Author Topic: Snow Load question.  (Read 1663 times)

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Offline Jeff922

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Snow Load question.
« on: February 03, 2013, 11:07:06 AM »
I'm continuing the design process of my barn and I got to thinking about snow loads.  I can't seem to find a clear answer on calculating snow load as it relates to roof pitch.  In my town in Maine, we have a Ground Snow Load of 90psf.  It seems to me this should be adjusted according to the roof pitch.  As a skier, I know that above 45degrees of slope, snow will start to sluff off.  My roof pitch is 12:12 so it would seem that my load should be reduced somehow.  Most of the span tables/calculators don't seem to list pitch.  What am I missing?
"They don't grow trees so close together that you can't ski between them"

Offline flyingvan

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Offline Jeff922

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Re: Snow Load question.
« Reply #2 on: February 03, 2013, 12:58:20 PM »
That's awesome thanks for that link!   :D
"They don't grow trees so close together that you can't ski between them"

Offline UK4X4

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Re: Snow Load question.
« Reply #3 on: February 04, 2013, 07:52:17 AM »
From my first engineer that looked at design 1

PS=CsX PF

PF=90
Cs= 0.66 based on 8-12 roof- slippery
gave me aprox 76# /sqft

But when you then look at uneven snow distribution- eaves having stacked snow etc

In discussion with the truss engineer- when you you've done all the calculations trimmed them to be as small as you can get

Decided on the max theoretical load taking into account - ground load- roof material -exposure heated or non heated- un balanced snow load- eaves have higher snow load

Do you honestly want your roof strong enough to not collapse -under the worst conditions

We used max load on whole roof for my trusses- no worries down the road

This was graph I found somewhere for the Cs value, but later found out all the other effects that have to be concidered




Offline Jeff922

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Re: Snow Load question.
« Reply #4 on: February 04, 2013, 01:50:13 PM »
Thanks UK, I'm planning on cutting my own rafters out of 2x.  I'm drawing my barn plans on CAD and, according to the calculations I did (if I'm doing this right), I should be able to use 2x10s 16OC.  I'll have rafter-ties about 2'-9" above the top plate (ridge height is about 11'-8").  My adjusted snow load calculation gave me 53.31psf (down from 90psf ground load); assuming 12:12 pitch, asphalt shingles, semi-heated structure, and semi-exposed.  I adjusted the rafter span for the height of the rafter-tie above my top plate (Hc/Hr), giving me a span of 11'-2" (down from 14'-8").  Therefore, I should be able to span the width of my barn if it's 22' wide.  Did I do this right?  Sure seems like 2x10s 16OC with rafter-ties low in the truss would be more than enough.
"They don't grow trees so close together that you can't ski between them"

Offline davidj

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Re: Snow Load question.
« Reply #5 on: February 04, 2013, 02:32:47 PM »
We've got a 12:12 standing seam metal roof.  We didn't get to reduce the ground snow load for reasons I can't quite remember (but that didn't bother me as our official snow load was probably low anyway and I didn't mind the extra margin).  But for the last few year it seemed complete overkill as the roof never held more than a few inches of snow, and at most for a day or two (less if we had the fire going).

That was until a few weeks back when apparently it rained, then the temp dropped and it started to snow just as the rain froze.  And then the snow stuck to everything like glue for a two-week cold spell, including our roof and the trees.  Even after we'd been in the cabin for two days with the fire roaring it still stuck to the roof, forming vertical snow waterfalls at the edge, hanging down to the mid point of our windows.  I'd never seen anything like it in the Sierras!

Anyway, I'm sure the engineering calcs assume stuff like this, but it was certainly counter to my intuition on snow build-up.


Offline MountainDon

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Re: Snow Load question.
« Reply #6 on: February 04, 2013, 04:58:09 PM »

That was until a few weeks back when apparently it rained, then the temp dropped and it started to snow just as the rain froze.  And then the snow stuck to everything like glue for a two-week cold spell, including our roof and the trees.  Even after we'd been in the cabin for two days with the fire roaring it still stuck to the roof, forming vertical snow waterfalls at the edge, hanging down to the mid point of our windows.  I'd never seen anything like it in the Sierras!


Thank you David. I have seen that occur twice in the short 5 winters we've had a roof on our cabin in the Jemez of NM. 
Just because something has been done and has not failed, doesn’t mean it is good design.

If you didn't have the time or money to do it right in the first place, when will you get the time and money to fix it?

