TABLE OF CONTENTS Dr. Alan Williams
The allowable stress on flexural members depends on the shape of the section and the bracing used to prevent lateral instability [1]. Sections are classified as compact, noncompact and slender in accordance with the criteria given in AISC Table B5.1. Most rolled W shapes qualify as compact sections. The exceptions are indicated in the AISC Tables of Properties by denoting the value of the yield stress F y' and F y'" at which a particular shape becomes noncompact. The allowable bending stress for compact symmetrical shapes is given by AISC Equation (F1-1) as
F b = 0.66 F y
when the maximum unbraced length of the compression flange does not exceed the smaller of
where b f = flange width, and A f = compression flange area.
When the unbraced length of a compact shape exceeds L c, but is less than the greater of
the allowable bending stress is given by AISC Section F1.3 as
F b = 0.60 F y
where r T = radius of gyration of the compression flange plus one-third of the web area
C b = bending coefficient defined in Section F1.3 and Table 6
= 1.75 + 1.05 M 1/M 2 + 0.3( M 1/ M 2 ) 2, but not more than 2.3.
The value of C b conservatively may be taken as unity.
Examples...
