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Re: Brace connection at mid-point

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Bill,
 
You might want to look at the most recent SEAOC Seismic Design Manual, Volumn III that has steel CBF design examples (based upon the 2000 IBC  I think).  It does talk about the x bracing and determining the appropriate length factors based upon the end conditions of the brace at the beam/column connection (pinned or fixed connection) and at the "X" brace cross point (pinned or fixed connection).   There are some illustrations regarding what is considered to be fixed or pinned connection at each of these locations. It is possible to make the one brace member which is interupted by the other brace continous by using lap plates each side of the brace (fixed end condition) instead of a single knife plate through the continuous brace member (pinned connection).
 
I would agree about concerns over the effectiveness of the "pinned" x brace midpoint connection and the tension brace stabilizing the compression brace member. 
 
Hope this helps.
 
Mike Cochran
 
 
-----Original Message-----
From: William.Sherman(--nospam--at)CH2M.com
To: seaint(--nospam--at)seaint.org
Sent: Fri, 9 Mar 2007 7:38 PM
Subject: RE: Brace connection at mid-point

Thank you, Harold, for responding.  I had in fact read these references.


And in fact, these references support my concerns: 

Both the "theoretical study" and the "experimental study" state that
"the effective length of the compression diagonal is 0.5 times the
diagonal length, when the diagonals are continuous and attached at the
intersection point."  Note the words "when the diagonals are continuous"
- this is my concern.  Even the concept of an "effective length" of
0.5*L implies that the member is continuous over the length L - but that
is not true for a diagonal member that is interrupted at the midpoint
with only a gusset plate connection. 

The third reference also shows continuous bracing in each direction, and
it states that "the tension member, at small
displacement level, was incapable of preventing the lateral deformation
of the full compression diagonal."  While a continuous brace would have
inherent strength until higher loads are applied, a compression brace
that is effectively pinned at the mid-point would buckle at relatively
low applied loads - when the tension brace is ineffective in restraining
buckling. 

Thus, my question is not whether the effective length is 0.5*L, but
rather whether the length L must be continuous in order to apply the
theory that the tension brace laterally supports the compression brace.
Unless testing has been performed for such a case, I am suspicious of
claims that a compression brace that is pinned at the midpoint will
perform as intended. 


Bill Sherman
CH2M HILL / DEN 
720-286-2792 

-----Original Message-----
From: Harold Sprague [mailto:spraguehope(--nospam--at)hotmail.com] 
Sent: Friday, March 09, 2007 5:04 PM
To: seaint(--nospam--at)seaint.org
Subject: RE: Brace connection at mid-point

Bill,
No one else responded, so I have to blow the dust off some old
references.  
These are also available on the AISC web site.

I would suggest the following references:

AISC Engineering Journal 1987, 3rd quarter, "Design of Diagonal Dross
Bracings Part 1: Theoretical Study"

AISC Engineering Journal 1988, 4th quarter, "Design of Diagonal Dross
Bracings Part 2: Experimental Study"

AISC Engineering Journal 1986, 1st quarter, "Effective Length Factor for
the Design of X-Bracing Systems"

Basically, the tension brace acts as an out of plane brace for the
compression brace.  The compression brace buckles in an S shape with the
inflection point being the intersection of the X brace.  You infer that
it appears to be a bad idea, but that is how X braces work.  That is
what has been used in telecommunications towers also.

For double members, I like to make one of the members continuous from
each end.

Regards,
Harold Sprague

>From: <William.Sherman(--nospam--at)CH2M.com>
>Reply-To: <seaint(--nospam--at)seaint.org>
>To: <seaint(--nospam--at)seaint.org>
>Subject: Brace connection at mid-point
>Date: Thu, 8 Mar 2007 07:26:17 -0700
>
>In using cross-bracing connected at the mid-point and designed for 
>tension-compression loads, must both braces be "continuous" thru the
joint?
>  In tension-only bracing it is common to interrupt one member and 
>connect with a gusset plate at the mid-point, but it is not clear if 
>the same details can be used for tension-compression bracing.  In 
>looking thru references on the AISC website, I could not find direct 
>discussion of continuity of diagonal braces.  I have seen various 
>articles that discuss the use of the tension brace to laterally support

>the compression brace with respect to "effective length" - but a 
>discontinued brace would "encourage" buckling at the mid-point vs a 
>continuous brace.  If true full lateral support is provided at the 
>mid-point perhaps it should not matter, but reducing the buckling 
>capacity of the compression brace and relying solely on the tension 
>brace to prevent buckling at the mid-point seems like a bad idea.
>
>If possible, please provide references that address this situation.

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