The roof structure of a large span building is often composed from truss beams. Generally, its collapse phenomena are caused by the buckling of chord members in truss beams. However, each chord member has to be divided into some elements to calculate such unstable behavior with usual finite elements. As a result, the analytical model has much more degrees of freedom and it is extremely difficult for structural designers to simulate the collapse behavior of the roof structure during design processes. For this sake, one of us has presented the new type of a beam element to analyze the nonlinear behavior of a truss beam with in-plane buckling of its chord members by using a single element. To understand the nonlinear behaviors of a truss beam more strictly, the lateral buckling of the truss beam should be considered as well as in-plane buckling. Then, it is necessary to evaluate the out-of-plane buckling of chord members in the truss beams. In this paper, we present the analytical method which enables us to evluate the post-buckling behavior of the chord member conveniently and systematically.
