1. Ph.D. Student, Dept. of Structural Engineering, Civil and Environmental Engineering, Chalmers Univ. of Technology, SE-412 96 Gothenburg, Sweden (corresponding author).
2. Professor, Division of Structural Engineering, Lund Univ., Box 118, SE-221 00 Lund, Sweden.
3. Professor, Dept. of Structural Engineering, Civil and Environmental Engineering, Chalmers Univ. of Technology, SE-412 96 Gothenburg, Sweden.