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RE: Stitch plate in double angles, Re: seaint Digest for 24 Feb 2005

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Dear Mr. Lewis: Since I have done a bit of research on double angles and stitches ( my doctoral dissertation topic a few years back) I taught to have my 2-cents in if it helps. In my opinion you are correct in all accounts in your e-mail. My suggestion to solve the problem would be to weld a continuous plate to the bottom of the double angle section (if existing stitches are not sticking out of the tip of angle legs) and turn double angles to a kind of I-shape. Or, if stitches are sticking out, weld two plates, or two smaller angles, one inside each existing angle ,making the angles turn into closed shape triangular or rectangular boxes. You need to check the new section for all failure modes such as overall buckling including flexural torsional buckling (specially if it is still open shape) , segment buckling between the stitches, local buckling, stitch failure, failure of connectors (welds or bolts) between the member and the gusset plate (is this the end connection?) as well as connectors between the gusset plate and the rest of the structure.

If I were to weld plates or angles to an existing member, I would check to see if the member has gravity load in it and what level of stress is there before heating it up by welding a new piece to it. If stresses are high, I would shore the members that are delivering gravity load to this member and reduce or eliminate gravity stresses in it during the welding process. Just to be prudent. One last question: Is the thickness of "one size smaller" angle less than 1/4" specified for 3x3x1/4"?

Finally, may be the best solution is to stick with your guns and do not accept this change. Insist that they take the braces down and replace them with correct members as specified. If they choose this route, they have to make sure that removal and replacement of the wrong members are done safely and do not create any residual stresses or strains that might affect the structure in any adverse way.

Please notice that all of the above is only my opinion, is for information only, and does not necessarily reflect the views of my employer or the file server Seaint where this is sent. If you choose to use any of the information in this e-mail, you are totally and completely responsible for all consequences of such use.
Best wishes.
Abolhassan Astaneh-Asl, Professor
Web: www.ce.berkeley.edu/~astaneh  and www.astaeh.net





   At 12:00 AM 2/25/2005 -0800, you wrote:
From: "Lutz, James" <James.Lutz(--nospam--at)earthtech.com>
To: "'seaint(--nospam--at)seaint.org'" <seaint(--nospam--at)seaint.org>
Subject: RE: Stitch plate in double angles

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Yup
-----Original Message-----
From: Rich Lewis [mailto:sea(--nospam--at)lewisengineering.com]
Sent: Thursday, February 24, 2005 2:28 PM
To: seaint(--nospam--at)seaint.org
Subject: Stitch plate in double angles


I designed a double angle knee brace.  The one supplied was one thickness
size smaller than the one I specified.  The brace has already been erected.
The contractor has asked me if he could add stitch plates to reduce the
unbraced length so that the smaller on will work.  As I see it, the stitch
plate doesn't do anything for increasing the strength of this member.  The
buckling strength of the double angles (3x3x1/4) based on the overall length
of the 2 angles together in the weak axis, not the individual angles (page
1-77 of AISC 9th ed.).  The stitch plates do not restrain the buckling
behavior of the overall member.  It is still pinned at each end and will
buckle in a single mode shape about the weak axis.  I see stitch plates as
being spaced at whatever spacing is necessary to force the overall member to
buckle before an individual angle will buckle.

Am I looking at this right?


Rich Lewis




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