Practical Significance of LRFD Beam Buckling Factors

The load and resistance factor design (LRFD) specification introduces two parameters, denoted $X_1$, and $X_2$, which are a confusing conglomeration of material and section properties. These two parameters can be approximated by a simple function of flange thickness and section depth, which allows for a clearer interpretation of these two terms. It is shown that these two yerms describe the relative bending efficiency of a W-shape; as $X_1$ decreases and $X_2$ increases, bending efficiency increases. The approximations for $X_1$ and $X_2$ lead to a simplifying approximation for the rather complicated LRFD equation for the elastic lateral-torsional buckling moment of a standard W-shape. A family of curves based on approximate equations for nominal bending stress as a function of $L_b/r_y$ is plotted. These curves can be used to compute an approximation for nominal bending moment for a given W-shape.