Gable End Bracing
Gable End Bracing
(OP)
For a wood framed building, the typical design in our office is to run diagonal braces at the gable ends. This provides a brace and load transfer mechanism at the hinge point when a gable end truss sits on top of a stud wall. This works fine for lower sloped and minimal width buildings.
A recent project has arose where the height difference from the top plate to the roof peak is 22'. If using a 45 degree brace, this would amount to approximately a 31' long brace. This seems impractical and also requires very large members to meet slenderness requirements per the wood design manual. An option would be to have full height gable end stud framing, however this would become very expensive due to overall stud length and connections that will be required to achieve this. Wondering what others have done for design of large gable ends.
A recent project has arose where the height difference from the top plate to the roof peak is 22'. If using a 45 degree brace, this would amount to approximately a 31' long brace. This seems impractical and also requires very large members to meet slenderness requirements per the wood design manual. An option would be to have full height gable end stud framing, however this would become very expensive due to overall stud length and connections that will be required to achieve this. Wondering what others have done for design of large gable ends.






RE: Gable End Bracing
RE: Gable End Bracing
RE: Gable End Bracing
I think pioneer09 is talking about braces which slope UPWARD from the top of the wall to the roof.
pioneer09,
jayrod12 is talking about horizontal braces in the plane of the ceiling (truss bottom chords). I think this is a good solution.
Another solution, which I have used only once, is to create a plywood (or OSB) ceiling.
DaveAtkins
RE: Gable End Bracing
It's done all the time here.
RE: Gable End Bracing
I agree with this assessment completely. I used to work for the Wood Truss Council of America and helped develop some of their documents related to this subject. I also used to work as wood truss designer so I've seen a lot of this condition -- or the absence of it -- in the field. The diagonal bracing stuff is mostly wishful thinking in my estimation.
Some other ideas we've kicked around here before:
1) The plywood sheathed horizontal truss mentioned above.
2) A real horizontal truss with webs and plates and stuff.
3) A horizontal beam (girt) at the top of the wall. I've actually seen this executed a couple of times.
4) A shabby, sheathing interrupted, un-designed gypsum sheathed horizontal truss (just the ceiling drywall).
Due to the utter hopelessness of trying to implement #1-#3, I've taken to going with #4. I have horizontal blocking installed at the top of the wall two truss spaces in and then hope for the best. At least that way there's a load path, even if it's competence is suspect.
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.
RE: Gable End Bracing
So I then send them a copy of the BCSI document and direct them to the permanent bracing section. They're shocked to find out that the bracing is their responsibility.
RE: Gable End Bracing
I do the same on un-tall gable ends, but my conscience gets to me on the bigger ones. Balloon framing is my go-to solution. Unfortunately, I seem to be one of the only engineers in our area who even thinks about this aspect of a structure and I lose business accordingly.
RE: Gable End Bracing
My approach is to design everything by traditional design, show the system as completely engineered by us, and then include the following:
• THE USE OF A PRE-ENGINEERED ELEMENT TO REPLACE A COMPONENT OF THE SYSTEM SHOWN IS ACCEPTABLE ONLY WHERE REVIEWED AND ACCEPTED IN ADVANCE BY [the engineer] IN WRITING. WHERE SUCH SYSTEM IS SELECTED, THE DESIGN RESPONSIBILITY SHALL BE BY THE MANUFACTURER, INCLUDING ALL CALCULATION AND SPECIFICATION REQUIRED TO ENSURE SUFFICIENT STRENGTH, STABILITY AND STIFFNESS. UNDER NO CIRCUMSTANCES SHALL [the engineer] BE HELD RESPONSIBLE FOR ANY DESIGN, SPECIFICATION, COMPONENT REVIEW OR LOAD PATH REQUIRED FOR SUCH PRE-ENGINEERED SYSTEM. WHERE A SYSTEM REQUIRES AN ENGINEER OTHER THAN THE MANUFACTURER TO BE RESPONSIBLE FOR SOME ASPECT OF THE DESIGN OR INSTALLATION, THE USE OF SUCH SYSTEM IS PROSCRIBED. WHERE FOUND ON THE WORKSITE, [the engineer's] LETTER OF STRUCTURAL REVIEW AND RESPONSIBILITY WILL EXCLUDE SUCH ELEMENTS.
RE: Gable End Bracing
RE: Gable End Bracing
You would also be surprised how few of our jobs are fixed fee. I've discussed this on the board before, but effectively we turn down the jobs where this would be a problem. I like to think of them as someone else's head ache.
If you think about it, a raftered roof with a ridge beam quickly leads itself to canned solutions. They are very simple.
RE: Gable End Bracing
With regards to the diagonal bracing being attaching to the vertical members of the trusses as jayrod12 notes. Will this not induce biaxial bending in the vertical truss members that was probably not accounted for in design?
I thought I had once read in the IBC that gypsum framed diaphragms are not allowed when used in conjunction with plywood/osb sheathed diaphragms. Anyone recall or come across this?
It seems that an osb ceiling diaphragm is truly required as some have mentioned above. This becomes very costly and an unheard of construction practice.
DaveAtkins mentions horizontal braces at the truss bottom chord. Wouldn't you still need to transfer this load into a diaphragm at the ceiling like the plywood/osb sheathing mentioned above?
S
RE: Gable End Bracing
I don't see biaxial bending, but there is a small amount of induced bending. If your braces are truly continuous (or close to, i.e. offset for splice) then I don't have much of a concern on that front. The worst is the significant uplift at top chord where the brace is trying to dump the lateral load into the roof sheathing.
I'm not sure about the IBC, but the Canadian code indicates you can't count for the drywall strength when installed overtop of wood panel. But provided they're installed on different members (i.e. wood panels on top chord, gypsum on bottom chord) then they can both be counted on. Otherwise when would you use gypsum diaphragms?
RE: Gable End Bracing
RE: Gable End Bracing
I doubt you will get any other system to work out.
You could possibly balloon frame with LVL's for the taller portion.
RE: Gable End Bracing
RE: Gable End Bracing
I was in a warehouse recently with a similar sized gable end. No ceiling, just a few diagonal braces. Don't know why it has not had a problem yet. Shows how much we know :>
RE: Gable End Bracing