Impact loads due to sliding snow

[slaterem]

Does anyone have a good reference for calculating or estimating the design load due to snow sliding off an upper roof onto a structure below?

The ASCE 7-05 reference, 7.9, for loads due to sliding snow, severely underestimates the load in my case. This was for snow falling 28 feet from an upper roof (10 feet from eave to ridge) onto a lower deck. Design flat roof snow load was 100 psf. The 5"x24" glulam beams of the deck structure were smashed by the impact, though there was ample structural capacity according to ASCE guidelines.

Thanks.

 

[cvg]

I think the issue is a severe point impact load and for ice, not snow. I have observed this personally when a large ice slab fell off the roof, hit the 2x12 redwood deck below and snapped the 2x12. There was little snow involved, all ice and it hit vertically, on the edge of the slab. So you had nearly the full weight of the slab (200 - 300 lbs at least) impacting an area of maybe 20 square inches. This works out to a lot more than 100 psf. Maybe 10 times that or more.

 

[ishvaaag]

http://www.slpoa.com/Articles/big_beam_babies.htm

http://cemsig.ct.upt.ro/astratan/didactic/bef/c09-bef.pdf

"...does NOT give guidance on specialist aspects of snow loading, for example:
– impact snow loads resulting from snow sliding off or falling from a higher roof"

http://eurocodes.jrc.ec.europa.eu/doc/WS2008/EN1991_3_Formichi.pdf

 

[Gumpmaster]

Slaterem,

I had a similar situation in Alaska. There's some research and literature out there, but nothing definative that I found. Best way is maybe to run an engergy balance to find the impact load. It's really a dynamic analysis rather than static.

I don't quite remember how I analyzed it, but I came up with around 1500-2000psf (my eave to gable was on the order of 50 ft).

We changed the building orientation so that the snow was shed into an empty area. We added snow blocks so the snow couldn't slide off unimpeded, and we added heating to the roof to minimize the amount of snow accumulation.

You also have to think about someone standing on the deck when the snow slides off. It doesn't matter if the deck holds together but the person gets pulvarized.

cvg, it's not just ice that does the damage. A big chunk of snow will do some some serious damage after falling 28 ft.

Good luck.

 

[cvg]

I agree snow could also be dangerous - however any significant snow accumulation generally morphs into ice given enough time. And I almost got pulverized in the deck incident. Of course, it was my fault - I was trying to knock it off the roof with a shovel and luckily my reactions are quick that it didn't land on my foot...

 

[slaterem]

Thanks for the feedback.

The codes obviously recognize that impact analysis is difficult and do not include it. The dissipation of the kinetic energy is so sensitive to the amount of deflection and the time taken to bring it to a halt that the possible range of loads is very wide.

Fortunately this facility is closed during the winter, except for occasional inspection, so there was no life safety issue to be concerned with.

I am on the track of a program to use Roark's Formulas for Stress and Strain which I'm told has an impact analysis.