An Experimental Study on Rocking Response of Bridge Spread Foundations

Abstract eng:
Some recent studies have shown that rocking of spread foundation has beneficial effect on the dynamic performance of piers by decreasing the plastic deformation occurring at the plastic zone. This implies that the ductility demand of piers can possibly be reduced. However, rocking is still not an acceptable mode of response in design code generally. In order to gain better understanding of the problem of rocking, especially its ductility demand, and in turn have more confidence in taking the benefit of rocking mode into account during design and retrofit design process, a series of preliminary rocking experiments were performed. A total of three 3.8 m high circular reinforced concrete columns with spread foundation were tested. Two of these columns were the as-built ones with lap-spliced longitudinal reinforcement and inefficient transverse confinement, representing piers with low ductility capacity; the other one was identical to the as-built ones but wrapped with steel jacketing to shift its ductility capacity to a level fitting the requirements specified by design code. During the tests, one as-built test column and one retrofitted column were rested on rubber pad to allow the mechanism of rocking to take place; another as-built column was constrained to the strong floor during testing to represent a benchmark test with fixed base condition. Using pseudo-dynamic test and cyclic loading test, the rocking behavior of these columns undergoing different level of earthquake accelerations, including a near field ground motion were obtained and discussed. From the benchmark test, the difference between the response behavior of rocking base and fixed base foundation was highlighted. By comparing the experimental responses of retrofitted column with that of the as-built one, the interaction effect of rocking mechanism on the ductility demand and strength demand of columns was also identified.

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