Scaling of the modal base shear in ASCE7-05.

[nour75]

ASCE7-05 gives provision (12.9.4) which addressing the scaling of the combined response for the modal base shear (Vt) when it is less than 85 percent of the calculated base shear (V) using the equivalent lateral force procedure. But, there is no statement for the case when the modal base shear (Vt) is greater than 85 percent of the calculated base shear (V).
Are there anyone can give a scaling recommendation for the second case?

 

[JAE]

I don't think you have to scale down. Only up. The idea is that the code writers wanted to limit how much benefit someone could massage into a model by using a modal analysis.

If the modal base shear is greater...the logic is: why don't you use the acceptable static analysis which is lower?

 

[nour75]

Thanks JAE for your answer.
You said “I don't think you have to scale down. Only up”, but in older version codes like UBC-97 in section 1631.5.4, this item is trying to reduce the elastic response parameters for design. This implies that this code tries to scale down not scale up the modal base shear.
I consider modal analysis is more accurate than the equivalent lateral force procedure in getting the base shear. So why we have to scale up or down?

 

[prsconsultant]

Follow thread487-204481 we had this discussion before.