1. Postdoctoral Fellow, Dept. of Civil Engineering, Univ. of Toronto, 35 St. George St., Toronto, ON M5S
1A4, Canada.
2. Bahen/Tanenbaum Professor, Dept. of Civil
Engineering, Univ. of Toronto, 35 St. George
St., Toronto, ON M5S 1A4, Canada (corresponding
author).