Comparison of the stiffness properties for various cross frame members and connections

Cross frames improve the stability of steel bridges by providing lateral and torsional restraint along the girder length. In order to be an effective brace, the cross frame must satisfy both strength and stiffness requirements. While cross frames are often constructed using steel angles to form an X-type brace, improved stiffness and fatigue behavior may result by using members with better compression strength, allowing the braces to be fabricated with a single diagonal. A TxDOT sponsored research project at the University of Texas at Austin is investigating multiple details which would allow the use of a single diagonal cross frame. The T-stem connection is comprised of a tubular member welded to the flange of a WT section. The stem of the WT can then be welded to the gusset plate. A second detail being studied is a cast steel connection. The steel casting fits to the outside of a round tubular member and tapers to a flat section, helping to minimize stress concentrations. A third detail consists of a gusset plate, or knife plate, which is inserted into a precut slot in the HSS member. Finally, the use of a double angle member along the diagonal is under consideration. This paper highlights results from axial tension experiments, with an emphasis on how the connections affect the individual stiffness of the tubular member as well as the overall cross frame stiffness.