Response spectrum loading generated as per ASCE

Applies To
Environment: ALL
Area: Analysis
Subarea: Response Spectrum Analysis
Original Author:Sye Chakraborty, Bentley Technical Support Group


Question 1 :

I am doing a response spectrum analysis in STAAD as per ASCE. The section 12.9.2 says the modal response parameters should be divided by R/I. Does STAAD do that automatically or do I have to do it ?


Response :

The division of the spectra by R/I is not done by STAAD.Pro automatically. Input the value I/R as Scale Factor.


Question 2 :  

The ASCE suggests the displacement and drift to be multiplied by factor Cd/I. How to account for this ?


Response :

Use the factor Cd/I as an additional multiplier for the seismic load in the combinations that are used to check displacements/drifts.


Question 3 :

The section 12.9.4 says if the response spectrum base shear (Vt) is less than 85% of the base shear obtained from Equivalent Lateral Force procedure ( V), the forces should be multiplied by 0.85 (V/Vt). How to account for that ?


Response :

You may increase the base shear by using a Direction factor for this case. However please be aware that this effect is only for the forces and not for drift calculations. So when using the seismic force as part of a combination that will be used for drift calculations, be sure that you divide the seismic load by the same factor to remove this effect from the drift calculations.


Question 4:

I have calculated the response spectrum curve from the ASCE 7-10 clause 11.4.5. Should I use g as the Scale Factor ?


Response :

Yes since these are normalized values, you need to use the g value in the Scale factor.