum is defined to be Design , and is input in manual way in Etabs, I dont like to perform automatic Seismic Case, because is not includet as well in Etabs(by EC8) - I THINK SOcan anybody help me

regards

Check Out Our Whitepaper Library. Click Here.sreichwein (Structural) 25 Jul 08

20:06 Some seismic codes require the base shear of the modal analysis to be within a certain percentage of the code calculated base shear. This value is 80% in ASCE 7-05. The scale factor is used to scale the calculated ETABS base shear value to at least 80% of the code value.

Jordan2008 (Structural) 7 Jul 08 6:11

Etabs gives quite large moment and shear at the bottom of the shear wall (bottom of the pier). As a result reinforcements are large. at the top of the pier 3m above results are reasonable. What value I used top or bottom of the pier.

Check Out Our Whitepaper Library. Click Here.slickdeals (Structural) 11 Jul 08

17:23 Bottom of the pier. Because the top of the pier is equal to the bottom of the pier above. The forces at the floor level are captured at the bottom of the pier. sreichwein (Structural) 25 Jul 08

19:58 Design the piers as you would a concrete column. The largest moment is going to govern, which appears at the bottom of the pier.

rejoicer (Civil/Environme) 22 Jul 08 12:18

According to 10.11.1 of ACI318-05, concrete shear wall should use 0.70Ig in the model. How to consider 70% reduction in Etabs? Reduce modulus of elasticity to 70% or set stiffness modifiers to 0.7.

Thanks. structuresguy (Structural) 22 Jul 08

13:22 Set stiffness modifier to the appropriate amount, 0.7 in your case for walls, unless they are

cracked, then it would be 0.35 for walls. slickdeals (Structural) 23 Jul 08

14:42 The best way is to use stiffness modifiers

Use f11,f22 and f12 on wallsuse m11,m22 and m12 on slabs sreichwein (Structural) 25 Jul 08

19:53 I set the modifier for f22 equal to 0.70 when modeling the cracks of a shear wall. For framing elements (like coupling beams and moment frames), I set I33 equal to 0.35 to model the cracking.

zstone (Structural) 19 Jul 08 19:43

Hi EverybodyI have four Story building, Flat slab with perimeter beams,loads>Dead - self weight of structureLive - 3 kN/m2When i convert loads to masses, i use 2 and 3 options2. From Loads Dead - 1, and Live - 0.53. From self and specified mass and loads Live - 0.5

and booth cases give me diferent results,

I cant UNDERSTAND THAT

zstone slickdeals (Structural) 19 Jul 08

21:33 I am surprised that this is happening. I have tried that before never had a problem. Did you report it to tech support? zstone (Structural) 20 Jul 08

6:34 In fact the diference is small.

From case 2. (From Loads Dead - 1, and Live - 0.5) i occur period T1=0.738 secFrom case 3. (From self and specified mass and loads Live - 0.5) - i occur period T1=0.742 secI want just to be sure, about cases 2 and 3which is prefered way to convert loads to mass 2 or 3,because im new user in ETABS, i have make it before with other softwares, and i never havent problems. But in Etabs i like to find most prefered way

zstone

slickdeals (Structural) 20 Jul 08

8:43 I don't think there is a preferred way. I would use option 2 if you don't have any additional masses (point or area) specified. And 99.6% (0.738/0.742) is close enough for any engineering we do.

HTH

zstone (Structural) 3 Jul 08 18:21

Hi Everybody

Im new in Etabs, I'd like to define a linear release on edge of slab, for example: conection of slab with wall to be pinned, not fixed,With Robot Millennium this is posible and very easy, is this posible in Etabs or SAP

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Why not define the wall using membrane type area? That's easy enough, and it gives you the pinned connection at the slabs. zstone (Structural) 4 Jul 08

17:19 thanks stressed for helpyou idea is correcti dont know what will hapen with bending moments in slab,(moment values in case of pinned edge - and moment values in case of wall modeled as membrane - dhe values o bending moments in slab must be same in to cases of modeling)i will try it jugurtha12 (Civil/Environme) 5 Jul 08

8:52 I do it with sap and i think that is very easy with sap we select the area element then we right clik then we dot it with joint offset overwritesory for my english drile007 (Structural) 16 Jul 08

13:17 Quote (stressed):Why not define the wall using membrane type area?

OK, but when try to do a dynamic (response spectra) analyisis walls defines as membrane type will vibrate perpendicular to their own plane! First dozen mode shapes will be just described shapes and we don't want that! Can we prevent this effect?


Since it now appears both zstone AND drile007 are experiencing a near identical predicament, I'll offer my $0.02. As with any problems you experience with a structural software package, if you're getting strange results, then don't be shy about getting creative and experimenting with some alternatives, for example 1)define or assign property modifiers to a shell type wall bending properties such that it acts like a membrane to give you the desired pin connetion 2) mesh the wall and assign only a small bottom sliver of the wall connecting to the slab to be membrane with the rest of the wall assigned as shell type 3) if #1 and #2 don't result in suitable results for both static and dynamic analysis, then create one model for static analysis and another model for dynamic analysis 4) If #'s 1, 2, or 3 don't work, then drink a bottle of Jack Daniels, put the gun barrel in your mouth and, oh wait....just teasing

zstone/drile007, let us know what you find. I'd be curious to learn for myself from your experience. Good luck. drile007 (Structural) 17 Jul 08

3:33 Quote (stressed):1)define or assign property modifiers to a shell type wall bending properties such that it acts like a membrane to give you the desired pin connetion

I already tryed this and I got the same results as I described in my previous post.

Quote (stressed):2) mesh the wall and assign only a small bottom sliver of the wall connecting to the slab to be membrane with the rest of the wall assigned as shell typeI think that this kind of assignements doesn't describe real behaviour of the wall state. civilhak (Structural) 17 Jul 08

5:31 Hi people,should this not work if you define walls as shell elements and plates as membrane elements???? stressed (Civil/Environme) 17 Jul 08

11:28 drile, by releasing moments at the wall/slab intersection, the physics of the problem would seem to dictate a (possibly significant) change in the dynamic modal results, whether or not you model with frame elements or shell/membrane elements. Could you explain what results specifically that you are concerned with that is causing you problems? drile007 (Structural) 17 Jul 08

15:37 What happened if you define walls as membrane type:Such (membrane) elements don't have any stiffness perpendicular to their own plane, or with the other words; they can't "possess" any moments because their inertia moment is zero. Furthermore, they can't "resist" to any force perpendicular to their own plane. And what happened in dynamic analysis? When we mesh the wall we automatically add mass at each wall node. When we add horizontal accelerations (response spectra) we got horizontal forces perpendicular to walls in each node (mass x acceleration = force). Since wall is very soft to forces perpendicular to their own plane, we got huge displacement (mode shapes) perpendicular to their own plane. I hope that I didn't miss the point and I use the right words since my mother language isn't English.

To avoid this problem I do what civilhak suggest, I define walls as shell elements and plates as membrane elements!

MOHAMMEDIDREES (Structural) 22 Jan 08 2:25

When we perform dynamic analysis in the out put of building modes we have Modal Participation factors and Modal Participating mass ratios what are thsese and where do we use them in our calcsdrile007 (Structural) 22 Jan 08

9:52

In short: Modal Participation factor means what percent of defined mass is vibrating in particular mode shape.

Drile007Diquan (Civil/Environme) 1 Mar 08

8:20 be carefull if you're not familiar with "Dynamics of structures". I recomend reading books from Clough, Penzien, Chopra, Paulay and Priestley, they have really good books to introduce you into dynamics of the structures

sorry for my english

DiquanWhiffle (Structural) 19 Mar 08

14:18 You should really understand what you are doing before you just "jump" into dynamic analysis in ETABS (or any program). Dynamics of Structures by Chopra is a very good textbook with good examples. He goes through a Response Spectrum Analysis and a Response History Analysis and explains the steps very clearly.civilhak (Structural) 17 Jul 08

5:47 Hi,Modal Participating mass ratios indicates how much mass is activated in your modes...according to EC8 this value should be atleast close to 90%...if you are undir this value, you should run more modes undir til you reach this value.Lets say your MPMS is 60%, then your resulting forces are to low, if you increase the number of modes then the resulting forces increase...this 90% is a good indicator in showing that you are running enough numer of modes. Diquan (Civil/Environme) 17 Jul 08

7:24 Correct.

Most of the programs of analisys reach 100% very easy, because is something of equations and the response spectrum it's using.

The number of modes are very easy to obtain, in a building each floor have 3 DOF (degrees

of freedom), 2 displacements 1 rotasional, so let's say a usual building of 8 floors or storys is going to have 24 DOF that is 8 floors * 3 DOF = 24 DOF

Again, sorry for my english, i speak spanish.

Diquan

radger49 (Structural) 19 Jun 08 22:26

How do we know that the concrete column, beam, shearwall passed after running the concrete design? I have no problems with steel design because there is option to verify if the structure passed the design, unlike the concrete structure it has no option for that, thanks! TtheD (Structural) 24 Jun 08

16:53 The easiest way I have found to check if Concrete shearwalls fail (rather than right-clicking on each one individually) is to do to File>Print Tables. Print the output summary of the shearwall design table, and it will tell if each pier fails in shear or flexure. For concrete beams and columns just look for the red ones. You can change the utilization limit at which they turn red (0.95 is default) by going to Options>preferences>Concrete frame design.

lebanque66 (Structural) 5 Jun 08 21:15

I am new in using etabs to analyse the rc building frame. I find the software very user friendly. However when it comes to steel trusses (gable type), I am bit confused as to how to model it and load it (i.e, windload).

Hope you can give me advise or specific examples with regards to this matter. stressed (Civil/Environme) 7 Jun 08

19:02 You may have to add intermediate gridlines between stories using reference planes: Edit>Reference planes. Draw point objects at end point locations of your truss and "connect the dots" to draw your truss, including sloped top chords. Activate your 'snap to perpendicular' snap tool to help draw vertical truss web members connecting to the chords. You don't have to divide your chords into smaller sections to do this

Vonx (Structural) 31 Jan 08 15:59

Hi all, I've got a question about the base shear. After you do the response spectrum analysis in Etabs, how do you find the total base shear of the structure(multi-storey building to be more specific)? This is to work out the scaling factor. I was told that you can look at the "Story force" section of the 1st storey of the structure in the output report. Is that correct?

stressed (Civil/Environme) 2 Feb 08

13:17 No, that's not correct. Under Display menu>show tables, checkbox reactions/support reaction. There is a "summation" base shear reported along with individual restraint reactions. You can control which cases are to be reported using the options on the right side of the show tables.

drile007 (Structural) 2 Feb 08 16:59

You can get Story shears:1) Display/ Show Story Response Plots.../ Story Shears2) Display/ Show Tables.../ Building Output/ Story Shears

Drile007Vonx (Structural) 8 Feb 08

15:07 Thanks guys, that helped.Vonx (Structural) 12 Feb 08

16:09 Hi stressed:I was able to do the show table thing, thanks for that. However, I could not find the "summation" function mentioned in your last post. I had to manually add all the reaction forces together under same direction to find out the base shear. Would you please give some instructions on this one?

stressed (Civil/Environme) 26 Feb 08 20:04

Vonx, apologies, I was mistaken in my earlier reply regarding what the "summation" option offers you. I have been working more with SAP lately and I gave you some misleading advice. Here is what you need to do to obtain the summed base shear that you want - First, select all joint restraints at the base of your model, as well as the first floor columns and walls which are connected to those restraints, and assign them all to a group. Next, use Define menu>Section cuts and define a section cut based on the group you just created. After you've run your analysis, using Display menu>Show tables>Section cut forces, you should have the summed base shears. Let me know if that technique doesn't do what you're looking for.Vonx (Structural) 18 Mar 08

17:07 Hi to stressed/drile007:

Thanks for the replies. I have done a comparison using both the story shear method and the section cut forces summation method to calculate the base shear. In my case, the values are very close:

Vx (story force) : Vx (section cut) = 1 : 1.03Vy (story force): Vy (section cut) = 1: 0.98

As you can see, the differences are within 5%. I tend to believe both methods are valid in terms of calculating the base shear using Etabs. Please give your comments.GarthW (Structural) 26 Mar 08

15:29 Vonx

You should have a look at Display -> Show Tables -> Modal Information -> Table: Response Spectrum Base Reactions. They h

ave had to introduce a special routine to determine base reactions since all results from Response Analysis are positive. Summing base reactions or column shears would be wrong.

GarthW

Skicat (Structural) 24 Mar 08 11:58

Does anybody have much experience importing revit files into etabs and exporting etabs files to revit? It's not as intuitive as I thought it would be. Also, any opinions about Revit Structure in general? Thank you. prsconsultant (Structural) 25 Mar 08

5:25 We have been using REVIT as a tool to convert RAM files to ETABS. It has been working fine. We have no major issues converting back and forth between ETABS & REVIT.

I have heard that RISA is more compatible with REVIT than any other software.

Vonx (Structural) 2 Mar 08 16:53

Hi guys, when I draw a diagnal member such as a bracer or a long cord in a truss in Etabs, very so often it goes through more than one storey levels defined. The annoying thing about it is Etabs sometimes (not always) automatically divides the line at the storey levels. It causes troubles, for example, when I try to sub-divide this line into segements. I tried to use the "Join Lines" function and it did not work. All the "Splice Point" in the Story Data were turned off. Is there any way to avoid this automatic line divide? Thanks in advance.

Check Out Our Whitepaper Library. Click Here.slickdeals (Structural) 3 Mar 08

20:54 No, ETABS is a building analysis program where stories are pivotal to design. It will break any line that is a story. If you want to get around it, define reference planes instead.alexbzeta (Structural) 21 Mar 08

19:00 I think there is more to this. I have also had columns and braces that span stories. Sometimes they break and sometime they don't. I'd like to know why.

slickdeals (Structural) 21 Mar 08 19:04

They will always break at stories

aquacata (Structural) 7 Mar 08 11:37

After I ran my ETABS building model, I got a warning message of zero stiffness at a joint. Also it specified my model is unstable and ill-conditioned. In your experience, could you tell what is the most possible reason for that?

I suspect that my ramp area is not connected to its edge beams. I did select the ramp object and one of its sloped edge beam and meshed the ramp area. But I couldn't tell if it is meshed or not. Do you know a way?

Ramp cannot be assigned to a diaphragm. How can I trick it into a ridig diaphragm?

Thanks a lot for your help!Check Out Our Whitepaper Library. Click Here.

slickdeals (Structural) 9 Mar 08 9:27

Any sloped member in ETABS can never be assigned to a diaphragm. For a diaphragm to be assigned, the member has to be in a horizontal plane. What kind of elements have you assigned to the ramp? (shell, membrane?).

The way to see your final mesh is to go to set building view options and select "show mesh". This will show you the mesh and associated nodal connectivity. Do you have a curved ramp by any chance? More info would help.

aquacata (Structural) 9 Mar 08 12:15

Thank you, slickdeals! In your way, I saw my ramp is not meshed. I will make it meshed.

My ramp is straight. I assigned it as a shell since it needs to take out-of-plane gravity load. Could you please let me know to make it behave like a rigid diaphragm, what stiffness modifier need to change and change to how much?

I am excited that you replied my post!slickdeals (Structural) 9 Mar 08

12:45 There is no way you can make it behave like a rigid diaphragm using diaphragm constraints in ETABS. How thick is your concrete slab for out-of-plane loads? If it is 8" thick or more, it is pretty rigid itself and would not need any additional modifiers.

However, if it is not very thick, then you can increase the in-plane stiffness (f11,f22 and f12) to make it behave like a rigid body. Remember that these modifier values are a percentage, meaning if you make it 2, then it will be 2 times more stiff in resisting in plane loads.

