Snow Drift Requirements of Old Building Codes
Snow Drift Requirements of Old Building Codes
(OP)
I am currently evaluating an existing building structure. There is a small office space, about 30'x70'x12' tall, tacked onto the front of a warehouse space that is 25' high and a roof length of 550' that is dumping snow drift on the office roof. The ground snow load is 20 psf (Ohio). The building, both office and warehouse, were built in 1971. The drawings say the building was designed for 25 psf live load. In evaluating the office roof joists for their ability to carry the drift load, they fail spectacularly, just like seemingly every existing building I run into. We are working for the contractor on this job, who is working for the building owner. I informed the contractor about the structural issue, and that additional joists should be installed. Of course the contractor doesn't want to spend more money, so his answer was that as long as the building met the building code when it was built, and we don't add any load to it, we don't have to reinforce the roof. So my questions are: Why do all the existing buildings I evaluate always fail to carry the snow drifts (according to the calcuations, they work in real life)? Did old building codes not include snow drifts? If so, when did snow drifts become part of the code? Apparently Ohio didn't adopt a national code until the late 70's, and I can't find a copy of the Ohio Building Code from 1971 to see if it included drifts.






RE: Snow Drift Requirements of Old Building Codes
The other two pages of the attached file are from the BOCA/Ohio 1978 code. The special snow loads are shown in an appendix (which didn't exist in 1970). The BOCA snow drift loads of the time were a function of the step height and the ground snow load ONLY. There was no allowance for upper length. That made them rather unconservative for large upper building lengths such as you have and sometimes overly conservative for very large roof step dimensions with small upper roofs.
The 1978 version was the earliest Ohio code I had in my archive.
RE: Snow Drift Requirements of Old Building Codes
M.S. Structural Engineering
Licensed Structural Engineer and Licensed Professional Engineer (Illinois)
RE: Snow Drift Requirements of Old Building Codes
RE: Snow Drift Requirements of Old Building Codes
RE: Snow Drift Requirements of Old Building Codes
The historical notes indicate that the 1953 NBC considered using ground snow loads on all flat roofs with reductions permitted only for sloped roofs.
The 1965 NBC adopted a more rational approach to snow loads, allowing for varying influences which could cause accumulations of snow loads on roofs. I do not have the details of that as I foolishly tossed that code when the new one came out...oh, the rashness of youth.
BA
RE: Snow Drift Requirements of Old Building Codes
RE: Snow Drift Requirements of Old Building Codes
RE: Snow Drift Requirements of Old Building Codes
RE: Snow Drift Requirements of Old Building Codes
Same nomenclature in Section 2305(c) of the 1973 UBC.
Mike McCann
MMC Engineering
http://mmcengineering.tripod.com