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Modal Response Spectrum Analysis

Modal Response Spectrum Analysis

Modal Response Spectrum Analysis

(OP)
I have a 7 stories of cold-formed bearing walls with cold formed joists on top of a 1 story concrete 2-way podium. no basement. The building is in seismic design category "D". I have special reinforced concrete shear walls the full height of the building for lateral support. Due to the concrete podium, I have a Type 2 vertical irregularity. Due to this Irregularity, I must use a Modal Response Spectrum Analysis. Is there a prevision in the code that will disregard the change in weight from the concrete slab to the lighter floors above?

RE: Modal Response Spectrum Analysis

Isn't the difference in weight causing the vertical irregularity? If you disregard it, you will be eliminating the irregularity, which although it would be desirable, I don't think this was the intention of the code writers. I am not aware of any loophole in the code that permits you to ignore this irregularity.

RE: Modal Response Spectrum Analysis

If you can demonstrate that the period of the structure up to the podium is much stiffer than the structure above the podium, I believe that you can analyze the upper bit on its own with ELF and then do a separate ELF on the lower structure using the base reactions from the upper as applied load. There's an ASCE7 section on this that gives the criteria. This might be helpful if you've got a bunch of extra walls in the main floor layout that will stiffen that level.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Modal Response Spectrum Analysis

(OP)
What if the upper structure is over 65ft? Am i still able to separate the upper and lower structures?

RE: Modal Response Spectrum Analysis

What clause has got you worried about the 65'? No need to make me go rooting through ASCE for it. If it's the permissible lateral system table, I think that's a separate kettle of fish. You should see what Hildebrandt has to say about this. There's not much that guy doesn't know.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

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