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RE: Truss Diagonals in Compression (AASHTO)

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Bill,

I always go to the panel points!

Mike Ritter, PE

> -----Original Message-----
> From:	Polhemus, Bill [SMTP:wlpolhemus(--nospam--at)sbinfra.com]
> Sent:	Monday, April 24, 2000 1:59 PM
> To:	'seaint(--nospam--at)seaint.org'; 'structx(--nospam--at)topica.com'
> 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.
>