1. Researcher, Dept. of Engineering Science, Univ. of Oxford, Parks Rd., Oxford OX1 3PJ, U.K (corresponding author).
2. Researcher, Institute of Structural Engineering, Univ. of Kassel, Mönchebergstraße 19, 34109 Kassel, Germany.
3. Professor, Dept. of Engineering Science, Univ. of Oxford, Parks Rd., Oxford OX1 3PJ, U.K.
4. Professor, Institute of Structural Engineering, Univ. of Kassel, Mönchebergstraße 19, 34109 Kassel, Germany.