Cyclic behavior of ellipse and peanut-shaped perforated buckling-restrained braces

Auxetic (effect of negative Poisson's ratio) metamaterials have increasingly attracted attention recently. Buckling-restrained braces (BRB) are commonly used as structural components to resist lateral loads and dissipate earthquake input energy. This study introduced the concept of auxetics to develop novel types of BRB. Two forms of perforated auxetic structures, including ellipse and peanut-shaped structures, were used in this study to develop new types of BRB core plates. Both experimental and numerical studies were conducted to evaluate the hysteretic performance of the two newly developed types of perforated BRBs. The test program included two ellipse-shaped BRB and three peanut-shaped BRB specimens. Each specimen had a fracture that occurred at the ligament between neighboring holes. Out-of-plane deformation was insignificant in each specimen. Test results showed that the perforated BRBs had a stable hysteretic response and good energy dissipation capacity. Finite element modeling illustrated that plastic strain was localized at the ligaments between neighboring holes. Parametric studies were also conducted on the effects of two key parameters: the unit cell number and the porosity. With a given dimension of the yielding segment, an increase in the unit cell number increased the BRB load-carrying capacity. With a given unit cell size and number, an increase in the porosity increased the equivalent viscous damping ratio, and decreased the load-carrying capacity and effective Poisson's ratio. For either ellipse or peanut-shaped structures, the effective Poisson's ratio may become negative as the porosity larger than 0.4. A negative Poisson's ratio may be beneficial for BRB energy dissipation capacity. The limitations of the numerical models and future research directions were also discussed in this paper.