Snow Drift Loads for flat roofs

[sportracer]

I am sizing these long spanning (53') roof trusses for a building in Chicago. The building code says to to follow ANSI A58.1-1982 for snow drift calculation, but I understand that standard has been replaced by ASCE 7-05.

Does someone have experience in calculating the portion of a flat roof that may experience snow drifting from the aerodynamic shade effect on the lower potion of a building. I understand that some of the trusses will be affected by higher snow loads caused by the drifts, but I am interested in determining how many of the trusses will be affected. And yes, I do not own a copy of ASCE 7-05. Thanks!

 

[RVSWA]

Generally a code specified standard is still the standard unless an amendment has been made by the local jurisdiction. What code are you using? IBC? ASCE sells a book that explains the provisions of the snow load chapter quite nicely.

 

[MiketheEngineer]

Might have to go buy one...Sorry

 

[msquared48]

Try your local chapter of the SEA (Structural Engineers Association).

Mike McCann
MMC Engineering

 

[cg3375]

We are seeing on the east coast a lot of interest in simply running an unbalanced load case of 1.5/.3 of the LL or ground snow load about the center axis of the truss.

 

[Ron]

ANSI A58.1 was the predecessor document to ASCE 7, which first appeared in 1988. Subsequent revisions to ASCE 7 occurred in 1992, 1995, 1998, 2002, and 2005. RVSWA is correct that code references may remain until changed by statute or ordinance in the case of adopted model codes; however, this is an old one and it is likely that is has been superseded by a subsequent ordinance or change that perhaps isn't so apparent. As an example, look for an ordinance that adopts the International Building Code or other model code. If you find one, you have a potential conflict that must be resolved by the Authority Having Jurisdiction (AHJ), usually the Building Official or an advisory committee.

Unless specifically dictated to use something this old by an AHJ, most engineers would consider its use to be a deviation from the accepted standard of care.

I get a sense from your query that you are perhaps not licensed as an SE in Illinois. They are picky about that and require specific SE licensing.

I would suggest that you buy a copy of ASCE 7-05 to be reasonably current. Some codes have not yet adopted ASCE 7-05 and are still using ASCE 7-02, but change will likely occur with the next code change cycle. Most model codes are revised on a 3-year cycle.

 

[cg3375]

Oops, I meant to also add before hitting enter that this is assuming that there is no parapet nor close proximity to a taller structure.