Return to index: [Subject] [Thread] [Date] [Author]

Truss Diagonals in Compression (AASHTO)

[Subject Prev][Subject Next][Thread Prev][Thread Next]
*You* are right, Bill.  As long as you don't use a shorter distance than 
AASHTO states, you are in compliance with AASHTO specs.  You have looked at 
what AASHTO says, applied engineering judgement that is in compliance with 
AASHTO, and came to a logical conclusion.  Now, if the peer reviewers want to 
absolve you of *all* responsibility, then that is a different story.

BTW, I am still of the "old school" and limit kl/r to 120 in primary members 
and 200 in secondary members.  Buckling happens too quickly and without 
warning to stretch limits.

When I was still in school (1958-59), we (the ASCE student chapter) had a 
presentation on the construction of the arch bridge at Glen Canyon.  We were 
told that the 120 ft long columns that supported the roadway level were 
unbraced to give a clean, uncluttered appearance to the bridge.  In our steel 
design class we had a discussion (initiated by the class, not the instructor) 
about the slenderness of the 120 ft long columns, did some Q&D calculations, 
and concluded that they were too slender to be unbraced.  About a year after 
the bridge was completed, the Bureau of Reclamation went in and installed 
braces on the longer columns because, IIRC, "they were sensitive to the 
wind."  (So much for aesthetics!)

Stick to your guns!

A. Roger Turk, P.E.(Structural)
Tucson, Arizona

Bill Polhemus wrote:

>>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?<<