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Decommissioned NZ bridge used for seismic research – Bridge design & engineering

The 90-year-old, 1.1km-long Whirokino Bridge on State Highway 1 south of Foxton was recently replaced by Waka Kotahi NZ Transport Agency with a wider US$43 million structure over the Manawatu River and Moutoa floodplain.

Deconstruction of the old bridge is giving Dr Lucas Hogan from the University of Auckland the opportunity test how a long bridge behaves in an earthquake.

“We have done a lot of bridge testing in the lab, and this is a unique opportunity to put a real bridge through its paces and even push it to failure,” said Dr Hogan, whose research is the result of a partnership between government, academic and corporate organisations.

The work is funded by the Earthquake Commission (EQC) and QuakeCoRE, carried out in cooperation with Waka Kotahi as the owner of the bridge, and demolition subcontractors working alongside lead contractor Brian Perry Civil. The lead contractor, which is a division of Fletcher, is fine-tuning the deconstruction programme to fit with the scientific needs of the University of Auckland team.

“A big focus will be on how the piles holding the bridge up behave in earthquake conditions,” said Hogan. “These types of piles are used in around half of all bridges in New Zealand, and many internationally, so it’s very practical science.”

Dr Jo Horrocks, head of strategic research and resilience at EQC, said that the commission invests over US$10 million each year in research to create stronger homes and infrastructure to reduce the impact of natural hazards. “We’re really pleased to be part of the team on this project. That we have so many organisations involved shows the importance of ensuring New Zealand has resilient infrastructure.”

Waka Kotahi senior manager project delivery Andrew Thackwray said that there are more than 4,500 bridges on New Zealand’s road networks and so strong, safe bridges are vital to keep the country moving. “New Zealand’s engineers have created a terrific network of bridges that have proven to be extremely robust despite all the natural hazards we are exposed to in this country,” he said. “This research will let us continue that proud tradition and build bridges that are even more resilient.”

The Whirokino research project has been planned for over two years. Hogan had to wait until the Covid-19 lockdown was lowered to Level 3 to join his colleague Dr Max Stephens in their new extended bubble to start the first phase of the research.

Above: Lucas Hogan with colleague Max Stevens

“That first stage includes installing instruments on the bridge to find out how it moves dynamically,” said Hogan. “Because seismic waves travel at a finite speed, one end of a bridge will start shaking before the other. In a long bridge, this can potentially cause a whipsaw effect. While many computer models have shown this effect, there is very little physical testing to prove it.

“The Whirokino Bridge provides an opportunity to see how these long bridges behave, which is very important given that there are many such bridges over braided rivers in the South Island.”

The research will take place over 10 weeks and will include removing sections to test at the University of Auckland, as well as testing the piles on site to simulate the stresses of earthquake shaking by pushing and pulling in a controlled manner.

“This will tell us a lot about how these bridges behave after 90 years in service,” said Hogan. “Having the whole bridge means we can also test potential fixes for making the columns and piles more robust which could be used on any similar bridges if needed.”