Are Unbalanced Snow Loads required for Single Family Residential Homes in the USA

[woodman1967]

From my understanding unbalanced snow loads are to be considered when designing trusses for single family residential homes (ASCE7-10 Section 7.6). I work with a home manufacturer who designs their trusses as monoslope trusses. On site, when the modules are put together the monoslope trusses form a gable truss with support wall/beam at midpoint.

I recently received the construction package for a home going to Maine. A coupe days later I received an updated package for the same home. Upon review I noticed the difference was the truss calc sheet.

The first calc sheet stated it was designed in accordance with the 2015 IBC section 2306.1 and referenced standard ANSI/TPI 1, this one stated that the unbalanced snow load WAS considered.

The second calc sheet stated it was design in accordance with the 2015 IRC sections R502.11.1 and R802.10.2 and referenced standard ANSI/TPI 1, this one checked uniform snow loads only.

Both calc sheets were stamped by the same Engineer. The second truss design was lighter than the first with much less reaction values.

Does the IRC allow for uniform snow loads only?

 

[phamENG]

Correct. I don't think the world unbalanced ever appears in the IRC.

Keep in mind that the IRC is based on ground snow loads, not flat roof or sloped roof snow loads. ASCE 7 has us using I*p[sub]g[/sub] for unbalanced loads on rafter framed roofs spanning less than 20ft, so there is some consistency of philosophy buried in there if you squint hard enough.

 

[STpipe]

Check the commentary for guidance perhaps? I'm not familiar with the IRC, but the Canadian equivalent (Part 9 of the building code), you only need to consider the simplified snow load applied as a balanced snow load on the entire roof.

There is a provision that states that in situations where the truss may have an intermediate support (like an interior bearing wall), then pattern loading needs to be considered, but it's simplified compared to the unbalanced snow load that the roof would need to be designed for under Part 4 (equivalent to Chapter 16 of IBC). The code further goes on to state that if there are any significant projections on the roof, or if the roof reaches a certain size, then the design should be done based on Part 4.

If I'm not mistaken, in certain situations the snow loads from Part 9 can be lower than the more detailed engineering design (since the Part 9 calculations don't take into considering snow sliding off the roof), so if the IRC is similar, it could also potentially explain the lighter design.

 

[lexpatrie]

I'm gonna hazard the guess that Maine may modify the IRC, Secondly, as the trusses are "engineered",

The unbalanced snow load doesn't know it's on a roof of a residential structure and spreads out and refuses to blow around. P.E.'s ignore reality at their peril.

The added load on the walls, maybe that won't alter the design or the foundation but I'd check it. If you check it and the existing design is fine, we'll, everybody learned something for next time.