[1] Northwestern Univ, Dept Mech Engn, Evanston, IL 60208 USA
[2] Columbia Univ, Dept Civil Engn & Engn Mech, New York, NY 10027 USA
[3] Univ Calif San Diego, Dept Struct Engn, San Diego, CA 92103 USA
[4] Univ Porto, Dept Mech Engn, Rua Campo Alegre 823, P-4100 Oporto, Portugal