1. Lecturer, Dept. of Civil and Structural Engineering, The Univ. of Sheffield, Mappin St., Sheffield S1 3JD, U.K. (corresponding author). E-mail: p.reynolds@sheffield.ac.uk
2. Professor of Vibration Engineering and EPSRC Advanced Research Fellow, Dept. of Civil and Structural Engineering, The Univ. of Sheffield, Mappin St., Sheffield S1 3JD, U.K.