huangyhg

response spectrum analysis & its resullts
hi,
when we use the response spectrum analysis, because of the mode combination methods (cqc, srss) all the results such as forces and moments have the same sign and all are positive. my question is how we can find out which one is positive and which is negative? which one is tension force and which is compression force? . . . there are some procedures that we can use to find out this, but none of them is practical to use in software. i need a guide that can be used in structural design software such as sap & etabs and use analysis results directly to design and iterate to achieve convergence in final design in shortest possible time.
thanks a lot.
before the modes are combined in all three directions, you should be able to get the signs of each component if you are using the cqc method (that is, before the final srss of the x y z components). you should also get some sign back when you combine your seismic loadcase with the self-weight analysis. however, it is not straightforward to extract signs and even more difficult to justify how/why you did it. hence, in most cases you can just take the maximum values say fmax, mmax, sigmax, and design the structure for both the tensile and compressive for each, i.e. +fmax, -fmax, etc.. this is usually conservative but nonetheless sound in terms of nuclear design for instance.
-- drej --
i agree with drej's response. the only way you can get the exact nature of the forces (+/-) is by doing a time history analysis.

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