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Re: Truss permanent bracing

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I've read that about the web members in compression
and that the TPI has tried to delegate that
responsibility back to the engineer.  In at least one
case, I have asigned this responsibility directly to
the truss manufacturer in the design spec, knowing
full well that I probably won't have a chance to see
those forces.

(On Soap Box)
If the web members sized and laid out by the truss
manufacturer are overly slender, then it should be the
truss manufacturer's decision to either strengthen
them or to brace them.  Pushing that responsibility
back on the engineer will ultimately cost the
contractor additional work because they can't bid it
up front, and it will force the engineer to perform
additional designs after the project has been bid.

But that argument isn't new, and I believe I've heard
it here before.
(Off soap box)

Jim

--- Neil Moore <nmoore(--nospam--at)innercite.com> wrote:

> Actually, the truss manufacture identifies the web
> members that have to be 
> braced for buckling.   He should be giving you the
> compressive force in 
> each of these web members.   You, in turn, take some
> rule of thumb 
> percentage of that force to determine the required
> load to resist the 
> buckling.  From there you multiply this force times
> the number of trusses 
> to determine a total load that somehow has to be
> transferred either to the 
> building or to the roof or to the gable wall.   Or
> you add 2x's to each of 
> the subject web members in the field to eliminate
> the buckling problem.
> 
> This is one of the little gotcha's that is in the
> TPI code.  The UBC refers 
> you to the TPI code.
> 
> I just noticed that the 2003 IBC requires in section
> 2304.4.1 item 17 that 
> the truss shop drawings are to provide the maximum
> axial compressions 
> forces "to design the size, connections and
> anchorage of the permanent 
> continuous lateral bracing".   The code didn't say
> who was to design this, 
> but you will probably find it in the TPI code.
> 
> Maybe Charley Truax can fill you in better on this
> than I have.
> 
> Neil Moore, S.E.
> neil moore and associates
> 
> 
> 
> At 10:28 AM 8/17/2005, you wrote:
> >Is it valid to summarize that a simple and
> rectangular
> >single-story hip roof structure with shear wall
> >construction would probably not require any "truss
> >permanent bracing"?  As I interpret it, the
> permanent
> >bracing is the structural component(s) required to
> >transfer loads (generally lateral) from one point
> in
> >the structure to another through the roof trusses. 
> In
> >this case, all lateral load goes directly to the
> >diaphragm.
> >
> >I agree with the argument that in-truss bracing
> >required to brace compression members from buckling
> is
> >the responsibility of the truss manufacturer.
> >
> >An inspector has identified a building design plan
> as
> >deficient because no such bracing is defined.  (The
> >project was designed and detailed 100% by the
> >architect, but that's another argument.)
> >
> >Regards,
> >Jim Wilson
> >
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