Etabs Response Spectrum Analysis - Walls attracting too much load

[krod013]

Hi All,

This is my first model in Etabs and it is giving me some very strange results. I have done a response spectrum analysis to determine the seismic loads on the concrete core shear walls. One wall (C/6-10) consistently gets excessively high shear loads compared to the rest of the core walls. Sure there is a bit of torsion in the model, but I wouldn't expect that much. I have been working on this for a while now and have tried many things to fix the model but nothing has worked so far.

I would be so grateful if someone could quickly open up the attached model and see if they have any ideas on fixing the problem? I've been looking at the load combination "Spec Envelope Max" and "Spec Envelope Min" to get the loads.

Thanks in advance.

 

[jvvse]

Sorry for the late reply.

I don't usually download and check models, but I made an exception and ETABS said it could not open the file. Oh well.

If it's your first model, you have a lot to learn about modeling in general. Most engineers are not taught what they need to know and it can be very hard to identify problems. Here are some pointers to help track down problems.

Check mode shapes for irregularities. (1st mode should be displacement in one direction, 2nd mode should be displacement in perpendicular direction, etc.)
Check weight of structure vs hand calcs.
Check mass of structure, x g, vs hand calcs of seismic weight.
Modal Mass Participation should be > 90% in the SumU.
Check COM, COR of diaphragms. Points, lines and areas can all be assigned separately to diaphragms but you should just stick to one.
Check Base Shear vs hand calcs, scale as needed.
Shear/Moment Diagrams that make sense. Forces should generally increase as you go down the structure.


Also, here are some notes on proper meshing/modeling that most engineers I meet don't know at first. If you are using automatic mesh then display the mesh under Display Options. These are taken straight from the ETABS manual:

Use 4-sided/quadrilateral elements or 3-sided/triangular elements.
Quadrilateral elements are more accurate than triangular.
Triangular elements are reasonably good for stiffness but poor for stress.
Best: each angle = 90 degrees. 2nd best: angle between 45 and 135 degrees. NEVER more than 180 degrees.
Aspect ratios are best at unity, 2nd best < 4, NEVER more than 10.
Aspect ratio for triangular element is the long side to the short side.
Aspect ratio for quadrilateral element is different lengths between opposite midpoints.
Membrane = in-plane only
Plate = oop only
Shell = both

 

[krod013]

Thanks for all your advice jvvse, that's really great - much appreciated. I have completed the checks that you recommend and these checks make sense. The base reactions for the response spectrum analysis are exactly as I expected them to be. I did a manual mesh - this was the first time I've done this as I usually use Robot Structural Analysis which does this for you.

I think the problem is in how I have set up the modal analysis because when I checked the equivalent static method results, the results on these walls are sensible. However with the modal checks, the walls are attracting very large loads as described above - even though the base shears are as expected.

I'm sorry you can't open the file - This was done in Etabs v 13.1.5 so maybe not compatible, but make sure to extract the documents from the zip folder.

 

[blihpandgeorge]

Hi

one common trap is that the load cases 'spec envelope max/min' don't give results at the same exact point in time, they give the max/min result at each point you are looking at which is typical of an envelope. Thus if you sum any results from these load cases you will get silly big answers. Similarly, the result in one direction, say axial, is not necessarily co-incident with the other forces such as shear, moment etc.

this may or may not be the issue you are seeing...

good luck