1. Ph.D. Candidate, Dept. of Engineering, Univ. of Cambridge, Trumpington St., Cambridge CB2 1PZ, U.K. (corresponding author).
2. Reader in Civil Engineering, Dept. of Engineering, Univ. of Cambridge, Trumpington St., Cambridge CB2 1PZ, U.K.
3. Head, Laboratory for Mechanical Systems Engineering, Swiss Federal Laboratories for Materials Science and Technology (EMPA), Überlandstrasse 129, 8600 Dübendorf, Switzerland.