1. Member, ASCE
2. Assoc. Prof., Engrg. and Envir. Mech., Massachusetts Inst. of Technology, Cambridge MA 02139. E-mail: ulm@MIT.EDU
3. Res. Dir., Laboratoire Central des Ponts et Chaussées, Department for Engineering Models, 58 Bd. Lefebvre, 75732 Paris Cedex 15, France.