Offline Jeff922

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Re: Snow Load question.
« Reply #7 on: February 04, 2013, 05:26:07 PM »
You really got me thinking David, you illustrated an excellent point.  Do you recall the Ground Snow Load you had to use in your calculations?  I still think it would be overkill to use 90psf to design my roof system.  A cubic foot of water weighs 62.4 lbs (ice slightly less).  Even a dense snow/ice mixture would only weigh a fraction of that.  A 12:12 pitch will only hold snow and ice up to about 18" worst case scenario, then it starts to slide off.  I really think I'll be safe using the reduced weight of 53.31psf. 
"They don't grow trees so close together that you can't ski between them"

Offline Don_P

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Re: Snow Load question.
« Reply #8 on: February 04, 2013, 09:13:55 PM »
Do think about it carefully before reducing the ground snow load. Design is for resisting a maximum load that might occur at any time in the service life of the building. Yup, it's a best guess based on past experience. My home has exceeded design wind and snow loads as have several I have built, it's a best guess remember, that door swings both ways. In that snow storm we had several collapses. The local Nautilus plant had a roof collapse, that broke the gas line, which then caught fire. I quit trying to reduce the ground load after I got a phone call from Terry Peak in the Black Hills. We had built a 12/12 metal clad roof with 2x12's 12" on center. He had 3' of consolidated ice and snow on it and an ice dam going on... How in the world? Well, thought I, it happens. Over the life of the structure, at some point, might the load stick up there? I've seen vertical ice. Might someone over the life of the structure install a porch roof? on the lee side? What's around the building? Go carefully when reducing loads. I'd rather reduce spans than reduce loads and strength. I do think the span reduction you took is incorrect if I'm understanding right. I've always understood the span to be measured horizontally between points of bearing, not bracing.

Offline Jeff922

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Re: Snow Load question.
« Reply #9 on: February 05, 2013, 03:50:00 AM »
Thanks Don_P, I'm definitely going to take your words seriously and rethink this.  The way I calculated the span is along the horizontal projection from the top-plate to the ridge; half the width (in my case 11' for a 22' wide structure).  My rafter span calculation gave me 14'-8" BUT I also needed to apply the reduction factor because I don't have a ceiling joist extending from top-plate to top-plate.  In my case, a rafter-tie is used 2'-9" above the top plate (reducing the roof systems ability to resist outward pressure).  Adjusted, I end up with 11'-2".  This distance will span from the top-plate 2" past the center line.  If I'm doing this wrong please correct me.  Maine is an extreme climate and ice is a VERY real concern thanks for your advice.

There is a reason why skiers call New England the "Ice Coast"


« Last Edit: February 05, 2013, 07:11:00 AM by Jeff922 »
"They don't grow trees so close together that you can't ski between them"

Offline UK4X4

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Re: Snow Load question.
« Reply #10 on: February 05, 2013, 08:02:10 AM »
Joist spanning 11 ft 2"
2 x 10's on 16 centers

hem fir no 2 fails
hem fir No 1 fails
southern pine number 1 passes-----Just it has 0.532" live load deflection- limit is 0.54"

This is using Forte 4 software
110mph
exposure C
360/240

Using 90# snow and the softwares default live and dead loads

So depending on the availability of the timber and quality available the software says your good...............just


To stop the spread- that will be some one else to step in !

- looking at a beam for that length 35.3" span - its beyond my knowledge and software

a triple 24" deep LVL fails on everything for example !


Please note I'm not an engineer !- well i am , but Instrumentation !


Offline SouthernTier

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Re: Snow Load question.
« Reply #11 on: February 05, 2013, 08:21:02 AM »
Jeff:

I just went through this for my ongoing design of my cabin.  I found a copy of the ASCE section 7 (or whatever it is called as referenced in the code) for calculating the snow load.  I set up a spreadsheet to run all the calcs here: https://docs.google.com/file/d/0Bw8bBpiOXV21S0x6eVhlenl1Ykk/edit  (let me know if you want me to e-mail it to you if it doesn't download).  You still need the ASCE 7 to get the figures and graphs although UK4x4 posted one of them here.

I didn't know about that Cornell web calculator, but doing by "hand" also lets one take into consideration unique cases, such as for me I have one side unobstructed, but the other side obstructed by a porch roof.

That all said, Don_P gave me the same advice, namely to go with the ground snow load even though the code, through ASCE 7 allows you to go with less.

I also took his advice on shifting from a triple 9-1/4" LVL to a double 11-1/4" LVL so those calcs above are out of date, but it allows for easy switching of assumptions.  Plugging in the full ground load for where I am still works for me when I use the 11-1/4" LVLs (note, I was sizing a ridge beam not the rafters.  I am going with the full 2 x 12 rafters for insulation space reasons.