I would suggest that you model the ramp explicitly and not assign any diaphragm constraints. For seismic loads, the masses are calculated automatically and for wind loads, it should not matter because the diaphragm width is calculated by the building width extents and hopefully, the ramp extents are within the building extents. Hope that makes sense.aquacata (Structural) 9 Mar 08

13:12 Yes that makes a lot sense. Thanks for your valuable help!

cityboy (Structural) 19 Feb 08 16:19

When checking the reaction forces I found that there were a few points numbered as negative (like -100) by ETABS, and more wired was that I got force reported at these negative points, the total force including these negative points was balanced with applied loads. However, no such negative points were reported from building model! No coordinates were associated with these negative points so that I couldn't track down them. Anybody has this problem before? I had 3 different building models and 2 out of 3 have the negative point number problem. How can I get rid of this negative point numbering? Thanks

Find A Job or Post a Job Opening Click Here.cityboy (Structural) 20 Feb 08

9:06 ok, the problem is solved. the negative point numbering comes after the auto mesh of the wall as ETABS adds a restraint to the mid-node by default.

drile007 (Structural) 25 Jan 08 6:45

What's the quickest way to determine the principal direction of the building?

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Murali27 (Structural) 29 Jan 08 11:10

Dirle

I am not sure of what is the structural system in the building,

I would say, quickly calculating section properties

Cheersdrile007 (Structural) 29 Jan 08

15:49

I was thinking about any system which is modeled by the Etabs.Can I help myself with the modal participating mass ratio.

Drile007Murali27 (Structural) 5 Feb 08

8:22 Drile

Yes, its correct

cheers

GarthJ (Structural) 9 Feb 08 20:14

Try to determine the angle of the base reaction for the first fundamental frequency. Check out the CSI web site. There is an article posted there with the title something like "Dynamic Analysis to Satisfy Building Codes" (or something like that)drile007 (Structural) 10 Feb 08

15:46 Thanks GarthJ...great readings. Now I see that looking for principal direction is meaningless if I do the right combination of modal maximums and spectra accelerations. RafikCGS (Structural) 14 Feb 08

8:16 One way is to perform a preliminary modal analysis; the principal direction corresponds to the direction of the first mode translational direction, translational first mode of vibration corresponds to the most flexible direction.

Skicat (Structural) 28 Jan 08 15:29

A senior engineer asked me if ETABS can model a structure with offset expansion joints. The EJ at the first level occurs in a different bay then the EJ's in the levels above. Has anyone had any experience with modeling this condition in ETABS? Thanks.

Eng-Tips Forums is Member Supported. Click Here to donate.Murali27 (Structural) 29 Jan 08

11:13 skicat

possible, but depends on the stability of the part which is on the other level. Not sure of the structural system what you have.

Cheersstressed (Civil/Environme) 2 Feb 08

13:03 Define a new membrane type slab to use for your expansion joint. Membrane type has in-plane stiffness only so as not to transfer moment. You can adjust the area property modifiers in order to transfer shear only in 1 direction


Few months ago I've import DXF file to Etabs with no problem. Walls and floors were Polyline entities. Now I made DXF file where walls and floors are Lwpolyline entities. Importing doesn't work. I tried with command PLINETYPE in Autocad to "convert" Lwpolyline to Polyline but it didn't help. Can you help me out?

Thank you in advance

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slickdeals (Structural) 28 Jan 08 12:42

For floors use a polyline, but make sure it is a closed polyline.For walls, import them as beams (draw them as lines). Trace the walls in plan on the beamlines.

JohnB965 (Structural) 5 Jan 08 15:12

I am currently using ETABS and UBC Code to model buildings in the middle east. Although ETABS calculates lateral drifts I am not sure of the following :-

1. ETABS calculates lateral drifts based on the SRSS earthquake load case. Does this value need to be multiplied by 0.7 R to obtain maximum inelastic displacements, or is this already included in the analysis. The maximum value of this drift has an upper limit of 0.02

2. This is the part I am not sure about. I also want to limit service drifts to H/500. Do I simply divide the drift by 1.4 to get service loads or do I need to multiply the ETABS drifts by 0.7 R / 1.4

Check Out Our Whitepaper Library. Click Here.drile007 (Structural) 5 Jan 08

16:03

I'm not familar with UBC code, but 1. I think you shouldn't multipied the drifts with 0.7 R (I supposed this is ductility of the structure), since drifts are independent from ductility2. Same as 1.

Drile007slickdeals (Structural) 9 Jan 08

8:03 John, It is very difficult to control your building deflections to H/500 for a seismic load case. H/500 is more for wind load at service level. For seismic, you can use the allowable interstory drift in ASCE which can of the order of a few inches.

ahmedhegazi (Structural) 11 Jan 08 16:23

John the values you get from Etab should be multiplied by 0.7R to get the maximum inelastic response displacement (UBC 1630.9.2) and it is interstory drift not story drift

the value for H/500 is used only for wind service limit and it should not be multiplied by 0.7R

Upper limit for seismic drift is function in building period (UBc 1630.10.2)slickdeals (Structural) 11 Jan 08

19:52 Please note that for seismic loads the displacement is always calculated at design levels per ASCE. You should not reduce your it to service levels. In addition the displacement should be multiplied by deflection amplification factor (Cd) to get the maximum inelastic deflections. HTH


Sorry, my claim in previous post was wrong (slickdeals and ahmedhegazi are right)...you should multiplied the lateral drifts with ductility to get real (inelastic) drifts which one you should check by the code.

Drile007Murali27 (Structural) 23 Jan 08

10:00 John

With UBC code,

1. Lateral drifts (displacements) are to be multiplied with 0.7 * R to obtain the maximum inelastic displacement, which would be useful in checking pounding of structures, expansion joint, etc,.

2. Seismic story drift demand is required to allow the movement in cladding design. I am not sure of H/500, which is really uneconomical to achieve.

UBC gives the limit 0.02 x story height, (as calculated above). If your story drift ratio is less than 0.02, then it is fine.

If you look at Eurocode, there is a limit of story drift ration 0.005 at service level, i.e., H/200

Hope this helps

drile007 (Structural) 21 Aug 07 8:33

1) I model a beam under the slab in Etabs. What kind of connection does Etabs presume between them, rigid or no connection at all? 2) How can I "tell" Etabs that the beam is under or above slab? I need this info. for exporting the exact model to Safe (or I can't do this in Etabs, but I can correct the vertical position in Safe).

Drile007slickdeals (Structural) 21 Aug 07

10:52 You can specify the location of the beam by using the "specity frame insertion point" By default ETABS places the top of the beam at the center of the slab. You can change this by changing your insertion point. And I think ETABS assumes a rigid connection between the beam and slab. I know it assumes composite action in case of a steel beam and concrete slab.ASCE07 (Structural) 2 Sep 07

17:57 Remember that ETABS transfers the loads only at the end points of elements.

You need to provide as many segments to beam and slab (meshing) in order to achieve

near uniform/accurate mass distribution in the model.I would recommend at least three divisions on each elements.If you do not segment slab and beams load from slab will directly pass to columns, thus you will get a beam under designed.

Connections will be "rigid" (although this is not a connection a tall, the RC structure are monolithic)

hope that helps.TSReddy (Civil/Environme) 5 Sep 07

4:25 Mr.ASCE07"ETABS converts the object based model into element based model for analysis and results are reported back on the object based model". If this statement is true, then what is the necessity of doing many segments to beams and slabs?As per my knowledge ETABS can do this process internally and no need of segments.amadeus (Structural) 16 Jan 08

14:55 If the slab has been defined as SHELL then meshing it will be necessary for the correct transfer of vertical loads. A minumum of three segments per side is often enough mesh refinement. If the slab has been defined as MEMBRANE you won't have to mesh it. The beams DO NOT need to be segmented in order to achieve correct load transfer.

Skicat (Structural) 4 Jan 08 16:16

I'm beginning to learn ETABS at our office and I've been trying to determine the most efficient way to model a structure. I concede that if the model was going to be simple, I would just create it using the drawing tools in ETABS. However, as the structures become more and more complex, it seems that it is easier to draw the model in 3D in AutoCAD and import a .dxf file. A friend in the office creates every model in ETABS by first modeling the first floor and then copying it up and modifying as necessary. Given that often times floors stack, I can see how this method works also. What is your preferred method to model in ETABS? Thank you. EBARRERA (Structural) 5 Jan 08

1:51 Both methods are good for modeling structures in ETABS. The first one will be better for structures with irregular shape, while the second one will be better for regular multi-story buildingsdrile007 (Structural) 5 Jan 08

15:57

I allways create a model with Allplan (AutoCad) and never with Etabs, since Etabs is very clumsy. It's also the third option Revit-Structure, but I never used it and I never heard any commnets on behaviour Revit-Etabs.

Drile007Skicat (Structural) 11 Jan 08

10:00 I've been tinkering with importing a 3D autocad .dxf file of a 17 story concrete building into etabs. As you know, ETABS allows you to initialize a model with a default.edb file in which

you can pre-set all of your materials, frame and area objects as well as your design defaults and load cases / combos. In theory, it's a great idea. In practice, it's kicking my butt. If I start a new file, and go directly to file -> import -> etc, etc, it prompts me to open the 3D .dxf model with a default.edb file if I please. When I do so, the program is reading the story data from the default file, which is not consistent with the number of stories in my model. I talked to CSI and they recongnize the problem. OK, no big deal. The way to work around that is to, as a first step, open the defualt.edb file and then import your model into that file. You have to promptly save the file as a new name to avoid over-writing your original model. No big deal. Well, even though I took the time to input the correct story data into my default file, when I go to modify the stories in any way (add, delete, revise height), the story data form is all messed up and nearly impossible to deal with. Ok, ok, still no big deal. When you import your model, ETABS assigns section properties to columns, beams, slabs, etc. I thought this would be extremely handy. I'm not sure it is. It seems to be a huge pain to go story by story editing the frame elements to make them match your structure. So the conclusion I've come to is that as efficient as it seems to be to import a 3D .dxf, in the end it seems easier to just import the grid and start from scratch. Sorry for the long post, you can tell I'm frustrated. All of that aside, I still love CSI :)

dorinad (Structural) 8 Nov 07 12:25

Can somebody help me with some values for the Scale factor in the definition of the response spectrum ? Especially for the definition of romanian p100 - 2006 seismic code (or EC8).

When defining a response function in etabs / sap2000 the values for the accelerations represent fractions of G (gravity acceleration) ?

any help is apreciated, especially from design practice.

Thanks

Daca ma poate ajuta cineva cu informatii despre modelarea spectrului de proiectare din P100 in etabs. mie, din ce am calculat, mi-a rezultat o accel max de o,16 m/s2. e corect ??? dorinad (Structural) 9 Nov 07

14:22 problem solved. just in case, if someone wants to know:

for Romania, ag=0.16 G, reinf. concrete building, ductility factor 4.725 the starting value for design spectrum is 1.6 m/s2 at T=0, with 0.9 m/s2 between 0.07 and 0.07 seconds.

scale factor "1" for N m unitsMonkeytz (Structural) 25 Dec 07

6:05 Sal...te pricepi la ETABS ? As vrea sa ma ajuti si pe mine cand am ceva probleme. Sunt unele lucruri pe care nu le inteleg...dc vrei putem sa vb pe messenger. Id meu e : [email protected] Multumesc. Sal

doantonminhphu (Structural) 16 Nov 07

5:21 I want to view the weight of a storey (include from self and load on floor), Thanks

Check Out Our Whitepaper Library. Click Here.drile007 (Structural) 16 Nov 07

8:40

It should be in Display/Output tables...

Drile007doantonminhphu (Structural) 18 Nov 07

20:38 But I calculate in handle,not right compare with etabsdrile007 (Structural) 19 Nov 07

5:16

Did you properly define masses...Define/Mass source?

Drile007

nickershocker (Structural) 8 Nov 07 11:35

Dear Potential Helpers,This is my first time using ETABS (other than the tutorial). Right now I am attempting to make sure that the gravity loads are getting to the columns. My dead load and unreduced live load seem to be accurate. However, my live load does not seem to be reducing. I am hoping my problem is simple (maybe I am just not looking in th right place). Here are a few details about my model:

- The structure is concrete and my slab is defined as a membrane. (well, where I am checking columns it is a membrane, it is defined as a shell elsewhere)- The live load in which i wish to take a reduction is called REDLIVE and I defined it as load type REDUCE LIVE.- Under options>preferences>live load reduction, I have selected influence area ASCE7-95.- I am viewing the results by selecting a single column, running an analysis, and going to display>show tables, then selecting the appropriate loads and checking Table: Column Forces under Frame Output.- I am running ETABS Plus V9.1.5

Any help you could offer would be greatly appreciated.

~NickFind A Job or Post a Job Opening Click Here.

nickershocker (Structural) 8 Nov 07 15:33

I was looking in the wrong place....apparently LLR isn't reflected in frame forces until after a design is run.

Thanks anyway, ~Nick

RCraine (Structural) 25 Oct 07 10:09

Can't I just model a spring stiffness as related to my soil sub-grade modulus of reaction, and review the results to determine if I'm having any uplift forces, instead of "jumping through hoops" to create tension/compression only springs/links? Is this not correct? I'm trying to get loads that are to be carried down to my 'mat' foundation under my shear wall frames.Let me know if this is not correct.Thanks.

RCAll that is necessary for the triumph of evil is that good men do nothing. Edmund Burke

www.tbastructures.comCheck Out Our Whitepaper Library. Click Here.

stressed (Civil/Environme) 3 Nov 07 14:34

Yes, you can do this easily. Model the basemat foundation or footings as a slab type area element using section properties equivalent to the actual foundation or footing thickness. Then select the basemat or footings and Assign>Shell/area>Area springs. If you had a subgrade modulus of reaction of .25 Kip-in, for example, you would assign it as .25 Kip-in area spring in the local 3 direction (vertical). You would also need to add soil springs in the local 1 and 2 directions. Either use springs, or restrain it in those in-plane directions.

Even if your mesh changes, ETABS should automatically handle those area springs as you assigned themRCraine (Structural) 5 Nov 07

10:35 Thanks stressed for the response. I know we've done this in RISA3D before, which you can model with compression only, but I figured as long as review of the soil springs showed no tension, then it would act as it should. We did just that with the sub-grade modulus of reaction. However, when applying the reaction to different size areas per Terzaghi's research, we found conflicting formulas for how you apply this. Do you know what the formulas are? I have a Bowles foundation book, as well as a 5th edition Das Foundation book. One states to multiply the sub-grade reaction by the width and one states to divide by it. I feel like it should be somewhere in the middle. Which one is correct?Thanks.


www.tbastructures.com


I'm not sure that I understand your question..the subgrade modulus represents k, right? a pressure vs deformation relationship. Subgrade modulus is an area value, so it's handled on a per area basis, and engineering programs use these values to calculate nodal spring constants. On a large mat foundation, you may have variable soil conditions supporting the foundation, but you could mesh your basemat elements and make multiple assignments to account for that. Since you're modeling the basemat foundation using area shell finite elements, the basemat is not treated as rigid.

CSI has a special purpose program for design and analysis of foundations called SAFE. RCraine (Structural) 6 Nov 07

9:54 Sorry for a confusing question.

Yes, I agree k is a pressure vs. deformation relationship ksi/in, or whatever units you want to use. This relationship of the k value with the shape and size of the foundation, as discussed in Das' Foundation Engineering, 5th Edition, doesn't have a linear relationship, as one would assume. Das claims that Terzaghi found a relationship with different areas (because k is based on a 1'x1' plate and this could change as the size of the footing changes) and in Das this relationship is stated as k*(1/B) where B is the width of the footing (ft). In Bowles' 2nd Edition book, it states the relationship is k*B, where B is the width. In both texts, they claim the formulas are derived from Terzaghi's research in the 50s.

