Wellington Central Police Station

There are many different types of base isolators used in New Zealand to reduce the impact of earthquakes on buildings and lead extruders were used in the Wellington Central Police Station.

This main administration building has 10 storeys above ground with a floor plan measuring approximately 38 by 31 metres. The building was constructed with reinforced concrete using a base isolating system to minimise the effect of ground shaking. This was done to make sure the police could operate from the building immediately after a major earthquake in the Wellington region.

The isolation is achieved by founding the building on 26,800 millimetre diameter reinforced concrete piles which are separated from the surrounding ground by 1,600 mm steel cylinders over much of their length. The bases of the piles are founded 15m below ground in greywacke rock. When the ground vibrates from side to side the building responds with a period of vibration much greater than if it were not separated. The result is that the intensity of movement of the building is much reduced.

In order to dampen the building motion, 24 specially designed lead extrusion dampers were installed at basement level, connecting the top of the piles to the ground. These dampers comprise 200mm diameter steel tubes filled with lead, with constriction near their mid-point. An internal double-acting steel plunger acts to force the lead past the constriction in the tube. The force required to do this is relatively constant and provides sufficient damping to the building motion and also prevents any lateral movement of the building in Wellington’s notoriously high winds. The maximum axial movement of the end of a damper is 400mm.

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Further reading

‘Base isolation for new police station’, New Zealand Engineering Vol.42:8 (September 1987), p.40.

AW Charleson, PD Wright, and RI Skinner, ‘Wellington Central Police Station – Base Isolation of an Essential Facility’, in Proceedings of the Pacific Conference on Earthquake Engineering, New Zealand (5–8 August 1987), pp.377–88.

Eileen McSaveney, 'Earthquakes - Building for earthquake resistance', in Te Ara - the Encyclopedia of New Zealand (updated 9 July 2013).

L.M. Megget, 'From brittle to ductile: 75 years of seismic design in New Zealand,' in Proceedings of the New Zealand Society for Earthquake Engineering Conference 2006.

CR Smart, ‘Wellington Central Police Station – Base Isolation Maintenance’, Bulletin of the New Zealand Society for Earthquake Engineering Vol.46:3 (September 2013), pp 141–56.