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Plainsboro (Structural)
26 Jan 05 11:04
I have 50 feet span W24x131. Due to columns and outriggers coming on this beam, I have placed a number of Parallel and Perpendicular stiffener plates. Is the beam top flange laterally braced or not?
Lutfi (Structural)
26 Jan 05 12:18
Generally stiffener plate will provide local compression flange lateral support. However, this will not reduced the unbraced length of the beam. You have to have another member, floor, or deck attached to the top flange to cut the unbraced length.

Having said that, more details on the parallel and perpendicular stiff plates and how they are connected to the beam before a final assumption is taken.  

Regards,

Lutfi
www.cdeco.com

UcfSE (Structural)
26 Jan 05 13:29
It is not braced.  The AISC 3rd Edition LRFD lists requirements for strength and stiffness of bracing.  Stiffeners might help strengthen the beam against buckling but they do not reduce the unbraced length.
MikeE55 (Structural)
26 Jan 05 17:15
Pardon me, Plainsboro, I would like to expand on your question.  If the beam is continuous across a column, and the top flange is continuously braced by roof framing, will stiffener plates at the column allow you to assume the bottom flange is braced at the column?  Of course, since the beam is continuous over the column, there is a stress reversal and the bottom flange becomes the compression flange.  I usually add an angle brace from the bottom flange of the beam to the roof framing to make sure the bottom flange is considered braced against lateral torsional buckling, but is that really necessary?  The stiffener plates are full length between the top & bottom flange.
UcfSE (Structural)
26 Jan 05 18:19
Transverse stiffeners are for local stability problems or for shear strength.  They do not help the overall stability of the beam according to the methods we use today.  The compression flange can still buckle out-of-plane.  To count as braced, you must provide strength to handle the bracing force and stiffness so that no deflection, or extremely small, takes place in the appropriate direction.  It isn't enough to provide a strong connection or brace that is too flexible or a stiff connection that is too weak.  Longitudinal stiffeners increase the resistance to buckling not by reducing the unbraced length but by increasing the buckling load, similar to how you can increase column capacity by either increasing "r" or reducing "Lb".  Keep the angle brace IMHO.
DaveAtkins (Structural)
27 Jan 05 12:52
MikeE55,
I believe stiffener plates directly over a column, with a four bolt cap plate on the column, essentially create a continuous column from the footing or floor below to the top flange of the beam.  So, yes, the beam is braced at the column, because this "continuous" column is able to resist the lateral buckling force through bending.

DaveAtkins

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