100 or 100 + 25?

[msquared48]

For years I have been using only 100 psf live load for public assembly areas on a roof, not adding snow (25 psf here). Professionally, I do not have a problem with this knowing that it is not very likely that people will be standing shoulder to shoulder on the deck in two feet of snow.

Anyone know of an IBC or ASCE 7 code provision where the 25 psf snow load should be added in addition to the 100 psf assembly load? I can see just using the snow load and forgetting the assembly load for snow loads in excess of 100 psf too. In the mountains here, 400 psf is not unheard of at 4000 feet.

Just trying to verify that the codes are logical here

Mike McCann
McCann Engineering

 

[msquared48]

Haynewp:

Sorry for the belated response - just noticed it.

Consider that according to section 12.7 of ASCE 7, the 100 psf live load to the roof would not need to be included in the seismic calculations. However, to support the 100 psf load, the dead load of the roof would be substantially increased above the 15 to 25 psf norm, thus adding to the seismic force.

Also consider that for snow loads above 30 psf, 20% of that load is added to the dead load for seismic.

As to the question of whether a portion of the 100 psf load should be added to the roof dead load although it is not directly required, we have to consider the intent of the code. The reason for the snow load inclusion is the time of duration factor as to how long that load will remain on the roof, essentially part of the weight structure over a longer than normal period of time, relative to the chance of occurrence of a seismic event. One might consider that if the roof of a building is in full use of the 100 psf loading one to two months over a years time, then maybe adding 25 psf to the roof dead load might be warranted. Such might be the case of an open air rooftop auditorium for concerts, weddings, graduations, etc. It becomes a judgement call.

Note, too, that all or a portion of "permanent equipment", "party walls" and "storage areas" are also included in the seismic dead loads. Live loads, but having longer duration of use, or loading.

You can factor the equations any way you want to. I am not really concerned with that as that's just the momentary code requirements and they will change eventually anyway. The engineering logic with how you approach the problem should not.

Mike McCann
McCann Engineering

 

[haynewp]

Mike,

I wasn't thinking about whether to include the 100psf in the seismic weight, but rather that the 100psf has to be included in the load combination with seismic. The live load effects still have to be added to the seismic even if you don't include the live load in the seismic weight calculation. Agree? (See footnote #1 under the combinations)

 

[msquared48]

Agree.

Mike McCann
McCann Engineering