DEDICATION v DEDICATION To my family, and Raquel vi DEDICATION ACKNOWLEDGMENTS vii ACKNOWLEDGMENTS

I would like to express my honest gratitude towards my thesis supervisor, Prof. Sílvio Abrantes, for his initial encouragement, his timely orientation and contin-ued support. I thank him for his humane character and accessibility, for alleviat-ing the pressure and for being patient when the pace of investigation became slow. I am indebted with him. I also thank my family for being there at all times. Without their incentive and comfort this thesis would be hardly possible. I also cannot forget Raquel, whose dedication and appreciation brought new inspiration to my work. Finally, I have to thank two institutions: 1) the Faculdade de Engenharia da Universidade do Porto (FEUP), Porto, Portugal, for approving my Ph.D pro-gram and providing electronic access to research literature; and 2) the Ministéri