Hi, i am trying to apply a seismic load to my structure using the equivalent lateral force procedure ASCE7-16 12.8. I believe this can be done simply by adding a seismic load definition ad selecting IBC 2018 ASCE 7-16.
A response modification factor of 3.0 was selected from ASCE7-16 Table 12.2-1. The approximate fundamental period (PX & PZ) were manually calculated using ASCE7-16 eq.12.8-7, withcoefficient values from ASCE7-16 Table 12.8-2. This is a snapshot of my input:
In the output file I found these values (highlighted in yellow) for the seismic parameter for which I also manually hand calculated exactly the same values.
However, I found the Cs upper limit value to be larger than what I calculated as per ASCE 7-16.
Assuming that STAAD used correct Ta period (1.29), R factor (3) and importance factor (1), the SD1 value is back calculated using ASCE7-16 eq. 12.8-3 for T<TL:
SD1= 0.066* (1.29 * 3/1) = 0.255
Coincidently, SM1 = Fv*S1 = 0.257
and SD1= 2/3 SM1
I theorize that STAAD incorrectly used the SM1 value instead of the SD1 value when calculating the upper Cs value. However, there is no way for me to verify this as there is not equation output for how STAAD calculates the Cs limits.
Where can I check which equations STAAD used to calculate the seismic parameters?
You may check the equations in the verification document in STAAD.Pro Help:
If that does not help, please attach the model and let us know the exact STAAD.Pro version being used.
Without the actual .STD model it is difficult to say for sure but there was a BUG 487670 that was causing the program to incorrectly apply the exception clause on some models as given in cl 11.4.8 wherein even when the conditions were not met (i.e. site class not equal to D & S1 not greater than equal to 0.2) the exception was still applied and you ended up with upper limit of Cs which was 1.5 (3/2) times what actual value of upper limit of Cs should have been. The description that you have provided here looks somewhat similar to this issue. BUG 487670 has been subsequently fixed in V22 U10 (Build 22.10.00.153) so my suggestion will be to check if you still find any discrepancy in build V22 U10 or later (latest build is V22 U11 build 22.11.00.156) if you are on any older build
Answer Verified By: Patrick Cheung
Thank you this was very helpful
Thank you this was very helpful.