Snow drift through web of truss above roof

[vandede427]

Q1: Does anybody know of any snow load provision that takes into account the “openness” of a roof wall type element in regards to snow drift. I’ve got a new beam on top of an existing roof that acts like a transfer beam for a column to be removed, but it creates snow drifts that overstress the existing roof joists, and new joists are needed in between existing ones. If the beam could be replaced by a truss or joist girder, would that member be open enough through its “web” to allow snow to pass through and not create a drift? The new drift created by the new beam overstresses the existing joists so new joists need to be added in between the existing joists. If the truss did not obstruct the snow and no new drift was created then the existing joist will still work. I've braced the top of the beam with angle kickers at each joist and I would need to do this for the truss as well.

Q2: Can joist girders have bottom chord bearing?

 

[GoBizon]

A few questions: 1. What is the ground snow and balanced snow height? 2. What is the depth of your new beam? 3. Can a shallower, heavier beam be used so that the beam depth is less than the balanced snow height?

 

[jike]

Q2....Joist Girder can definitely be built for bottom chord bearing......just show them what you want.

 

[DaveAtkins]

Is your beam less than 15' long? If so, it does not cause drifted snow (per the ASCE 7 Code).

DaveAtkins

 

[vandede427]

See the attached pdf. It's a new 52' long W36x150 directly above two existing 26' long W18x46 beams with 28' long 20H6 joists on either side. The new beam was designed for the point loads that pick up the existing structure so the existing column can be removed.

It's in Fredericksburg, VA

Here are my snow loads:
ground snow 25 psf
flat roof snow 20 psf
obstruction height 3.5 feet
drift snow 40 psf at high end of triangle
drift length 11'

I like the idea of using a shallower beam. What I'll do is back calculate the maximum drift the existing joists can handle and from that determine a maximum beam depth.

 

[vandede427]

I don't think I attached the file correctly. Let me try again.

 

[vandede427]

ok, with the W36 transfer beam and no new joists in between the existing joists, the existing joist were 30% overstressed.

when I found an equivalent W27 size, the existing joists were only 2-3 % overstressed.