Ohba ohashi pile-supported bridge experiment revisited: paradoxes and new interpretations

Abstract eng:
The seismic response of an actual bridge supported on groups of steel piles in extremely soft clay (NSPT = 0) is investigated. The pier and its foundation were thoroughly instrumented in the early 1980’s by the Institute of Technology of Shimizu Corporation. This pioneering project provided several earthquake recordings, with peak accelerations in the free field as high as 0.11g. Two important observations were made: First, rock-to-surface amplification of peak horizontal acceleration varied significantly in the two horizontal directions, which is not compatible with vertically-propagating one-dimensional waves. The existence of a multi-dimensional wave field in Ohba Ohashi valley has been successfully demonstrated by earlier investigators. Second, the pile cap and bridge deck responded with a maximum acceleration of only 0.05g – as compared to an expected 0.15g or higher – a unexpected behavior that has not been explained/reproduced in a satisfactory manner to this day. An attempt is presented in this paper to reproduce the structural response using analytical tools. The relatively low level of peak strain in the soil (γ< 10-4), in conjunction with the high plasticity index of the clay material and the lack of damage in the structure justify the use of linear elastodynamic models. The effects of soil amplification, kinematic interaction, foundation compliance, pile-to-pile interaction and superstructure dynamics are considered. Pile group effects are treated both by simplified and rigorous elastodynamic numerical models. It is shown that records and predictions compare reasonably well when pertinent assumptions are introduced. Implications of valley effects, soil-structure-interaction, multiple-support excitation and structural connections on seismic response are discussed. The influence of kinematics of the movable bearings connecting the deck and the piers is investigated. A possible scenario for the deamplification of earthquake motion at the structure is proposed.

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