1. Professor, College of Civil Engineering, State Key Laboratory of Disaster Reduction in Civil Engineering, Tongji Univ., Shanghai 200092, P.R. China (corresponding author).
2. Research Student, College of Civil Engineering, Tongji Univ., Shanghai 200092, P.R. China.
3. Professor, Vibration Engineering Section, Univ. of Exeter, Exeter EX4 4QF, UK.