huangyhg

simply supported beam design
i am reviewing a design that has a beam subjected to 200k-ft max moment and a 32 ft span, the designer has specified a w21x44. when i checked the beam via asd the compressive flange would have to be braced at least every 4.7-ft to meet the flange criteria. i think the designer is just selecting the beam by mr method without checking the unbraced length. am i interpreting this correctly?
please provide some additional information. what is the beam supporting - concrete deck, metal deck, etc? are there
there are purlins bolted to the bottom flange of the beam, the roof deck is then attached to these purlins. the purlin spacing varies from 4' to 12'. i am also concerned about the roof deck pans to purlin connection which is via a beam clamp. do you know of any load capacity information on this type of connection?
using aisc manual of steel construction, ninth edition, asd:
page 2-168: w21 x 44 (a36 steel) is good for maxiumum moment of about 163 kip-ft, no matter how short the unbraced length. the limiting factor is the allowable shear strength of the web.
however - for steel with fy = 50 ksi, the w21 x 44 is the lightest section for the 200 kip-ft moment (page 2-202). the unbraced length would have to < or = about 6.2 ft (the intersection of the line for the w21 x 44 and 200 kip-ft moment).
based on the information that you have given, it does not sound that the compression flange of the beam is braced. if this is the case, i think a w16x89 or so is the lightest section that will work (that's for 50 ksi steel). i don't know anything about the beam clamp connection you are talking about.
sorry i failed to mention the beam has an fy=50 ksi
concur with broekie.

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