Seasonal Buildings, Snow Load and Risk Category 6

[skired]

I am designing a group of buildings for a summer campgrounds in a high (~60 PSF) snow load area. The buildings will be unoccupied and winterized/unheated during the winter.

Buildings are risk category II

My thoughts regarding snow loads
1. Design snow load using a Ct of 1.0, Risk Category II
2. Use a Ct for unheated structures since the buildings will be unheated in the winter.

My reasoning to use Ct of 1.0 is that when the building is unheated it'll be unoccupied and arguably Risk Cat I. If the buildings end up getting used during the winter (not unoccupied) then they'll be heated.

Curious to hear others thoughts.

 

[DoubleStud]

Ct unheated will be 1 too. am I missing something? Ct is only used to reduce strength of wood because of high temperature.

 

[skired]

Ct unheated will be 1 too. am I missing something? Ct is only used to reduce strength of wood because of high temperature.

I mean the Ct factor in determining snow loads from ASCE 7

 

[phamENG]

Ct from ASCE 7, not the NDS. Thermal Factor for snow load calculation.

I'd use 1.2 for my Ct. If one of these collapses, the owner (or their insurance company) will still be looking for a pound of flesh to offset their loss whether it was occupied or not. Skimping on this puts a real nice target on your back.

 

[DoubleStud]

Thanks! Forgot about the ASCE one and so used to only use 1.0 or snow load dictated by the building department.

 

[phamENG]

Doublestud - for houses, are you not doing any with attics? If you have an unoccupied attic, it should be 1.1 in most cases. Around here, the minimum R value for attic insulation is R-38 (though I think it just went up with the code change). So truss and conventionally framed gable roofs with unoccupied attics would fall under 'cold ventilated roofs'.

 

[Eng16080]

I tend to be on the conservative side, so I would most likely use Ct=1.2. I wouldn't even think about it unless it made a big difference with the roof framing, like if you couldn't get 2x12 rafters to work and now had to use LVLs, or something like that. Still, I'd probably just stick with Ct=1.2 regardless.

The flip-side of this is that your argument is logical, and the commentary to ASCE 7 mentions this exact scenario!

Some dwellings are not used during the winter. Although their thermal factor may increase to 1.2 at that time, they are unoccupied, so their Importance Factor reduces to 0.8. The net effect is to required the same design load as for a heated, occupied dwelling.

Now, if I was going to take this approach, I would still use Ct=1.2, but would use Is=0.8 for Risk Cat. I. Using Ct=1.0 and Is=1.0 makes it confusing/misleading if you'll be communicating that information, like if it's including with your general notes.

 

[DoubleStud]

phanENG,

It is one of those things where my mentor failed to explain to me. So if the building the department list it on their website the roof snow load, do I multiply this Ct? I notice the snow load they list on their websites are typically higher than the snow load if I were to compute it from ground snow load from ASCE. I will sharpen my pencil next time.

 

[Eng16080]

The point by phamENG is a good one. I also often use Ct=1.1 for insulated roofs.

If this is a summer cabin with no insulation (which is likely) then Ct=1.0 makes sense. But, if the cabins will at some point be used in the winter, I think there's a good chance that the owner would insulate the roofs making them Ct=1.1. If they're paying for the heat, they'll definitely have a strong incentive to do this. And if that's the case, all of a sudden the roof will no longer meet code if you design it per your method. If you do proceed this way, I would add a note on the plans describing this scenario.

 

[phamENG]

If the building department lists a ROOF snow load, then you should use the higher of their value or your calculated value. Most jurisidictions where I have worked follow IRC Table R301.2, though. That is Ground Snow load. In that case, I would use the higher of their listed ground snow load and ASCE 7 mapped ground snow load to enter into your calculated roof snow load.

It's important to recognize the distinction between Roof snow load and Ground snow load.

 

[phamENG]

Eng16080 - good call on the commentary reference. I was not aware of that.

 

[Eng16080]

DoubleStud, I second the comment above. I was about to write exactly the same thing. Building departments often provide incorrect/misleading information. Taking the more conservative of their value and your calculation makes the most sense.

 

[dauwerda]

Many of the jurisdictions here in Colorado specify a roof snow load, that roof snow load is typically equal to the ground snow load. Basically they are just saying, we don't care how steep your roof is or what material (how slippery) it is, you don't get to reduce from the ground snow load.

 

[DoubleStud]

dauwerda, yes! I am in Colorado too. For example in Vail, the town of Vail does specifies 145 ground snow load. If you use SEAC ground snow load, you will get about 85 ground snow load. I am hesitant to add anything else to increase the snow load. I feel like they added a lot of fluff to account different kind of roof and drift.

 

[jerseyshore]

Sometimes I forget the commentary even exists in half of these books. I want to thank all of the code writers for making the main text hard to understand and hiding the actual useful, clearly written sections for the commentary that no one looks at. Good find Eng16080.

 

[skired]

Sometimes I forget the commentary even exists in half of these books. I want to thank all of the code writers for making the main text hard to understand and hiding the actual useful, clearly written sections for the commentary that no one looks at. Good find Eng16080.

Especially now that 7-16 is split into two books. I like the ACI 318 format with the commentary in line

 

[jerseyshore]

I love reading the old NYC codes any time I have to look back for an older building. They are just so simple. They say wind load = 30 psf. That's it. The whole city, every building.

And they're written in such a way that is easy to follow (with some funny old-timey language in there too).

But you're right, let's bog down ASCE 7 so much that we need two books, of which most professionals won't read more than 5% of either one. Don't get it.