Pole Frame building with scissor trusses?

[kareng]

Hi all - I'm fairly seasoned with wood design, but have only done small-ish pole frame buildings. This one is 66'x136' single story and they're asking for 6x6 posts @8' ocwith an 16' tall eave and scissor trusses spanning 64'. There are a couple of interior + end walls I can use for lateral, but the scissor trusses add a new wrinkle. Usually with scissor trusses I limit the lateral movement to H/400 and tell them to provide temp bracing and wait to connect the trusses to the walls until all dead load is in place. Anyone have any tips?

 

[jayrod12]

I appreciate that they are asking for 6x6 posts at 8ft that are 16ft cantilevering, but that's just not reasonable.

Regarding the scissor trusses. I am of the opinion that regardless of the size of wood post, they'll be flexible enough at the peak to avoid constraining the trusses so they should still work with the pin-roller assumption. Confirm with a truss guy what level of horizontal movement is expected though, it might be bigger than we think.

Are you putting trusses at 8 ft on-centre too? Or beams between the posts?

 

[kareng]

Thanks jayrod12 - I was thinking that the walls would do the heavy lifting for lateral loads so I don't have to rely on fix-free posts. But I'm still not convinced this will all work once I take a closer look. Beams between the posts.

 

[msquared48]

I would look to two scissors trusses at each 6x6 post, one to either side of the post with bearing blocking under the trusses that is thru-bolted to the 6x6. Uplift straps over the trusses nailed off to the 6x6. Just use a two or three foot depth at the 6x6 for the scissors truss with 4:12 top and 2:12 bottom pitch.

Mike McCann, PE, SE (WA, HI)

 

[jayrod12]

Edit: didn't realize you mentioned there are some interior walls.

If you don't need to rely on the posts for lateral, I'm not sure what your concerns are then with the scissor truss idea? are you worried about them pushing outwards? I think that you won't really be able to mitigate that in a construction scheme like this. Like I said, ask a truss guy what the horizontal displacement is expected to be. I'd bet it's not as bad as you think and when you check it versus the height it's possibly pretty close to your H/400 number you're shooting for.

 

[msquared48]

When you analyze the trusses, pin both ends with no rollers to get the horizontal kick generated, then verify that the 6X6 verticals can take this load in bending and deliver your l/400 deflection limitation.

Mike McCann, PE, SE (WA, HI)