STAAD seems to be using ASCE 7-16 11.4.8 Exception 2 because T>1.5Ts even though S1 < 0.2. Site class is D, other parameters and calculations are in the attached Excel sheet.
Cs Calculation.xlsx
Is there something I am missing or seeing wrong? Thanks for the help!
The multiplication factor 1.5 on seismic response coefficient Cs is only applicable for site class = D and S1 >= 0.2 according to exception 2 Cl 11.4.8. Since S1 = 0.059 < 0.2 the multiplication factor of 1.5 is not applicable here for Cs. BUG 487670 is in the defect backlog and the development team is working to get this bug addressed as soon as possible.
As a workaround if you go to the seismic load cases in your model, you’ll notice the seismic load items have a factor (load factor). This factor is 1 by default, but you can change it if you want. In this model you can counteract the 1.5 factor being applied to Cs by making seismic load item factor equal to 0.6667 (1/1.5)
Answer Verified By: Damon Macioce