Timber/Steel Pavilion

[grogannc]

I have a client using an Architect that wants a pavilion ~13'x18', 6:12 hipped roof, that doesn't want any kickers/knee braces at the tops of the columns. They are allowing me to use HSS columns though. Thinking of running the HSS up to the top of the perimeter beams and using beam pockets welded to the HSS with enough bolts in the paralm/glulam to take the moment couple from the eccentric roof loading and theoretical moment from the vertical loads on the beams. Any problems conceptually with this? The loads are fairly light-I haven't sized the HSS yet but I have a feeling its going to end up a 6x6.

Also they want the whole thing vaulted. With a hip roof I don't know of any way to make it entirely vaulted. What I have done in the past is treat the ridge/hips as beams and support the reactions at the end of the ridge through a separate beam/column. Anything I am missing here?

 

[KootK]

Were it feasible, I would flagpole the columns rather than make a moment frame of the beams. It may simplify some things. I also have an instinctive mistrust of the stiffness of wood moment connections made using conventional fasteners. This is a minor preference though.

If you go with moment frames, I'd lean towards a dimensionally stable engineered wood product over glulam. Moment connection probably means grabbing the beams high and low. That can be problematic for members that can be expected to shrink and potentially split at the bolts.

You're thinking manufactured wood trusses, right? I did some two way vaulted hips as a truss designer. They work fine at small scales like this if you can keep the ceiling pitch under half of the roof pitch. It will lay out just like a regular hip roof in plan.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[jdgengineer]

One solution I often employ is a kinked steel beam. If the loads are small this could be a flitch plate or HSS. You can make one of the hips dominate as the steel beam and frame the other hip into it. Then furr out to conceal as required.

 

[jdgengineer]

Or if you have a main ridge between the hips you can add two steel beams. One at each hip/ridge intersection.

For these situations I always use cantilevered columns. Like kootk I have similar hesitation to a wood moment frame.

 

[grogannc]

I'm thinking of doing both-at least within reason. Not sure what the footing sizes will end up being yet but the Architect was nice enough to show them currently 3'x3' so if I had to bump them I don't think it would be a problem. If I use a Glulam or Paralam Plus PSL I was going to slot the steel in the top row of fasteners for swelling/shrinkage but not sure how effective this will be. I wanted to stay away from LVLs since this is an exterior application and 18' span. Any other alternatives? Flitch plate?

They literally want the entire thing vaulted with no verticals. Even with trusses-I can't imagine its not going to sag over time?

 

[grogannc]

jdgengineer-thats a great idea...so one hip steel, ridge steel, opposing side hip steel? And frame the rest into the steel?

 

[KootK]

Steel columns. Steel ridge beams. Maybe steel perimeter beams is the logical next step. Easy moment connections. Excellent point about not using LVL outside.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[grogannc]

Thanks for the ideas guys. Think I am going to go with a steel bent across the middle spanning from short side to short side and then make the two supporting beams steel (easy moment connections in the weak direction) and since the steel will be a frame at that point, just fill everything else in with timber like normal. Should be pretty easy.

 

[grogannc]

Convinced the owner & architect this morning to go with a more standard gable vault and use steel beams so I can have semi-moment resisting connections. What do you guys usually use for something with light residential type loads. A shear connection with clip angles top/bottom enough?