# RE: through-truss without top chord bracing

• To: <seaint(--nospam--at)seaint.org>
• Subject: RE: through-truss without top chord bracing
• From: "Majid Sarraf" <msarraf(--nospam--at)unl.edu>
• Date: Wed, 13 Feb 2002 00:37:41 -0600
It would be interesting to know if the 2% force is supported by any test
data on pony trusses? Which journal and what page?

Stability analysis is made based on stiffness of lateral supports and not
their strength. However, normally lateral bracing for compression flange of
beams designed based on the 2% rule results in a sufficiently stiff braces.
In the case of pony truss lateral bracing is provided through out-of-plane
flexural stiffness of the verticals, partially restrained at the base. Is
the lateral stiffness of a cantilever designed for a given force, equal to
that of steel strut designed for the same force?

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

-----Original Message-----
From: Matthew Stuart [mailto:m.stuart(--nospam--at)aespj.com]
Sent: Monday, February 11, 2002 10:19 AM
To: seaint(--nospam--at)seaint.org
Subject: RE: through-truss without top chord bracing

I have seen design guidelines for this condition before but can't recall
where. I think if you were to ask Dr. Yura (steel bracing guru at University
of Texas/Austin) he would probably say design the web to resist a lateral
force = 2% of the axial force in the top chord and you can assume that the
top chord is braced at each panel point.

-----Original Message-----
From: 	Ken Peoples [mailto:kspeoples(--nospam--at)lvta.net]
Sent:	Monday, February 11, 2002 11:09 AM
To:	Seaint
Subject:	through-truss without top chord bracing

Can any of you recommend a reference for the design of a through-truss (pony
truss?) without top chord bracing?  I have seen trusses - such as gangways -
where the top chord "appears" to be unbraced.  In actuality, they must use
the stiffness of the vertical web members to resist the lateral buckling of
the top chord because there is no way for the top chord to be considered as
unbraced for the entire span of the truss and still be able to take much
compressive load.  Are any of you familiar with a design procedure for this
type of system?
Ken

Kenneth S. Peoples, P. E.
Lehigh Valley Technical Associates
Northampton, PA 18067-9039
Phone: (610) 262-6345
Fax: (610) 262-8188
e-mail: kpeoples(--nospam--at)lvta.net

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