100 or 100 + 25?
100 or 100 + 25?
(OP)
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 :)
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






RE: 100 or 100 + 25?
RE: 100 or 100 + 25?
Mike McCann
McCann Engineering
RE: 100 or 100 + 25?
You apparently have 100 psf FLOOR live load and 25 psf snow load. Floor live and roof snow ARE added together in the combinations.
RE: 100 or 100 + 25?
What constitutes a room would be the only argueable issue here, but you have to consider the intent of the code. I believe that a space does not have to be enclosed, either partially or completely, to be constituted a "room". It gets down to the "public" access issue here. Hence I believe that section 1605 would apply here.
Mike McCann
McCann Engineering
RE: 100 or 100 + 25?
IBC 1607.11.2 specifies the live loads to use for roofs and says to use 100 psf for assembly areas. So the 100 psf is in fact a roof live load and does not need to be combined with snow.
RE: 100 or 100 + 25?
I think that there is a subtle but important distinction.
That is in fact a live load that is applied to a roof, but it is not a roof live load.
RE: 100 or 100 + 25?
LIVE LOADS. Those loads produced by the use and occupancy
of the building or other structure and do not include construction or environmental loads such as wind load, snow load, rain load, earthquake load, flood load or dead load.
This definition identifies the scope of the type of loading included in Section 1607. Generally, live loads are not environmental loads or dead loads, but are transient in
nature and will vary in magnitude over the life of a structure.
LIVE LOADS (ROOF). Those loads produced (1) during
maintenance by workers, equipment and materials; and (2) during the life of the structure by movable objects such as planters and by people.
This definition is needed for the proper application of the load combinations in this chapter. This definition clarifies that roof loads, such as snow loads, are not live
loads.
I would interpret this to mean is the 100 psf live loads are greater than the 25 roof live load, I would use 100 psf.
Don Phillips
http://worthingtonengineering.com
RE: 100 or 100 + 25?
I disagree. A live load that is applied to the roof is by definition a "roof live load" (as set forth in IBC Section 1607.11). The purpose of labeling it as a "roof live load" as opposed to any other kind of live load is so that it can be combined appropriately in Section 1605. That is, the effects of the live and snow loads are considered separately and the worst case is used for design. The probability/reliability-based load combinations do not require the cumulative effects to be considered. In other words (as Mike originally said) the roof deck is not going to be packed with people standing shoulder-to-shoulder immediately after the 50-year design snow storm hits.
RE: 100 or 100 + 25?
my apologies, I mis-read the posts. I meant to agree with you and disagree with JAE.
RE: 100 or 100 + 25?
"Occupancy related loads on roofs are live loads (L) normally associated with the design of floors rather than roof live loads (Lr)"
Although it makes sense to not include snow with such roof occupancy loads, I don't think it is allowed. I am not sure they want you to take a reduction as a floor load and include it in the roof load only combinations.
RE: 100 or 100 + 25?
RE: 100 or 100 + 25?
But that said, "practically" I would wonder how you'd ever get a full assembly load on a roof with 12 to 20 inches of snow on it as well.
I guess this is where engineering judgement comes in...yes?
RE: 100 or 100 + 25?
Mike McCann
McCann Engineering
RE: 100 or 100 + 25?
What have you been doing when it comes to seismic? The roof live (Lr) does not show up in the strength combinations with seismic; 1.2D + 1.0E + L + 0.2S
So if you were consistent with treating it as Lr, then it would not be combined with seismic either. Which doesn't seem correct to me. If there were assemblies on the roof as frequently as the typical floors below, wouldn't that give the roof live load the same chance of occuring simultaneously with an earthquake as any other floor?
Or have you just been disregarding roof assembly live load in combination with snow?
RE: 100 or 100 + 25?
RE: 100 or 100 + 25?
RE: 100 or 100 + 25?
RE: 100 or 100 + 25?
RE: 100 or 100 + 25?
Exactly, that's what this whole discussion is about...
RE: 100 or 100 + 25?
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
RE: 100 or 100 + 25?
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)
RE: 100 or 100 + 25?
Mike McCann
McCann Engineering