1. Graduate Student, Dept. of Civil Engineering, K.U.Leuven, Kasteelpark Arenberg 40, B-3001 Heverlee, Belgium.
2. Professor, Dept. of Civil Engineering, K.U.Leuven, Kasteelpark Arenberg 40, B-3001 Heverlee, Belgium.
3. Professor, LMSSMat, Ecole Centrale de Paris, Grande Voie des Vignes, F-92295 Châtenay Malabry, France.