Response Spectrum analysis with a braced frame.

[3doorsdwn]

I have a question about response spectrum analysis (as per ASCE7) in general. I have a short (20 foot tall frame) that is braced in both directions. A equipment fabricator wants to do a response spectrum analysis as per the code to get as low as possible seismic load (for his piece of equipment). Originally, I would not permit it because I thought we would use a moment frame for lateral resistance, and since any change in the frame's stiffness (i.e. changing a beam or column size here and there before the issue date, which is months down the road) would alter our response. But now, with a braced frame, I would think changing beam member sizes would not significantly impact our response. I would think the first few modes would clearly dominate. Thoughts? Opinions?

 

[Qshake]

A single story braced in both directions would merely behave as if it were a block setting on the ground or rock. The first mode would be the predominant response and the coefficient would be as high as allowed by the code because of the stiffness.

There would be no benefit to the equipment fabricator at this point to perform a RSA.

Regards,
Qshake

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[SHBH]

Even changing the type of frame (moment or braced) is not going to change the design of supplier's equipment. As far as i know these eqp supplier neither ask you about the natural freq of your structure nor they model the structure in their design. They simply use the equation given in the code ASCE07/IBC/UBC which make use of Sds (design sprectral response at a period of 0.2s.

 

[3doorsdwn]

Qshake, that is kind of my p.o.v. on the subject as well. Thanks for your input.

 

[3doorsdwn]

By the way qshake, the reason he wanted to do a response spectrum (in the first place) is because he got too much load from using equation 13.3-1 (in ASCE7-05) for non structural components. But even using response spectrum there are certain code minimums (for the base shear).