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RE: Truss Diagonals in Compression (AASHTO)

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Bill-
Let's see if I understand the effect of your "peer (?) reviewer's" position:

	If you go from member end to member end instead of panel point to
panel point to figure buckling capacity you can ignore the statics that
there is a force at one panel point that must somehow get through the
deformed set of gusset plates (which apparently aren't subject to buckling),
through the member (which is probably stiffer than the gusset plates but can
buckle because the "peer reviewer" has so decreed), through another deformed
set of gusset plates (which also aren't apparently subject to buckling) to
the next panel point where the force is passed to the rest of the structure.


It must be an example of the new math they are teaching the kids these days.
:<(  

Myself, I prefer panel point to panel point.

Regards,
Bill Cain, S.E.
Oakland  CA


	-----Original Message-----
	From:	Ritter, Mike [SMTP:mritter(--nospam--at)lgt.lg.com]
	Sent:	Monday, April 24, 2000 12:41 PM
	To:	'seaint(--nospam--at)seaint.org'
	Subject:	RE: Truss Diagonals in Compression (AASHTO)

	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.
	>