Re: Snow load evaluation

[Marc_F_Hult <MFHult@xxxxxxxxxxxxxxxxxxxxx>]

On Tue, 13 Feb 2007 11:49:10 GMT, nobody@xxxxxxxxxxxx (Dave Houston) wrote in
message  <45d1a42c.2072748375@xxxxxxxxxxxxx>:

>L. M. Rappaport <nospam@xxxxxxxxxxx> wrote:
>
>>On Sun, 11 Feb 2007 11:58:05 -0500, "Robert Green"
<ROBERT_GREEN1963@xxxxxxxxx>
>>wrote (with possible editing):
>>
>>>Anybody doing anything automated in terms of detecting an excessive snow
>>>load on a structure's roof?   I saw a news report about upstate NY where a
>>>guy said "when your doors begin to stick, that's when it's time to clear
>>>the snow off the roof.  It's good to know one of the warning signs but
>>>surely there's got to be a better way!
>>
>>You could put stress gauges on the roof rafters, but you'd still have the
>>problem of calibration.  There are just too many variables - rafter size,
>>>rafter spacing, decking, shingle material, water content of the snow, ice,
>>>etc - to be able to do this in advance.
>
>I think it would be difficult to use strain on the rafters as an indicator
>with a rather evenly distributed load like this. Even wind might have as big
>an effect as snow.

Perhaps Dave means strain _of_ rather than "strain on".

>If you could place load cells to measure the weight on the roof, you should
>get reliable data that correlates with snow load.

Physics gets in the way of Dave's reasoning. If wind has "as big an effect as
snow" on strain of the rafters, it would have a proportionally equal effect
on the stress because, Hooke's Law pertaining, strain is directly
proportional to stress.  So load cells would be subject to the same
limitation as "using strain on the rafters as an indicator".

But more importantly, I doubt that this would even approach practicality in a
in a typical stick-build US house.  Dozens of load cells might be required
even if the roof was not also partially supported by the walls.

Much more doable and probably more accurate would be to estimate the mass of
water on the roof by obtaining a top-to-bottom sample of the snow pack with a
pipe and weighing the amount in the tube. This would be repeated for each
range of thickness on the roof (2-3ft, 3-4 ft etc) and multiplied by the
estimated square footage of the roof that corresponds to each range of snow
thickness. This gives the total load (weight) in pounds.

It also leads to the "small dog corners large cat question": "What's next?"
Ok. So you know that the load is x pounds or y pounds/ft^2. So what?

I think you'd end up with Frank's answer/approach.

... Marc
Marc_F_Hult
www.EControl.org