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RE: through-truss without top chord bracing

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It is important to remember that using Dr. Yura's approach considers brace
strength and stiffness.  In my opinion I would be very careful in using the
web members to restrain the lateral movement of the top chord because of the
relative lack of stiffness that could be provided by a cantilevered
(assumably small section size) web member.  This could be used anyway but be
careful and double check your numbers!

If I recall correctly his general recommendation is 1 to 1.5% of the axial
load plus stiffness requirements with the 2% general rule of thumb covering
some of these situations (IIRC this works < 60% to 70% of the time)...

==> His current recommendations are in section C3 of the 3rd edition of the
LRFD manual (replacing the 2% rule I believe).

Another issue with Yura's theory that should be considered in this problem
is the fact the for each extra brace (web member in this case) the stiffness
required to provide a brace point increases dramatically.  This is based
upon the assumption that each brace will force another mode of buckling.
Face value alone this appears to be conservative since each brace may not
need to force another mode of buckling but just keep the member from moving
very much.  However, since I have not actually done testing on this subject
I would have to default to Yura's recommendations.  There are also some good
papers from Jim Fisher, LeRoy Lutz, and others on this subject also.
Possibly the AISC Design guide on torsional design might be helpful (#9 I
think)

Pay great attention to the connection details as you will probably want to
make the web connections fixed at each end, maybe even have a redundant two
point fixed restraint on the bottom (i.e. at deck level and at xx' below
deck level... top and bottom of deck supporting structure).  Orient the web
member strong axis to be in the lateral direction for the chord member.

Just brainstorming, but another idea that might help (even though it seems
weird) you might try having the weak axis of the top chord vertical using
it's strong axis help prevent out of plane buckling.  For this case you
might even be able to design the web members as weak-axis and torsional
restraints only and minimize their requirement to brace the chord laterally.
This might work pretty well since a chord on a truss like this is typically
an axial load only member.  Design the chord with Lx=full Ly = Web member
spacing @ moment being checked (Lt could be this also if the web member
provides torsional restraint).

Hope this helps,
Greg Effland, P.E.
KCMO USA

-----Original Message-----
From: Matthew Stuart [mailto:m.stuart(--nospam--at)aespj.com]
Sent: Tuesday, February 12, 2002 6:19 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


-----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?
Thanks in advance,
Ken

Kenneth S. Peoples, P. E.
Lehigh Valley Technical Associates
1584 Weaversville Road
Northampton, PA 18067-9039
Phone: (610) 262-6345
Fax: (610) 262-8188
e-mail: kpeoples(--nospam--at)lvta.net


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