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steel gusset-to-brace connection...ignore beam end moments

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I forwarded a question from this list to the AISC Solutions Center.  
The issue: how to deal with fixed end moments at beam/column 
gusset plate connections for diagonal bracing.  The question was 
from pgce(--nospam--at)maine.rr.com and, as far as I know, listizens never 
answered his question.

AISC's reply: KISS-ignore the rotational rigidity of the connections 
and assume beams are pin-ended (at least for connection design), 
regardless of actual rigidity.  

My comments:  

In Gross's report on three connection tests (mentioned in the 
Thornton article cited in AISC's reply), they found the gussets gave 
"nearly fixed (beam end) conditions."   In those tests, fixed end 
moments from beam gravity loads were not included, but end 
moments due to frame action were included.   The way the test 
was arranged, frame action deformation tended to have a beneficial 
effect. Gross and Thornton did not comment on the fact that fixed 
end moments from gravity loads on the beam would have had the 
opposite effect. Furthermore, frame action may not always produce 
the beneficial effects noted in the tests.  But...we ain't seen no 
dead bodies....

Anyway, here is the correspondence:

(I shortened the original question and used numerous abbreviations 
so that the questions complied with the AISC 1000 charater limit.)


------- Forwarded message follows -------
From:           	AISC Steel Solutions Center <solutions(--nospam--at)aisc.org>
To:             	"'devans(--nospam--at)tnh-inc.com'" <devans(--nospam--at)tnh-inc.com>
Subject:        	RE: Comments:  Design-Connections: 
Date sent:      	Fri, 1 Nov 2002 09:57:37 -0600 

If you look on page 13-4 of the 3rd Edition LRFD Manual, there is a 
description of the Uniform Force Method, which is self explanatory. 
The object is to convert the detail to a statics problem while 
keeping it simple.  So far, we have had no complaints that it 
doesn't work and have hundreds of buildings done that way.  

For a more in-depth discussion of the Uniform Force Method and 
the assumptions made, we would recommend reading the following 
Engineering Journal paper:  

Connections: Art, Science, and Information in the Quest for 
Economy and Safety	 William A. Thornton	 12/01/1995	  
AISC Eng Journal

Bill Liddy 
AISC Solutions Center 

American Institute of Steel Construction, Inc. 
One East Wacker Drive, Suite 3100 
Chicago, IL  60601-2001 

-----Original Message-----
From: devans(--nospam--at)tnh-inc.com
Sent: Wednesday, October 30, 2002 12:03 PM
To: solutions(--nospam--at)aisc.org
Subject: Comments: Design-Connections: 

The following question, from the SEAINT listserv, deals w/ a gusset
welded to col & bm. The issue also arises w/ a bolted/welded conn:
LRFD Fig 13-2(a), shows no external moment on the right-hd bm 
end, whereas I expect an end moment due to the rotational rigidity 
of the large gusset and extended gusset/bm/col conn. Obviously, 
there's more restraint if the gusset is welded to both bm & col, but 
there's substantial restraint even if the gusset is bolted to the col, 
w/ the LRFD fig geometry. Would appreciate response to the 
original Q & to my comments. Thks, D Evans


Frm:pgce(--nospam--at)maine.rr.com 
To:seaint(--nospam--at)seaint.org
Sub:Gusset to Brace Conn 
Sent: 22 Oct 02 

"Analysis & design of conns" by W Thornton & T Kane, ch 2, fig 
2.8 Ex. 2 brace conn design shows a "concentric" brace conn w/ 
gusset welded to bm & W col flange. Considering rotational rigidity, 
gusset stresses from bm end rotatation should be accounted for, 
yet they're not addressed in the example.


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