Seismic response of column/cap beam tee connections with cap beam prestressing

Concrete bridge joints in California were detailed with no shear reinforcement in the 1950s. The probable consequence of such poor detailing was demonstrated on an as-built bridge tee (interior) joint from the Santa Monica Viaduct in Los Angeles. When a redesign of this joint was considered with the conventional design methods based upon the maximum joint shear forces, it resulted in a considerable amount of joint reinforcement, causing congestion of steel within the joint. This has been identified as a major construction problem in concrete bridge construction. Three redesigns of the prototype joint were sought with the objective of reducing the amount of reinforcing steel within the joint, thereby ensuring constructability. In all three designs, force transfer models were employed in determining the appropriate amount of joint reinforcement rather than using the joint shear forces as the design parameter. The first tee joint was designed with conventional reinforcement while the cap beams of the other two units were detailed with partial and full prestressing. Considering that the cap beam prestressing assists in transmitting the shear forces across the joint, the joint reinforcement in the latter units was further reduced when compared to the first unit. The suitability of the redesigned details was verified by subjecting half scale test models to simulated seismic loading. The response of all three redesigned units was satisfactory and complied to the current seismic design criteria. Some damage occurred to the joint of the reinforced concrete unit while the joint damage of the prestressed units was limited to only minor cracking. There was a clear indication that cap beam prestressing proportionally enhances the seismic performance of the joint. When the cap beam was designed with full prestressing, it was also shown that precast construction can be adopted as an alternative to cast-in-place construction for building multi-column concrete bents. The details of the joint design, seismic performance of all three redesigned units, and some design recommendations are presented in this report.