Rocking shallow foundations to improve bridge performance under earthquake loadsLijun Deng Experimental and numerical studies of the seismic performance of bridges supported by rocking foundations are reported. Sophisticated models of bridge systems (including soil, footing, column, deck, and abutment) as well as relatively simple models of isolated bridge bents were tested at the UC Davis centrifuge. Numerical simulations using OpenSees were validated against model test results and used to perform parametric studies to compare the stability and performance of systems with rocking footings to those with hinging columns. The experiments and analysis suggest that deck masses supported on rocking foundations are more stable (they are more difficult to topple) than the deck supported on hinging columns. The main reason for superior stability of rocking systems is their recentering properties. When a rocking system rotates, a gap opens under the footing. When the rocking loads are released, the closure of the gap returns the footing toward its initial position. On the other hand, systems with strong foundations and hinging columns force all the ductility into one structural mechanism (the hinging column) and have inferior recentering properties. The work also shows that the moment capacity of rocking foundations for bridges can be reliably calculated and that settlements are acceptably small for rocking foundations on good soil conditions. New design procedures for symmetrical two-span bridges supported on seat-type abutments and shallow foundations are proposed and presented with example calculations. It is suggested that further work be done to verify that rocking foundations can also be used for other bridge types and soil conditions. Subsequently, the Seismic Design Criteria could be modified to allow uplift due to rocking for new and retrofit bridges. |
