Brace Stiffness and Forces of X-Type, K-Type, and Z-Type Cross Frames in Steel I-Girder Bridge Systems

The stability of steel bridges is improved by using cross frames, which provide lateral and torsional restraint along the girder length. In order to be considered an effective brace, the cross frame must satisfy both strength and stiffness requirements. Cross frames can utilize a variety of layouts: the X-Type and K-Type cross frames are commonly used in current practice for steel I-girder bridges, while the single diagonal Z-Type cross frame is being researched at the University of Texas at Austin as part of a TxDOT sponsored research project.
Cross frame members are often designed following slenderness limits for tension and compression members given in the AASHTO Bridge Design Specification. Depending on the complexity of the steel bridge design, basic computer software may also be used to determine the member forces. While sizing cross frame members using these methods may result in a conservative design, slenderness limits may not be a good indication of the forces.
In order to further examine the forces in the cross frame members, as well as provide insight into the stiffness of the brace, results from two different experimental setups will be provided. The first setup investigates the stiffness of each cross frame type subjected to applied equal and opposite moments, representing the forces induced when the girders begin to buckle. The second setup looks at forces in the cross frame caused by the application of a live point load, such as when a heavy truck passes the brace location.

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