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# RE: Trussed arch effective length

• To: <seaint(--nospam--at)seaint.org>
• Subject: RE: Trussed arch effective length
• From: "Haan, Scott M." <HaanSM(--nospam--at)ci.anchorage.ak.us>
• Date: Wed, 19 Jan 2005 17:44:14 -0900

I found out I was slightly wrong and that the girts brace the interior
and exterior chords.  I agree that the length for compression strength
out of plane should be the distance between girts. I can believe the
continuous compression members could be considered fixed in the plane of
the web at the panel points and could use a K=.5 in that direction and
the braced length would be the distance between panel points.  In the
narrow direction K may be .5 but the braced length should be the
distance between girts.  The designer indicated that they have done a
finite element analysis that shows the web members brace the chords out
of plane at panel points and I look forward something that seems
counterintuitive to me.

I only have the 4th edition and it has stuff on latticed columns in
plane but have not read the whole thing yet.

Scott Haan.

-----Original Message-----
From: Gale45man [mailto:gale45man(--nospam--at)yahoo.com]
Sent: Wednesday, January 19, 2005 3:55 PM
To: seaint(--nospam--at)seaint.org
Subject: Re: Trussed arch effective length

For the top chord, it doesn't make sense to me to use out-of-plane KL =
1/2 the distance between the panel points.  I'd use KL = the distance
between the girts.

My first pass at evaluating stability of the entire
arch, including the bottom chord and web assembly,
would be a 2nd-order elastic analysis, perhaps with
expected out-of-straightness included in the model.

Galambos "Guide to Stability Design Criteria for Metal Strutures" 5th
edition may have some pertinent info.
There's a chapter on arches.

d @ v e e v @ n \$

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