Does anyone know which formula is correct? Thanks.

Also, if you want to design your shear walls correctly, the footings in etabs should have soil properties below to allow for shear to be transferred to the floor slab diapraghms and not all to the footing. This also helps reduce the possibility of having uplift on the your footings because most of the load on your walls is then transferred to the diaphragm and basement walls as a result of the allowed movement in the footing below your design in ETABS. We do utilize SAFE for footings once we have the controlling loads on them from the ETABS analysis.

Thanks for your help.


www.tbastructures.com

phuocqn (Structural) 15 Oct 07 22:39

I am analysing a air traffic control tower. The client requires us give them the maximum acceleration of tower. How can I get the acceleration from software Etabs of Sap ?

Find A Job or Post a Job Opening Click Here.drile007 (Structural) 19 Oct 07

17:40 Hello,

I think, that you can get acceleration by Display/ Show output tables...response spectrum accelerations. They are listed by the mode shapes.

Hope that will help you

Drile007stressed (Civil/Environme) 3 Nov 07

14:37 drile007 is correct, you can get this info in the output. But before analysis, you will need to define response spectrum function and RS case(s). The "factor" is to scale to G in current units, and also to apply additional factors to match with base shear

cenju (Structural) 17 Oct 07 14:27

Hello folks,

I have a situation here: I was checking the foundation reaction of a 9-story concrete flat plate structure. I was only interested at the gravity loading (DL and LL)

The reaction from a column load take-down (manually calculated) appears to be (20% to 75%) less than the reaction from ETABS output. I checked this on a basic 1x1 bay 4-storey structure in ETABS and turns out the reaction is 10% more than the traditional tributatry load-take-down. It appears as if the program internally applies a load factor.

Has anyone ever encountered this situation or am i missing something? ....I used to consider myself a comfortable user of ETABS but this is kind of frustrating!!

Your response is greatly appreciated.

drile007 (Structural) 19 Oct 07 17:42

I think, that the answer could be in how did you mesh the floors.

Drile007johnjeed (Structural) 30 Oct 07

3:05 If you model your floors as membranes you will find that the distribution of reactions in the columns matches that if you performed the calculation manualy using tributary areas. The difference in reactions is due to the in plane stiffness of the floor when modelling using shell elements.

TomKC (Structural) 12 Sep 07 7:54

I have generated my static base shear in Etabs with "Time period Ct(ft)Programm

calculated" and the result are different base shears in x- and y-direction. Befor that I have used "Time period Method A" and the result were same base shear values for both direction. Does anybody know why I get differt results for my base shear and which is the correct way of generating the base shear?

Eng-Tips Forums is Member Supported. Click Here to donate.AhmedR (Structural) 14 Sep 07

14:58 that is expected, cuz u r using the period from model analysis, and Etabs taking the period which have the maximum mass participation in that direction

for example1st mode 2.5 sec with UX=70% UY=10%2nd mode 2.0 sec with Ux=10% UY=12%3rd mode 1.7 sec with UX=9% UY=60%

the period which is etabs taking to calculate the static base shear in X& Y direction will be:

Tx= 2.5 secTy=1.7 sec

TomKC (Structural) 18 Sep 07 5:17

Hallo AhmedR,

thanks for your helpful reply. My problem is now that Etabs takes the 10th mode for y-direction and that results in an enormous base shear which is beyond a designable value! As I have some Null Lines moving in the animated view I guess this could be the reason for the high mass activation in the 10th mode. I'm going to check this. Did you have similar effect anytime?

AhmedR (Structural) 21 Sep 07 1:49

Hi TomKC

10th mode is not reasonable to me , usually 1st 3 modes are govern. If there is no problem in the model ,I guess you structure have is separated or flexible or slender. if it is ,so it is better to use response spectrum analysis. for static analysis you may take one of the 1st 3 modes which has the larger mass participation in each direction and apply it as user defined period in each direction ,or use method A static analysis is approximated method anyway

regards

Corey7320 (Structural) 22 Aug 07 9:10

What is the most common way to model a one-way concrete slab in ETABS?drp181 (Structural) 28 Aug 07

13:27

Use a membrane element and check the "Use Special One-Way Load Distribution" box in the Wall/Slab Section dialog. Make sure the program has chosen the correct direction -- there should be an arrow shown in the direction of the span.ASCE07 (Structural) 2 Sep 07

17:53 I second drp181.Corey7320 (Structural) 12 Sep 07

8:30 Why use the membrane option and not shell? I thought membrane only allows in plane stresses. You can still choose the one-way load distribution with a shell element.WillisV (Structural) 12 Sep 07

13:44 Selecting a membrane option for a floor element in ETABS triggers ETABS to use its internal tributary width algorithms to calculate the load distributions as opposed to using true bending distribution as with a plate or shell element. This (using a membrane element and thus ETABS built in algorithms) results in much faster run-times than using a plate or shell element but can lead to incorrect answers if your floor plate does not behave as their algorithms assume (e.g. if the continuity effects are great etc).

tony19 (Structural) 29 Aug 07 16:10

I need to model a structure with waffle slabs. The structure is very irregular and using the template for waffles slabs (i.e. new model, waffle or Edit add to model) will probably do nothing good. Any idea of approximations or tips of how to build the model. It's a 14 story building. If I use waffle slabs template it'll make the model very difficult to work with. Will take great time computer wise for the design.Any ideas are highly appreciated.drile007 (Structural) 30 Aug 07

4:25 Try to build a 3D model in Cad and then make a DXF file, which one you can import in to Etabs. I follow this way and it's working. You can also try with Revit...

Drile007ASCE07 (Structural) 2 Sep 07

17:52 Are you going to design the waffle slab using ETABS ?

I doubt if it does that. You can put the self weight to zero for a regular slab, and put equivalent SDL on the slab enements for reaction purposes.

Also, you need to provide cracking MOI -"Icr" for deflection purposes.

I hope that helps.

CivilTanju (Structural) 6 Jul 07 13:10

hi all,

yesterday I have encountered a very strange outputs of etabs. I was analysing single storey structure, load case was only with gravity uniform loads on floors (rigid diaphragm), everything went so normal run the analysis and after that i run the design/check( concrete frame,EuroCode-2) i get some longitudual reinforcement outputs for columns on the screen thinking that these are normal results program supposed to create. Then I added some additional(4) storeys on top of the ground floor (load case and code was same). But even now the structure was comprised of 5 floor, the longitudual reinforcement was same with last result on the ground floor columns.

It made me think that longitudual reinforcement is provided only with min percentage specified on code(Eurocode-2) as long as concrete cross section bears all the axial-load capacity.

Any idea??

Best Regards,Check Out Our Whitepaper Library. Click Here.

Murali27 (Structural) 19 Jul 07 16:22

Of course, you set the min & max value for the percentage of reniforcement.

Pls change the min value to 0.001 and check the steel requirement.

Hope this helps

CheersASCE07 (Structural) 14 Aug 07

21:12 you need to check the min values for column. If that is not being exceeded your output will not change. try reducing column size.

johnclarion (Civil/Environme) 18 Jul 07 1:52

Hi Dear friendsI need to model shear wall for pushover analysis . Does anybody know how can I do it ?in general, how can I do pushover analysis shear wall structure in or ETABS?

drile007 (Structural) 19 Jul 07 2:54

In a few words:

You must model a wall with Frame Sections and then define Frame Nonlinear Hinge Properties. Then you assign this Hinges prop. to prior define frame sections prop.. Next step is to define Static Nonlinear/Pushover Cases...

Drile007johnclarion (Civil/Environme) 19 Jul 07

8:48 Dear DrileHow can I define wall as frame section? may you please explain for me how?

for example if I define wall as column, should I define a column insted of wall with that dimension?help me please

drile007 (Structural) 19 Jul 07 9:15

Yes, if the wall thicknes is 20cm and lenght 600cm then you define column 20/600cm. Otherwise you can't model the RC walls. Be careful with local axes...I could send you a large model where I done pushover analysis on old masonry structure strengthened with reinforced concrete walls.

Drile007johnclarion (Civil/Environme) 21 Jul 07

1:33 Dear Drile,As you know, I model shearwall as column, but when define it, I could not understand what should I do with length of beams which are connected to the walls. because in this case, the length of beams should prolong!!!may you please help me about this?

Regardsdrile007 (Structural) 22 Jul 07

7:01 You should model a beam with two sections:1) Rigid body in the area where beam crossing the wall (inside the wall)->that's right, prolong the beam to the center of the wall.2) Regular beam outside the wall area.

PS: Sorry for my English...

Drile007

jimka (Civil/Environme) 19 Jun 07 10:53

I have a beam and i want to assighn two uniform loads,The first is for all the length of the beam but the second is onlyfor a part of the beam(in the middle of the beam)Is there an easy way???? slickdeals (Structural) 23 Jun 07

7:31 There are 2 things you can do. You can either superimpose the loads or use 2 different load cases. You will have to divide the beam into smaller parts and assign the loads over them.For example, if you have a beam with 1 klf load over the entire length, and an additional 0.5 klf over the middle third. Divide the beam into 3. Select line -> Edit -> divide. Once the beams are divided you can choose to superimpose and you will end up with 1 klf each on the 1st and 3rd thirds and 1.5 klf over the middle third.

HTHAnantha

jimka (Civil/Environme) 23 Jun 07 16:42

Thank you very muchstressed (Civil/Environme) 16 Jul 07

15:07 Actually, you can assign two uniform loads without having to either divide the frame or define another load case. Whenever you assign distributed loads to a frame, the dialogue gives options for both trapezoidal load and uniform load. You can assign the trapezoidal load (which can be uniform) for part of the frame, and the uniform load applies to the entire frame in addition to the trapezoidal option.

Gunblues (Structural) 1 May 07 11:19

Hello,

I'm hoping you can help. I have a building modelled with a movement joint running through it, so essentially it's two separate buildings. We're looking at ways to connecting the two to provide a shear connection to shore up one of the buildings when they move side-by-side. However it will still allow movements "away" from each other.

It is possible to model the building points along the connected face in such as manner as to account for these effects?

Thank you. Find A Job or Post a Job Opening Click Here.

shaheed (Structural) 4 May 07 7:44

Hi

I have not had much experience with this myself but I dare to suggest that would need to use some sort of "link" element at the points where the buildings are joined. Also I believe if you use a link system you need to carry out a non linear analysis. Maybe some one else would have a better/simpler way of achieving what you require or could elaborate on the above suggestion.ASCE07 (Structural) 25 May 07

6:09 I fail to understand your question. You want to "shore" up the buildings while allowing to "move"? Could you elaborate your question ?thanks Murali27 (Structural) 4 Jun 07

15:59 Gunblues

How about pounding each other? Did you consider that?

Is it for thermal expansion only?

Just see, if possible by external restraint at those points?

I understand that you are trying to use shear walls along this plane to resist the shear from

both the buildings

Is it correct

Cheers

inju (Structural) 23 Apr 07 14:05

Hi everyone,

I was wondering if anyone has tips on how to modela steel frame on ETABS only for gravity loading and estimating foundation reaction. (no lateral force)

The structure is: -1 storey steel frame (high and low roofing)-roof joist and deck system

I modelled the structure as:- all beams & joists = pin-pin - columns = cantilever gravity columnsrunning the model this way generates a number of errors and to termination of analysis.

I will be requesting techincal support on this issue from CSI. But, i will appreciate your help.

ThanksInju

ryansteel (Structural) 6 May 07 2:45

i know your cantelever columns are fixed at the bottom(for steel, pin support is default), right? take time in removing warnings and errors its part of life in ETABS.ASCE07 (Structural) 24 May 07

18:52 numerous errors indicate that there are shortcomings in your model...you are apparently a beginner...ytr going to CSI website and use watch and learn for ETABS...I hope that helps..Murali27 (Structural) 4 Jun 07

15:55 Inju

Could you please look at the filename.$og file in the run folder. Just check the part "Percentage of Error"Global Force Balance, which will give you fair idea on Print Static/Dynamic check.

OrLook at the deformed shape, whether it makes any sense?

or

Look at the mode shapes, if you are asked for it, it may also give some indication

Cheers

GarthJ (Structural) 1 Jun 07 22:52

In a building which has some levels below grade the ultimate level of footings would be, of course, at the basment level. The soil around the basment would have, potentially, a large impact on providing restaint against lateral earthquake movement. I'm wondering how others have modelled the horizontal restaint of the soil. Horizontal springs?

Eng-Tips Forums is Member Supported. Click Here to donate.Murali27 (Structural) 4 Jun 07

15:50 Hi GarthJ

Yes soil springs can be assigned to represent the passive resistance of soil.

Please review your assumption, is it really required bcoz, lot of basement retaining walls will be there to resist lateral shear.

Cheersmatrixeric (Structural) 19 Apr 07

17:02 Hello All:

I am modelling a 10 story concrete 2-way slab/concrete columns/concrete shear wall model in ETABS. I have defined the concrete 2-way slab as a shell. Now coming to the problem, the reactions I obtain at columns are <50% of what I obtained from column (made sure that column is far from the core) load take down for couple of columns. I have tried checking every possible cause for error but haven't found any logical explaination of why this is happening?

Any help will be greatly appreciated !!!inju (Structural) 23 Apr 07

9:45 Hi,

As I understand the situation it seems like the load from the shell is not distributed to the beams (columns) in a two-way fashion. I would suggest that you mesh the shell element after you assigned the surface loads (self wt, live load, snow load .. or whatever)If you are using ETABS version 9.0.8, try ASSIGN>FRAME/LINE>AUTOMATIC FRAME SUBDIVIDEASSIGN>AREA/SHELL>AUTO MESH(make sure to keep a temporary back-up of the file before meshing)

Murali27 (Structural) 4 Jun 07 15:47

Hi matrixeric

I would suggest to go ahead with inju's suggestion if we are aiming for realistic distribution. Especially it happens for corner/edge columns, where force will be very less compared to manual trib area calculations.

If we want to match with manual method, please go ahead with membrane option and connect the beams (null) on each vertical element. It matches with manual calculation.

Please also check with Tributary area for each column in the ETABS output.

Cheers

hannes964 (Civil/Environme) 11 Jan 07 10:38

Hi folks,

i’m still really unhappy by meshing floors with areas. I handled to mesh the geometry manually, which is irregular, in six big quads. So far the model runs without warnings. I tried to generate a finer mesh by dividing the areas into sub-elements to get the proper deformation of my floor. But then I receive that some areas are too close by checking the model. Someone gave me the hint to delete these areas and rebuilt these areas manually (thanks civilungo). So far so good if the number is small. In my example the number of areas which are too close is about 100. So I have no chance to redraw these areas manually. My question is actually really simple, how can I mesh irregular floor areas in etabs? Or are these warnings harmless??? I’m really depressed…I’m wondering that nobody has any problems by meshing floors in etabs (I use version 9.0.7)

Thanks a lot

hanneszeemas (Structural) 22 Jan 07

5:18 HiIn my opinion if you have very regular building then go for Auto meshing, because as you know that manual meshing makes the life so much difficult specially Architects are involved too much. Any way if you want to go for manual mesh, I have one more method which i usuall do. Make a very basic model and export to ROBOT or AUTOCAD, generate the mesh if you are using ROBOT by number of meshing method or just create manual mesh in AUTOCAD and then import back to ETABS, Assign properties and loads and annalyse, this is my experience that chances of warnings are very low. Also for warnings i tell you one thing, if error is less than Exp-11, then I think you can neglegt the warnings, in this case check the reactions and other things carefully.opa123 (Structural) 28 Jan 07

12:11 you can also build your floor model in safe and export it as dxf abd then take it to etabs.

zeemas (Structural) 29 Jan 07 23:26

can do with SAFE, but many places mesh aspect ratio becomes very bad, which can cause these warnings again.

