Appropriate Seismic Cs Values

[carnahanad]

Hello,
I'd like to clarify a design issue we are having. We want to make sure we are applying the appropriate Cs value to our building mass.

We've run through the numbers and the basics. We are using ASCE 7-05.

Cs = 0.311 (12.8-2)
Csmax = 0.947 using just Ta = .156

Due to some irregularities, we have to use dynamic analysis.
Our analysis gives us Tdyn = 0.273
CsTdyn =0 .542
Csmax(CuTa) = 0.676
0.85CsCuTa = 0.575

Since Csdyn < 0.85CsCuTa my first thought is we use 0.85CsCuTa = .575. However, this value is still greater than the Cs = 0.311 we calculated in the beginning.

Is 12.9.4 of ASCE 7-05 written under the assumption that Csmax is less than Cs? It seems like it should only be putting a lower limit on the maximum. Or since Csdyn is greater than the Cs value, should we use CSdyn unfactored for our dynamic analysis design?

The basic question is what do you do if Cs, with no period in the equation, is still less than Cs using CuTa or Tdyn?
I look forward to the responses,
Alex
.

 

[wannabeSE]

Your base shear using the equivalent lateral force (ELF) is still governed by equation 12.8-2. The ELF base shear is not affected by the period in this structure. Use the ELF base shear (an ignore the upper limit from eq 12.8-3) in 12.9.4 to scale the dynamic forces if the dynamic base shear is less than 85% of the ELF base shear.