1. Structural Engineer, Mott Macdonald Ltd., St. Anne House, Wellesley Rd., Croydon CR9 2UL, U.K. (corresponding author).
2. Lecturer, Dept. of Engineering Science, Univ. of Oxford, Parks Rd., Oxford OX1 3PJ, U.K.
3. Reader, Dept. of Engineering Science, Univ. of Oxford, Parks Rd., Oxford OX1 3PJ, U.K.
4. Research Fellow, School of Engineering, Univ. of Surrey, Guildford, Surrey GU2 7XH, U.K.