GarthJ (Structural) 1 Jun 07 23:34

I generally use automesh and control the meshing at special locations with grids and/or null lines.I would look at your force balance of your model and then I would look at how many digits of accuracy are being lost. They don't seem to worry too much if there are six digits or more lost.You must gain confidence in your results. Remember you can use the process of elimination to get rid of your problem, study the forces and deflections of this new model and then compare with the model that has the trouble. This may be a way to gain some confidence in results.

Hope this helps

ryansteel (Structural) 1 May 07 9:44

Hi there everyone,

Code says elements/odjects not part of lateral force resisting system needs to be deformation compatible.i'd like to know if there is a logical way to model for Deformation Compatibility?

Eng-Tips Forums is Member Supported. Click Here to donate.GarthJ (Structural) 1 Jun 07

23:19 I believe what is meant by "deformation compatible" is that the member/element will perform or survive after the extreme event. There can be various detailing requirements for members that are part of the lateral load resisitng system (LLRS) but these requirements don't necessarily apply to members that are not part of the LLRS. My understanding is that members not part of the LLRS can't fail during an earthquake. So I beleive two anlyses must be done. One with, and one without, the elements that are not part of the LLRS. These members must still be able to resist the factored force effects so they don't collapse during an EQ.

Hope this helps

hippo11 (Structural) 4 Jan 07 17:42

I am modeling a steel-framed building that has some non-structural masonry partitions that will be infilled floor-to-floor (as opposed to continuous) after the steel skeleton is built.

How can I model these walls correctly? I’d like to have the mass represented correctly, but I don’t want the masonry partitions to contribute to overall lateral stiffness, or to local steel beam stiffnesses.

Would it be proper to use a membrane object but with an f12 release?

I guess I could just account for the nonstructural masonry as Line Loads but those loads could be tedious to input on all the individual beams.

Thanks!Check Out Our Whitepaper Library. Click Here.

WillisV (Structural) 5 Jan 07 7:41

Just use the line loads or convert it to a reasonable uniform load - it will be less hassle in the long run. tpist (Civil/Environme) 5 Apr 07

16:50 A good idea to convert the masonry partitions to a surface load, because when an earthquake occur, the collapse of these partitions will be in the first seconds, so these elements are really don't contribute by their lateral stiffness, they contribute by their mass. I have read about this from a great scientist (to convert partitions to a surface load on the slab or deck). So if u don't want to model the slab you can use a horizontal element like slab ( i forget about the name of this element) which don't mean anything (no mass, no stiffness) after u assign you surface load on this element, then loads will be transfered to beams with the software processes.This feature give you an accurate results after VICTOR DAVIDOCCI.ASCE07 (Structural) 25 May 07

6:15 I'd use a line load convert from psf to plf !

CHARLES76 (Structural) 30 Apr 07 18:37

Hi everybody,I'm new to this forum and it seems very useful.I have a question.How can you design a slender column in etabs? Or which is the factor that I should check to get a correct design? Thanks everybody in advance.

Eng-Tips Forums is Member Supported. Click Here to donate.zeemas (Structural) 3 May 07

0:33 ETABS has capability to check slenderness of column according to selected code. In the design chart ETABS doesnt show the slenderness ratio but it checks accordingly, and even if you have some double story height column, ETABS will consider that length.

Try itZeeryansteel (Structural) 6 May 07

2:56 YUP! Etabs takes care of slenderness(its the only FEA sofware I know that does it). But "sometimes" unrealistic slenderness ratio happens, better check on objects that gives-off veryhigh(unrealistic) D/C ratio by clicking "overwrites".

rmedina1 (Structural) 4 May 07 9:57

I've been modelling a steel frame building with moment connected beams acting as the lateral system. When I run the program with the auto select list and run a steel design, I

get the largest member in the auto select list assigned to the lateral beams. I tried removing that member size from the list, and rerunning the analysis and design, only to get the next largest member. Checking the strength and deflection ratios of the member, I am well below the limit for failure.

Does anyone know why the program isn't designing the beams appropriately, is there something i need to check with the overwrites. Any help would be greatly appreciated.

Check Out Our Whitepaper Library. Click Here.ryansteel (Structural) 6 May 07

2:35 Hi there, I think when u use Autoselection in design the section that gets selected has something to do with the controls of drift/displacement of a jiont, and if no target drift specified it will select the section the gives smallest drift/displacement.

try this http://www.csiberkeley.com/faq_ETABS.html#sfdo4

LeonRestrepo (Structural) 15 Feb 07 17:06

how do i calculate the drift with etabs , when there is no common points between adyacent stories.

when i enter a multi-tower building etabs only gives me thedrifts of stories that have common points with the next story.

Find A Job or Post a Job Opening Click Here.radugrosu (Structural) 21 Feb 07

5:35 You really have two options. 1. Find points that are reasonably close to the same vertical on each storey of interest and compute the drift yourself.2. Try harder in finding points on the same vertical and let Etabs compute the drifts. There's no need to find corresponding points from the base all the way to the top, you just need to find them on groups of storeys.

Radu

Turkishengineer (Structural) 15 Jan 07 8:09

Hi everybody,I'm new to this forum and it seems very useful.I have a question.How can you (or can you )design a built-in column in etabs?Thanks everybody in advance.

Check Out Our Whitepaper Library. Click Here.zeemas (Structural) 23 Jan 07

7:57 Hi,For defining built up column go to frame definition and select add I/W section or any other section type which you want to use and input the properties. Also if you want to define tapered section or section with variable depth, define two section with different depth and select non prismatic section, define start section, end section and the length. Turn on the

extrusion in case to see the exact direction you want. Hope it will work.

Zeevera82 (Civil/Environme) 10 Jan 07

9:15 Hello to everybody! I've just discovered your forum and I think I'll join it very frequently, it's rally interesting....I have a problem with etabs... I create the model of a building using both frame and wall elements, but when I open the menu "display-set output table mode" I can't choose the option "wall forces" because it is not written in bold character.What did I do wrong?

Thanks a lot for you helpzeemas (Structural) 23 Jan 07

6:45 Hi Vera

For getting wall output you have to assign them as Pier and spanderal (See technical document in ETABS manual to know about the Label option) do the analysis, run the wall design, after this procedure if you check the ouput table you will be able to get wall forces.

Best!!Zee

rmedina1 (Structural) 17 Jan 07 16:54

Hey, i've built up a fairly large model with a curved joist roof. It's composed of about 10 bays and i'm trying to model the roof using a series of null areas to try and approximate the roof's actual curvature. I'm also inputting my own wind loads, not using the wind coefficients or any automatic windload.

Now when i run the model even though i've applied the wind loads to the curved roof areas the structural frame doesn't respond to the loading. Do i have to attach the areas to each node of the roof individually, or do i need to give the roof some sort of property such as slab on deck.

I'm currently using ETABS v.8

any help would be appreciated.Eng-Tips Forums is Member Supported. Click Here to donate.

stressed (Civil/Environme) 20 Jan 07 11:48

You have to draw/mesh the null areas at joints to distribute load to the joints.

Alternatively, you can define your own deck membrane area, but with a material with no weight or stiffness (tiny modulus value), and ETABS will distribute area loads that you assign for wind, as distributed loads along the frames.

civilungo (Structural) 28 Sep 06 1:39

hi:i'm modeling a 30 floor building in viña del mar (chile), i assigned membrane as a floor section, i have used a spectrum function as require NCh433of96 chilean seismic code. my problem is that i've assigned rigid diaphragm in all floors then i run the program an when y look joint displacement i watch in load case joints moves together but when i watch the joints displacement on any load combination it does not keep the condition of rigid diaphragm.

regardsDiquan (Civil/Environme) 1 Oct 06

20:54 You have to assing the diaphragm to each level, so at the end you will have 30 diaphragms. Click on the "plan view" and go one by one, the name of the diaphragm have to be different for each floor

Tiene que asignarle el diafragma a cada nivel por separado, al final tendra 30 diafragmas asignados. Le da click a "plan view" y va uno por uno, los nombres de los diafragmas tienen que ser diferentes en cada piso

Diquan

drp181 (Structural) 3 Oct 06 3:31

The results from a response spectrum analysis represent the maximum values at any time. The results for displacements of the points in the rigid diaphragm will have peak values that occur at different times, meaning that the deflected shape will appear to violate the rigid diaphragm constraint, though it does not.

Before proceeding with the design, be sure that you fully understand the fine points of response spectrum analysis. It is not the same as simple static analysis, and should be treated accordingly. For instance, all signs are lost as a result of the combination of modes, and therefore the code checks must be done assuming the worst combination of different forces (i.e. axial force and moment).civilungo (Structural) 4 Oct 06

1:08 thanks, but i don´t understand well, if i look the modal displacement (each one the different modes) the structure shows the displacement of the joint taking into account the diaphragm rigid condition... all of these movements are represent as the result of the spectrum analysis for each mode... but the question is: what does the displacement of each node that the structure shows represent in the spectrum analysis response (all modes superimposed by CQC) ? is the spectra response displacement case the maximum value of displacement of any node in each mode? i think it should be the superimposed by CQC response from the different mode shapes

Diquan : thanks, but the rigid diaphragm definition can be used the same in all floors because it associates all the joints selected into a master joint in each floor.. but if i want to use two rigid diaphragm in each floor i need to use two diferents rigid diaphragm assignment.

stressed (Civil/Environme) 31 Oct 06 8:52

The joints on your floors will move/deflect together reacting to in-plane lateral loads with a rigid diaphragm, but will not move together with load combinations if load components of the combination include non lateral loads, such as Dead or Live loads in gravity (-Z) direction. The rigid diaphragm links joints together that are reacting to in-plane forces (membrane area local 1 or 2 direction), and does not link together joints reacting to loads coming from other directions. That is likely why you see joints on the floors moving together reacting to response spectrum lateral loads, but not to load combinations including other load cases.plopez (Structural) 15 Nov 06

16:49 Parece que estas considerando combinaciones de cargas que tienen estados de carga sismicas y el CQC te hace una distorsion de los desplazamientos que pareciera que el diafragma se deforma. Hay que verificar las deformaciones que se producen por cada modo separado.civilungo (Structural) 25 Nov 06

23:42 si plopez, estoy considerando cargas sismicas obtenidas con CQC mas cargas estaticas es cierto que se deforma con ellas y tienes razon, verifique las deformaciones por modo y luego aplico la proporcion con la masa modal de cada uno de los modos.

gracias

thanks everybodyopa123 (Structural) 21 Dec 06

8:05 well should i get design load from

Mx max and its corresponding My and N maxorMx max and My max and N max ?shdma (Structural) 12 Jan 07

8:29 Hi?all

Rigid diagram reduces the degrees of freedom and makes the model run much faster. If you don't include shell element with the rigid diaphragm, you can't obtain bending moments and shears for slab design,and the bending moments and shears for the beams is samller than the real situation, the bending moments and shears for the columns is more than the real situation.

usually I modify the I33 stiffness for the beam to simulate the real situation. According to some CODES amplify 2-3 times.

Good luckShdma

rmedina1 (Structural) 9 Jan 07 15:53

I modelled and designed a complicated curved truss on etabs and now i'm trying to get an output of the design forces. I manage to get the tables for all the members, but due in part to the curvature and having to model it as a series of straight lines i end up with a lot of data.

Is there anyway where i can make Etabs just give me the worst case loading cases used for design?

Find A Job or Post a Job Opening Click Here.drp181 (Structural) 9 Jan 07

19:27 When you print output data, there is an option to print "Envelopes Only." If you select only the design load combinations for the output, this will give you the envelope loads, both maximum and minimum, which are the same as those used internally by ETABS for design.

jeepman5 (Structural) 6 Jan 07 16:16

What does a o/s #3 message mean on etabs when doing concrete design?drp181 (Structural) 9 Jan 07

19:24 O/S #3 signifies "Shear stress exceeds maximum allowed" - i.e., a shear failure.

yabi (Structural) 24 Dec 06 4:34

Sorry if this is a simple question, but I am new to ETABS.In analysing a building after defining the loads (dead, live, wind and earthquake, we also have to define load combinations and choose the sever case for each element based on these load combinations.How can I define load combinations in ETABS?

Eng-Tips Forums is Member Supported. Click Here to donate.yabi (Structural) 24 Dec 06

5:04 I find "Load Combination" command under define menue. Now my problem is how to assign these load combinations to the building?

civilungo (Structural) 25 Dec 06 22:33

you must see your design code, for example one of the ACI318-99 load combination is 1,4D + 1,7Lthen to define this load case you must ADD new combo... then select DEAD Static Load and select as Scale Factor 1,4 then press ADD Button, then select LIVE Static Load and select as scale factor 1,7 then press ADD button

yabi (Structural) 26 Dec 06 3:08

Dear civilungoThanks for your reply. I was able to define a load combination as you mentioned. My problem is how to assign this load combination to the structure?

Thanks in Advance

civilungo (Structural) 28 Dec 06 17:45

load combination is just a array of loads cases in your structure that serves to calculate forces, stresses and deflection but you don't have to assign load combination to the structure, you just have to assign loads (DEAD, LIVE, EARTHQUAQUE, ETC) and the load combination are independent from the structure.yabi (Structural) 30 Dec 06

1:12 Thanks. I didnt know that. Therefore assigning basic loads (DEAD,LIVE,EQ and wind) will be enough and load combinations will be done automatically.thanks again

hannes964 (Civil/Environme) 23 Dec 06 9:49

Hello,

I try to mesh manually a floor of an irregular geometry by dividing the area at lines (shell elements). When I check the model, I get the warning message: areas "are too close at points...".Has anyone an idea how I can fix this warning. I'm not able to mesh this geometrie proper. In the meanwhile I'm totally helpless. Thanks a lot.civilungo (Structural) 25 Dec 06

22:27 i think you should run "merge points" comand and "check Model" in analyze menu the first comand will find any joint and join into one when they are nearest (less than the tolerance) the second comand will find any irregularity from your structure like overlaping areas or lines and it will select all the elements with problems... then i use the comand "Show Selection Only" in view menu to show just the elements that have problems.

sorry by my grammarhannes964 (Civil/Environme) 27 Dec 06

7:28 thanks for the answer. the command merge points is a powerful command which i have not known. thanks. but i'm still not able to fix the following warning:

2. UG, F4 & F413 are too close. Check at (179,622,296,385,3,15)2. UG, F413 & F414 are too close. Check at (161,803,262,212,3,15)Check Selected Objects.

i obtained this warning after dividing the area with the command mesh areas...i can select them, but then i have no idea what the next step is. i tried to use the command merge points after selecting all objects and points. but this command don't fix the problem.

thanks a lotcivilungo (Structural) 28 Dec 06

17:40 no! merge points just wants for any piont near from other point less than your tolerance and it convert into just one point, bt in your case i preffer erase and create new sub-

elements F4 F413 F414 ... i think is more easy

bye

blihp (Structural) 28 Nov 06 18:11

hi

i have a building where i have a couple of columns relatively close to another and am finding that they are attracting axial loads under a lateral load only case (ie no gravity or live loads).

The columns are pinned and thus it cant be frame action (and the results show they are carrying no shear). So the columns only have axial load in them, no shear.

I have modelled the slabs as shells (ie 6 dof); do you think this may be a cause of the problem? (i dont want to spend a day trying something that is unlikely to be the problem...)

I have also tried refining the mesh, which did help but didnt alleviate the problem. The only thing that did help was reducing the stiffness of the slab to a very low value of modulus (ie 1% of full E), but im unhappy with this for other reasons.

