A built-up shape that is symmetrical about the y-axis BUT NOT symmetrical about the x-axis consisting of

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A built-up shape that is symmetrical about the y-axis BUT NOT symmetrical about the x-axis consisting of a 3/4 x 18 top flange, a 5/8 x 10 bottom flange and a 3/4 x 60 web is used as a beam (dimensions in inches). A572 Grade 50 is used. Lateral bracing is continuous. What is the Nominal Bending Strength for Flange Local Buckling (FLB) consideration (Mn_FLB)?