3.7 The modification factor, cb

Introduction to cb

$C_b$ is a modification factor used in the equation for nominal flexural strength when determining Lateral-Torsional Buckling . The reason for this factor is to allow for non-uniform moment diagrams whenthe ends of a beam segment are braced. The conservative value for $C_b$ can be taken as 1.0 as according to Chapter F2a of the Specification Section of the Manual (page 16.1-32) for all cases. Also, the Manual gives a table of values for $C_b$ for some loading conditions of simply supported beams. This is located in Section 5 of the Manual (page 5-35) and is named, Table 5-1.

Equation for cb

If the value of $C_b$ is not given in Table 5-1, the equation in Chapter F2a of the Specification Section of the Manual (page 16.1-32), given here, can be used to find $C_b$ .

Where:

• $M_{max}$ = absolute value of maximum moment in the unbraced segment, kip-in.
• $M_A$ = absolute value of maximum moment at quarter point of the unbraced segment, kip-in.
• $M_B$ = absolute value of maximum moment at centerline of the unbraced segment, kip-in.
• $M_C$ = absolute value of maximum moment at three-quarter point of the unbraced segment, kip-in.