1. Assistant Professor, School of Engineering, Univ. of Warwick, Coventry CV4 7AL, U.K. (corresponding author).
2. Professor of Vibration Engineering, Dept. of Civil and Structural Engineering, Univ. of Sheffield, Sheffield S1 3JD, U.K.
3. Research Student, Dept. of Civil Engineering, Technical Univ. of Denmark, 2800 Kgs. Lyngby, Denmark.