Wellington’s Te Papa museum is internationally renowned for its fine collection as well as its ingenious earthquake-proof technology – the 135 base isolators (lead-rubber bearings) on which the building rests. The story about the evolution of those bearings is perhaps less well-known. LAWRENCE SCHÄFFLER explains.
BASE ISOLATORS – large, rubber blocks with lead column cores – create a cushion between a building and its foundations. The lead core softens under seismic pressure, absorbing energy that would otherwise be transferred to the building.
The concept was conceived and developed in the early 1970s by Dr Ivan Skinner and Dr Bill Robinson, two scientists with the then Department of Scientific and Industrial Research (DSIR). Its real baptism of fire came in 1982 with the construction of Wellington’s William Clayton building. The world’s first office building to use the technology, it rests on 80 bearings.
Since then base isolation has been used in thousands of buildings worldwide, including the San Francisco City Hall and the approaches to the city’s Golden Gate Bridge. In many cases the technology has been retrofitted to existing structures in seismically-active areas.
The base isolation principle was born when Skinner, head of Engineering Seismology at DSIR, told Robinson he proposed to use steel dampers to seismically isolate the new office building being planned for the Ministry of Works (it was later renamed the William Clayton building).
Robinson thought there had to be a better damping material than steel and his investigation eventually arrived at lead. But his experiments with lead shear dampers weren’t particularly successful: the lead failed fairly quickly – it needed to be supported in some way. His solution was placing a lead plug through the layers of steel and rubber.
Dreaming up the base isolation principle was one thing – simulating and proving its effectiveness on a realistic scale was something else. That problem fell to Cameron Smart – another DSIR scientist.
As was a common problem for the era, research funding was tight, so Smart designed a test rig which used – as its central component – an old D8 Caterpillar bulldozer scrounged from the Ministry of Works. Its 100kW diesel engine was capable of providing a vertical load of 300 tonnes and a displacement of up to 91mm.
The following extract is paraphrased from the book Smart wrote about his family in 2011: Baby Boomer – A Life to be Bettered.
“In the late 1960s Ivan Skinner conceived of base isolation to defend buildings and bridges against earthquakes. Ivan developed the mathematical theory but needed a machine to test the full-size dampers – I was to design it!
“The Ministry of Works could make an old Caterpillar D8 bulldozer from a quarry available to us. The deal was done for $1000, and an area in one of the wooden sheds built by the US Marine Corps in 1942 was partially cleared for the machine.
“I decided to take the power from the left rear sprocket through an eccentric, using this as the first link in a four-bar chain to change rotary to reciprocating motion. The eccentric was to take the form of one inside another, so that by rotating the inner with respect to the outer the two stroke could be varied between tests.
“The eccentrics and connecting rod were to be made of spheroidal graphite cast iron, and I found some elderly patternmakers in Porirua to make the pattern equipment. The other parts were to be steel weldments.
“I made a 1/10 scale model of the reaction frame in thin sheet steel and took this to several fabricators, telling them I wanted one of these but 10 times as big. William Cable’s foundry in Gracefield and their fabricating shop in Kaiwharawhara proved to have the best combination of price and competence, so I awarded them the contract.
“We could not afford a proper paint job, so we scrubbed off the worst of the dirt and flaking paint with wire brushes and brushed the Caterpillar parts yellow and the PEL parts orange, a colour scheme very popular in the 1970s. I called it the Masher – Machine for Simulating Earthquakes.”
Robinson tested numerous bearings on the Masher, progressively increasing the diameter until, at a lead diameter of 170mm, the D8’s gearbox failed after two cycles. This result was deemed a success, the William Clayton building received its bearings.
Robinson was honoured with a Queen’s Service Order in 2007 in recognition of his contribution to the reduction of the earthquake vulnerability of communities. He died in 2011.
The first base-isolated structures
While the William Clayton building was the world’s first office building to receive the lead-rubber bearings, it wasn’t the first structure to use the DSIR’s base isolation concept. That honour rests with the North Island’s soaring Mangaweka Railway Viaducts, completed in 1981, a year before the William Clayton building. They are the world’s first base isolated bridges.
The three massive viaducts, among the highest in New Zealand, were constructed between 1973 and 1981 to move the Main Trunk Line away from geologically unstable land. The South Rangitikei viaduct is a 78 metre tall, 315m long triple-span structure, while the Kawhatau (73m high) and North Rangitikei (81m high) viaducts are both 110m long single-span structures.
All use concrete twin-shafted vertical piers carrying a continuous prestressed hollow box. In an earthquake the pier bases could lift up to 13cm to allow energy and pressure to shift from one pier leg to the other. The rocking action is controlled by large “energy dissipaters” installed in the pier bases.