Base shear in core walls

[goph101]

Hello all,

I am working on an 8 story building with 4 levels of parkade, and trying to determine seismic forces in core elements using etabs. I am new to etabs, and I have gone through the tutorials but still cannot find my answer. There are 3 cores, 2 are C-shaped, 1 is a box.

I build the core walls initially as pier and spandrel elements, and ran my analysis. When looking at forces in y-direction, loads in pier and spandrels, match with base shear. In x-direction the loads in pier and spandrels are significantly less that total base shear.

I rebuild the walls as 3-d elements each core as a single pier type. Same results. Forces in cores in x-direction are significantly less that the seismic base shear demand.

Additionally, given that there are 4 levels of parakde, I set my seismic so that it considers loads from level 8 to main(ground level), and I set the base of core walls to be pinned.

Any help would be appreciated.

 

[asixth]

Is there anything else in the x-direction which is taking some of the shear such as columns or other walls. Response spectrum results might not be able to be summed because the results are provided as positive.

I would be assigning the cores as individual piers, say core 1, 2 and 3 and use those results. Depending on the orientation of the piers the V2 and V3 components may need to be transformed based on axis orientation.

 

[goph101]

Is a response function required? I was under the impression that I could simply define static load cases and have it use a base shear. Is this incorrect? I'm new to the program, and don't remember seeing that in the tutorials.

There should not be any other components taking shear in my design, columns are specified pinned.

I'll try changing the spectrum, and changing the core definitions, and repost if issues.

 

[pkjr618]

For your 8 storey bldg., you can get away with just the static base shear procedure if the seismicity in your area is average to none and some other things like structure regularity as well as consideration of the system used to resist the seismic forces. Consult your local code for these items.

 

[luungoclinh]

How do you mean when you say "loads in pier and spandrels", and also "base" in " When looking at forces in y-direction, loads in pier and spandrels, match with base shear. In x-direction the loads in pier and spandrels are significantly less that total base shear."
Where do you take the value that you call base shear,and also loads in pier and spandrels.
Lisu

 

[PicoStruc]

For seismic purpose, I start with 0.7 for F11 and F22 for seismic analysis, as prescribed by Concrete code A23.3-04 in Canada.
For wind, it is iterative depending of the cracking state as described in previous posts.

 

[PicoStruc]

Sorry, wrong thread. How to delete it ?

 

[goph101]

Sorry for the late reply,

I have been working to get the correct results. After much review of the model I am fairly confident that the model is behaving correctly. I can now be more specific for my issue.

1. When I generate results output with wind or seismic and I total up the story shears I get a total base shear (at main floor) that is what I expect for the building of the size and shape. I would expect to see shear in each of the cores that adds up to the total base shear, with any torsional forces included. However, I get a total base shear that is far less than the total shear in walls (if I add them up). My torsion should not be thigh as the CR is very close to CM, and in the middle of the structure.

2. How to I get the total tension and compression in each of the cores for overturning? I want to have a case with Factored wind and dead load applied to the structure to get the total overturning moment at the base. I want to use 1.25 Dl on one side tot get max compression, and .9dDL on opposite to get max tension. Is this possible?

I am still working on this. If anyone can provide assistance I would greatly appreciate the help.

Thanks,