This report presents the results of a comprehensive component testing program on a type of buckling-restrained brace known as the Unbonded BraceTM. This commercially available seismic brace enhances the earthquake resistance of building structures by providing supplemental strength and energy dissipation.
The report introduces the braces by presenting an in-depth analysis on their stability against: (a) global flexural buckling, (b) buckling of the inner core in higher modes, and (c) plastic torsional buckling of the inner core. After establishing the formulae that describe the brace capacity the report proceeds with a survey on past experimental studies conducted on unbonded braces.
Subsequently, the report presents the results from a two-phase experimental testing program at UC Berkeley that tested five buckling-restrained braces with various configurations. The unbonded braces tested are representative of the braces designed for use in two major building projects in Northern California. The test results established that the unbonded braces tested deliver stable and repeatable behavior. Their plastic deformation capacity exceeds the specified requirement both in terms of ultimate deformation and in terms ofcumulative plastic deformation.
The behavior of the unbonded brace is characterized at the macroscopic, force-deformation level with the Bouc-Wen model, which is found to predict the brace behavior with fidelity. The parameters of the macroscopic model are related to the geometric characteristics of the brace and the mechanical properties of the steel and can be used with confidence to characterize the behavior of the braces.
A nonlinear analysis on the response of a single-degree-of-freedom structure equipped with the mechanism of the unbonded brace illustrates the benefits of added strength and dissipation in association with the ductility demands on the brace. This analysis also demonstrates that a bilinear force-deformation relation produces results in close agreement with the Bouc-Wen model.
The study concludes that the unbonded brace is a reliable and practical alternative to conventional lateral load-resisting systems, and is capable of enhancing the earthquake resistance of new and existing structures.
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