Application of Low Damage Tension Braces in Top Extensions of Existing Buildings: A Case Study for Seismic Upgrade

Abstract

Tension-only braces are generally used as diagonal cross members (rods, flat bars or similar) to resist lateral loads. The application of these braces has been limited to low-rise buildings (generally up to two stories) by most standards and is not recommended for seismic applications. This is firstly because of the lack of damping and ductility and, secondly, the absence of a reliable over-strength mechanism. When these braces are combined with resilient self-centring devices, the product is a low-damage system with considerable hysteretic damping that eliminates the issues mentioned for traditional tension bracing systems. This paper describes a unique case study where innovative tension-only braces are used for seismic upgrading and refurbishment of an existing concrete building. The 48 Greys Ave is a 10-storey office building in central Auckland undergoing a major upgrade that includes adding a new upper level to the structure. Although the seismic risk at this site is relatively low, the addition of the new floor meant that it could experience high accelerations in a design-level earthquake. The paper describes the project and elaborates on the innovative approach adopted and the analysis method. It also includes information on the resilient devices and physical tests of them before they are installed. This paper's findings will interest engineers and researchers passionate about low-damage design and innovative seismic upgrade solutions.