1. Technology Licentiate, Division of Structural Engineering, Luleå Univ. of Technology, 971 87 Luleå, Sweden (corresponding author).
2. Professor, Division of Structural Engineering, Luleå Univ. of Technology, 971 87 Luleå, Sweden; presently, Division of Structural Engineering, Technical Univ. of Denmark, DTU-Building 118, Brovej, DK-2800 Kgs. Lyngby, Denmark.
3. Lecturer, Division of Structural Engineering, Luleå Univ. of Technology, 971 87 Luleå, Sweden.