gable end wall suppor

huangyhg

gable end wall support
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).

i think you've hit on the two most typical options.
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.

the kickers are my prefered way of doing it.
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.

i agree with the others here...
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
thank you guys.
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?

with regard to your last question, yes.  you will find no horizontal diaphragm values for any sheetrock or gyp sheathing anywhere.
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
kicker method
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.
i hope i understand the question completely.  the way i usually tell them to frame the gable end is using balloon framing.  so i would think if you wood can span to the top of the gable (probably using lvl), then your load will transfer to the sheathing.  will that solve your problem?
never, but never question engineer's judgement
ah yes, option 3 that the rest of us forgot to mention.
balloon framing is taking the studs up to the roof (variable height studs). contractors don't like it but engineers do.
i disagree with most of you.  this may be more a geographical thing than technical, but i would never use roof sheathing on such a steep roof as two diaphragms when i can install a flat diaphragm at ceiling level.  the ridge of this thing is 30' high!  long studs for balloon framing.  if you put the diaphragm where it wants to work, then you only need battens and a metal roof on top.
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.  
done all the time here hokie, in a moderate wind and high seismic area too.  i've seen some pitches of 14/12 in high snow load areas.  still standing.
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