Return to index: [Subject] [Thread] [Date] [Author]


[Subject Prev][Subject Next][Thread Prev][Thread Next]

I did some calcs over the weekend wrt to your falling ice chunk.  Based
on my analysis you're going to have a very difficult time "catching"
20,000 lbs of ice falling from 20 feet.  Of course it depends on the
desired performance;

life safety? framing & roof deck damage? roof deck damage only?

   I think it should be approached from an energy calculation.  You have
4.8 x10^6 in-lbf of potential energy to deal with.  A timber roof system
with limited deflection capability will not be able to "store"  or
absorb  this amount of energy elastically.   In addition  you have the
problem of the chunk "bouncing" into the parking lot.   A steel framing
system that can plastically deform is a possible solution.

Can the ice build up be reduced or controled --- like avalanche

Robert Kazanjy, PE  **Disclaimer: I speak for myself not UC-Irvine**
Senior Development Engineer
Civil & Environmental Engineering
UC Irvine

Randy Vogelgesang wrote:

> Neil,
> This topic is along the lines of a problem that I will soon have to
> address.
> Several months ago Kirkwood Ski resort called me to look at a roof
> that had
> a failure.  It seems that a large glacial like chunk of ice (rough
> guess 10
> tons) fell about 20 feet from an upper roof and shattered some rafters
> at
> the front of the general store.  It turns out this is the second time
> that
> this has happened.
> A little aside, it seems that architects rarely listen to warnings
> explaining all the possible affects of sixty feet of annual snow fall.
> The lower roof was originally 3x6 rafters at 2' oc spanning about 5'.
> The
> first time they failed they replaced them with 4x6 at 2'oc.  Five or
> six of
> these 4x6's at the impact area were reduced to splinters.  The snow
> load at
> this area (Pf) is 300 psf, the funny part of the story is that this
> roof
> never has any snow on it.  The upper roof tends to acculimate snow at
> an
> adjacent valley area and then gradually creep until it eventually
> falls to
> the unloaded roof below, then bounces off and lands in the parking
> lot.  I
> gave Kirkwood a temporary repair last the winter, and told them to
> call me
> this summer for a permanent repair.  I think that I will have fun
> trying to
> come up with a reasonable but safe impact load for the new roof.
> Randy Vogelgesang S.E.
> South Lake Tahoe