By Laurie Winkless
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Designing a structure that can withstand an earthquake is, unsurprisingly, very complicated. For smaller structures, it’s possible to ‘tie’ the roof, walls, floor and foundation together, so that they form a rigid box that can withstand the lateral (sideways) forces produced in a quake. For taller structures, though, you need to take a different approach to structural protection – you need to let the building move a little, while maintaining its structural integrity. For that, engineers often turn to seismic base isolation. These systems work to decouple the building from the ground it sits on, by separating them via flexible, energy-absorbing support structures. That way, when a quake hits and the ground begins to move, some of that energy is directed into the base isolation structures, rather than into the building. These isolation systems can take lots of different forms, including heavy-duty springs that contract and bend, and padded cylinders that roll when a seismic event occurs. But it was a kiwi engineer, Dr. Bill Robinson, that invented the most ubiquitous base isolation system in the world; the lead-rubber bearing (LRB).
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In 1975, Robinson’s lead-rubber bearing design was released, and it shared some similarities with a car’s suspension system. Each pillar was made from laminated layers of rubber and steel, which acted like a spring, pulling the bearing back into shape after each shock. This multilayered sandwich of steel and rubber surrounded a central core of lead, which was the shock absorber. It stretched sideways when the earth shook, stopping most of the quake’s energy from passing into the building. The combination of flexibility and damping quickly made LRBs the isolation system of choice, and their design has hardly changed since.
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Is there a way to make those structures more earthquake-resilient, without the hefty price tag of LRBs? That’s a question that Dr. Gabriele Chiaro from the University of Canterbury found himself asking several years ago. And in 2018, he and a group of colleagues were awarded a $1 million grant by the NZ government to find an answer. Their proposal, titled ‘Eco-rubber seismic-isolation foundation systems’ aims to make use of a major source of waste – tires.
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Let’s start with the raft. This would sit immediately beneath the building, acting somewhat like a traditional foundation. Rubberized concrete is used because it can be more easily deformed than standard concrete – this makes it less susceptible to large cracks, but the rubber also reduces its mechanical strength. That’s where the tiny steel fibers come in. They act as micro-reinforcing – a scaled down version of rebar – which hold the mix together, and stop even the smallest cracks from forming.
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