Need a book? Engineering books recommendations...

Return to index: [Subject] [Thread] [Date] [Author]

RE: Residential Truss Bracing

[Subject Prev][Subject Next][Thread Prev][Thread Next]
We have similar difficulties.  The TPI-95 document is referenced by the state statues here in Florida. By default the document requires the EOR to design lateral bracing as located by the truss design engineer.  The individual truss drawings should indicate the location of any required lateral truss bracing. Diagonal bracing and gable end bracing seems to fall to the EOR entirely.
However, the document also states that it is not intended to superceed the contract documents (drawings/specs/etc).  This can be an important clause to defending your intent on how the design responsibilities/liabilities should be assigned. 
The truss designer/supplier, however, may argue that they are only responsible for doing what is in their sub-contract and are not bound by the contract documents.  It would then fall to the GC to make sure that the requirements of the contract documents had been put into the truss-designers sub-contract. There is a helpful discussion about this in the May 2003 Structure magazine.
15                               Message:0015                           15
From: "Thor Matteson, SE" <
To: "SEAINT" <
Subject: Residential truss bracing

Some earlier comments on this topic reflect my experience as well.  This is
one of those issues that has bugged me for a long time, mostly because the
truss industry seems to have a really different idea of how truss bracing
systems are designed and installed than what seems to actually happen in the
real world.

Clients for residential projects seem to think my job is completely finished
when they get a building permit.  I cannot recall ever seeing a set of truss
calcs for a house, even though our notes say that they are to be submitted
for our review.  The truss engineering notes ALSO usually say that the
"building designer" (that's us) should be given a copy of the calcs.

When I became aware that the Truss People (TPI, WTCA) expected me to design
permanent lateral bracing (PLB) for truss designs, I immediately put a
"weasel note" into my Structural Notes.  The note says that if PLB's are
required by the truss engineer, then the _truss manufacturer_ will pay the
costs to design them, or the PLB's will be designed by the _truss_ engineer.
The latter conflicts with ANSI/TPI standards, but seems more likely to
result in PLB's actually getting designed.

Question for Building Officials:  If a house otherwise meets the
Conventional Light Frame requirements of the code but has a trussed roof,
who is the "building designer" that is responsible for PLB design?  The
contractor? The framing sub?   IMO, a trussed roof  is NOT conventional
light-frame construction--is there any mention of metal-plate connected wood
trusses in the conventional framing section of the codes??

Our local jurisdiction only requires that truss engineering be on site at
the time of the framing inspection.  IMO--and I believe according to the
Codes--the truss engineering constitutes a "deferred submittal" (per IBC, similar in the UBC/CBC) that should come from the building
designer's office with the designer's approval.  (Oops, "approval" is a bad
word... )

I read an article a few years ago where some "piggy-back" trusses over a
gymnasium collapsed under only 5 inches of snow.  I have seen piggy-back
trusses that had only about a third of the PLB's installed that the calcs
called for, and no diagonal braces at all--and the builder said that was how
they always installed them!  This from a contractor with 20 or so years of
experience, some of which was in Southern California--where all contractors
know _everything_ about building.

Truss roofs with proper PLB's (and diagonal bracing for the PLB's) installed
often look more complicated than a stick-framed roof.  Maybe that's why
builders tend to leave out the bracing--it looks like overkill!

The plated truss industry should take a hard look at how truss roofed
systems are _really_ constructed.  Passing the PLB buck to some phantom
"building designer" (that usually does not exist, in the case of residential
trusses) does not help the homeowner whose roof has just collapsed.

So, have any of you Colorado engineers seen a correlation between roof
failures and PLB mis-installation?  Or anyone in New England, after last
weeks storm??


Christopher A. Banbury, PE
Vice President
Nicholson Engineering Associates, Inc.
PO Box 12230, Brooksville, FL 34603
7468 Horse Lake RD, Brooksville, FL 34601
(352) 799-0170 (o)         (352) 754-9167 (f)