[1] Department of Structural Engineering,
[2] Technical University (Politecnico) of Milan,undefined
[3] Piazza Leonardo Da Vinci 32,undefined
[4] I-20133 Milan,undefined
[5] Italy,undefined
[6] Department of Engineering Mechanics,undefined
[7] Tsinghua University,undefined
[8] Beijing 100084,undefined
[9] P. R. China,undefined