aDepartment of Civil and Structural Engineering, Hong Kong Polytechnic University, Hunghom, Kowloon, Hong Kong, China b Department of Civil Engineering,Tsinghua University, Beijing 100084, China Accepted 8 August 2002 民用和结构工程学,香港多科技大学, Hunghom, Kowloon,土木工程, Tsinghua大学,北京100084...