Elastoplastic model for steel buckling-restrained braces

Buckling restrained braces (BRBs) permit exploiting the hysteretic properties of their core material both in tension and compression. However, the global behaviour of the BRB does not fully replicate the local behaviour of the core material and important differences have been observed in experimental tests, such as an higher bearing capacity in compression and a significant role of isotropic hardening. These observations have thus promoted the use of specific constitutive models for BRBs. Besides a proper description of behavioural aspects, a BRB model should include some highly desired requirements: explicit computation of the plastic component of the deformation as required in BRB capacity models, smoothness of the elastic-plastic transition to improve convergence rate, simple implementation and limited number of input data parameters to facilitate its implementation and use. In order to address the above behavioural and modelling issues, an elastoplastic model for steel BRBs is presented in this paper following a consistent approach based on a rheological scheme that leads to a quite simple constitutive law involving only one internal variable. The adopted formulation allows a straightforward physical interpretation and identification of the constitutive parameters of the model as well as explicit computation of response quantities related to failure and dissipated energy. Response results obtained using the proposed model are compared to the outcomes of experimental tests available in the literature and involving different BRBs under various strain histories, showing good predictions of the experimental behaviour