SNOW DRIFT 4

[Aldoman]

So I'm doing a job in South Carolina where the ground snow load is 5 psf and I have a canopy resting up against a taller building that's over 300 ft long. When I apply the snow drift requirements of ASCE7 to it I end up with about 45 psf maximum snow. Now something seems implausible here. I can't believe that such a drift is realistic when the ground snow load or the balanced snow load on the higher roof is so small. Where is the logic in this analysis or am I missing something?

Aldoman

 

[JAE]

San Antonio, Texas, 1984 or there-abouts. Something like 13 inches of snow fell. Carports and warehouse roofs all fall down.

Grocery stores run out of camera film and toilet paper (go figure).

Structural engineers in the area see an uptick in revenue.

Light snow, dry snow that is, blowing across a 300 ft. roof can and will pile up at obstructions.

 

[Aldoman]

Sounds like a horrible storm. But would you have designed for it? 13" of snow is about 15-17 psf and San Antonio couldn't be more than 5 psf ground snow. This wasn't a once in 50 year storm from the magnitude of the snow fall. Put it another way, if only 5 psf fell that day, or about 4", would the snow drifts had been that huge?

 

[msquared48]

45psf drifted is nothing. Try designing for 400 psf uniform, let alone the drifting.

In Seattle, I have seen photos from theturn of the century of 4 feet in downtown Seattle. We normally do not get accumulations from any storm much above 8" or so, but one year recently we got two storms within 48 hours that each dumped 12 inches. We did get a lot of carport and boat dock cover collapses, and a few local code upgrades for snow design requirements. Point being things outside the norm do happen, and we have to design for them.

Drifting, with any wind blown snow storm, is normal though, not out of the ordinary.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[dcarr82775]

Doesn't seem out of the realm of possibility, but yes it requires a 'perfect storm' with wind blowing from the right direction for a while.

Look at it this way, you have 300' of roof upwind of the canopy with a couple of psf of snow on it. The wind is moving some of that snow down towards your canopy where the disturbed air flow will drop that snow onto your canopy. It doesn't take much snow off that large upwind area to get to 45 psf.

Unlikely? sure

Impossible? I have never been to SC, but it doesn't sound impossible to me.

 

[Archie264]

msquared48,

400 psf of uniform snow load? Could you elaborate? I'd love to hear about it. Thanks.

 

[dik]

45 psf is approx 4' of snow or maybe 2' if 'waterlogged' (some will say 1')... unless there is a minimum snow drift allowance, then it may govern. In many jurisdictions, snow accumulation is 'capped' at 3x the GSL.

I have no idea of what your snow loading conditions are in South Carolina... didn't know you even had snow <<G>>.

Dik

 

[Aldoman]

To dcarr82775,

Large snow drifts and their possibility of occurring is not my issue. I sense that although it can happen, is it in the 1 in 50 year storm likelihood range? With a ground snow of 5 psf in South Carolina or about 4" is it really within that probability criteria to expect a 3' to 4' drift from that snow to land on the lower structure? The Code says yes. But I believe their criteria was based on areas where ground snow was more substantial. Maybe I should design for a higher wind speed? ;-)

Aldoman

 

[woodman88]

Per the ASCE 7-05 Section C7.7 it states "...Case studies show that, in regions with low ground snow loads, drifts 3 to 4 ft. high can be caused by a single strom accompanied by high winds."

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[dcarr82775]

We looked at a metal building in the arctic once. Interestingly enough the area was very dry so the ground snow was surprisingly low. I want to say 20psf, no more than 30psf. The building collapsed under a drift that was equivalent to 3* that ground snow load. Why? Because the wind just kept blowing in one direction. How long the wind blows is really what matters. No wind no drift.

If you are arguing the code is too conservative, that is a different argument altogether.

 

[msquared48]

Archie:

Yep. 400 psf at Stevens pass in Washington where we commonly get heavy wet dry snow bofore the dry coloder snows come later, and later with rain on a heavy snow pack. This is a favorite local ski area at 4000 feet in elevation.

Recentlyu, I ran across another site in Oregon, a ski area too, with a snow load of 480 psf. Almost 5000 feet there.

These are loads where pretty anything you design in wood is going to be shear critical and very inefficient.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[dik]

Woodsman... if the ASCE recommends 3 or 4' for areas of small GSL, then 45 psf is not out of line... and, I'd run with that... In areas of drifting, you can often end up with a 'bulb' of snow overhanging the actual lower roof area... I'll try to dig up some photos of this...

Dik

 

[woodman88]

Dik - The "C" section that I quoted is from the commentary to the standard. Their recommendation is to use the equations, etc. of the standard. The comment about the "3' to 4'" is one of their reasons for the equations, etc. being used.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[JAE]

woodman88 - good quote - good find to help with this post.

 

[humanengr]

Previous building code editions did not require snow drift for GSL < 10 psf.
Recent ASCE stndards now require snow drift for any GSL values.
I recently examined bldg. in 10 psf GSL area. A couple of years ago the bldg.
had 6, yes six, feet of snow on the roof. Again, way beyond code required loading,
but thank Goodness, there was no collapse, although there was some damage to repair.
The redundancy and strength reserve from other load cases probably saved the day.
Of all bldg. collapses, how many do you recall were due to snow vs. other load conditions? Probably by far majority of collapses in US were due to snow.
Maybe that's why the Codes are conservative when it comes to snow drift.

Note: If you use snow guards to reduce drift, make sure you use the correct Cs
coefficient. It might increase depending on conditions.

 

[kylesito]

I do not have the code with me but I believe there are some maximums to design for. You may want to check those.

However, 45 psf drift is not unrealistic. I do a lot of work along the Eastern seaboard and have seen some drifts in VA get up to 65 psf pretty regularly. We are a little colder here...but yes, it seems in the ballpark.

PE, SE
Eastern United States

"If a builder builds a house for someone, and does not construct it properly, and the house which he built falls in and kills its owner, then that builder shall be put to death!"
~Code of Hammurabi

 

[chicopee]

Have you research snow loads have triangular distributions which many uneven roof lines would have?

 

[Archie264]

Msquared, a belated thanks for the information. Very interesting...

 

[berkshire]

One of the things you have to consider in Georgia and South carolina is freezing rain.
I had a building with a flat roof in North Georgia, where I had to go out and rent scaffolding jacks to support the roof beams that were cracking after a week of freezing rain in 1983. We had a measured accumulation of 8" of solid ice. I believe that was equivelant to over four feet of snow.
B.E.

The good engineer does not need to memorize every formula; he just needs to know where he can find them when he needs them. Old professor