1. Assistant Professor, Dept. of Civil Engineering, Technological Univ. of Panama, Panama City, Panama 20297.
2. Associate Professor, Dept. of Civil Engineering, Univ. of Puerto Rico at Mayaguez, P.O. Box 9000, Mayaguez, PR 00680 (corresponding author).
3. Professor of Construction Engineering, Dept. of Civil, Construction and Environmental Engineering, Iowa State Univ., Ames, IA 50011.