1. Professor, Univ. of Massachusetts, Amherst, MA 01003 (corresponding author).
2. Professor, Dept. of Civil Engineering, Shanghai Jiao Tong Univ., Shanghai 200240, PR China; formerly, Visiting Scholar, Univ. of Massachusetts, Amherst, MA 01003.
3. Professor, Ecole Centrale de Nantes, Nantes, France.