A Practical Approach to Design Supplemental Damped Isolation Systems

Abstract

With an update to the NZS1170.5 design hazard expected in 2023, New Zealand designers will need innovative solutions in-order to manage potentially significant increases in design hazard, particularly around the lower North Island.

Seismic base isolation has been increasingly used in New Zealand over the past decade, typically adopted for new buildings of high importance, in regions of moderate to high seismicity. Reductions in seismic base shear are proportional to increased displacements of the isolation system, and with an increase in seismic hazard this inherent a trade-off becomes more challenging. Increased isolator displacements require larger moat widths, challenging flexible service connections across the isolation plane and larger stabilising structures to sustain the roller-over demands at each isolator bearing; therefore, a careful balance must be achieved with a holistic overview to balance costs.

Base isolation with supplemental damping is an efficient means to gain the benefits of both increased superstructure period and additional energy dissipation to control displacements. This has been implemented on a recent high importance project in Wellington, New Zealand by Holmes – and is set to be the first of its kind in New Zealand.

This paper outlines a methodology for designing supplemental-damped isolation systems to meet both base shear and isolation displacement targets. This captures conceptual design through to validation via time history, and a proposed compliance pathway. The authors also provide interim recommendations for adapting the approach in near fault regions, as well as early design steps to de-risk common concerns of highly damped solutions.