1. Dept. of Civil and Natural Resources Engineering, Univ. of Canterbury, Private Bag 4800, Christchurch 8140, New Zealand (corresponding author).
2. Dept. of Civil and Natural Resources Engineering, Univ. of Canterbury, Private Bag 4800, Christchurch 8140, New Zealand.
3. BRANZ, Private Bag 50908, Porirua City 5240, New Zealand.