Trust Web Bracing

[XR250]

Has anyone ever witnessed wood trusses with their webs braced correctly? 90% of the time, all I see is just a straight run of 2x4's attached to the webs with no diagonals bringing the load to a diaphragm. Without the periodic diagonal bracing, the webs can all sideways buckle the same direction. I was recently reviewing a new townhouse for a buyer where the truss webs were not braced properly. I asked that the tract builder to have his engineer design the diagonal bracing. Enclosed is the response from the builder's engineer (yes he is a P.E.).
I have subsequently asked for him to provide calculations as to why the diagonals are not needed. This should be fun.

 

[oldrunner]

Sometimes the 1x4 is connected correctly to the studs in the gable walls. But one has to provide the calc for the connection and the adequacy of the gable wall stud bending to keep the collective braces from buckling. For each braced web member, the force to keep the web member from buckling ranges between 2 to 3 percent of the calculated maximum compressive stress in the web times the number of trusses you are bracing.

 

[dhengr]

XR250:
Ask for the truss supplier’s std. submittal package that they give to the AHJ, the EoR, the Arch., the builder, for approval; and to the builder upon/with delivery of the trusses and to be used for truss erection. This package should have an industry std. literature piece on bracing which is to be followed during erection and for the final structural bracing. It instructs the truss installer to follow the bracing instructions therein, or to get a local PE to design a bracing system for the job. Furthermore, the bldg. dept. should be inspecting for these details, get them involved.

 

[CELinOttawa]

Time will tell, and I am *very* keen to know how this turns out.... BUT, it is likely fine. The 2% bracing force (as mentioned above) is very easy to handle in narrow town home style construction.

Also, you may find that the roof was done as an overall system, and that the diagonal brace is in an adjacent town home. I cannot imagine how anyone passes such construction, but I've seen it before. Such a "party" element should not be allowed to count for all the disparate properties!

 

[XR250]

@CEL:
Yeah, I guess it doesn't amount to much over 7 or so trusses. It is fun just to mess with these people, however.
The tough ones to brace are the flat top chords of piggyback trusses. They carry a bunch of compressive load. I never see those done properly either.
I recently designed a warehouse and required that the truck manufacturer design the trusses such that bracing was not req'd. They had to up some of the web sizes, but it the job alot cleaner.

 

[KootK]

Yeah, I guess it doesn't amount to much over 7 or so trusses. It is fun just to mess with these people, however.

You were on the right track in your original post. The webs aren't braced and their capacity is around 1/4 of what it was designed to be. There may be dozens of impossible to quantify reasons why nothing ultimately goes wrong here but engineering design won't be one of them.

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.

 

[jfmann]

From my experience over many years........ reading trade articles on this subject, reviewing architectural plans, reviewing truss-design submittals, inspecting roof-truss assemblies and even preparing reports for legal cases involving truss-assembly collapse and personal injury........it is quite (painfully) clear that proper "design" of temporary (especially) and permanent truss bracing occurs about as often as that "blue moon".

Some comments;
Here in New Jersey (as well as Pennsylvania and just about every location I have been involved with)...until "recent" changes in regulations, architects invariably took the position that they were not responsible for design of permanent truss bracing.........irrespective of what had been relatively clear code provisions instructing otherwise.

Although, several years ago per my recollection (maybe more)........code provisions were "modified" to allow instructions for permanent truss bracing to be communicated to the builder by truss manufacturer.......I doubt that this has improved as-built installation of truss bracing......though of course my observations are limited in this regard.

Building inspectors ......at least in this area.......rarely bother to check truss bracing, other than perhaps a brief "look".

Primary reason there is not (as best I can tell) more actual damage due to lack of properly-designed bracing is abundance of safety factor.......with design loads, truss-design methodology and material properties.

John F Mann, PE

http://www.structural101.com

 

[jfmann]

To clarify......architects continue to claim they have no responsibility for design of truss bracing (either directly by them or via consultation with structural engineer).....it is just that now they can point to revised regulations that (as I believe is the case; someone correct me if mistaken) allow them to avoid responsibility.

