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RE: Unbraced Length

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Another good source is the AISC 1999 LRFD spec.  This is spec in the 3rd
Edition of the LRFD Steel Manual.  Section C3 gives both the stiffness and
strength requirements for bracing.  Your end goal here is to prevent
buckling of the compression flange.  This can be done by direct bracing of
the compression flange or by preventing twisting of the section.  Your
situation sounds like it can provide twisting restraint, if it is detailed
properly.

Ed Haninger
Fluor
Aliso Viejo, CA



                                                                                                                      
                    "Kestner, James                                                                                   
                    W."                     To:     <seaint(--nospam--at)seaint.org>                                               
                    <jkestner@somervil                                                                                
                    leinc.com>              cc:                                                                       
                    12/10/2003 09:16                                                                                  
                    AM                      Subject:     RE: Unbraced Length                                          
                    Please respond to                                                                                 
                    seaint                                                                                          . 
                                                                                                                      
                                                                                                                      




I agree that Dr. Yura is a good source. I doubt if the web resistance has
much potential for reducing beam size. Perhaps stiffeners installed on only
one side of the web may have some potential. Would a single stiffener have
enough stiffness and strength to brace the top flange? This would be
dependent upon the width and height of the girder plus the amount of force
in the top flange and how often it is braced. Perhaps you can develop some
guidelines or design charts.

A couple of practical examples come to mind:
     An upset balcony girder in a theater.The bottom of the girder is
     torsionally braced by floor beams framing into it.
     A thru plate girder bridge. The floor beams torsionally brace the
     bottom of the plate girder. For highway or railway applications, an
     additional load may want to be considered with a vehicle accidently
     striking the unbraced top flange of the plate girder.
Good Luck!

Jim K.


     -----Original Message-----
     From: Effland, Greg [mailto:geeffland(--nospam--at)butlermfg.com]
     Sent: Wednesday, December 10, 2003 10:46 AM
     To: 'seaint(--nospam--at)seaint.org'
     Subject: RE: Unbraced Length

     You might look into the Stability Bracing literature from Dr. Joseph
     Yura (U of Texas- Austin).  I am pretty sure the literature he hands
     out at his seminars cover the issue of tension flange bracing.



     HTH,
     Greg Effland, P.E.
     KC, MO USA


          -----Original Message-----
          From: Udall, Jeffrey D [mailto:JDUdall(--nospam--at)babcock.com]
          Sent: Wednesday, December 10, 2003 10:36 AM
          To: seaint(--nospam--at)seaint.org
          Subject: Unbraced Length



          I'm presently doing a literature search as part of a university
          thesis project.  I'm hoping this list can give me some
          assistance.


          I'm trying to determine the critical bending moment of a beam
          when it is subjected to a uniform load.  The beam is continuously
          supported laterally and torsionally on the tension (bottom)
          flange.  The top flange (compression) is not restrained.  Current
          practice is to consider the unbraced length of the compression
          flange as the full span of the member.  In order to reduce this
          span, stiffeners are added at appropriate intervals that
          essentially tie the compression flange back to the laterally
          braced tension flange.


          However, these stiffeners are costly in terms of labour,
          especially when there are large numbers of them and the spans are
          significant.  I am trying to determine the effective span of the
          compression flange given that it really is attached to the
          tension flange through the web. The stiffener connects the two
          flanges together by treating the stiffener as a column, and is
          therefore quite stiff. The web serves the same purpose but acts
          only in bending in the weak axis.  This is not nearly as
          effective, but it does provide some degree of strength that is
          being ignored. I am hypothesizing that the effective unbraced
          length of the compression flange is less than the full span
          length, and determination of such will reduce 1. the need for
          stiffeners, and/or 2. the size of the beam.


          I'm reviewing some of the texts by Nethercott, Bleich, Galambos,
          and a few others, but I can't seem to find my specific situation.
          Galambos (in his Stability book 5th ed.) makes mention of it but
          does not elaborate.


          Has anyone looked at this arragement before?  I'm looking for
          references if you've got them.


          Thanks.
          Jeff Udall
          Cambridge, Ontario (Canada)









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