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

 

[Taro]

The IBC load combinations in section 1605 all use roof live load OR snow, not both together.

 

[msquared48]

Nice to verify that the code is logical here. Thanks.

Mike McCann
McCann Engineering

 

[JAE]

Except that there is no 100 psf Roof Live Load.

You apparently have 100 psf FLOOR live load and 25 psf snow load. Floor live and roof snow ARE added together in the combinations.

 

[msquared48]

I believe if the area is a public assembly area, as a public roof deck where residents or the public can congregate, then the 100 psf value must be used - see 2006 UBC Table 1607.1, occupancy #28, hotels and multiple family dwellings, specifically "public rooms".

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

 

[Taro]

JAE,

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.

 

[Lion06]

Taro-
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.

 

[DonPhillips]

This is taken from 2003 IBC Commentary, 1602 Definitions

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

 

[Taro]

StructuralEIT,

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.

 

[Lion06]

Taro-
my apologies, I mis-read the posts. I meant to agree with you and disagree with JAE.

 

[haynewp]

See the ASCE 7-05 commentary:

"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.

 

[ctcray]

I'm not looking at the code, but in this example, wouldn't you look at 0.75 x (100psf LL + 25psf SL) = 93.75psf for concurrent transient loads, but this can't be less than one acting alone, ie 100LL. So, in effect, you don't need to consider it as 125psf.

 

[JAE]

Taro - good point. Also the follow ups. I would say that "technically" the 100 psf as a live load (an assembly on the roof) would, in my eyes, be more like a floor live load.

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?

 

[msquared48]

Yes.

Mike McCann
McCann Engineering

 

[haynewp]

msquared;

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?

 

[Taro]

Great point, haynewp. For the seismic load case, I would probably also use the worst case of the assembly live load or the snow load, but not both. This is obviously an area where the code needs some clarification. A former coworker of mine is currently chairing an SEA committee that is trying to sort out this exact issue. Until they get it sorted out, I guess we have to fall back on engineering judgment (or opinion).

 

[UcfSE]

Following ct's post, I would put it in the load combinations and sort it that way. Those should already include factors to account for two or more transient loads occuring at the same time. As to whether its a roof or floor load, I suppose I would try to evaluate the frequency this load, or conditions warranting this load, would occur. If the frequency of occurance is closer to that of a floor load, I would call it a floor load, and vice versa.

 

[Taro]

Unfortunately, the 0.75 factor used to combine multiple transient loads only occurs in the basic ASD load combo section, not LRFD combos or alternate ASD combos. The code guys will need to sort it all out.

 

[UcfSE]

The effect of transient loads is taken into account in the strength design combinations in the ASCE 7. Of course, the 0.75 isn't there but some reduction is used. You don't apply roof live load with snow for instance, and roof live load and floor live load are not both multiplied by 1.6 in the same combination.

 

[Taro]

"You don't apply roof live load with snow for instance, and roof live load and floor live load are not both multiplied by 1.6 in the same combination."

Exactly, that's what this whole discussion is about...