1. a Laboratoire de Mécanique des Solides (CNRS UMR 7649) , Ecole Polytechnique , 91128 , Palaiseau cedex, France
2. c Department of Civil Engineering , University of Minnesota , 500 Pillsbury Drive S.E., Minneapolis , MN , 55455 , USA
3. b Laboratoire Jacques-Louis Lions , Université Pierre et Marie Curie , 75252 , Paris cedex 05, France