The remarkable earthquake isolation system at Wellington's Parliament House
To make sure Parliament House and the Parliamentary Library are able to withstand an earthquake of up to 7.5 on the Richter Scale, a special engineering system of base isolation was used in their foundations during the strengthening and refurbishment project of 1992-1995. Most masonry buildings constructed prior to the Napier earthquake in 1931 were built with minimal lateral resistance. They are considered to be brittle structures, which are likely to collapse in a moderate earthquake. These parliamentary buildings came within this category.
The original building foundations of Parliament House were laid in 1912 and consisted of a mix of gravel and cement. The Parliamentary Library consisted of two wings of a similar construction built in two phases, 1883 and 1899. By the 1980s, the foundations of both buildings were showing signs of deterioration and with the Wellington Fault within 400m of the parliamentary site, there was concern as to the likely impact of an earthquake on these buildings. Not only were the buildings dangerous to work in but the office accommodation was shabby and needed to be modernised. As both buildings had Historic Places Trust “A” classifications requiring permanent preservation, a balance of conservation and strengthening had to be struck. In 1989, the New Zealand Government made the decision to strengthen and refurbish the buildings.
The brief for strengthening the buildings.
Part of the briefing process was to ascertain the level of earthquake risk and what loads were likely to be placed on the structures in certain conditions taking into account the close proximity of the Wellington Fault and the impact of near-fault effects. It was determined by the Department of Scientific and Industrial Research that the risk of an earthquake up to 7.5 Richter Magnitude could occur every 500 to 600 years. A Geological Survey showed that the fault had not moved for 350 years and so the likelihood of it doing so on this scale in the 150 year design life of the buildings was predicted at 10% to 50%.
Based on these predictions, a system of base isolation was devised to separate Parliament House and the Parliamentary Library from their original foundations and place them on lead-rubber bearings.
1992 saw the beginning of the biggest strengthening and modernising project ever undertaken in New Zealand.
Base isolators used for earthquake proofing
417 base isolator bearings were placed within the existing foundations under Parliament House and the Parliamentary Library and the buildings above separated from their original foundations by a 20mm seismic gap. The aim of base isolation is to reduce the transfer of earthquake forces from the foundations to the building above, thus significantly reducing the need for extensive and intrusive strengthening concrete shear walls throughout the building.
Installing the base isolators meant propping up the whole building section by section, cutting out large 7 tonne blocks of the existing foundations and building new concrete foundations to hold the base isolators. Original foundations above and around the base isolators have been considerably strengthened by new heavily steel reinforced concrete sandwich beams.
Strengthening Parliament House's foundations with base isolators, August 1992.
Source: Parliamentary Services
Base isolator bearings were invented and developed in the 1970s in New Zealand by Dr Bill Robinson. They consist of many sandwiched layers of bonded rubber and steel with a central core of lead. They are designed to take the weight of the building and let the foundations move sideways - up to 30cm each way - going with the earthquake. The use of a solid lead core is significant because lead is able to soften when under pressure thereby absorbing energy that would otherwise be transferred as movement. Because the base isolators act as shock absorbers, the buildings themselves move much less. There is much less risk of damage to the buildings or injury to the people in them.
The internal structure of the buildings was reinforced by new shear walls, strengthening critical areas with reinforced concrete, and tying walls and floors into the total structure. All this was carried out while minimising the amount of disturbance to spaces and finishes and maintaining a sensitive balance between strengthening and conservation.
The buildings on their base isolators have so far stood the test of time although Wellington has not experienced an earthquake of the magnitude of the 1855 earthquake that measured 8.2 or of the magnitude 7.1 and 6.3 experienced in Christchurch in 2010 and 2011.
This video shows how the whole thing was done:
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