SAP2000 Response Spectrum Problem 1

[canwesteng]

I´m having an issue with response spectrum analysis in SAP2000. I´ve defined a response spectrum with a constant value of 1g and am comparing to a linear acceleration of the structure of 1g. For the case of a frame of constant mass cantilevering from a support, these give the same answer. If I add a cantilever section to the frame, (think an upside down L with one fixed support), I get a base shear from the response spectrum that is less than the base shear from the linear case. Am I misunderstanding how a response spectrum functions?

Note I have 100% mass participation in all directions

 

[JoshPlumSE]

Take a look at the attached image (from a RISA-3D training manual I put together) than shows modal forces for the first three modes in an RSA analysis.

Notice how the shear for the first mode all go in exactly the same direction (hence base shear matches). But, if you look at the higher modes, there is switch back based on the mode shape.... Hence, lower base shears.

Does this help?

 

[canwesteng]

Yes, lots. I was able to build a simple model in STAAD with similar results to illustrate what's happening. SAP is a little to black box to really get a feel for this, or maybe just because i'm new to the software.

 

[stressed]

Typically response spectrum load case scale factor is adjusted to match static code base shear.

Base shear is calculated for each mode and then modes are combined.