huangyhg

unbraced length of beam supporting platform
i have a beam spanning about 20 ft. supporting a platform. can the platform be counted on as providing lateral support for the beam at each location where the platform is connected to the beam? (thus, keeping the unbraced length of the beam down)
yes, as long as the platform has a diaphram shear stiffness such that lateral translation of your top chord will be resisted. for an edge beam supporting a floor as such, if the beam were to deflect laterally, there would be a curved shape along the edge of the platform. the platform - whatever its made out of - may or may not be stiff in-plane to resist this movement. an example of a material that has questionable disphragm stiffness is certain types of bar grating which can deflect in-plane and thus may not provide you with the needed stiffness.
the platform is solid diamond tread and is bolted to 4 w10 columns. i think its' pretty safe to say lateral translation of the top flange of the beam will be resisted by the platform.
bagman,
it is my opinion that your assumption is correct.
for example if we have a platform with two 20 foot span beams that are 6 feet apart and we place three 6 foot long beams between them at quarter points, then the un-braced length of the 20 foot span beam is 5.0 feet. the presence and connection of the 6 foot beams to the web of the main beams will restrain the top flange from buckling. this is what i have been taught and used in my practice.
diaphragm would also provide lateral bracing of the compression flange at point of attachment as well. it does not take a whole lot of force to provide lateral stability. i have heard form a california colleague once that presence of filing cabinet next to a column provided adequate lateral stability during a seismic event!
it is safe to assume that your un-braced length is at every location where the top flange is welded/or bolted to the floor plate.
regards,
lutfi

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