It is recently more important to grasp the collapse behavior of structure, to make a better structural design. Though large span structures like dome are designed so that these members retain elastic properties for Level 2 earthquake motions, it is so useful to predict the collapse behavior against unknown large earthquakes. However, the roof structure of large span structures consisted of truss beams has so many members, therefore it is not realistic for structural designers to analyze with the full discrete model which can express in-plane and out-of-plane post-buckling behavior of truss chord members. Therefore, the simple and systematical analytical tool which can evaluate these collapse behavior of truss chord members is needed. In this study, we consider the section of the truss beams separated by lateral stiffening members as one element section and propose a convenient beam element which can evaluate the lateral post-buckling behavior of truss beams.