Assessment of an Earthquake Resilient Bridge with Pretensioned, Rocking Columns
AuthorMantawy, Islam M.
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The seismic performance of a new bridge system is studied, tested and improved. The new bridge system: 1) reduces onsite construction time by using precast components, 2) eliminates major earthquake damage by utilizing rocking column and confinement of the column ends with a steel tube, and 3) maintains the system functionality after a strong earthquake by minimizing residual drift through the use of prestressing strands in the columns. Furthermore, it uses only conventional materials. The shaking table performance of a quarter-scale, two-span bridge constructed using the new system was compared with that of a conventional cast-in-place bridge with similar geometry tested in 2005. The new bridge system was constructed in about 20% of the time needed for the conventional cast-in-place system. In the tests, the conventional bridge suffered major concrete cracking and spalling, whereas in the new system, damage to the concrete was only cosmetic. In the conventional bridge, the longitudinal bars buckled and both the longitudinal and spiral reinforcement fractured, whereas in the new system the damage to the reinforcement was limited to longitudinal bar fracture, and that occurred only under excitations larger than the design level motion. The residual drift of the new system was essentially zero for all motions, whereas one of the exterior bents of the conventional bridge was so badly damaged and out of plumb that some of the supplemental mass on the bridge had to be removed and testing was stopped shortly thereafter. The only substantial damage that the new bridge system experienced was longitudinal reinforcing fracture. Therefore, ways to delay fracture were developed analytically. Reinforcement fractures were audible during the shaking table tests of the pretensioned rocking system. Reinforcement fractures were estimated in three ways using: 1) audio recorded during each test, 2) measured rotations at column ends and 3) analytical models, which included a fatigue material. This analytical model was then used to explore methods to improve the performance of the system by delaying reinforcement fracture. The analytical parametric studies on the scaled model showed that increasing the bar size and the locally debonded length of the reinforcement were both effective strategies to reduce and delay bar fractures. For the shaking table experimental model configuration, the analytical model showed that increasing the longitudinal bars by one size and increasing the debonded length by 44% would delay bar fracture until an excitation 67% larger than the excitation where reinforcing bars first fractured in the physical experiment. The parametric study also was conducted for a prototype bent; this recommended values for longitudinal bar size, debonded lengths for longitudinal bars and effective prestressing for prestressing strands to delay the fracture of the longitudinal bars and the yielding of the prestressing strands until after the 150% design level motion.