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# D1.C.5.3 Flexural Design Strength

In LRFD, the flexural design strength of a member is determined by the limit state of lateral torsional buckling. Inelastic bending is allowed and the basic measure of flexural capacity is the plastic moment capacity of the section. The flexural resistance is a function of plastic moment capacity, actual laterally unbraced length, limiting laterally unbraced length, buckling moment and the bending coefficient. The limiting laterally unbraced length L

_{r}and buckling moment M_{r}are functions of the section geometry and are calculated as per the procedure of Chapter F. The purpose of bending coefficient C_{b}is to account for the influence of the moment gradient on lateral-torsional buckling. This coefficient can be specified by the user through the use of parameter`CB`(see D1.C.6 Design Parameters) or may be calculated by the program (if CB is specified as 0.0). In the absence of the parameter`CB`, a default value of 1.0 will be used. The procedure for calculation of design strength for flexure also accounts for the presence of residual stresses of rolling. To specify laterally unsupported length, either of the parameters`UNB`and`UNT`(see D1.C.6 Design Parameters) can be used.