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RE: Brace connection at mid-point

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That sounds prudent.  Difficult and expensive, but prudent.

Regards,
Harold Sprague





From: <William.Sherman(--nospam--at)CH2M.com>
Reply-To: <seaint(--nospam--at)seaint.org>
To: <seaint(--nospam--at)seaint.org>
Subject: RE: Brace connection at mid-point
Date: Wed, 14 Mar 2007 11:18:21 -0600

I was able to locate a copy of the 2000 IBC Seismic Design Manual with a design example of a concentrically braced frame. It states that a single plate connection at the intersection point (for tube steel braces) is not recommended due to the significant out-of-plane displacement needed to achieve sufficient restraint. It references an experimental study of cross-braced frames using HSS - I am currently working on getting a copy of that study. It doesn't seem like allowing such large displacements is a good idea for seismic loads with load reversals. Thus, I am moving more and more towards the conclusion that the radius of gyration should be maintained thru the center connection of cross-bracing designed as tension and compression bracing, (unless otherwise laterally restrained out-of-plane, such as by a floor).

Bill Sherman

________________________________

From: Harold Sprague [mailto:spraguehope(--nospam--at)hotmail.com]
Sent: Tue 3/13/2007 6:13 PM
To: seaint(--nospam--at)seaint.org
Subject: RE: Brace connection at mid-point

Bill,
I agree that the means of "continuity" is subject to debate.  I avoid cross
X braces. I think the last time I used one was in a single angle in which I
designed them for tension only.  I like the 2 story X braces.  The problem
with X braces that cross in the middle of a story is that the cost of the
gusset plate is huge, and if the brace is large, it is very time consuming
to construct.

I don't have an answer to your more fundamental question.  My personal
practice is to design continuity at the intersection by various means.
Towers have a peculiar looking tesion compression member that is validated
by testing.

Sorry I couldn't offer more help.

Regards,
Harold Sprague

>From: <William.Sherman(--nospam--at)CH2M.com>
>Reply-To: <seaint(--nospam--at)seaint.org>
>To: <seaint(--nospam--at)seaint.org>
>Subject: RE: Brace connection at mid-point
>Date: Fri, 9 Mar 2007 20:38:19 -0700
>
>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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