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RE: Truss Diagonals in Compression (AASHTO)

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If it were a double angle, the gusset would much stiffer than the angles
in one axis, and using the member length for "l" may be reasonable,
however, in the other direction, perpendicular to the plate, I would use
the distance between panel points for "l".  Since you have an single
angle, and neither major axis is parallel to the gusset plates, I would
use the panel to panel point distance for "l" in both directions.


Charles F. Espenlaub, III, P.E.
Martin-Espenlaub Engineering



-----Original Message-----
From: Polhemus, Bill [mailto: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.