I can only assume that they are acting as some sort of coupled truss through the slab. As i am using ETABS to design the cores/walls, i want to have all the stability to be taken by the cores/walls and not be lost in other elements. I will ensure these other elements are ok under lateral loads separately.

any suggestions?

many thanks in advance

"a wise man know his limits" anon.blihp

daniela123 (Civil/Environme) 23 Nov 06 14:59

I'm a newcomer and I'd like to say you Hello.Now, I have a question for you, hope you can help me: what's the diferrence between modelling concrete shear walls like shell or membrane? Which is the most convenient solution?

Thank you and have a good day.Check Out Our Whitepaper Library. Click Here.

civilungo (Structural) 25 Nov 06 23:37

you may model shear walls with shell elements, because they represent shear and flexural deformation like frame (try making some examples). membrane i use like slab to distribute the loads from floor to walls.

byedaniela123 (Civil/Environme) 26 Nov 06

3:44 Thank you for your answer civilungo.Now I have another question for you: I use for the slab modeling shell elements and after runing the program I can obtain the stresses on the slab. How do you proceed for calculate slabs?

And, another question: do you use auto mesh for meshing the walls and slabs or do you make it manually?

Have a good day.civilungo (Structural) 6 Dec 06

7:15 there is no reason to make meshing manually, what do you mean about calculate slabs? stresses? forces?

¿hablas español?civilungo (Structural) 6 Dec 06

7:56 when i have used mebrane i make just one slab per floor, but when i use shell i prefer to use a meshed floor to refine the results in it. the problem with automesh is that the program mesh with square shape and then we could have problems with arquitecture... sometimes it seems a good option... but only in shell floors. if you test with some examples you can see that there is no reason to mesh walls.. it makes more complex model.

i attach an example. this is a 29 floor apartment buildind in concon- chile http://www.gigasize.com/get.php/217111/costa_de_montemar.rar

asterixUC (Structural) 3 Nov 06 9:04

Hi, i'm from Chile and i'm designing a 26 storey bulding that needs to be modelated taking in acount the flexibility in plane of the floor. Normally i wold use several (2 or 3)diaphragms per storey when important discontinuities in the floor are present, but i have heard of people using semirigid diaphragms instead to do that. I have read every manual i have found to understand the modelation, but i still don't find a formal explanation that can make me trust in that thing. How is the mass matrix assembled, not lumped? How are lateral forces aplyed (accidental torsion) to a center of mass taking in acount the in-plane deformation of elements in the diaphragm? How are the cinematic ecuations of the constraint, if there are any?

If someone can answer me these cuestions or send a CSI document whit some aclarations, please do so, i'll be deeply thankful.

P.S.: Sorry for my english, i don't write/speak it very frequent ;)stressed (Civil/Environme) 4 Nov 06

11:48 You don't have to assign a semi-rigid diaphragm to account for that flexibility in each floor. By default, the program uses the stiffeness and mass of area finite elements and frame

elements modeled on each floor, and spreads mass based on material and section properties. You may or may not have to refine your floor area mesh to create more joints in order to obtain reasonable mass distribution.

As I understand it, assignment of area "semi-rigid" diaphragms is a new option and is used for one purpose, and one purpose only: to tell the program where the diaphragm extents are located for automatic generation of wind loads. That's it. Assignment of semi-rigid diaphragm does not create any rigid diaphragm at all. If you're not using auto-wind load generation based on diaphragm extents, you don't need to make the 'semi-rigid' area assignment in order to account for flexibility of the floors, and the program will by default use flexible diaphram based on material and section properties of what you have modeled.

If you have doubts, simply create a simple, but irregular one or two story structure with floor areas, beams and columns, and see for yourself the period and reactions to lateral loads, checking with and without rigid diaphragms. After assigning rigid diaphragm, you probably already know that in order to delete them, you'll then have to assign area rigid diaphragms again, but with diaphragm property = none. Let us know if that answers your question.asterixUC (Structural) 6 Nov 06

10:16 Thanks, but i allready know everything you just explain to me. Thank you anyway. I also readed that this semi-rigid diaphragm was only to automaticaly calculate and assign wind loads, but how are these loads aplied to the structure? then it is no longer posible to do that at the center of mass. Does the program automaticaly calculate how much load must be assigned to each node of the finite element diaphragm in order to represent te total lateral force? How does it? Does it also work for static torsion loads? Diaphragms are very usefull to assign loads representing sismic accidental torsion, thats why i'm interested in these new diaphragms.I want to know if someone has allready experimented with these. I will build some simple models to experiment my self, but it's allways interesting and usefull to hear other opinions.thank you again for answering

WillisV (Structural) 6 Nov 06 10:50

I have discussed this same issue with CSI - their answer being:

"Yes, lateral loads can be applied to semi rigid diaphragm and the load will be distributed to points based on their stiffnesses and point locations on slab."asterixUC (Structural) 7 Nov 06

14:36 That's the answer i wanted to hear, thank you. I would like to have a complete explanation of how the program does it, but this is enough for me to start testing these in simple models.WillisV (Structural) 7 Nov 06

15:59 I would too but that's about all I could get out of them for now.

stressed (Civil/Environme) 19 Aug 06 16:35

I've recently started to work with the tension/compression limit feature, and I'd like to share some of my experience so far, but I'd also like to hear from some of the more experienced users regarding their suggestions in how to best use this capability.

Ok, first of all tension only starts as a frame assignment with compression limit = 0 or some small negative number. Next, after you have your load combinations for design, go to Define>Convert combos to nonlinear cases and select the combos (the ones with lateral loads?) to convert to nonlinear. I am not sure if I should then delete my linear load combos (?)

Next, run an analysis, and after it's finished, go back again under Analyze menu and analyze "run static nonlinear analysis" which can take a while to converge. Check interactive frame member forces for axial forces to see that the braces, or whatever frames you assigned to be compression limit=0, are acting only in tension. I see that ETABS defaults to two nonlinear steps in reporting output with different results. Worse case final step seems to look reasonable.

I'm now trying to sort out if ETABS is designing properly with the nonlinear combos. I'm curious if those of you with 'Plus' version have access to this tension only nonlinear analysis or is it only available with nonlinear? I'm just getting familiar with how to use this capability, so any advice would be appreciated.

WillisV (Structural) 20 Aug 06 22:37

FAQ802-1169masomenos (Civil/Environme) 22 Aug 06

13:50 Thanks Willis! A star for your excellent FAQ. A couple of comments though - first, tension/compression limits seem to be working in ETABS 9, so that saves you from having to manually define nonlinear hook-type links. Also, your suggestion in your FAQ to Define>static nonlinear/pushover cases seems to work well, but it appears you can also use that dialogue to define combos too by adding 2 or more load cases. In other words, the static nonlinear/pushover "case" dialogue can be used to define combos as well as individual nonlinear load cases. There is also the alternative to Define>Convert combos to nonlinear cases. This ETABS feature seems to work well now, although it's a little more complicated than doing the same thing in SAP.masomenos (Civil/Environme) 31 Aug 06

12:10 I forgot to post that I heard back from CSI technical support last week.. they confirmed that it's best to define static nonlinear/pushover cases as "combos" rather than individual cases. Otherwise, if you don't define as nonlinear combos, you risk getting in trouble with the + and - directions, and furthermore, frame members assigned as tension only (compression limit = 0) may take compression unless ALL combos are defined as nonlinear, not just lateral loads.

WillisV (Structural) 31 Aug 06 12:37

Any non-linear type of analysis should always be performed on the full combination in question as superposition of loads is no longer valid.

joanrizu (Structural) 19 Aug 06 0:55

I am designing a building without a column in the first story. I´ve found that if I run an elastic analysis and then I define a sequential construction case, etabs plus v.909 makes an analysis and gives results that makes sense. But I am not sure that etabs plus results are correct, because manual refers to etabs nonlinear when you ask about sequential construction. Could some body tell me more about this topic.

Find A Job or Post a Job Opening Click Here.stressed (Civil/Environme) 19 Aug 06

15:23 I believe you need to have ETABS nonlinear version to run a sequential construction analysis. However, when you first install ETABS, it defaults for some time period (30 days?) nonlinear version, even if you purchased a lesser version, or until you get a permanent license file. I assume that you went under the Define menu to Add a sequential construction case. The thing is, with ETABS, to do sequential construction, you must make two analyses. First run your regular analysis, then you have go back again under the Analyze menu to run the Contruction sequence nonlinear analysis. In your situation where you are using a tranfer girder on the first story, that is a typical application where the stage construction might give some insightful results compared to a linear Dead case analysis. Keep in mind that ETABS stage construction (to my knowledge) is only additive, in that you can add stories and/or groups, but not remove elements or group as part of the sequential construction analysis. good luck.joanrizu (Structural) 24 Aug 06

18:57 I think Etabs Plus has an opendoor to the sequential analysis if you define thedead-sq after you run the static analysis, that is thestatement I wrote by first. Please follow this stepswith any model you have: delete any dead-sq case, runstatic analysis, define a dead-sq case, run thesequential construction case, and you will obtain thisanalysis run log:

Program ETABS Version 9.0.9.0 File:modelo secuencial similar al tutorial.LOG BLOQUE 5

B E G I N A N A L Y S I S 2006/08/18 22:58:53

MAXIMUM MEMORY BLOCK SIZE (BYTES) = 31.962 MB

E L E M E N T F O R M A T I O N 22:58:53

NUMBER OF JOINT ELEMENTS FORMED = 79 NUMBER OF SPRING ELEMENTS FORMED = 0 NUMBER OF FRAME ELEMENTS FORMED = 159

E Q U A T I O N S O L U T I O N 22:58:53

TOTAL NUMBER OF EQUILIBRIUM EQUATIONS = 207 APPROXIMATE "EFFECTIVE" BAND WIDTH = 33

NUMBER OF EQUATION STORAGE BLOCKS = 1 MAXIMUM BLOCK SIZE (8-BYTE TERMS) = 6447 SIZE OF STIFFNESS FILE(S) (BYTES) = 51.191 KB

NUMBER OF EQUATIONS TO SOLVE = 207 NUMBER OF STATIC LOAD CASES = 4 NUMBER OF ACCELERATION LOADS = 6 NUMBER OF NONLINEAR DEFORMATION LOADS = 0

J O I N T O U T P U T 22:58:53

G L O B A L F O R C E B A L A N C E R E L A T I V E E R R O R S

PERCENT FORCE AND MOMENT ERROR AT THE ORIGIN, IN GLOBAL COORDINATES

LOAD FX FY FZ MX MY MZDEAD 3.26E-15 .000000 1.18E-13 0.478953 6.80E-14 0.015486TRIBUTAR .000000 .000000 .000000 .000000 .000000 .000000SX .000000 .000000 .000000 .000000 .000000 .000000SY .000000 .000000 .000000 .000000 .000000 .000000

SPEC FX FY FZ MX MY MZSX .000000 .000000 .000000 .000000 .000000 .000000SY .000000 .000000 .000000 .000000 .000000 .000000

E L E M E N T J O I N T - F O R C E O U T P U T 22:58:53

NUMBER OF JOINT ELEMENTS SAVED = 79 NUMBER OF FRAME ELEMENTS SAVED = 159

E L E M E N T O U T P U T 22:58:53

A N A L Y S I S C O M P L E T E 2006/08/18 22:58:53


B E G I N A N A L Y S I S 2006/08/18 22:58:54

MAXIMUM MEMORY BLOCK SIZE (BYTES) = 31.962 MB

E L E M E N T F O R M A T I O N 22:58:54

NUMBER OF JOINT ELEMENTS FORMED = 79 NUMBER OF SPRING ELEMENTS FORMED = 0

L O A D R E - S O L U T I O N 22:58:54

NUMBER OF STATIC LOAD CASES = 4

E L E M E N T J O I N T - F O R C E O U T P U T 22:58:55

NUMBER OF JOINT ELEMENTS SAVED = 79 NUMBER OF FRAME ELEMENTS SAVED = 159



N O N L I N E A R S T A T I C A N A L Y S I S 2006/08/18 22:59:28

MAXIMUM MEMORY BLOCK SIZE (BYTES) = 31.962 MB NONLINEAR STATIC CASE = DEAD-SQ CONTINUE FROM PREVIOUS CASE = 0 LOAD CONTROL TYPE = FORCE METHOD TO USE WHEN HINGES DROP LOAD = UNLOAD TYPE OF GEOMETRIC NONLINEARITY = NONE SAVE POSITIVE INCREMENTS ONLY = YES MINIMUM NUMBER OF SAVED STEPS = 1 MAXIMUM NUMBER OF SAVED STEPS = 50 MAXIMUM NUMBER OF NULL STEPS = 50 MAXIMUM NUMBER OF TOTAL STEPS = 200 MAXIMUM NUMBER OF ITERATIONS PER STEP = 10 RELATIVE FORCE CONVERGENCE TOLERANCE = 0.000100 RELATIVE EVENT TOLERANCE = 0.010000 STAGE NUMBER = 1 ENTIRE STRUCTURE IS ACTIVE = NO CHANGES TO ELEMENTS IN STRUCTURE = NONE

STEP 0

STEP 1, INCREMENT = 1.000000, SUM = 1.000000, MAX = 1.000000

Unbalance/tolerance = 4.39E-12, Iteration 1 converged Saved as Output Step 1

TOTAL NUMBER OF CONVERGED STEPS SAVED = 1 TOTAL NUMBER OF CONVERGED STEPS NOT SAVED = 0 TOTAL NUMBER OF NULL STEPS = 0

---------- TOTAL NUMBER OF ALL STEPS = 1

TOTAL NUMBER OF ITERATIONS CONVERGED = 1 TOTAL NUMBER OF ITERATIONS DISCARDED = 0 FOR CONVERGED STEPS ONLY: AVERAGE NUMBER OF ITERATIONS PER STEP = 1.00 MAXIMUM NUMBER OF ITERATIONS PER STEP = 1 STAGE NUMBER = 2 ENTIRE STRUCTURE IS ACTIVE = NO CHANGES TO ELEMENTS IN STRUCTURE = ADD ONLY

STEP 0



TOTAL NUMBER OF CONVERGED STEPS SAVED = 1 TOTAL NUMBER OF CONVERGED STEPS NOT SAVED = 0 TOTAL NUMBER OF NULL STEPS = 0 ---------- TOTAL NUMBER OF ALL STEPS = 1

TOTAL NUMBER OF ITERATIONS CONVERGED = 1 TOTAL NUMBER OF ITERATIONS DISCARDED = 0 FOR CONVERGED STEPS ONLY: AVERAGE NUMBER OF ITERATIONS PER STEP = 1.00 MAXIMUM NUMBER OF ITERATIONS PER STEP = 1 STAGE NUMBER = 3 ENTIRE STRUCTURE IS ACTIVE = NO CHANGES TO ELEMENTS IN STRUCTURE = ADD ONLY

STEP 0




TOTAL NUMBER OF ITERATIONS CONVERGED = 1 TOTAL NUMBER OF ITERATIONS DISCARDED = 0 FOR CONVERGED STEPS ONLY: AVERAGE NUMBER OF ITERATIONS PER STEP = 1.00

MAXIMUM NUMBER OF ITERATIONS PER STEP = 1 STAGE NUMBER = 4 ENTIRE STRUCTURE IS ACTIVE = YES CHANGES TO ELEMENTS IN STRUCTURE = ADD ONLY

STEP 0




TOTAL NUMBER OF ITERATIONS CONVERGED = 1 TOTAL NUMBER OF ITERATIONS DISCARDED = 0 FOR CONVERGED STEPS ONLY: AVERAGE NUMBER OF ITERATIONS PER STEP = 1.00 MAXIMUM NUMBER OF ITERATIONS PER STEP = 1 STAGE NUMBER = 5 ENTIRE STRUCTURE IS ACTIVE = YES CHANGES TO ELEMENTS IN STRUCTURE = NONE

