Gable end wall support
Gable end wall support
(OP)
I have been asked by a contractor to engineer a wood garage. The plan dimensions of the garage are 40'-0" x 60'-0" (clear span) with a required clear height of 16' and a 8:12 roof. I have already proposed a Metal Building, which doesn't appear to be an option right now.
Before I accept the job, I want to make sure I can get everything to work. The only problem I can't resolve is how to transfer the loads from the gable ends into the wood roof diaphragms.
I could sheathe the underside of the trusses with plywood (gypsum board is not strong enough) but I don't think the contractor would like that. I could provide kickers at 2'-0" o.c. but I am not sure if this is the best way to go.
I am just wondering what is the best way to resolve this problem (see attached).
Before I accept the job, I want to make sure I can get everything to work. The only problem I can't resolve is how to transfer the loads from the gable ends into the wood roof diaphragms.
I could sheathe the underside of the trusses with plywood (gypsum board is not strong enough) but I don't think the contractor would like that. I could provide kickers at 2'-0" o.c. but I am not sure if this is the best way to go.
I am just wondering what is the best way to resolve this problem (see attached).






RE: Gable end wall support
1. diagonal kickers from the top of the lower wall up to the sloping roof diaphragm.
2. Develop a horizontal truss or diaphragm in the plane of the ceiling line along the bottom of the trusses. Either the whole ceiling is sheathed and serves OR the first several truss spaces are utilized as a horizontal truss or diaphragm to transfer the lateral force to the side walls.
RE: Gable end wall support
JAE is right. you probably wont have to sheath the entire ceiling area, just a few truss spaces in from the gable. design that sheathing like a beam.
RE: Gable end wall support
2X, 3X or $X Kickers are normally the best option, but use a 4 foot spacing, not 2 feet. 4 to 8 feet is normal, depending on the loads and geometries involved here. You should be able to develop the connections at the plate and roof levels with no problems with Simpson or similar connectors.
FYI: Never, but never rely on sheetrock applied to the ceiling structure to resist any horizontal loads. This is not allowed by code.
Mike McCann
MMC Engineering
RE: Gable end wall support
I have dabbled in wood before but never have gotten a chance to do a full design. I am figuring this would be an easy structure to start out on.
Msquared48:
I was looking at using a Simpson GBC (Gable Brace Connector) for my end walls. The information I have states that these are good for 275# - 355# (depending on the angle of the brace). This is what I was basing my 2'-0" spacing on. I'm not sure if there is a better connection than a Simpson GBC?
Also, I tend to agree with the note about the Gypsum board, however, if the code gives me allowable diaphragm values then why can't I use those values? Is this because the code gives values allowable shear in walls and not ceilings?
RE: Gable end wall support
With regard to the GBC, I normally use 1 or 2 A34's, A35's or equivalent that can give me up to 900# per connection. I have not used the GBC.
Mike McCann
MMC Engineering
RE: Gable end wall support
He's right, kickers at 24" o.c. is too tight, 48" should work just fine. I've used a variety of Simpson connectors at the intersection of the brace to the wall; doesn't really matter except that it's easy to construct and meets the load requirements. You don't need a connector where the brace meets the roof. Just run some blocking between trusses, and face nail the kicker to the blocking. This gives the framer some leeway.
Bottom chord beam method
If you go this route, don't sheath the underside of the trusses. This might interfere with finishes and it's harder to nail something while holding it in the air. Provide blocking at 24" o.c. between truss bottom chords and lay the sheathing on top of that. The framer will have to notch around the truss webs, but that's not that bad. Call out a nailing pattern (edge/field) for that decking similar to a diaphragm/shearwall and nail to the blocking AND truss bottom chords. This method also creates some storage area in the attic.
RE: Gable end wall support
Never, but never question engineer's judgement
RE: Gable end wall support
balloon framing is taking the studs up to the roof (variable height studs). contractors don't like it but engineers do.
RE: Gable end wall support
This will be tricky to erect and maintain stability until the sheathing is in place. I once saw a similar structure reduced to a big pile of broken lumber when a storm hit at the wrong time.
RE: Gable end wall support
If you want to use plywood on top of the bottom chord of the roof trusses, I have no problem, but the framers will scream.
Mike McCann
MMC Engineering
RE: Gable end wall support
RE: Gable end wall support
Never, but never question engineer's judgement
RE: Gable end wall support
RE: Gable end wall support
I'm not sure how much work the contractor has done with engineers in the past. I would like to have all of my ideas/requirements for building this thing on the table before I start with the final calculations and drawings. If he doesn't like what I am saying needs to be done then he is free to go somewhere else and I have learned a little about wood framing.
Also, in regards to the roof diaphragm, I know I have to amplify the loads to account for the diaphragm being at an angle (this is in regards to the horizontal wind loads) but do I consider the roof as 2 diaphragms 24'x60' (accounting for the slope) or 1 diaphragm (48'x60')?
RE: Gable end wall support
Now at the top of the 2 king studs you need to be able to transfer it to roof diaphragm. You can do this by putting some transfer beam at the same 2 locations to connect several trusses until you have enough length to transfer the load with nails. Since the wind can go both ways, you need to have good simpson connector to connect the king studs to the transfer beams for axial. I hope I am making any sense. Like I said, as long as you can justify your load paths then you are good to go.
Never, but never question engineer's judgement
RE: Gable end wall support
I was just looking at the simpson A34 and A35 and noticed that they did not give an allowable tension/uplift value for these types of connections. Since wind loads are produced in both directions how do eliminate the tensile value in the connection of the kickers?
RE: Gable end wall support
Then the top plate (at 16' height)could be beefed up to span horizontally between columns and carry either direct wind or suction pressure. Bolt top plate to steel gussets projecting from columns.
Best regards,
BA