1. Res. Asst., Dept. of Civ. Engrg., Tsinghua Univ., Beijing 100084, China.
2. Prof., Dept. of Civ. Engrg., Tsinghua Univ., Beijing 100084, China.
3. Asst. Prof., Dept. of Struct. Engrg., Politecnico di Milano, P.za Leonardo da Vinci 32, 20133 Milan, Italy. E-mail: pisani@hp735.stru.polimi.it