1. Associate Professor, Institute for Mechanics of Materials and Structures, Vienna Univ. of Technology, A-1040 Vienna, Austria (corresponding author).
2. Research Assistant, Vienna Univ. of Technology, A-1040 Vienna, Austria; presently, Univ. of West Bohemia, Pilsen, Czech Republic.
3. Professor, Dept. of Civil and Environmental Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139.