[Strawbale] Death in collapsing SB house

[Jeff Ruppert]

Harry,

Engineering based on codes does not cover all scenarios.  Snow load 
numbers in the IBC or IRC are based on 50-year snow events.   From a 
practical standpoint, you should not need to get on your roof very often 
and shovel snow.  But paying attention will never hurt.  The most 
critical areas for a roof are the valleys.  Snow tends to accumulate in 
these areas due to drifting.  This can create huge loads on a valley 
roof member, requiring extra connections and larger members.  Buildings 
that are not symmetrical and of odd shape also need extra attention due 
to imbalanced loading.

Depending on the slope of your roof, shoveling can be hazardous.  I have 
heard of snow shearing nearby trees off as it comes off the roof.  I 
have also heard of snow removal people being trapped against a home when 
they slid off the roof with the snow they were clearing.  One of them 
was next to a door and managed to get inside and rescue his co-workers.

Areas that experience moderate snow fall can occasionally see much 
higher snow loads.  Most codes and they way we build these days have 
redundancies that we don't even account for when doing calculations.  It 
is always a good idea to have someone who knows what they are doing size 
the most critical members and connections so those redundancies are left 
intact.  Many non-builders or owner-builders are not familiar with codes 
and therefore use their common sense.  This is ok to an extent. 

Jeff Ruppert, P.E.

<http://www.odiseanet.com/>



Harry Kits wrote:
> Interesting discussion after the winter of snow we have had here in the
> Ottawa area. Our first Ottawa winter after over 20 years of Toronto winters;
> those being much milder snow wise [except of course the one that the mayor
> called out the army to deal with ;-)  ].
>
> We have a steel roof at a 4/12 slope with all hips.  Total of 4 valleys.
> The slopes without valleys regularly had that swoosh sound of the snow
> sliding.  The ones with valleys never let go and only in the last couple of
> days has it all melted.
>
> Everything was engineered of course so I wasn't too concerned this winter.
>
> I was curious though about how we could know when the snow load might be too
> much or too extreme despite the engineering.  Does the engineering cover all
> worst case snow load scenarios?
>
> Any thoughts out there?
>
> Harry
>
> On Fri, Apr 11, 2008 at 11:18 AM, Matt Chase <mattchase at canoemail.com>
> wrote:
>
>   
>> Hi Sherwood
>>
>> Not sure where you're coming from, but here in Canada, the national
>> building code has a feature for calculating snow loads that is as
>> follows:
>>
>> There is a slope factor (given the symbol Cs) that is multiplied by
>> the snow load to reduce it if one can count on sliding off of snow to
>> occur.
>>
>> For "normal" roofs, it ranges between 1 (when the roof slope is less
>> than 30 degrees, approximately a 7:12 slope) and 0 when the roof is
>> steeper than 70 degrees (a very steep roof indeed!, a 33:12 slope).
>> The factor Cs changes linearly with the angle, so right in the middle
>> (i.e. 50 degrees) would give a Cs factor of 0.5, reducing the snow
>> load by half.
>>
>> For unobstructed, slippery roofs (i.e. your steel roof), the range is
>> between 15 and 60 degrees (3:12 to 21:12 slope).  For your 12:12
>> slope (45 degrees) the factor Cs is equal to 0.33 which means that
>> the snow load is reduced by 1/3 - a considerable reduction.
>>
>> Building codes where you are from probably have a similar means of
>> treating the snow load.  So, you could probably build a lighter truss
>> by making it steeper.  However, to make the roof steeper, you would
>> also need some extra material simply to get up that high.
>>
>> There is a trade-off involved...
>>
>> Cheers,
>>
>>
>>
>> Matthew Chase
>> MSc. Candidate
>> Biosystems Engineering,
>> University of Manitoba
>> Winnipeg
>> (204) 880-8403
>> (204) 237-5315
>> Sign up today for your Free E-mail at: http://www.canoe.ca/CanoeMail
>>
>> _______________________________________________
>> Strawbale mailing list
>> Strawbale at listserv.repp.org
>> http://listserv.repp.org/mailman/listinfo/strawbale_listserv.repp.org
>>
>>     
>
>
>
>   
> ------------------------------------------------------------------------
>
> _______________________________________________
> Strawbale mailing list
> Strawbale at listserv.repp.org
> http://listserv.repp.org/mailman/listinfo/strawbale_listserv.repp.org