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FW: through-truss without top chord bracing

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Everyone,

Take a look at the new LRFD 3rd edition, which contains a new section on
bracing.  Bracing design is comprised of two components; strength and
stiffness.  The 2% rule will provide strength, but with the problem at hand,
the stiffness of the bracing might be the tougher nut to crack.

Curt La Count
Jacobs Engineering
Portland, OR

-----Original Message-----
From: Matthew Stuart [mailto:m.stuart(--nospam--at)aespj.com]
Sent: Tuesday, February 12, 2002 4:19 AM
To: seaint(--nospam--at)seaint.org
Subject: RE: through-truss without top chord bracing


Dr. Yura's bracing research has shown that the 2% rule-of-thumb is
conservative. Obviously you would use 2% of the maximum top chord axial
force at the midspan of the truss to design all of the web members providing
even more redundancy.

-----Original Message-----
From: Majid Sarraf [mailto:msarraf(--nospam--at)unl.edu]
Sent: Monday, February 11, 2002 10:45 PM
To: seaint(--nospam--at)seaint.org
Subject: RE: through-truss without top chord bracing


Ken,

I suggest that you look at Timoshenko and Gere's Theory of Elastic
Stability, pages 111 and 112.

I do not believe that using 2% of axial load applied as lateral load for the
design of top chord is a conservative assumption.  Axial force in the top
chord varies from 0 at the ends to its maximum at the mid-span where the
stability against out-of-plane buckling of the entire top chord can be
provided only through bending stiffness of the web components and rotational
restraint of the floor beams.

Majid Sarraf, Ph.D.
Senior Bridge Engineer
Imbsen and Associates Inc.
Sacramento, CA



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