Modal Response Analysis Shear Scaling

[EugeChumbita]

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?

 

[WillisV]

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

 

[EugeChumbita]

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.

 

[WillisV]

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.

 

[EugeChumbita]

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!

 

[ishvaaag]

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.

 

[haynewp]

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.

 

[EugeChumbita]

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.