I was watching the news at the gym last night, and they were showing some nasty collapses. I bit my tongue at some of the things people were saying and the terms they were throwing around. Why can the media not find a decent structural engineer to get on camera to discuss these things, so the general public fully understands why these things happen? Any news story about medicine has a doctor or three on there giving their opinion....
One funny thing is that as you drive down the road, you can see which roof systems are new.... and which ones are old. That is, what roofs have the proper ventilation/insulation and which ones do not... by evidence of the icicles hanging from the eave. Makes you kind of wonder that as the energy conservation increases if the snow loads should increase as well.
I was looking at my roof this morning thinking that I have to try and do some work this weekend. With 19' eaves, I'm not sure I'm going to be able to do much except end up on the internet.
I'm curious to know if the failures where design problems or construction problems. I've seen enough "interesting" problem solving by the truss installation crews to marvel at the fact more roofs haven't collapsed simply due to poor construction and sloppy inspection.
It's easy to raise the snow loads in the codes, but if mistakes and poor workmanship keep slipping through the cracks, we'll have 2"x12" roof trusses held together with twine and wishful thinking.
The last time we had these types of failures back in the late 70's, it seems that most failures that we looked at were either prefab metal buildings or other buildings that had design or construction errors. One that I recall had a column relocated to a location different than the design plans showed without apparent change of the beam size.
Every once in a while hard times have to weed out the trash.
That picture you attached is from Lead South Dakota USA? Is it true that the ground snow load for the area is 40psf? If so, I would say that the design load of that roof is greatly exceeded and that I don't feel so bad about my roof.
Assuming a powder snow with 1" of water per foot of snow, and 5 to 6 feet of snow in the pictures, that's a minimum of 5" of water or about 30 psf.
Now, if it's not powder... that's when it gets interesting - double that. 60 to 80 psf and rising...
Collapsed a few marina float dock roofs that were designed for 10 psf show - sank the finger piers too. We saw a total of 24" of snow then in two storms. Have seen as much as four feet at the turn of the last century, but I'm not quite old enough to remember that...yet.
Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask
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