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Omega Factor

Omega Factor

Omega Factor

(OP)
From everything I have read in ASCE7-10,the Omega Factor for seismic loads only applies to horizontal forces, not vertical.

Am I missing something?

Mike McCann, PE, SE (WA)


RE: Omega Factor

Nope, you got it. The overstrength factor is required for Eh only, not Ev.

RE: Omega Factor

See section 12.4.3, its Qe times overstregnth. The forces this applies to can apply at any angle. If you are talking purely about Ev then you are correct, unless your system is very unique.

RE: Omega Factor

Quote (MS^2)

Am I missing something?

Nope, you've got it. The code rightfully assumes that the overwhelming majority of buildings will:

a) have a very short period for vertical seismic motion. Basically just axial stretching and compressing of the vertical gravity resisting elements.

b) have no energy dissipation built into the elements that make up the vertical load resisting system. Pretty tough to have a vertical system that would yield during an earthquake yet remain stable for support of gravity loads.

In summary, it would be irrational to apply over strength to the vertical seismic load because that load was never reduced by R in the first place. Right out of the gate, Ev represents a short period elastic response which is pretty much as bad as it gets.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

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