STEP 0

* * * W A R N I N G * * * THE APPLIED LOAD IS ZERO IN NONLIN-STATIC CASEDEAD-SQ , THE ANALYSIS WILL PROCEED BUT THE STRUCTURE WILL NOT BE LOADED


Unbalance/tolerance = .000000, Iteration 1 converged Saved as Output Step 1


TOTAL NUMBER OF ITERATIONS CONVERGED = 1 TOTAL NUMBER OF ITERATIONS DISCARDED = 0 FOR CONVERGED STEPS ONLY: AVERAGE NUMBER OF ITERATIONS PER STEP = 1.00 MAXIMUM NUMBER OF ITERATIONS PER STEP = 1

TIME FOR INITIALIZING ANALYSIS = 0.03

TIME FOR CONTROLLING ANALYSIS = 1.14 TIME FOR FORMING STIFFNESS MATRIX = 0.07 TIME FOR SOLVING STIFFNESS MATRIX = 0.24 TIME FOR CALCULATING DISPLACEMENTS = 0.00 TIME FOR DETERMINING EVENTS = 0.00 TIME FOR UPDATING STATE = 0.04 ---------- TOTAL TIME FOR THIS ANALYSIS = 1.52


qrexus (Structural) 15 Aug 06 4:06

Anybody have the Etabs version changes from v850 to v857 or greater. Please post them.Thank you,Regards,Q

Find A Job or Post a Job Opening Click Here.Diquan (Civil/Environme) 22 Aug 06

22:07 http://www.csiberkeley.com/etabsv9/

--------------------------------------------------------------------------------

New Features in ETABS Version 9

ETABS Version 9.0.0 is a new version, and is a direct upgrade from Version 8.5.6. New features include the following:

· Added Semi-rigid diaphragm option· Added Design output to Database· Added Story vertical load, shear and overturning plots· Improved plan display of most design quantities· Improved analysis model creation time· Enhanced Model Building of Walls with openings· Added IBC 2003 seismic and wind loads· Added auto-permutation of Wind directions and eccentricities· Added Open-structure wind loads· Added Export to SAFE V8 with poly areas· Updated Concrete Frame Design to ACI 2005· Updated Concrete Shearwall Design to ACI 2005· Updated Steel design to AISC-ASD 2001 (Seismic Provisions 2002)· Updated Steel design to AISC-LRFD 2001 (Seismic Provisions 2002)· Updated Steel design to AISC 2005 (Not in initial release)· Added Import/Export from Autodesk Revit Structure· Added Import/Export from ProSteel· Added Import/Export from IFC· Updated CIS/2 Import/Export· Added Import from STRUDL· Added Import from STAAD

· Updated Help· Updated Manuals

Diquan

shaheed (Structural) 13 May 06 8:40

As an extension to the question already asked by "Sepultura" how would one model the soil spring as compression only - using ETABS. Any hints/thoughts would be extremely appreciated.

Check Out Our Whitepaper Library. Click Here.stressed (Civil/Environme) 19 Aug 06

15:37 There is no automated way to do this like in SAFE. However, you can define Link properties, Gap type, and do joint assignments to put them at joints on your basemat. The stiffness values for the gap links would be determined by soil subgrade modulus, size of element and tributary area. The gap type links are nonlinear compression only joint springs. If you can live without the nonlinear compression-only analysis, ETABS lets you assign area springs on your basemat (modeled as slab/shell) in which you input subgrade modulus for vertical springs. Thing is, you have to remember to restrain the basemat in the local 1 and 2 lateral directions too, using area or joint springs or joint restraints, to prevent instabilities.

qrexus (Structural) 10 Aug 06 9:49

Anybody have the Etabs version changes from v907 to v909 or greater. Please post them.Regards,Q

lmcid (Structural) 10 Aug 06 10:23

Significant Changes from Version 9.0.0Changes from Version 9.0.9 to Version 9.0.9/1 (issued 07/26/2006)- Minor modifications to Revit imports.- Corner/Edge rebar in rectangular concrete columns can now be of different sizes.Changes from Version 9.0.8 to Version 9.0.9 (issued 07/19/2006)- An export capability has been added to export element connectivity to program Perform3D.- Import of models from other programs has been enhanced.- Minor other bug fixes and enhancements.Changes from Version 9.0.7 to Version 9.0.8 (issued 06/16/2006)- The Line Constraint stiffness has been changed back to what was being used in V8. Thestiffer constraints in earlier versions of V9, even though giving better stress results, werecausing unnecessary numerical sensitivity problems in some models, especially larger ones.- Minor modifications to Revit and IFC imports.- Other minor design bug fixes.Changes from Version 9.0.6 to Version 9.0.7 (issued 05/29/2006)

- Redundancy factor Rho for seismic loads was missing from load combinations for ACI-02and ACI-05. Corrected. These designs should be verified.- Part of the code related to creation of analysis model has been optimized for time. Shouldallow larger jobs to be run faster.- Several new tables added to the database display.- Interface with Autodesk Revit Structural updated.- Interface with IFC updated.- Import from STAAD updated.- Changes to Steel Design based on the Chinese code.- Other minor bug fixes to design.- Other minor bug fixes.Changes from Version 9.0.5 to Version 9.0.6 (multiple limited releases)- 2GB limit on some intermediate analysis files removed. Should allow larger jobs to be run.Changes from Version 9.0.4/1 to Version 9.0.5 (issued 04/02/2006)- Export/Update capability added for Autodesk Revit Structural Version 3. This dataexchange is still under development. Imported/Exported models should be checked.- Design database tables for Shearwall and Composite beam designs added.- Other minor bug fixes and enhancements.Changes from Version 9.0.4 to Version 9.0.4/1 (issued 03/24/2006)- Support for Autodesk Revit Structural Version 3 (Beta) has been added. This is still underdevelopment. Imported models should be checked. Export is currently not supported.- Sections database for AISC 13th Edition have been added and are the new program default.- Other minor bug fixes and enhancements.Changes from Version 9.0.2 to Version 9.0.4 (issued 03/22/2006)- Database display times have been significantly improved.- Other minor bug fixes and enhancements.Changes from Version 9.0.1 to Version 9.0.2 (issued 01/23/2006)- A bug that in some cases caused User specified Wind Loads to diaphragms to be reset tozero has been corrected.- A bug that caused export to SAFE not to export slabs has been corrected.- A bug that caused Static Nonlinear analysis results not to be available in Database view hasbeen corrected.- Minor enhancements to Composite beam design.- Minor enhancements to Line (Edge) Constraint formulation.- Other minor bug fixes and enhancements.Changes from Version 9.0.0 to Version 9.0.1 (issued 01/07/2006)- A bug that caused DXF import not to work has been corrected.- Minor enhancements to import from Autodesk Revit Structural.- Minor enhancements to import from IFC files.- Minor enhancements to Chinese steel design.- Minor enhancements to database and graphic display of numerical quantities.- Other minor bug fixes and enhancements.-5. New Features in ETABS Version 9ETABS Version 9.0.0 is a new version, and is a direct upgrade from Version 8.5.6. New featuresinclude the following.· Added Semi-rigid diaphragm option· Added Design output to Database· Added Story vertical load, shear and overturning plots· Improved plan display of most design quantities· Improved analysis model creation time· Enhanced Model Building of Walls with openings

· Added IBC 2003 seismic and wind loads· Added auto-permutation of Wind directions and eccentricities· Added Open-structure wind loads· Added Export to SAFE V8 with poly areas· Updated Concrete Frame Design to ACI 2005· Updated Concrete Shearwall Design to ACI 2005· Updated Steel design to AISC-ASD 2001 (Seismic Provisions 2002)· Updated Steel design to AISC-LRFD 2001 (Seismic Provisions 2002)· Updated Steel design to AISC 2005 (Not in initial release)· Added Import/Export from Autodesk Revit Structural· Added Import/Export from ProSteel· Added Import/Export from IFC· Updated CIS/2 Import/Export· Added Import from STRUDL· Added Import from STAAD· Updated Help· Updated Manualsqrexus (Structural) 10 Aug 06

12:14 Thank you very much.

ingemor (Structural) 8 Jul 06 21:33

A couple of days ago I posted in Murali27’s “Export from Etabs” thread in the SAFE forum more or less the following:

I have heard that there is another trick to solve the problem of exporting spectral analysis results from Etabs to Safe besides using equivalent static seismic load cases. I think that the idea behind the trick could be something like this:

1.) Run a static quake and the required spectrum analysis in the same Etabs model.

2.) Analyze the different output formats for both results in exportable Safe.f2k text file.

3.) Extract the spectrum analysis results from Safe.f2k text file.

4.) Change and match format of the spectrum analysis results to the format produced by ETABS for static quake analysis (eliminate excess data).

5.) Re-incorporate the spectral results in the modified format into the SAFE .f2k file.

6.) Delete the static quake results from that file.

7.) Import the modified f2k file to SAFE and run it.

I understand that the changing and matching format process is tedious but, again, I have heard that it can be expedited by using Excel.

I’ll like to know comments and recommendations both from those who have used this trick or a similar one, as well as from those who are interested in try it out. I’ll be trying it out in couple of weeks.

Regardsingemor

Check Out Our Whitepaper Library. Click Here.DENSAL55 (Civil/Environme) 10 Jul 06

14:48 exporting spectral analysis results from Etabs to Safe for slab foundation analysis or beam foundations analysis has no sens because results given by spectral analysis are always in absolute value this means that they are always positive or if multiply by -1 always negative.And by this way it doen't give the behavior of overturning effect.DENSAL55 (Civil/Environme) 10 Jul 06

14:57 There is anothere way to solve this problem.Make spectrale analysis then take sheare story result and there position, and make a static analysis with the previous sheare story as static force in the new analysis at there respective position, the reaction given at the base of the structure is given with there sign, and at this point we can use this result for safe analysis or othere one.ingemor (Structural) 12 Jul 06

21:20 Thank you DENSAL55 for your comments and recommendation.

I have limited knowledge of dynamic analysis, nevertheless I’m documenting my self to digest the reason you posted that makes the proposed trick not viable.

As for your recommendation, its is clear and I’ll try it out.

Regards and thanks again.ingemorMurali27 (Structural) 21 Jul 06

10:35 Thanks DENSAL55 & Ingemor for your thoughts. Spectral story shears are considering the contribution from all the modes. Hence, can these story shears be applied as a static force? I will also do some comparison.

Thanks againMurali27DENSAL55 (Civil/Environme) 21 Jul 06

12:13 It's what i say .A combination or SRSS give always an absolute value and we can't use this result for a calculation of slabe fondation or beam fondation it can't give the overturning effect . So we can use an equivalent static model by using story shears applied as static forces. DENSAL55 (Civil/Environme) 25 Jul 06

13:57 To confirm what i said before, you can see that Etabs use the Story shears value to calculate oveturning moment Mx and My.at differents levels and at the base of the structure.this overall moments is equivalents in value to those you obtain by the equivalent static case using story shears.

Murali27 (Structural) 15 Jun 06

15:42 Dear All

Mass moment of inertia changes with respect to mesh element size. When i take a plate of 4mx4m, ETABS performs the analysis and MMI is 3 times more than the element when meshed 1mx1m.

Will it calculate the self weight of column/wall in the computation of MMI?

ThanksMurali G

Eng-Tips Forums is Member Supported. Click Here to donate.DENSAL55 (Civil/Environme) 17 Jun 06

4:23 Yes it will calculate the self weight of all object if the option in mass source are checked. Murali27 (Structural) 17 Jun 06

16:20 Thanks DENSAL55.

My question is that floor meshing makes the change in value of MMI. When it gets divided into many parts i.e fine meshing, then the result will be close to the exact one.

Thanks againMurali G

sepultura (Structural) 16 May 06 17:32

I want to modelate solid slab of concrete, and the program give me streess (kg/cm2), and I want to design it with moments (kg-cm). how to realize that convertion?

Find A Job or Post a Job Opening Click Here.DENSAL55 (Civil/Environme) 17 May 06

16:44 Go to Display >> Show Member Force/stress diagram >> Shell Stesses/Forces if we select Forces we have All Forces M11 M22 which are out of plane moment in kg-cm/cm of lenght.sepultura (Structural) 17 May 06

18:20 Thans DENSAL55, but my questin is: How I could do it manually, if I have not the option of M11 or M22?The stress (kg/cm2) I want to convert to moment (kg/m), manually. (in flexural design).Diquan (Civil/Environme) 22 May 06

8:49 Simple:

S=(M*c)/I

S = StressM = Momentc = distance from the outer fiber to the center of the member

I = inertia of the section

[email protected]

Diquan (Civil/Environme) 22 May 06 8:55

... ou i forgot, when calculating the inertia in the equation:

I=b*h^3/12

"b" is 100cm or if you use inches "b" equal to 12in (1feet)

so, when calculating the amount of steel is going to give you "cm^2" for every "cm" - (cm^2/cm or in^2/in)

[email protected]

DENSAL55 (Civil/Environme) 22 May 06 16:08

in the case of slab it is not so simple, because we have many type of stress S11=F11/th-(12*M11/thb^3)*x3. S22=F22/th-(12*M22/thb^3)*x3. S12=F12/th-(12*M12/thb^3)*x3 etc... x3 represente the variation of thickness of slab.

to obtain M11,M22 etc...we must integrate the stress s11,s22,s12 along the thickness of the slab. see "Internal Force and Stress Output Page 144 in the sap reference manual " The Shell element internal forces (also called stress resultants) are the forces and moments that result from in tegrating the stresses over the ele ment thick ness. Theseinternal forces are etc...DENSAL55 (Civil/Environme) 22 May 06

16:12

- x3 represente the variation of thickness of slab from midplan of slab to the point where the stress is calculated .

stressed (Civil/Environme) 23 May 06 19:11

For designing the slab, don't forget about drawing section cuts to obtain resultant forces and moments for design. If you can get a copy of SAFE, you can export the slab from ETABS to SAFE for design of reinforcement and punching shear checks

sellsam (Structural) 2 May 06 11:06

In both ETABS and SAP2000, when I declare response spectrum case data, how to declare scale factors, do I put scale factors in both x and y direction, or must I declare two seismic functions in x direction with it is scale factor, and the same in y direction.

thanks. dawn836 (Structural) 2 May 06

12:27 you should declare two functions, every direction must has its one function with its own scale factoryou may have force apllied in two directions in same function if you have to consider orthognal effectssellsam (Structural) 8 May 06

11:05 hello and thanks dawn836,

it is more correct if we proceed with the second proposition , so we can have orthogonal effects, do not we.

thank youMurali27 (Structural) 12 May 06

13:32 Sellsam

Yes, if your structure calls for orthogonal effects. Otherwise, you may need to define in two principal direction of structure to understand the distribution of modal shear.

Muralisellsam (Structural) 21 May 06

9:48 thanks murali.sepultura (Structural) 10 May 06

23:43 Which parameters of the soil I must to know to modelate it with springs in the ETABS?Could you explain me?Murali27 (Structural) 12 May 06

13:38 Sepultura

Soil springs - Structure directly on soil: Then, Subgrade modulus of soil.

