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Modal Response Analysis Shear Scaling

Modal Response Analysis Shear Scaling

(OP)
I am performing a Modal Response Spectrum Analysis using a design spectra developed for the site. ASCE 7 Section 12.9 requires that the value of the base shear not be less than 85% of the base shear obtained using the procedures of Section 12.8. If I follow section 12.8, I end up calculating a static equivalent base shear based on MAPPED values of Ss and S1.
Is this correct or should i use the values of Sds and Sd1 from the SITE SPECTRUM to calculate the static base shear to compare with the dynamic value?

RE: Modal Response Analysis Shear Scaling

Per 11.4.7 you are allowed to use a site specific study for accelerations for the equivalent lateral force method procedure.   

RE: Modal Response Analysis Shear Scaling

(OP)
Thank you very much for your answer WillisV. Now, 11.4.7 allows me to use it. But do I have to? I am being required to do so and I do not see where it says it is mandatory.

RE: Modal Response Analysis Shear Scaling

Of course you don't have to.  You are allowed to use the mapped values in lieu of the sight specific study for use in the ELF if you wish; however, I do not see why you would do that as it will most likely increase your base shears and require higher scaled forces from your modal response spectrum.   

RE: Modal Response Analysis Shear Scaling

(OP)
Because in this case using the site spectrum accelerations results in a higher value of the static base shear. So I had to redo my analysis using that higher value..
Thanks again!   

RE: Modal Response Analysis Shear Scaling

As you did, the sound and more professional practice is to choose the safer option at hand; other thing is that who started the investigation was searching for a lesser cost, and be able to impose that criteria. But I think the professional stance is to support the locally found values for design.

RE: Modal Response Analysis Shear Scaling

If it is Site Class F you have to use the site specific values. You can use a Tfundamental for the ELF based on eigenvalues as long as it is not greater than Cu*Tapproximate. This may help to reduce the minimum base shear requirement according to the ELF.

RE: Modal Response Analysis Shear Scaling

(OP)
Thank you all for your answers. My site was class D. I followed the code because it was my first time doing this kind of analysis but now i know that next time i'll use the site values right away.  

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