1. Professor, Dept. of Mechanical Engineering, Univ. of Minnesota, 111 Church St. SE, Minneapolis, MN 55455 (corresponding author).
2. Technical Manager, Vestas Wind Systems, Hedeager, 8200 Aarhus N., Denmark.
3. Professor, Dept. of Mechanical Engineering, Univ. of Minnesota, 111 Church St. SE, Minneapolis, MN 55455.
4. Professor, Dept. of Civil, Environmental and Geo-technical Engineering, Univ. of Minnesota, 500 Pillsbury Drive SE, Minneapolis, MN 55455.
5. Structural Engineer, Barr Engineering, 4300 Market Pointe Dr., Minneapolis, MN 55435.
6. Professor, Faculty of Engineering, Hokkaido Univ., 5 Chome Kita 8 Jonishi, Kita Ward, Sapporo, Hokkaido Prefecture 060-0808, Japan.