Using "roof snow load" as a design load
Using "roof snow load" as a design load
(OP)
I've been engineering small pole-type buildings for about 5 years now. 3 years ago, we switched over to the IBC from the 1997 UBC. Our company requires that our customers obtain snow loads from the counties for us to use in design. Many counties provide them with "roof snow loads", and I know that the "norm" of many civil/structural engineers has been in the past to take the ground snow load, reduce it, and use this as the "design" load for roof components. In fact, most counties accept this from other engineering companies.
The problem is that I use unbalanced snow loads in my design, which is usually 1 to 5 psf above ground snow, and considerably more than that for roof snow load. I can find no justification in either the old UBC or 2003 IBC to "stop" at the roof snow load calculation and use this. But I'm developing a reputation for "over-engineering" these "low-cost" structures (trust me-adding a few more purlins is a big deal to these contractors!). I've asked some older (that is "more experienced") engineers how they justify doing this, and no one has an answer except "I don't know-that's just the way it's always been done".
I'd really like some input from some senior civil/structural engineers on this.
Thanks
The problem is that I use unbalanced snow loads in my design, which is usually 1 to 5 psf above ground snow, and considerably more than that for roof snow load. I can find no justification in either the old UBC or 2003 IBC to "stop" at the roof snow load calculation and use this. But I'm developing a reputation for "over-engineering" these "low-cost" structures (trust me-adding a few more purlins is a big deal to these contractors!). I've asked some older (that is "more experienced") engineers how they justify doing this, and no one has an answer except "I don't know-that's just the way it's always been done".
I'd really like some input from some senior civil/structural engineers on this.
Thanks






RE: Using "roof snow load" as a design load
RE: Using "roof snow load" as a design load
RE: Using "roof snow load" as a design load
Dik
RE: Using "roof snow load" as a design load
I get the impression that many jurisidictions get confused by the term "ground snow load" so they don't properly specify it. In their infinite wisdom, they think "but we need to specify ROOF snow load!"
RE: Using "roof snow load" as a design load
DaveAtkins
RE: Using "roof snow load" as a design load
dik-where does GSL=RSL/0.8 come from? I've never seen this in the UBC or IBC. When would you use just this, and not account for drifting snow over the ridge on a gabled roof?
RE: Using "roof snow load" as a design load
Typically in Canadian Codes, the normal design roof load is 0.8 X GSL or Ss. We also include Sr which is a rain load in conjunction with the snow loading. Formulae for unbalanced conditions usually refer to Ss and Sr. We have a Ca for accumulation that uses Ss and Sr to determine the unbalanced conditions. It was based on this that I suggested using the 1/0.8 as a means of replicating the GSL condition.
Dave... thanks for the plug.
Dik
RE: Using "roof snow load" as a design load
S = Ss(Cb*Cw*Cs*Ca)+Sr
Where, Ss is the ground snow load; Sr is the associated rain load.
PBengineer, I think the roof snow load may certainly exceed ground snow load. But it is better to consider the load case by case.
In my cases, according to NBCC, S is smaller than Ss since
Cb=0.8; Cw=wind exposure factor, typically use 1.0, but can be 0.5 to 0.75; Cs=roof slope factor, typically 1.0, to be reduced when slope larger than 30 degrees; Ca=the snow accumulation factor, typically 1.0 to 1.25 (larger when the slope is steeper, but it is compensated by Cs at the same time), for large flat roof Ca may exceed 1.0 as well; Sr is usually smaller than 10% of Ss.
For flat and shed roofs or shallow gable (less than 15 degrees), arched and curved roofs you don’t need to consider unbalanced loading, just apply “full and partial loading” with “the specified uniform snow load in Sentence 4.1.7.1.(1), computed using C a = 1.0,distributed on any one portion of the loaded area, and half of this load on the remainder of the loaded area”. For steeper gable, arched and curved roofs you’ll consider the unbalanced loading (e.g., snow on half of a gable)as well, Ca can be up to 1.25
More likely, the roof snow load can be higher when you have snow drifting and sliding situations.
Hope this helps.
RE: Using "roof snow load" as a design load
The ground snow-load measurements on which this
Standard is based contain the load effects of light rainon-
snow. However, since heavy rains percolate down
through snowpacks and may drain away, they might not
be included in measured values. Where pg is greater
than 20 lb/ft2 (0.96 kN/m2), it is assumed that the full
rain-on-snow effect has been measured and a separate
rain-on-snow surcharge is not needed.
In addition, the sloped roof snow load is based on Pf and Ps, not Pg (although there is the beta factor that has Pg in it, you could be conservative with this factor)
RE: Using "roof snow load" as a design load
All of the roof collapses that I'm familiar with were as a result of unbalanced snow loads. Don't let the money folks talk you out of checking unbalanced loads to save a few dollars.
Also, I would caution you, about getting snow loads from the counties. It's very likely that whoever is feeding you these numbers don't know their butt from a hole in the ground. Look at the snow maps and be sure the values they give you pass the sanity check. If it's in one of the blacked out areas . . . good luck.
RE: Using "roof snow load" as a design load
RE: Using "roof snow load" as a design load