Seismic Load Comb, Chapter 12 of ASCE 7-05

[LannyBudd]

General question about the load combinations used in Chapter 12 of ASCE 7-05. What is the difference between the load combinations in section 12.4.2.3 where Qe is multiplied by rho (redundancy factor) and section 12.4.3.1 where Qe is multiplied by omega (overstrength factor)? When should I use one over the other?

 

[jdgengineer]

Which combination you use depends on the element of the structure that you are designing. You use the "standard" seismic load combinations unless directed to use the overstrength combinations in ASCE 7 or the material specific detailing requirements.

In general you are required to use the overstrength combinations for elements of the structure that are designed to behave essentially elastic or would have significant detrimental effects to the structure if they were to fail (collectors, elements supporting discontinuous vertical elements, etc.).

The requirements of when you need to use the over strength factor are unfortunately littered throughout ASCE 7 chapter, and the other material standards, and not in one specific location. For instance Section 12.3.3.3 and Section 12.10.2.1 both require the use of the overstrength factor.

Hopefully that helps somewhat, but the codes unfortunately are becoming increasingly complex.