-MuraliDENSAL55 (Civil/Environme) 12 May 06

14:47 It's same as when you calculate a beam on elastical supports, you represent soil by coefficient of reaction. Soil - loose k=0.1-0.5 kg/cm3 - medium dense k=0.5-5 - dense k=5-10

- compact k=10-20 - hard k=20-100you select joints at base then assign->point spring->kx ,ky,kz...etc.You must add your fondations as a shell model (slab) the joint correspond to shell mesh. i hope this help.

sepultura (Structural) 16 May 06

17:29 Is there any relation between coefficient of reaction (kg/cm3) of the soil and resistance soil support (kg/cm2)?DENSAL55 (Civil/Environme) 17 May 06

16:55 Yes there is a relation between coeficient of reaction and resistance of soil p=-k*y where y is the displacement p is the pression on the soil k is the coefficient of reaction.DENSAL55 (Civil/Environme) 10 Apr 06

8:12 When columns and beams are modelised with Piers and Spandrels what's more than when they are modelised by frames elements.DENSAL55 (Civil/Environme) 18 Apr 06

4:15 Normaly columns and beams are modelised with frames elements. Etabs allow modelisation of these elements with piers and spandrels. is piers and spandrel disigned for shell elements only or not ?

Thanks.Murali27 (Structural) 21 Apr 06

9:39 Piers and spandrels can be assigned for frame elements also.DENSAL55 (Civil/Environme) 23 Apr 06

3:16 thank you Murali27

civilhak (Structural) 31 Mar 06 6:34

hi hi.The thing is that I made a model with rather large shell elements in ETABS that runs ok, but after meshing the walls further to obtain more accurate results the model doesnt runs :S, an error message says that I should check connectivity and such for element 1242, my question is, how an I find that element quicly??

Thank you in adcane boysregerdscivilhak

Find A Job or Post a Job Opening Click Here.civilhak (Structural) 31 Mar 06

6:46

sorry, I forgot to say that in SAP you can select an item by "label", can´t we do that in ETABS?, do I have to browse through the model to find my element?

regardscivilhakMurali27 (Structural) 14 Apr 06

0:08 Finite element numbering is being done with respect to stories that is the different from other standard FEM softwares. Hence the same label will be repeated for all the story levels.

The only way to find the location by looking at the respective story level. In the error message, X,Y&Z coordinates will be given and this is the way to find the location.

Hope this helps, Muralijen4950 (Civil/Environme) 22 Apr 06

12:04 It also gives you the coordinates of the element. Turn on the area labels, and get close to it by trying to match the coordinates in the lower right hand side of the window

rook1 (Structural) 11 Apr 06 21:50

Im a newbie on etabs.. id like to design a 5storey hotel.the walls will be made from 10" CMU and 8" CMU on the top 3 floors.. The CMU are to carry all vertical loads including the lateral forces.. the slab of the entire bldg is hollowcore concrete planks.any suggestions on wat properties to assign to the wall and slab?help much appreciated!Murali27 (Structural) 14 Apr 06

0:01 Hi rook1

Hollow core concrete planks -> Membrane properties

Wall -> Assigned membrane or shell propertiesCan be idealized as equivalent thickness by using stiffness reduction factors.

Hope this helps,Muralirook1 (Structural) 17 Apr 06

18:34 Can be idealized as equivalent thickness by using stiffness reduction factors <-- please be specific with this. thanx!i saw a post with the equivalent thickness. but the CMU we are using are 10" and 8".. the post only mentioned 6" CMU..

should all walls be assigned pier section then be assigned as "should be checked"?Murali27 (Structural) 21 Apr 06

9:37 Hi Rook1

Using stiffness reduction factors, we can assign the exact properties of 10"CMU wall as a equivalent rectangular concrete wall. (Area, Ix, Iy,etc).

Yes, we need to assign pier labels to extract the forces out and the same needs to be designed as per CMU design procedure,

Hope this helps

ryansteel (Structural) 11 Apr 06 8:15

is there a way to modify M33 of Shell elements, for crack section requirement, or my is it logical to modify it. i am confused

Eng-Tips Forums is Member Supported. Click Here to donate.Murali27 (Structural) 11 Apr 06

9:55 Is that for shell element or solid 3D element?

M11 - Bending xM22 - Bending yM12 - Warping xy

Hope this helps

DENSAL55 (Civil/Environme) 27 Mar 06 4:01

What's difference between deck and slab.Eng-Tips Forums is Member Supported. Click Here to donate.

Murali27 (Structural) 9 Apr 06 16:35

Hi Densal 55

Deck is specifically for one way spanning pre cast slabs and the rest will be slab category. In slab category, oneway distribution can be enforced.

Hope this helpsDENSAL55 (Civil/Environme) 10 Apr 06

7:04 thanks Murali27

DENSAL55 (Civil/Environme) 27 Mar 06 10:46

In the results table below, I find that for the first mode the value in Ux direction and Ry are very important nomaly only one value should be expressive.is this good result.

Mode Period UX UY UZ SumUX SumUY SumUZ RX RY RZ SumRX Su

mRY SumRZ1 0,910045 75,2861 0,0007 0,0000 75,2861 0,0007 0,0000 0,0011 99,4974 0,0053 0,0011 99,4974 0,00532 0,430470 0,0016 72,1128 0,0000 75,2877 72,1135 0,0000 98,7345 0,0011 0,0815 98,7356 99,4985 0,08683 0,375553 0,0096 0,0385 0,0000 75,2972 72,1519 0,0000 0,0469 0,0081 70,9066 98,7825 99,5067 70,99344 0,197741 10,2317 0,0011 0,0000 85,5289 72,1530 0,0000 0,0016 0,1217 0,0081 98,7841 99,6284 71,00155 0,104799 0,0003 10,7728 0,0000 85,5292 82,9258 0,0000 0,7230 0,0000 2,8893 99,5071 99,6284 73,89086 0,088809 6,1742 0,0010 0,0000 91,7033 82,9268 0,0000 0,0001 0,2856 0,0017 99,5072 99,9140 73,89257 0,079276 0,0003 3,0483 0,0000 91,7037 85,9751 0,0000 0,2256 0,0001 11,7153 99,7327 99,9141 85,60788 0,059936 0,0001 0,1440 0,0000 91,7037 86,1191 0,0000 0,0062 0,0000 0,0271 99,7390 99,9141 85,63499 0,052271 3,3125 0,0002 0,0000 95,0162 86,1193 0,0000 0,0000 0,0421 0,0004 99,7390 99,9562 85,635310 0,048579 0,0004 4,7498 0,0000 95,0166 90,8691 0,0000 0,1507 0,0000 1,3878 99,8897 99,9562 87,023111 0,038883 4,8055 0,0002 0,0000 99,8220 90,8693 0,0000 0,0000 0,0434 0,0009 99,8897 99,9997 87,024112 0,036300 0,0003 2,2210 0,0000 99,8224 93,0904 0,0000 0,0687 0,0000 4,5193 99,9584 99,9997 91,5434

Find A Job or Post a Job Opening Click Here.Murali27 (Structural) 3 Apr 06

3:26 Absolutely good result, With the inclusion of 12 modes, modal mass participation achieved more than 90% in two translation & rotation degrees of freedom.

-MuraliDENSAL55 (Civil/Environme) 3 Apr 06

4:31 ThanksDENSAL55 (Civil/Environme) 3 Apr 06

4:56 My question is "is it normal to find in the same mode two values with high ratio, in my case for the first mode i have - Ux 75,2861 - Ry 99,4974 ."

thanks.WillisV (Structural) 3 Apr 06

7:51 That just means the first fundamental period of the building is a combined translational/torsional mode. It is not necessarily normal or not normal - it just depends on the layout of the lateral system in the building versus the mass distribution. You will want to make sure to design for any torsional irregularities per code. DENSAL55 (Civil/Environme) 3 Apr 06

11:24

Thanks WillisMurali27 (Structural) 8 Apr 06

7:00 Hi DENSAL55

As per your results, the modes are really uncoupled. i.e the first mode Ux = 75% & Ry (Over turning moment about y-y axis = 99.4%), Second mode Uy = 72.11% & Rx(Over turning moment about x-x axis = 98.75%) & Third mode Rz = 71% (Torsion mode)

Hope you understand.DENSAL55 (Civil/Environme) 8 Apr 06

17:10 Thanks Murali27dawn836 (Structural) 10 Apr 06

0:01 Murali27, what uncoupled modes mean?

Murali27 (Structural) 27 Mar 06 9:01

Dear All

In the computation of diaphram mass, the forces are not matching with the story shears. Is there any bug in ETABS?

ThanksMuraliDENSAL55 (Civil/Environme) 27 Mar 06

11:37 Story shear is based on modal participating mass ratio, the mass is the total mass of story.You must ask for more mode (number of mode) in calculation of your model to reach the matching value.ryansteel (Structural) 27 Mar 06

23:56 YUP! Some code says at least 90% of total mass should participate. Murali27 (Structural) 3 Apr 06

2:53 Thansk a lot

Murali27 (Structural) 2 Dec 05 9:56

To All

As per ACI code, the initial stiffness modifiers are to be assingned for wall/slab/column/beam structural elements.

Slab - 0.25 Ig Wall - 0.70 Ig

Beam - 0.35 IgColumn - 0.7 Ig

In ETABS

Slab -> Assigned with shell element -> M11, M22 & M12 as 0.25

Wall -> Assigned with shell element -> f11, f22,f12 are related to in-plane forces.

Do we need to apply 0.7 factor to f11, f22 & f12 (axial & shear area) ? M11,M22 & M12 are insignificant since they affect out of plane bending forces.

Please clarify this point.

ThanksMurali G

Check Out Our Whitepaper Library. Click Here.johnjeed (Structural) 2 Dec 05

22:04 As I understand the intention of the stiffness modifiers is to account for reduced stifffness of cracked structural elements. I would have thought that the flexural stiffness of members (i.e. bending properties) only would need to be modified.Murali27 (Structural) 3 Dec 05

9:16 Yes. Johnjeed.

You are correct. But in shear wall, the be section will undergo cracking due to in-plane moment.

How to assign stiffness modifiers for this condition? f11, f22 & f12 - 0.35Ig. But there are also related to axial stresses.

How to go about?

Thanks

WillisV (Structural) 5 Dec 05 7:53

Typically I initially assign a 1.0 factor to f11, f22, and f12 along with a 0.01 factor to m11, m22, and m12 (so that the shearwall essentially has no out of plane stiffness and acts as a classical in-plane only resisting element). I then run the model and check axial stresses against the modulus of rupture to see if the assumption that the wall is uncracked (f11,f22,f12 = 1.0) was correct. For those portions of the wall that are cracked, I reassign with the lower 0.7 crack factor to f11,f22, and f12.

1Quique (Civil/Environme) 1 Mar 06 23:01

Try and see. Model one slab of 240x240in, 4 beams and columns. Modify m22=.01, m12=.01 (don't use zero). Load slab with 0.4096 Kip-ft. Mesh it 6x6. Run model. See M33 on beams. See M11 vs. M22 on slabs. Good luckMurali27 (Structural) 2 Mar 06

4:00 Hi IQuique

Once you change the m22 modifier, the load is going to be transfered in X direction i.e along local axis 1. Please let me know what is your point here.

Hi WillisV

For serviceability studies, lower factor 0.7 crack factor would be ok. But for ultimate state (design purpose), 0.35 would be appropriate.

Please explain with respect to ACI 10.11.1

ThanksMuraliWillisV (Structural) 2 Mar 06

7:21 Murali - agreed my crack factors shown are for the serviceability/deflection analysis of the walls. 1Quique (Civil/Environme) 2 Mar 06

12:55 My point of view: Modifiers are very useful for driving the concrete behavior. Modifiers, concrete, steel bars, plans details and constructor technology are necessary guests to obtain the structure born in our mind. Then, initial modifiers suggested for ACI 10.11.1 must be adjusted when we know something about amount of bars on concrete. Another important question. Take in mind that walls are only connected to structure through 4 poits (or 3). In my example is impossible think in modifier m22=.01 because minimum fe2 requiered m22>.01. Forgive my english but my language is spanish. Bye

KuJayHawk (Structural) 3 Feb 06 11:55

I am working on the load rating of a historic truss bridge. All the diagonals are steel rods which can't take any compression. I tried to use RISA but the program can't handle Tension Only (even you turn on the Euler Buckling option) and keep the truss stable at the same time. Even worse, the program locked almost all the truss joints due to instability but DID NOT report the lock as it normally does for other models. I contacted the technical support at RISA and they told me the only way to let the model run is to let every truss member carry both tension and compression. I wonder if anyone here used other program to handle this type of problem? I have SAP2000 and ANSYS on my computer. ANSYS sure is powerful but the problem will be with the moving load generator. How about SAP? What do you do with tension only brace analysis uner building lateral forces?

WillisV (Structural) 3 Feb 06 19:26

Sap would be a good choice for this type of analysis, it has moving load application as well as nonlinear nlink framing elements which can be designated as tension-only. The procedure would be similar to that for tension only braces in ETABs per FAQ802-1169KuJayHawk (Structural) 4 Feb 06

22:56 Thanks WillisV! I never used SAP but will give it a try. I've never had ETAB either. Seems like it is not as simple as I thought of. No wonder RISA can't solve the problem.

useetabs (Structural) 4 Jan 06 16:56

I use etabs 8.5.6 to model a 35 story building. I model the slabs using shell and membrane and obtain different period. torsion x y shell 4.6 3.5 4.15membrane 20.3 7.5 15.6

the drafts using membrance are much larger than using shell.

please whether it makes sense or the mode has some problem?

Check Out Our Whitepaper Library. Click Here.Murali27 (Structural) 17 Jan 06

4:28 It looks like there is a problem with local modes. Please verify that whethe you had applied rigid diaphragm.

-Murali Guseetabs (Structural) 23 Jan 06

16:59 Hi Murali:

Thank you! this mode had been applied rigid diaphagm.could you give more suggestion

-useetabs jen4950 (Civil/Environme) 25 Jan 06

2:33 This makes perfect sense. Membranes have no bending stiffness, whereas shell elements introduce the stiffness of the slab into the system.

Be careful here- say for instance you have a shear wall primary lateral system and have the gravity columns modeled in the ETABS model, they will attract moment and forces if connected to a shell floor element, or if geometric or mass floor to floor inconsistencies exist and you have a rigid diaphram. The differential intrastory movement will engage the gravity columns.useetabs (Structural) 25 Jan 06

9:29 Hi,jen:thank you. then which mode should I use to design shear wall?jen4950 (Civil/Environme) 26 Jan 06

21:07 Well, it depends on what type of structure you are designing. Dual system? Devil is in the details..

I'd recommend a membrane element for a strictly shearwall lateral system. There are several different opinions about the subject though.

One guy I know would say model the whole damn thing and let the force go where it wants to; But this is risky- if you have a flat plate system your punching shear might control the capacity of the system and you might get unrealistic results.

Think pancake failure = bad.

But if you have a pan joist or slab and beam system, it might make sense to model elements that in reality have the capacity and stiffness to contribute to the lateral effectiveness of the system.

"Engineering Judgement in spades."useetabs (Structural) 27 Jan 06

16:27 Hi, jen:

You are right. As you said, I have a shear wall primary lateral system , the gravity columns system and a flat plate system modeled in the ETABS model. I export the slab model with safe export option "Export Floor Loads and Load from Above" and the punch shear obtained from safe is very large and unrealistic. could you give me further suggestion to do the ETABS model? Thank you very much!

useetabs. jen4950 (Civil/Environme) 28 Jan 06

1:02 I'd model the plate as a membrane element and do the flat plate as a seperate analysis in something like PTData or ADAPT if it is a flat PT plate.

Do not rely on the stiffness of the flat plate for your lateral system IMHO. It is very rigid in shear and as a collector mechanism but bending is not effective. I.E. gravity- it works; stiffness in regards to the lateral system- think paper with a toothpick thru it. It's just not realistic or safe. You don't see any flat plate moment frames without beams- so why include them as a contributing element just because you have shear walls?

This is the fundamental arguement for rigid diaphrams- Of course if you have a crazy geometry and structure like I'm dealing with right now it becomes clear that a rigid diaphram just might not be conservative.. but that's another thread all in it's own..

