EARTHQUAKE RESPONSE OF AN OPEN-TYPE WHARF WITH A PNEUMATIC CAISSON FOUNDATION

摘要

An open-type wharf with a pneumatic caisson foundation has strong earthquake resistance because of its high rigidity. Construction of wharves with stronger seismic resistance and a larger planned depth is indispensable to accommodate ever-increasing larger vessels. Considering this trend and strong earthquake resistance, open-type wharves with a pneumatic caisson foundation will be widely applied in the future. Consequently, it is necessary to establish a practical design method of the wharf. Existing practical earthquake resistant design methods for an open-type wharf with a steel pipe pile foundation follow the below-mentioned procedure: (ⅰ) To calculate acceleration response spectra at the position of 1/β below the virtual ground surface by means of a one-dimensional earthquake response analysis of the ground. Here, the virtual ground surface means a 1/2 gradient face in between the slope and the seabed, β is a pile characteristic value; (ⅱ) To calculate response acceleration corresponding to the natural period of the wharf. Here, the damping constant is 20%; (ⅲ) To divide the response acceleration by gravity acceleration to obtain seismic coefficients; and (ⅳ) To calculate inertial force using the seismic coefficients and verify the stability of the structure. To follow the design procedure described above may be appropriate. However, the difference in the rigidity of foundation between the two structural types should be considered. The authors conducted two-dimensional finite element earthquake response analyses, modelling both open-type wharves with pneumatic caissons and soil layers, and discussed their earthquake responses. As a result of this study, the earthquake response of the wharf with pneumatic caissons is different from that on a steel pipe pile foundation. The optimum evaluation positions of acceleration response spectra have proved to be foundation bottom levels, and the optimum damping constant for the evaluation of response acceleration to range from 5-20%.