Snow Load on Pergola

[mkberry]

Is there a code provision to reduce the snow load on a lattice type structure such as a pergola? (IBC or ASCE7)
When the joist members are far enough apart, the snow only collects on the surface area of the members. Other times, however, the members are close enough together that I feel the snow could bridge across the members and collect more weight than just the exposed surface area, but less than a full roof snow load.
ASCE7 has a force coefficient for wind on lattice frameworks, but I am not finding anything for snow.
If no code provision exists, what loads have you used in past experiences?

 

[blake989]

I'm not sure if my method is correct, but it is definitely conservative. I check for ice loads on lattice type structures. ASCE7-02 requires a whopper of an ice load that will typically be much larger than a snow load on lattice.

 

[mkberry]

Thanks blake989. In my area the ice thickness is .25" while I have a snow load of 30 psf. With 2x6 framing at 6" o.c., the snow load controls at 3.75 #/ft versus 1.81#/ft for the ice. The support beams carrying the joists are the controlling members. Nonetheless, I am going to design the joists with a load of 50% of the snow load over the tributary area (15psf or 7.5#/ft for each joist). The architectural requirements for the pergola allow me to use this conservative approach while still using the arch. requested member sizes. I think the actual term for what I was describing is "Snow Accretions" (ASCE7-02 Section C10.2). I've never used that section before, thanks for pointing it out! I will look into the references when Icing or Accretions are more critical to the project.

 

[vmirat]

I ran into this exact situation with a pergola. My project had 2x2 cedar slats at 3 inches on center supported by 2x8 joists at 2'-0" o.c., so there was 1.5" of slat and 1.5" of open space between them. I asked ASCE what they would do, but they were of no help. I then asked the Cold Regions Research & Engineering Laboratory (CRREL). They suggested I assume a 45 degree angle of snow from the top of the slat upward to the top of the snow layer. I also have a 30 psf ground snow load, so the total snow layer thickness would be about 1 foot. Since this is much greater than the spacing of the slats, I assumed a uniform snow load across the slats, which turned out to be 18 psf.

The other issue I ran into was lateral wind load. I did not want to assume that the wind would behave in a purely horizontal nature, but would attack the 2x2 slats and 2x8 joists at some angle, therefore adding to the lateral loading. I couldn't find anything about angled wind loading in ASCE 7 or IBC, but I did find a European code which required a 10 degree variance in wind attack.

 

[mkberry]

vmirat, thanks for sharing your method. I agree with your load of 18psf. Was it arrived at by a use of engineering judgement, or some function of the 45 degree angle? Assuming a 45 degree chunk of snow falls out between the slats leaves around 29 psf of snow. I know more than a pound of snow will slip through those openings before they close up. Thanks for pointing out the wind load. I have seen too many engineered designs that seem to disregard it. Another lateral load that you are likely to have on a pergola is a porch swing with three kids on it swinging as hard as they can! (Though it's safe to assume they won't be outside during a wind event)

 

[vmirat]

You can never count on kids to do the reasonable thing! Anyway, I assumed that the snow load would be as if the roof was solid - worst case senario type thing. The 18 psf is the flat roof snow load as determined by ASCE 7 using a ground snow load (Pg) of 30 psf, an exposure coefficient (Ce) of 0.9, a cold roof temperature coefficient of 1.2 (Ct), and an importance factor (I) of 0.8.

Pf = 0.7(0.9)(1.2)(0.8)(30) = 18 psf