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RE: Truss Diagonals in Compression (AASHTO)
[Subject Prev][Subject Next][Thread Prev][Thread Next]- To: "'seaint(--nospam--at)seaint.org'" <seaint(--nospam--at)seaint.org>
- Subject: RE: Truss Diagonals in Compression (AASHTO)
- From: Acie Chance <achance(--nospam--at)lacsd.org>
- Date: Mon, 24 Apr 2000 16:14:49 -0700
It seams to me that the length to be used is not that cut and dry. It may be the length from the center of the connection to the center of connection, zero stress to zero stress. If we take an extreme example of a truss with a W36 top and bottom chord we are adding more then 36 inches of length to the diagonal member by assuming the brace length from the panel points. If we had a brace of variable cross section, small at the ends and larger at the middle, the actual buckling strength is not based on the thin sections at the ends. It would be conservative to design for the smallest section but not completely correct. Look at chapter 11 of " Guide to Stability Design Criteria for Metal Structures". With all that said some " Engineering Judgement " can be used. You may want the "peer reviewer" to prove his point. Acie Chance -----Original Message----- From: Rick.Drake(--nospam--at)fluor.com [SMTP:Rick.Drake(--nospam--at)fluor.com] Sent: Monday, April 24, 2000 12:11 PM To: seaint(--nospam--at)seaint.org Subject: Re: Truss Diagonals in Compression (AASHTO) Bill: I'm not sure who is "right" in this case, but my practice is to go from panel point to panel point when considering AISC slenderness ratios. I then use this slenderness to determine buckling loads for both the member and the gusset plates. note that the L in the KL/r ratio is defined as the laterally unbraced length of the member. Unless the gusset is REALLY stiff, I don't see how it can provide enough lateral support to be considered a lateral brace. Rick Drake, SE Fluor Daniel, Aliso Viejo, CA *************************** "Polhemus, Bill" <wlpolhemus(--nospam--at)sbinfra.com> on 04/24/2000 10:59:29 AM Please respond to seaint(--nospam--at)seaint.org To: "'seaint(--nospam--at)seaint.org'" <seaint(--nospam--at)seaint.org>, "'structx(--nospam--at)topica.com'" <structx(--nospam--at)topica.com> cc: 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.
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