1. Structural Engineer, Skidmore, Owings & Merrill Inc., The Broadgate Tower, 20 Primrose Street, London, EC2A 2EW, U.K. (corresponding author).
2. Senior Lecturer, School of Civil, Structural and Environmental Engineering, Univ. College Dublin, Newstead, Belfield, Dublin 4, Ireland.
3. Professor of Vibration Engineering, College of Engineering, Mathematics and Physical Sciences, Univ. of Exeter, Exeter, Devon EX4, U.K.