Behaviour and design of generic buckling restrained brace systems

Abstract

The recent series of damaging earthquakes in Christchurch, New Zealand (NZ) has encouraged greater recognition of the post-earthquake economic impacts on NZ society and higher emphasis on low-damage earthquake resisting systems. Buckling Restrained Braces (BRB) are seen as a significant contender for such a system. They have been developed and used in both North America and Japan and are recognised for their superior seismic performance compared to existing concentrically braced systems due to the suppression of brace buckling in compression, and hence the development of equal strength and stiffness under tension and compression loading. However, the focus of development in those countries has been on establishing a testing regime to which companies produce patented systems. This has limited their application in New Zealand due to small demand and has generated interest in development of a generic solution.

This research project focuses on the development of a reliable design procedure and detailing requirements for a generic BRB system. This started with the development of a design procedure based on modifications of the concentrically braced frame (CBF) design procedure contained in HERA Report R4-76 (1995). This has been used to develop a representative design for a 10 storey building, from which a brace size and bay has been chosen for experimental testing. A series of dynamic sub-assemblage tests were performed at the University of Auckland on this BRB frame with two different brace connection configurations to gauge the performance of the designed system. The results are presented and discussed herein. An initial prototype model for analytical modelling of the sub-assemblage frame has also been constructed and subjected to inelastic time history analyses.

The experimental tests show stable hysteresis loops which is a principal feature of the BRB system, albeit with the occurrence of slack in the connections. These test results show the reliability of the proposed design procedure and detailing, especially after procedural modifications to prevent slack from occurring in the two different connection systems.