Gable End Bracing

[pioneer09]

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.

 

[jayrod12]

I would argue your brace length for slenderness calcs would be the truss spacing on the diagonal so essentially 3ft. And you would just splice the braces as required to keep them at regular length pieces of lumber.

 

[XR250]

I usually balloon frame them. The analysis and detailing of gable braces is always sketchy and are never installed or engineered properly in my experience. There is no free lunch. They put a huge up or down load on the attached truss and gable end. Fishing them through the truss webbing is another story. Lots of shiny metal to add (as Kootk would say). All those loads have to get down to the foundation somehow. I balloon framed a 29 ft. tall one recently using 2x10's. I have also detailed a plywood ceiling under the trusses to form a deep wind beam by having those trusses designed 1/2" shorter than the rest.

 

[DaveAtkins]

jayrod12,

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

 

[jayrod12]

I understand exactky what he's saying. My advice still stands. he still needs horizontal bracing but most (95% or more) trusses have similar vertical webbing from truss to truss. So you fasten the diagonal braces to each truss vertical on the way by. When your 8ft piece runs out, add another one.

It's done all the time here.

 

[KootK]

The analysis and detailing of gable braces is always sketchy and are never installed or engineered properly in my experience. There is no free lunch. They put a huge up or down load on the attached truss and gable end. Fishing them through the truss webbing is another story.

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.

 

[jayrod12]

It still surprises me when other engineer's look at a roof plan and don't understand why the bracing is called off. The typical response "It's a pre-engineered system, shouldn't the truss supplier be designing and detailing the 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.

 

[XR250]

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 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.

 

[CELinOttawa]

Jayrod: Early in my career I came very close to being bit by this steaming pile of liability avoidance you point out, and it is applicable to MANY, not just Truss, pre-eng systems.

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.

 

[jayrod12]

CEL how do you justify the fees to design your entire wood frame roof including the trusses? The truss design alone would add such an enormous amount of fees that we'd rarely get the work.

 

[CELinOttawa]

I don't design trusses, only rafters. We have pre-worked tables and canned CAD that makes it nearly no additional work. We also are rarely involved in small residential, most jobs are very large ($0.5M construction cost plus) for our residential work.

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.

 

[pioneer09]

To clarify, this is a very large gable end on a church with an approximate 37' tall wall at the peak. Balloon framing seems unrealistic with this length of stud.

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

 

[jayrod12]

You can do horizontal bracing in lieu of a ceiling diaphragm. They perform the same function if designed and detailed correctly.

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 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 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?

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?

 

[CELinOttawa]

With that large a gable, maybe look at balloon framing with PSL studs or some other alternative timber? Maybe deep CFS channels?

 

[XR250]

To clarify, this is a very large gable end on a church with an approximate 37' tall wall at the peak. Balloon framing seems unrealistic with this length of stud.

That is a big ass gable end. Probably should have some steel or glulam wind columns in it or use a plywood ceiling (that is not unheard of, actually).
I doubt you will get any other system to work out.
You could possibly balloon frame with LVL's for the taller portion.

 

[pioneer09]

Balloon framing would be nice, but like you note this is a very large gable end. Can you even get LVL's in a length this long? Multiple ply studs to achieve this extreme length seems unrealistic for conventional framing practices. It sure looks like a plywood ceiling is the rational approach.

 

[XR250]

LvL's go to 50 ft., I believe
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 :>

 

[CELinOttawa]

PSL, LSL, LVL, many of these products are available in very long spans... It is a transport problem, not a manufacturing problem.