1. Ph.D. Candidate, Dept. of Civil and Environmental Engineering, Imperial College London, London SW7 2AZ, U.K. (corresponding author).
2. Reader in Nonlinear Mechanics, Dept. of Civil and Environmental Engineering, Imperial College London, London SW7 2AZ, U.K.
3. Professor of Structural Engineering, Dept. of Civil and Environmental Engineering, Imperial College London, London SW7 2AZ, U.K.