Design of Cross-Braced Frames for Predictable Buckling Behavior

The relationship between axial load and end-rotational stiffness is derived. Reliable postelastic performance of concentrically braced frames requires that braces be designed for predictable buckling behavior. For cross-braced frames, restraint provided by the complementary brace (including both translational and rotational stiffness) can have a significant effect on buckling strength; this effect is complicated by the tension in the complementary brace, which increases its stiffness. Differential equations of stability are employed to determine upper and lower bounds of buckling strength, as well as to formulate the relationship between axial load and rotational stiffness. Given current detailing practices, it is desirable to control the plane of buckling so that the ductility of end-gussets is utilized; values of the effective length factor are provided to allow the design of braces for predictable buckling behavior. Out-of-plane buckling of systems with crossed torsionally stiff braces may place severe torsional demands on the end-gussets of the brace in tension; in-plane buckling is a viable alternative if it can be shown that it occurs at a lower load than out-of-plane buckling.