John F Mann, PE

http://www.structural101.com

 

[XR250]

Let's see if we can name some of the reasons we don't see more failures (in addition to jfmann's)…
1) Unintentional support from interior walls
2) Maybe half the webs want to buckle one way, half the other way and the forces cancel
3) In a narrow building, the roof diaphragm transferring the load to the gable ends



I am always amazed at the redundancy in a trussed roof system. I have seen plenty of instances where the two center webs of each truss were cut to create attic space and little if any sagging was observed.

 

[medeek]

The BCSI-B3 sheet produced by the SBC is fairly comprehensive in its coverage of permanent truss bracing. Is this the industry standard for truss bracing? or is there something more?

http://support.sbcindustry.com/images/publication_images/b3.pdf

As engineers I don't think we typically design MPC trusses or their bracing.

A confused student is a good student.

 

[jfmann]

Up until "new" regulations........which, as noted by medeek, resulted in BCSI-B3, which is an attempt to fill void caused by lack of responsibility by architects (due mostly to lack of knowledge than anything else)........TPI codes (referenced by building codes) had specified that "building designer" was to take full responsibility for design of permanent lateral bracing. For some (few) projects, architect (or builder) would assign this task to professional engineer (PE)........more likely large developments.

John F Mann, PE

http://www.structural101.com

 

[KootK]

I helped fabricate trusses like these at one time. Consider this representative anecdote:

On a larger span truss, the location of the peak might be off by as much as three to six inches. As the truss supplier, what you do is ensure than none the trusses ever gets flipped before they are bundled. You also tag the right ends (or left) so that the installers do not mix them up. The truss supplier does these things because a consistent peak offset won't get noticed but a peak that oscillates randomly about its theoretical position sure as heck will. Precision is much more important than accuracy.

The moral of this story: it's far more likely that defects and tolerances will be systemic rather than random.

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.

 

[bigmig]

The original poster is absolutely correct. I have seen good engineers casually cast this bracing requirement aside thinking that the truss guys take care of it, just like this story references. You need to brace your stiffened webs so they can't all go at the same time.

This is uncommon in practice, both with builders and most engineers, primarily because no one has taken a second to point out to the engineers that their trusses are in a vulnerable state of near collapse under design loads.

If you start reviewing your truss shop drawings line by line, with a calculator, you will end that exercise with an amazing increase in lack of faith for the truss engineer stamping their drawing. 90% of the information is input by a truss jockey on summer break, and glanced over by a guy in California doing the red light green light "the software told me so" review.

The moral is, the engineer of record needs to detail this x-bracing for braced webs on his drawings, and verify that the detail is installed by walking out there and looking at it.

 

[manstrom]

Careful,

The truss designer is responsible for the lateral bracing of their trusses.

YOU are responsible for the global bracing of the structure, i.e. the diagonals. The BCSI has an article on it. It is the same way that you are responsible for the gable end bracing.

Throw a few typical details on your sheets and you should be good in the future. On this project, have them throw some diagonals up there.

When I am working on a problem, I never think about beauty but when I have finished, if the solution is not beautiful, I know it is wrong.

-R. Buckminster Fuller

 

[XR250]

@Manstrom;

I am only a consultant to the buyer - not the building designer.
FWIW, standard details can be a problem as the loads in the webs, chords etc. are different on each job.

 

[KootK]

YOU are responsible for the global bracing of the structure, i.e. the diagonals.

I'll have to check out this BSCI article. I'm surprised by this. As EOR, I might know that diagonals will be required ahead of the truss shop drawings but I won't know where.

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.

 

[woodman88]

Which Building Code is the truss design under?

Under the 2012 IBC section 2303.4.1.2 "Permanent individual truss member restraint. Where permanent restraint of truss members is required on the truss design drawings, it shall be accomplished by one of the following methods:
1. Permanent individual truss member restraint/bracing shall be installed using standard industry lateral restraint/bracing details in accordance with generally accepted engineering practice. Locations for lateral restraint shall be identified on the truss design drawing..."

So a design is not required, as long as the above is met.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.