I'd only use the ETABS model for a gravity load takedown and column design; as well as shear walls. PT plates are an art that is out of ETABS land.. be sure to use dedicated PT software like PTData or ADAPT.

strucengr (Structural) 14 Apr 05 17:06

How do you normally apply line load on a wall? I added a dummy frame element at the top of the wall and applied the uniform load. However, it gives me a warning that the load has not been transfeered. does the line's property need to be modified?

Thank youCheck Out Our Whitepaper Library. Click Here.

amadeus (Structural) 15 Apr 05 15:08

Please, read one of the FAQs, it may give you the answer.strucengr (Structural) 15 Apr 05

18:58 The FAQ dealt with slabs supported on wall. But in my case, there is no slab. The line load comes from non-structural heavy glass. WillisV (Structural) 6 Jun 05

10:50 Apply the actual heavy glass weight as an area load on the wall element. 14159 (Structural) 30 Jul 05

21:50 I usually either break it into nodal loads or put a fake line element along the wall and apply the load to it.

DBDjen4950 (Civil/Environme) 25 Jan 06

3:15 The simple answer here is that area elements only recognize loads applied at their nodes, or on their surface- NOT EDGES or LINES.

So if you want to get a sheet of glass that's sitting on a wall to register, the reactions from the glass must end up at the nodes of the wall. Make a fake beam, unless you want to determine a nodal load.


To All,

In one of the structure, the fundamental mode is a torsional mode.

When i asked ETABS to perform static method as per UBC code. I assigned X direction as well Y direction. The first three time periods are

1). T1 = 1.963 sec - Rotation2). T2 = 0.85 sec - X translation3). T3 = 0.65 sec - Y translation

ETABS picked up the corresponding time period T2 & T3 in the calculation of base shears. It avoids fundamental time period T1. Is this correct?

Your input will be appreciated.

ThanksMurali G

Find A Job or Post a Job Opening Click Here.WillisV (Structural) 20 Dec 05

7:23 ETABS is correct in that it is finding the modes for each earthquake direction which have

the largest modal participation factors in that direction.Murali27 (Structural) 21 Dec 05

7:54 Hi Willis V

You are correct. But the modes are really coupled with respect to both the translations & rotation.

Say, T1 = 1.963 (33% - UX, 29% - UY & 17% - RZ)T2 = 0.896 sec (1% - Ux, 10% - Uy & 30% - Rz)

Certainly we need to go for response spectrum method to capture the realistic distribution of force.

This question comes exactly in the scaling the foces with respect to static base shears. i.e scale factor.

Which time period would be realistic to calculate the static base shears?

Hope you got my point

ThanksMurali G

WillisV (Structural) 22 Dec 05 7:38

Murali,

For the static method, the code is assuming a force distribution based on a perfect translational mode shape. Technically you can just use the approximate period equation in the code and use that to calculate the base shear, even if you do have the actual modal response of the building. I would feel fine with using the period values ETABS gives as having the most translation modal participation in that particular direction and ignoring coupling effects. Obviously the code approximate method ignores them.


Hi Willis

You are correct.

In this case, as per ETABS, the demand would be 35% more than the base shear calculated from approximate period (coupled mode).

My opinion is that the structure would behave as a coupled mode before it reaches a perfect translational mode.

ETABS automatic calculation of time period & base shear (as per static method) needs some engineer judgement when it comes to irregular building.

Murali G

dawn836 (Structural) 9 Jan 06 3:43

i have a question what coupled mode mean?WillisV (Structural) 9 Jan 06

10:05 A mode shape involving both twisting/torsion and direct lateral translation. jen4950 (Civil/Environme) 25 Jan 06

2:36 More specifically and simply: you apply a force or excitation in the X direction, and you get resulting forces in the X, Y and Torsional directions -> you have coupled modes.

Exciting a simple building in 1 direction will result in reactions in that direction.

shaheed (Structural) 19 Jan 06 10:10

I am trying to model a raft for a 20 story building using shell elements. I then grounded each node of the raft mesh using "gap" link elements to model the compression only spring stiffness of the soil support condition but there are a couple of things which dont appear to be correct with the model for the following reasons:

1) Firstly the gap link elements seem to take tension as well as compression, which is not what I want. I want compression only

2) I can not get a display of the forces in the link elements.

what am I doing wrong. Help please any comments/hints will be extremely appreciatedCheck Out Our Whitepaper Library. Click Here.

WillisV (Structural) 19 Jan 06 11:53

See my ETABS FAQ on the hoops you must jump through to make gap and hook type springs work in ETABS. The FAQ is for tension only elements but it applies just as well to compression only:

FAQ802-1169

KenyaCragrat (Structural) 27 Dec 05 17:58

Hi, can anyone please explain why some shear wall objects on plan are not visible in the elevation of that gridline in the adjacent window, whilst others on the same plan are visible?

I am using ETABS NL version 8.5.0. However, if I set the elevation window to 3-D view, then the missing shear wall is visible! I have tried the mouse right-click and enabled all views, etc, but still this problem persists! Thanks!!

Find A Job or Post a Job Opening Click Here.


Definition of grid lines to be revisited. It means that the shear wall objects not in line with the grid lines.

If this is the case, there is a option of defining developed elevation. You can use them to define grid lines along curve.

-Murali GKenyaCragrat (Structural) 29 Dec 05

6:25 Hi Murali, many thanks- yes, I have rechecked the gridlines, and even though the shear wall is on the grid, it does not appear.

On a related issue, I did add a developed shear wall remote from a gridline and that did not appear as well. Perhaps the section line needs to be exactly on the centreline of the shear wall?

Having said that, I just went back and looked at the coordinates of the wall object and they were some 3-4mm off the gridline. Correcting it sorted it out- many thanks!!

Do you know of a way to move points to nearest gridline as a menu edit item?

tab35 (Structural) 11 Oct 05 16:04

I've really just been introduced to ETABS and I was wondering if it's possible to set up individual inclined gridlines within a system of orthogonal grids.

Check Out Our Whitepaper Library. Click Here.WillisV (Structural) 12 Oct 05

7:27 Nope.


If you convert grids to a 'general' grid system, you can then use the Draw line tool (change from Frame type to Gridline) to draw gridlines in any skewed direction. CSI does a good job of adding useful features, many improvements in the past 3 years or so, but they do a poor job of documenting these featuresrcampbell (Structural) 28 Nov 05

5:49 You can also specify a diagonal grid system in Etabs by basically doing exactly what you would normally do in creating a rectangular grid system.

Simply go to edit grid system, create a new grid system (you can call it diagonal for ease of reference) then in the Edit Grid tab click on the "locate system origin" button and here you can specify the new origin and also the angle with which this grid system is to be located with respect to the global axis. Then just add gridlines as per normal.

Very easy to do and most helpful for those jobs with a diagonal grid system, best advice is to have a play with it to get the feel of it.

Hope this helps

Ryan.

mpirez (Structural) 12 Feb 04 11:07

Hi! Evertyime I run a model I get the following warning:"***WARNING*** THE SOLUTION LOST 6.7 DIGITS OF ACCURACY FOR DOF UZ OF JOINT 11484 LOCATED AT X = 1414.000, Y = 698.444444, Z = 1056.000, STIFFNESS MATRIX DIAGONAL VALUE = 2.6116E+10" Why does this happen? What can I do to prevent this error?Thanks in advance,Monica amadeus (Structural) 18 Feb 04

16:34 Normally, this warning messages dissapear when you mesh the shell elements into smaller pieces. Try reducing the size of the mesh elements and that error will probably go away.

Amadeusfafaf (Structural) 25 Feb 04

10:15 I would suggest that you check the displacements of the structure (on the display, and animating the displacement graph in 3d makes it easier finding the members making very large displacements) and find very large displacement.The remedy is to introduce more restraints or connect the offending member.

I hope this helps,andiron (Structural) 27 Jun 04

21:04 amadeus: just getting a hang of etabs 8.2.7..and was trying with various ways of modelling slab/shear wall/wind etc..

if one area is drawn for the whole floor (bldg has columns and c shaped shear walls ) and auto mesh area into structure is cliecked (with say 4'X4' meshing enforecd) and voila! etabs does it..as per mr habibulla you can enforce auto-line constrain and not worry about meshing not being compatible in the tradional sense..in fact as per habibulla the shell stress contour may jump across elements but largely ok..so far so good..but analysis will often yield some error due to, in my opinion, this "fudgy" meshing..

i tried inpputing random columns, with no walls, and did auto meshing and the program did not even acknowledge if some columns where there.. irritated, i did bay-by-bay auto meshing by manually drawing null lines connecting the columns and meshing looks much better..i merged all the areas to make one diaphragm..results seem much better on that..so the auto meshing isn't really auto..and meshing by the progrem is still quite often not correct...

tell me what you think about it..

ths andironRyanLee (Civil/Environme) 13 May 05

5:38 hi fafaf:how are you i had been did just as you said above, but i can't find large displacement, and the warning tell us that it's the point ,not the member (am i right?),so i am afraid that i can't get you meanning , can you tell me more ? thanks a lot!ryan

andiron (Structural) 2 Oct 05 10:45

amadeus is WRONG!!!Murali27 (Structural) 7 Oct 05

7:27 I think that when the stiffness value is in the order of +10e6 or more, no need to worry about this.

You can check gloabl force envelope to see the percentage of error in loading.

I can see this certainly meshing problem.

Minh102 (Structural) 1 May 05 10:22

Analysing complex model in Etabs (with part of the building in model is not parallels to main axes), I often get warning messages about losses in accuracy. I guess that the reason is Etabs' automesh function were not working properly. How to fix this problem? Anyone have the same problem with me?

Check Out Our Whitepaper Library. Click Here.WillisV (Structural) 4 May 05

10:34 Loss of accuracy warnings are normally indicative of improper restraints (too many releases). Make sure that your restraints are not leaving any nodes free to "spin" in the air. Loss of accuracy messages due to automeshing are certainly possible and are normally due to tight corners etc. where ETABS is forced to provide a very atypical (long and slender) mesh to meet the structure. Try "smoothing out" the edge of slab of the building to avoid this problem.

Minh102 (Structural) 6 May 05

9:22 WillisV, you are right. I have noticed that warning massages points to place in model where two beams intersect with very near but not equal 90 degree. I hope next version of ETABS will have more intelligent automesh, but now I have to draw null line to force automeshing which is really time consuming.

Another problem is that ETABS not designs wall resisting forces acting on it surfaces, so I have to design retaining wall at basement level manually. Is there any tip for modelling basement wall?WillisV (Structural) 6 May 05

16:32

Minh102,

I typically design basement and retaining walls seperately from the actual analytical model. Normally for basements the forces applied tend to balance themselves out from both sides of the building and there is no need to carry the force in the actual model. In cases where the earth loads are coming from only one face and cannot be effectively taken by the walls on the sides, I normally just placed the load on a ridig diaphragm master node at the center of pressure to distribute it to the lateral system. The actual design of basement walls is easy as they are normally treated as simply supported concrete beams. Minh102 (Structural) 8 May 05

9:54 Thank you for sharing experience. Like you I often design basement walls as simply supported beams, but sometimes when vertical loads are high I also check wall as columns using P-delta analysis and design under combined load.WillisV (Structural) 8 May 05

21:16 I agree Minh102, with high basement loads I will take a look atj more advanced P-delta analysis though this rarely adds more than 10% or so to the controlling moment.

Another alternate is to use the ACI method for slender precast wall elements (new as of ACI-99 - in chapter 11 on walls after the bearing wall empricial method). This method was placed into the code due to the fact that precasters were using taller and more slender wall sections without specific guidance for their design. Though originally inteded for precast it is equally valid for reqular walls and is essentially a p-delta method.

NADEEM2288 (Structural) 4 Apr 05 2:27

Hi:In ETABS when we model flat slab using template, to define drop panel area structural properties do we define over all depth of slab i.e. from the top of slab to the bottom of the drop panel or define depth from the soffit of the slab to the bottom of the drop panel. This confusion is because of the fact that the program shows two overlapping slab properties at the drop panel area. Does the program add stiffnesses of the two over lapping slabs? Your help will be greatly appreciated.Thanks.

Eng-Tips Forums is Member Supported. Click Here to donate.amadeus (Structural) 15 Apr 05

15:12 In SAFE, when you define a drop panel, the drop panel properties replace the slab properties in the drop panel area. It means that you can overlap both areas and there will be no problem. Since ETABS floors can be exported to SAFE, I can bet that the behavior is the same.NADEEM2288 (Structural) 19 Apr 05

5:10 Thanks Amadeus for your response. Based on research using a sample flat slab model in ETABS, the following conclusions can be made;

ETABS flat slab modeling using templates:

Slab thickness = 8“

Drop Panel Thickness = 12” (i.e. 8” + 4”)

For the above situation, define slab property with 8” and drop property with 12” thickness. Assign whole slab the slab property and the drops the drop property. ETABS ignores slab property as defined for the whole slab at drop location and instead uses drop panel property. ETABS also ignores drop panels if automatic meshing option is used.

ETABS flat slab Manual modeling:

Do not overlap slab properties in ETABS since it ignores the drop panel property and uses slab property instead. The best way is to mesh the floor slab in such a way that it creates mesh lines at drop panel location. Then pick up the broken drop panel areas and assign them drop panel property using full height as 12” for the above mentioned example.

SAFE flat slab modeling:

In SAFE, in case of overlapping areas SAFE replaces slab property with the drop panel property.

efighettib (Structural) 14 Feb 05 15:17

Hello All

My question is the next: If I do a shear wall design, defining the "Pier section type" as "Simplified T and C" I can get the Pier longitudinal reinforcing concentrated in both ends of the wall, and in the top and the bottom of it.

My problem is that if I have 2 perpendicular walls, the Pier longitudinal reinforcing is different in each wall (and off course, must be the same, because they are sharing the same point).

How can I get the real value for that point?. Thanks!!!Find A Job or Post a Job Opening Click Here.

joanrizu (Structural) 19 Mar 05 9:01

I think you must define the two orthogonal walls with the same pier name. Then you must assign it a general pier section to design it. Then your two walls will be really one wall with two orthogonal components.

joanrizu (Structural) 27 Dec 04 15:54

I am trying to design a small residential 2 story building in high risk seismic zone, but arquitect doesn’t want wide columns, he wants columns of the same width of the partition walls. Is possible to model the building like a frame with some slender walls replacing columns, but with wide beams for medium spans? Is this only a name change to avoid Code restriction about frame member dimensions? How is the behavior of the joint beam (35 cm wide) with a very slender wall (12 cm wide)? What about the reinforcement development at transverse beams? I am using Etabs to model the building and it seems there is no problem with the design, but I am worried about the actual behavior of the

structure.Find A Job or Post a Job Opening Click Here.

Yameen (Structural) 18 Jan 05 12:00

What I can suggest u is to make a hoop at the junction of beam and column so that u can control the shear coming on to column through beam, however it is practically acceptable to have slender columns with wide beamns, as far as the transverse reinforcement of the beam is concern, u can provide additional shear reinforcement but I can't comment how the cost will increase.

erenattos (Structural) 29 Oct 04 22:58

hi folks, i´d llike to know how can i modelate a flexible floor concrete diaphragm in ETABSctcing (Structural) 30 Oct 04

13:44 AS far as I know, you just need to create a shell property in "Define" "Wall/Slab/ sections"

The shell element should have the appropiate material properties and thickness

hope this can help

amadeus (Structural) 3 Nov 04 15:43

In a change from previous versions, from V7 ahead, flexible diaphragms are the default in ETABS. If you desire a rigid diaphragm you have to define it explicitly. So, essentially, if you do nothing you will end up with a "flexible" floor diaphragm.

Amadeus

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