Canopy Drift Loads 1

[rjstubbl]

I am designing a precast building with a small (cantilevered 5'-0") canopy at one end. The building is approximately 1300' long and 40'-0" tall.

The drift load would be around 200 psf and 40' long, then truncated to the 5' canopy according to what I can find in the ASCE. This would usually not cause any problems with the PC, but the canopy is on a 2'-6" spandrel panel which has a large amount of torsion due to the drift.

It doesn't make sense to me that the snow will stack up perfectly 11'-0" up the side of the building. Is there a maximum angle the snow will accumulate from the front edge of the canopy? Or any other way around using the 200 psf?

Thanks for the information.

 

[jimstructures]

What I was trying to show, with a real photograph of real snow is the snow can fall in a truncated drift pattern. The snow doesn't always taper to zero at the roof edge, even if that is what normally happens. That was pretty dry snow falling in a low wind situation that created the truncated drift. It does happen, so it is conservative to allow for it in the calculations. How often it happens that way is another question for another day. So I will continue to follow the formulas for drifting. I have seen canopies ripped off of buildings because the flanges were not stiff enough to support the canopy beams without stiffeners and the flanges deformed and the bolts pulled through.

Jim

 

[WCW]

You might want to get in contact with Dr. Michael O'Rourke. He is a professor at Rensselaer, and a snow load expert for ASCE. A few years ago, I listened to him present a webinar titled Advanced Snow Loads. In it he discussed the snow loading of canopies. He stated that canopies should be treated as roof steps and that he had observed failures in canopies where the roof step was between 5' to 15'. However, he also stated that if the horizontal extent of the canopy is small, the height of the drift will be be limited by the angle of repose for drifted snow. He gave this slope as 1 vertical to 4 horizontal. His take on these canopies was that they were not clearly addressed by the code and that some engineering judgement was warranted.

WCW