1. Associate Professor, Dept. of Civil Engineering, Tsinghua Univ., Beijing 100084, P. R. China.
2. Professor and Director, ACE-MRL, Dept. of Civil and Environmental Engineering, Univ. of Michigan, Ann Arbor, MI 48109.
3. Professor, Dept. of Civil and Environmental Engineering, Univ. of Michigan, Ann Arbor, MI 48109.
4. Research Assistant, ACE-MRL, Dept. of Civil and Environmental Engineering, Univ. of Michigan, Ann Arbor, MI 48109.