huangyhg

bending coefficient, cb, for allowable stress design
the use of the two approaches compared with the closed form formulation of the check should confirm or deny the validity.
i don't believe that you can simply adjust your unbraced length or your moment capacity with cb and enter the charts. if you look at the equations f1-6, f1-7, and f1-8 you will see the following:
the break points for the charts depend upon the square root of cb.
the allowable stress in f1-6 depends upon cb used in a denominator added to 2/3 and therefore is not a direct linear relationship.
the applicable allowable stress in some cases is the larger of f1-7 and f1-8, therefore a direct adjustment linearly by cb is not appropriate.
what you must do is use the equations and skip the charts. they were developed only for cb = 1.0.
just as the triple lux may not win you the gold, this method will not necessarily provide you with the correct section.
this method is a good approximation, however the shape should still be checked per the applicable formulas from aisc chapter f.
n-gin-ear (visitor)20 jan 02 10:48
the beam charts you speak of are not limited to use with cb=1.0. you can enter using lb/cb (in place of cb) for hyperbolic curve portions or lb/(cb^(1/2)) for parabolic portions of the curve. obviously, once your size is selected (preliminarily from the charts), do your final check based on the equations f1-6 through f1-8.
granted, cb=1.0 is conservative and 99.9 percent of the time is what is used for expediency in design, but using real cb is certainly not a prohibitive approach. using the charts with the "adjusted cb's" mentioned above is a lot quicker than trial and error checks of beams from the sx tables with lb>lu and real cb. the charts are the best starting point when lb>lu, whether cb=1.0 or cb>1.0.
steel structures: design and behavior", 4th edition, by salmon and johnson clearly addresses this topic. (page 527 for those of you playing along at home.)

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