1. Department of Civil and Environmental Engineering, Viale A. Doria 6, 95125 Catania, Italy E-mail: fvaglias@iiiga.ing.unict.it
2. Department of Civil and Environmental Engineering, Box 352700, University of Washington, Seattle, WA 98195-2700, USATel: (206) 543-7645 Fax: (206) 685-9185 E-mail: markbenj@u.washington.edu