IBC 1605.3.1 Exception 2 vs. ASCE 7

[JoshH726]

Can anyone provide an explanation why IBC 09/12 includes the exception that roof live loads less than 30 psf need not be combined with seismic loads, but ASCE 7-05 or -10 does not?

 

[frv]

It is in ASCE. See Section 12.7.2 (4).

 

[JoshH726]

frv, isn't ASCE 7 Section 12.7.2(4) considering "W" to be using for Eqn. 12.8-1, which in turns feeds E? I can see using snow for a component of the seismic load, but IBC seems to be referring to you do not need to consider 0.75*Lr or 0.75*S in seismic load combinations IBC ('09) Equation 16-13, if Lr or flat roof S is less than 30 psf?

 

[frv]

I'm not entirely sure what you just asked.

 

[JoshH726]

ASCE 7 load combination 2.4.1(6) includes the effects of both E and Lr or S. Given that 2.4.1(6) includes E, you are bounced to Section 12.4.2.3 to use a modifed equation of 2.4.1(6), 12.4.2.3(6) in this case, which is (1.0 + 0.105*SDS)*D + H + F + 0.525*rho*QE + 0.75*L + 0.75*(Lr or S or R).

Now reading IBC 1605.3.1 Exception (2) - "Flat roof snow loads of 30 psf or less and roof live loads of 30 psf or less need not be combined with seismic loads. Where flat roof snow loads exceed 30 psf, 20 percent shall be combined with seismic loads".

So does this mean that ASCE 7 12.4.2.3(6) becomes (1.0 + 0.105*SDS)*D + H + F + 0.525*rho*QE + 0.75*L ?