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
> our design calculations for retrofitting some overhead sign supports
> on a
> freeway project where the existing signs are begin replaced with very
> electronic message boards.
> In reviewing the existing structure, a truss supported on large
> columns, it was noted that while the compression stresses in MOST of
> diagonal truss members are fairly low, these members are comprised of
> 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,
> many years, and since we're putting much heavier signs on these
> and since my "okaying" smaller members means I think they'll work fine
> the new loading, AND since a plaintiff's attorney would have a FIELD
> with me in the witness box should one of these signs for whatever
> decide to part company with its support and attempt to occupy the same
> space-time as a human being passing unsuspecting in an automobile
> Well, I think you get my drift.
> Problem is, I typically calculate the "effective length" of truss
> from panel point to panel point. The peer review guys claim that "the
> 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
> aren't infinitely rigid.
> So, what do you think? Who's right?