The Connection Between a Steel Cap Girder and a Concrete Pier

At congested highway interchanges, horizontal and vertical clearance requirements may dictate the use of narrow piers and shallow depth cap girders to accommodate the various roadways and overpasses. In situations such as this, the state of Texas uses horizontally curved steel plate girders as the bridge structural system, supported on integral steel cap girders at single column piers. Two bearings are used to connect the steel cap girder to the concrete pier. Owing to the narrow pier, unbalanced loading may produce a transverse overturning moment at the pier; the bearings resist this moment my developing a couple, with one bearing loaded in compression and one bearing loaded in tension. When the unbalanced loading is caused by truck traffic, which is cyclic, the bearing resisting the uplift is subject to fatigue loading.

The standard connection used by the state of Texas is an in-house design that comprises a line rocker bearing, which accommodates the horizontal rotation, and embedded anchor bolts that are used to both resist potential uplift and to provide a positive connection from the cap girder to the pier. The behavior of this connection, however, is not well understood and the detailing is complex. The objectives of this research were to examine and categorize the behavior of the existing connection and to develop a new detail that is simpler and cost effective, and to develop design guidelines for steel reinforcement in the concrete pier cap.

The research showed that the standard TxDOT connection performs adequately with respect to horizontal rotation but that it cannot resist uplift because of the poor fatigue characteristics of threaded anchor bolts. While the fatigue problem may be mitigated by post-tensioning the uplift bearing, this option is not available for the standard connection. Two new connection details were developed, one to be used for situations in which uplift does not occur and one that is capable of resisting uplift. The new details replace the line rocker with a rolled wide-flange section, and the threaded anchor bolts are replaced with high-strength threadbar, which is specifically designed for post-tensioning. The new connections proved to be more cost effective than the standard connection. At the top of the concrete pier, a strut-and-tie model is recommended for designing the steel reinforcement to support the bearing loads.