1. Department of Civil Engineering, Padang Institute of Technology, Jalan Gajah Mada Kandis Nanggalo, Padang, Indonesia
2. Department of Civil and Environmental Engineering, Yamaguchi University, Tokiwadai 2-16-1, Ube, Yamaguchi 755-8611, Japan
3. School of Civil Engineering and Surveying, University of Portsmouth, Portsmouth PO1 3AH, UK