web stiffeners for compression flange bracing

huangyhg

web stiffeners for compression flange bracing?
we are designing a general aircraft hangar building with 50 foot simple span steel header beams that support prefab wood trusses spaced at 24 inches.  i am concerned about lateral support of the compression flanges.  using asd aisc 9th edition, for w27x94 (fy=50) beams with fb=.66fy, lateral bracing supports would need to occur at maximum 8.9 ft (lc) spacings and would be designed for capacity of 4.9 kips horizontal force (using 2% of the maximum compression flange force).
question 1:  could lateral bracing be provided by wood trusses?  if so, is it reasonable to use a design horizontal force of 4.9*2.0/8.9=1.1 kips per truss and provide adequate connectors from the truss to the nailer plate and from the nailer plate to the beam flange?  in other words, can a designer distribute the lateral bracing force along the length of the beam?
question 2:  in lieu of bracing by the trusses, is it permissible to use full fitted web stiffeners, spaced at lc or less, to brace the top flange by connecting it to the relatively laterally-stable bottom tension flange?
any code or text references would be appreciated!
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quote:
question 2:  in lieu of bracing by the trusses, is it permissible to use full fitted web stiffeners, spaced at lc or less, to brace the top flange by connecting it to the relatively laterally-stable bottom tension flange?
that doesn't work. you can't brace a top flange with a vertical stiffener.
if the wood trusses bear directly on the steel beam they will provide the lateral bracing for the top flange.
i don't see anything in the code that allows you to distribute the required strength by the ratio of the brace spacing over lc.  in addition to the required strength, appendix 6 of the new code has a stiffness requirement, something you should pay attention to, seeing you're bracing steel with wood.
if you want the trusses to provide bracing, you'll have to provide the truss engineer with bracing forces and design connections to handle the brace force.  this is unusual so i typically won't use trusses as bracing per se.  instead i feel comfortable saying that with trusses, my unbraced length, say l/2 instead of at each truss location.  by the time you go through everything it may prove easier to just provide more beam weight and leave it at that.  
read the 13th ed. manual, spec. appendix 6.
i have dealt with this same issue myself--on a project with a steel truss top chord (a wt) braced by wood roof trusses.  basically, i did what you are proposing--i figured out what my maximum unbraced length could be, and provided enough connections in that length for the 2 percent force.  so i did not distribute the 2 percent force across the entire truss span, but only across the assumed unbraced length.
in your case, if you assume the beam is braced 2' oc, i think you should design for the 2 percent force every 2' oc.  if you assume the beam is braced 6' oc, you should design for the 2 percent force every 6' oc.
daveatkins
i concur with the way you propose to provide bracing. but don't forget the bottom flange.  you will probably have net uplift under some wind conditions, so you have to brace the bottom flange accordingly.
it is unnecessarily expensive to design a beam spanning 50 ft to be unbraced.  the load will always brace the beam if adequate connections exist.
would anyone explain why a full fitted stiffener can't be used? i am trying to understand the rationale behind it. would it not prevent local buckling?
slickdeals:
the flange brace is intended to prevent lateral torsional buckling of the section - twisting.  the stiffeners are useless in the prevention of ltb.
yes, i should have been more explicit.  the vertical stiffeners, as marcbse says, do help to brace against local buckling (web or flange) but the original post was definitely talking about lateral torsional buckling of the overall