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Re: Question on wood Roof Trusses

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WEB BRACING ANCHORAGE TO WHAT?

The problem is that there are two phases to the design of any of the
commercial truss systems.  The truss designer indicates in his calculations
which web members must be braced and his small sketches that come to the
job site usually will indicate a 1x4 in the proper locations.  

The second step is for the engineer of record for the project to review the
truss design, note where the web members have to be braced and extract the
compression force in these web members from the truss designers
calculations.  Once the structural engineer has this value, he then will
determine the force required to keep the web member from buckling.  Usually
this is about 2% of the compressive load.  Then he counts the number of
trusses, multiply this times the 2% compression force and has an
approximate value for what the 1x4 web bracing members have to transfer to
an anchor point.

Our local county structural committee has discussed this problem.  In the
past, truss calculations and layouts were not being properly reviewed by
many of the local engineers.  The county started enforcing this a few years
ago.  At our last meeting, one of our prominent structural engineers said
he was amazed at the mistakes and problems that he has found in the
submittals by the truss companys.  (he didn't specify which one's, as there
are a number of them here).  Problems such as the final bracing points and
forces were easy; problems such as the trusses coming down on bearing walls
that weren't bearing walls was something else.  Problems with the proper
loadings, ect, were also found. 

Section 2321.1 of the 97 UBC specifies that the design of trusses will be
per ANSI/TPI 1-1995, which many people don't have.  In the TPI  code(Truss
Plate Institute) is the language about who's responsible for what, as I
stated in my earlier email. 

The dilemna here is that in the design stage, the E.O.R. won't have this
information.  This may have to be performed after the actual contract is
awarded.  A similar problem exists in sprinkler systems when the
subcontractor submits his design, which may be nothing like what was
envisioned during the design phase.

Make sure your client knows that there may be additional fees to perform
this work.


Neil Moore, S.E.


At 12:29 PM 1/12/2001 -0500, you wrote:
>Until the plated truss industry will provide me with the forces to be 
>resisted and locations of the braces while the project is still in the
design 
>stage, I am going to require that the truss designer be responsible for the 
>design and detailing of all bracing that is required as a condition of
design 
>of the truss.
>
>While the plated truss industry is reluctant to admit it, there *are* 
>self-equilibrating methods of bracing compression members that do not have 
>to be carried down into the basic structure.
>
>I would be very hesitant to modify anything that the truss designer
provided, 
>since they could then point to the modifications and say that that was the 
>cause of a problem.
>
>I have in my files (somewhere) a TPI addendum/letter, ca. late '60's, early 
>'70's, that says that the building designer is *not* responsible for bracing 
>that is a condition of the design; that the truss designer is responsible
for 
>this.
>
>A. Roger Turk, P.E.(Structural)
>Tucson, Arizona
>
>Neil Moore wrote:
>
>>>It appears that you have discovered one of the problems with the truss
>industry.  These member are supposed to be braced and this usually appears on
>the truss company's drawings that come to the site.  Apparently these were
not
>installed.  What many engineers that specify prefabricated trusses don't know
>is that THEY are responsible for the bracing of the web members.  The truss
>company design will indicate what web members are to be braced, but the
>ENGINEER (if there is one) has to complete the bracing design.  That is,
>possibly being braced off to the gable walls or strutted up or down to roof
>diaphragm or to walls below.  
>
>>From an older commentary in  the TPI-85 publication:  "The need for and
>location of lateral bracing that may be required to reduce the buckling of
>individual truss members is determined as part of the wood truss design
and is
>the only requirement for bracing that will be shown on the truss design. 
>Lateral bracing details, including method of connection and transfer of
member
>buckling forces to the structure, are to be determined by the building
>designer."  ...... there's more.
>
>In your case, you can probably add another 2x4 to the compression webs to
form
>a tee section.  The installation should be done with care to prevent
spliting.
>
>
>Neil Moore, S.E.<<
>
>