Bearing capacity in long-span tubular truss chords

Existing design specifications used in the United States and abroad do not address the limit state of bearing failure that may govern in the design of tubular truss chords at the supports. The lack of treatment of this limit state seems to represent an unsatisfactory situation given the fact that very large concentrated reaction forces are often applied transversely to the ends of chord members with slender circular cross sections; long-span overhead highway sign trusses are but one example of an affected application. The bearing failure limit state is manifest as a coupled failure mechanism, consisting of a plastic collapse of the chord end and local plastification of the chord wall. In order to better understand this limit state, two full-scale experimental tests are conducted at the University of Pittsburgh as part of the current research. The specimens considered are proportioned to be representative of the bearing region of an overhead sign structure that failed in the limit state under investigation. These tests yield important information regarding the manifestation of the bearing limit state and also afford an opportunity to verify finite element modeling techniques for use in a parametric study. The parametric study reveals that the bearing capacity is influenced by many factors including the geometry of the bearing connection, the adjacent intermediate truss member, the nature of loading, and the material properties. Using regression analysis on the parametric study results and concepts from the yield line method for analyzing collapse mechanisms, general equations for predicting the bearing capacity are developed.