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RE: Truss Diagonals in Compression (AASHTO)

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Bill,

The first truss that I designed for a project used a graphic model that was
drawn to scale and the forces were determined by measuring the diagram with
a scale.  (The point is that I have been designing them for a while.)  I,
too, assume the length to be panel point to panel point.

That said, the whole principle of the "special concentric braced frame" is
to assure that the gusset plate does NOT provide lateral moment restraint,
and there are checks to assure that the member in fact acts as if it is
pinned at the gusset for out of plane buckling.  The commercially available
design programs predicate the design on the panel point to panel point
dimension.

If a diagonal extends past the flexure point (as defined for a "special
concentric braced frame") the restraint is greater than a theoretical pin,
but it is by no means infinitely rigid. 

Regards,
Harold Sprague


> -----Original Message-----
> From:	wlpolhemus(--nospam--at)sbinfra.com [SMTP:wlpolhemus(--nospam--at)sbinfra.com ]
> Sent:	Monday, April 24, 2000 10:59:29  PM
> To:	seaint(--nospam--at)seaint.org
> Subject:	Truss Diagonals in Compression (AASHTO)
> 
> I'm in a sort of "dispute" with a "peer review" engineer. They're checking
> our design calculations for retrofitting some overhead sign supports on a
> freeway project where the existing signs are begin replaced with very
> heavy
> electronic message boards.
> 
> In reviewing the existing structure, a truss supported on large concrete
> columns, it was noted that while the compression stresses in MOST of the
> diagonal truss members are fairly low, these members are comprised of
> single
> angles, only 2.5" x 1.5" (a non-standard size, leading me to wonder if
> they're cold-formed). Since the l/r value I calculate for these members is
> greater--in some cases much greater--than 200, I called for them to be
> replaced with larger "standard" size angles.
> 
> My rationale is that, even though such members have "worked" for lo, these
> many years, and since we're putting much heavier signs on these supports,
> and since my "okaying" smaller members means I think they'll work fine
> under
> the new loading, AND since a plaintiff's attorney would have a FIELD DAY
> with me in the witness box should one of these signs for whatever reason
> decide to part company with its support and attempt to occupy the same
> space-time as a human being passing unsuspecting in an automobile
> below....
> 
> Well, I think you get my drift.
> 
> Problem is, I typically calculate the "effective length" of truss members
> from panel point to panel point. The peer review guys claim that "the
> code"
> allows you to go from actual end of member to actual end of member. I
> disagree strenuously with this, since I happen to know that gusset plates
> aren't infinitely rigid.
> 
> So, what do you think? Who's right?
> 
> Thanks.
>