1. Research Associate, Dept. of Civil and Structural Engineering, Univ. of Sheffield, Sheffield S1 3JD, U.K.
2. Lecturer, College of Engineering, Mathematics and Physical Sciences, Univ. of Exeter, Exeter EX4 4QF, U.K.
3. Professor of Structural Dynamics, College of Engineering, Mathematics and Physical Sciences, Univ. of Exeter, Exeter EX4 4QF, U.K. (corresponding author).