Theoretical and experimental analysis of dissipative buckling restrained braces

Buckling restrained braces are passive energy dissipators used for seismic protection of building frames; such devices consist of slender steel bars connected usually to the frame to be protected either like conventional (concentric) diagonal braces or like chevron braces. Under horizontal seismic motions, the interstory drifts generate axial strains in the steel bars beyond their yielding points, such tension-compresssion cycles constitute the hysteresis loops. Thebuckling of the steel bars (core) is prevented by embedding them in a stockiest encasing; it consists usually of a steel tube filled with mortar. A crucial issue is to allow sliding between the core and the encasing to prevent relevant shear stress transfer. This work aims to contribute to a better understanding of the behavior of buckling restrained braces, the final objective is to foster its mass use in developing countries (in earthquake prone regions), particulary for reinforced concrete building frames. The research approach consists of designing, producing and testing (in Argentina) five reduced scale dissipators (about 400 mm long) and of taking profit of the gained experience to design, to produce and to test (in Spain) four full size (near 3000 mm long) prototype devices. All these tests are a individual, i.e. no subassemblies (accounting for the building frames) are considered. The main conclusion is that it is possible to obtain a reasonably cheap (about 1000 US$ per unit; this amount corresponds to production in Spain (summer 2006) without optimizing the fabrication process), efficient, robust, low maintenance and durable prototype device requiring only a low-tech production process (suitable for developing countries). Moreover, the results show that the fatigue life of buckling restrained braces, even highly uncertain, can be significantly bigger than expected (according to some previously published results); it might allow extending the life of these devices after a number of strong seismic inputs. A numerical analysis of the buckling behavior of these devices is performed; it allows formulating some design recommendations. Further research needs are identified.