引言以往十年美国作了很大努力去改进新桥的抗震设计方法和制定现有桥梁改建的原则。这些努力在一定程度上是因1971年2月9日圣费尔南多地震引起桥梁破坏而激发起来的。这次地震表明过去设计方法的不适当,同时表明有必要提高按照1971年以前的方法设计的桥梁的抗震能力。 INTRODUCTION In the past decade, the United States has made great efforts to improve the seismic design method of Xinqiao and formulate the principles of the existing bridge reconstruction. These efforts were in part motivated by the destruction of the bridge caused by the San Fernando earthquake on February 9, 1971. This earthquake indicates that the past design methods were inappropriate and at the same time shows that it is necessary to improve the seismic resistance of bridges designed in accordance with the methods before 1971.