Cyclic Tests of Precast Pretensioned Rocking Bridge-Column Subassemblies

AbstractRocking columns reinforced with unbonded prestressing offer advantages for bridges constructed in seismic regions because they can recenter the structure after an earthquake. Under lateral load, the columns rock on the foundation and cap beam, and rotate as rigid bodies rather than deforming. This paper describes cyclic load tests of two subassemblies representing parts of a bridge bent constructed using precast pretensioned rocking columns, spread footings, and a precast cap beam. The subassemblies, representing two halves of a single column, were subjected to cyclic lateral displacements of increasing amplitude under a constant vertical load. The proposed column design has several advantages over conventional cast-in-place construction. The use of precast columns and cap beams reduces on-site construction time. The use of unbonded prestressing minimizes residual displacements after an earthquake. The strands are deliberately debonded through the clear height of the column and bonded to the column concrete at the top and bottom, where the column is embedded in the cap beam and footing, respectively. The columns are confined by steel tubes and annular end plates at their interfaces with the footings and cap beams to minimize concrete damage when the columns rock. In the tests, the columns returned to their initial location with essentially no concrete damage after being displaced to peak drift ratios exceeding 10%. At the end of the tests, the columns’ lateral strengths still exceeded 80% of their peak values. The tests also provided the opportunity to evaluate practical procedures for proportioning key details of the system.