1. Professor, Institute of Structural Engineering, Zhejiang Univ., Hangzhou 310058, P.R. China (corresponding author).
2. Graduate Student, Institute of Structural Engineering, Zhejiang Univ., Hangzhou 310058, P.R. China.
3. Professor, Institute of Structural Engineering, Zhejiang Univ., Hangzhou 310058, P.R. China.