Service Load Combinations - Section 1605.3.1

[KBVT]

Question concerning the ASD load combinations in Section 16-5.3.1 of IBC 2006. Exception 2 indicates "flat roof snow loads of 30 psf or less need not be combined with seismic loads. Where flat roof snow loads exceed 30 psf, 20% shall be combined with seismic loads.

Load combination 16-13 (or LC 6 of ASCE 7-05) is the only load combination that combines seismic effects and snow loads: D + 0.75L + 0.75S + 0.525E. Is this exception saying that for flat roof scenarios where pf is the snow load used in the load combination (in lieu of ps), that the snow load can be disregarded, and for flat roofs with greater than 30 psf snow load, the 0.75 load factor can be reduced to 0.20?

It is a little odd considering that the only LRFD load combination including both snow & seismic also uses a 0.2 load factor for snow. Anyone have any guidance?

 

[JAE]

For the ASD combination, wouldn't it be 0.15S?

0.20 x 0.75 = 0.15

 

[KBVT]

Is it 20% of the total service live load, or 20% of the 0.75 in the original load combination? It doesn't seem very clear to me.

 

[KBVT]

Check that, 20% of the total snow load.

 

[JAE]

Yes, that's how I read it - 20% of the base snow load.

 

[msquared48]

It makes no sense to me that no slope reduction for snow load is allowed when combining with seismic, at least that is what I am hearing here. I have always done this for highly pitched roofs. Some of the snow loads I have seen are in the order of 400 psf.

Am I hearing this correctly?

Mike McCann
MMC Engineering

 

[ajh1]

ASCE7 indicates that the "S" in the combination is either the flat roof snow load or the sloped roof snow load. IBC specifically references the flat roof snow, although as has been pointed out that doesn't make much sense on a very steep roof. The seismic W is based on 20% of the uniform design snow load regardless of roof slope (ASCE7 Section 12.7.2 item 4.) The basis for including snow in that calculation is the flat roof 30 psf limit, however that is not the number to plug into the process.
A definite disconnect between IBC and ASCE7 in this regard.