Timber Truss w/ Rotated Purlins?

[UTvoler]

Hi all,

Working on a little single story clubhouse, and the architect want's to incorporate timber trusses ~10-ft O-C. He's showing purlins spanning across the top; we could block between them at the trusses to provide rotational resistance but that seems to be a bit of a long span. We have 30psf snow, so decent loads and it makes me uneasy without really knowing how to analyze this framing. Any thoughts; anyone seen this done before?

 

[jayrod12]

This is done a fair bit. I agree about the rollover blocking at the trusses. Keep in mind that the sheathing will also help, provided you nail the sheathing to the blocking at each truss, to hold the purlins in place.

 

[phamENG]

Which are you trying to do - rafters with way too much blocking, or really closely spaced purlins with blocking (but called out as rafters)?

The purlins go between the rafters and the truss (typically at truss nodes along the top chord) and are beams that support the rafters. Not sure what I'm seeing in your section.

 

[UTvoler]

Thanks jayrod.

phamENG, not sure what you're asking/suggesting. The architect is showing 2x12 rafters/purlins at 2'-0" o-c over the tops of the trusses (to be able to insulate) and then 2x4 "furring" perpendicular to the 2x12's (for ventilation) and then roof sheathing. Says they've done this before, I don't love the 2x12's on the angle; seems rotation could be an issue.

 

[SWComposites]

rafters and purlins are two very different things (even this aerospace guy knows this). rafters go up/down the roof slope. purlins go lengthwise of the roof. what you show are "purlins".

 

[driftLimiter]

This is done all the time around here. I would put blocking each bay at every HT truss. Nailed into the roof sheathing those 2x12s wont go anywhere. I would design them as if they were flat and for the full vertical component of the roof loads.

If you're not convinced, you could calculate the component of the roof loading projected onto the truss top chord, and check your blocking connection to resist that load. A series of toenails is typical, but some Simpson connectors can get you more connection if you need it.

 

[phamENG]

Okay...I see it now. I mixed up the leaders. You just don't have any rafters...only purlins. No reason not to. Analysis should be pretty straight forward. You'll have bi-axial bending, which light frame wood isn't great for, but it's likely doable. Not common at all in my area, which isn't far from you. I'm guessing this is somewhere between Roanoke and Leesburg?

 

[UTvoler]

SWComposites, agreed!
driftLimiter, thanks for the input. Good thought on checking it flat-wise.
phamENG, you're right on it; the project is in Harrisonburg.

 

[driftLimiter]

@utvoler and @phameng.

My comment about design it as if it was flat I think is opposite of what your thinking, and a bit different than what phameng is suggesting.

WHat I meant was design it as if it was a *flat roof* and neglect any weak-axis bending.

Being that the purlins are nailed off top and bottom to sheathing and gyp, and blocked. I don't see any weak-axis bending needs. But I can understand arguments for the contrary.

I would suspect that the roof sheathing could support the 'lateral' component of roof loading without need for weak axis purlins.

 

[jayrod12]

I agree with driftLimiter's design procedure for this scenario.

 

[kissymoose]

I'd still put in blocking above the trusses. Attachment to the sheathing/furring will brace an individual purlin, blocking will stop all the purlins from rolling together.