Ice Rake Loads on Roof

[pbc825]

I originally posted this is in the Mechancial forum, but thinking about it further, this is really a structural scope.

Working on a tank for the Canadian prairies with an "ice rake" (as named by the client). The purpose of the ice rake is to retain snow and ice that might otherwise fall on someone's head around the base of the tank. The tank has an umbrella roof and the rim angle is roughly 45 ft from the ground. I'm having difficulty deciding on lateral loads for the ice rake. One thought is to add up the total weight of snow for the unbalanced condition (~31 kip in this case), multiply by the sine of the angle at the rake (a little under 0.5), and distribute this load over 90 degrees of ice rake. By my way of thinking this is the upper limit, but is an outrageously conservative load.

Does anybody have advice or literature which might point me in the right direction to estimate load on the ice rake?

Thanks in advance.
PC

 

[maxdistortion]

I've been looking around throughout the day to see if I can find what you are asking in a code or standard. A colleague designed some structures for ice protection and ice rakes at one of the large oil sands plants, he was using the Alberta Building Code commentary but I cannot find anything in there for you, it's starting to bug me now.

You remember that fatality from fallen ice a few years ago in an industrial plant? since that time we started to build shelters by building exits and retrofit old roofs with rakes and so on. The shelters (canopies) are quite hefty, I some are designed to 15kN impact force.

The clients haven't minded too much in being conservative in the rakes. The detail I have for the current ones we are installing is L3x3x1/4" angle welded to small plates, which are anchored onto the purlin with two heavy bolts, and also contain sheer lugs (10mm rod) that pierce into the cladding. This standard applies to all buildings of all slopes at this location, and the slopes range from 10 to 25 degree.

If you are interested I can give you snow loads and you can back calculate what loads they are using. My guess is they just picked 3" angle as standard. I am trying to find the exact slopes in those roofs.

 

[pbc825]

Much appreciated maxdistortion.

I was not aware of the fatality. I suppose that may be the reason ice rakes are becoming more popular.

I have a similar detail for the rake as proposed by the fabricator I'm working for (when compared to your description). I'll have a look at what ultra-conservative load looks like and post once I review the results.

PC