Capacity design of traditional and innovative ductile connections for earthquake-resistant CLT structures

Traditional connections in earthquake-resistant cross-laminated timber buildings are susceptible of brittle failures, even when buildings are designed and supposed to be ductile. This is mainly due to the large underestimation of the actual strength of the ductile components, with consequent increased strength demand for the brittle parts, which may fail if designed with insufficient overstrength. Recent studies demonstrate that the use of steel connections characterized by a well-defined mechanical behaviour can improve significantly ductility and dissipative capacity of cross-laminated timber structures and the reliability of the capacity design. In this paper, the conceptual model of capacity design is discussed, proposing some modifications to improve its reliability for traditional and high-ductility connections for CLT structures. Results from quasi-static cyclic-loading tests of an innovative ductile bracket are presented and the corresponding overstrength factors are computed using the proposed conceptual method and compared with values available in the literature for traditional connections. Finally, a comparative application of the capacity criteria to the design of the innovative bracket and of a traditional nailed connection is presented and discussed.