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RE: Stitch plate in double angles

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If the stitch plate runs the full length, then you change the member characteristics.  Now, instead of a single angle buckling, the double angle and plate are buckling and the member properties should be applied as such.  This also assumes that the combo section can withstand compression load.  If it can't, then you've still got nothing.  Then, you can look at reducing the length of the single angles in compression failure between the stitch plates and the combined section at the stitch plates.  At first glance, and reasoning, it seems this would work.  However, I haven't delved into it, so I can't say for certain.
 
The question that I have is, how difficult would it be to get new angles cut, drilled, and delivered???  And how many of them are there???  A replacement section, even if it is bigger, can still be gotten locally, most likely.  Try having the contractor look at the overall saving here.  Does he really think he can save money, and time, to have you go back to the office run the calculations to determine the correct spacing and size of the plates (which could become a trial and error situation) and then have to go through the effort of having plate or bar cut, get the correct locations on the members, and then go through the effort of field welding???
 
Either way, it seems that this becomes the responsibility of the fabricator as they supplied the wrong member size.  Was this an oversight on the shop drawings???
 
Personally, I'd have them replace the member.  It's the easiest solution right now and probably the less costly.  Then again, I am not in your situation and I don't know what you are dealing with entirely.  The point is, there is more than one way to skin a cat.
 
Dave Maynard, PE
Gillette, WY
-----Original Message-----
From: Rich Lewis [mailto:sea(--nospam--at)lewisengineering.com]
Sent: Thursday, February 24, 2005 3:28 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