Real-world seismic strength testing on decommissioned bridge
Deconstruction of the Whirokino Bridge south of Foxton gave Auckland University’s Dr Lucas Hogan and his team a chance to real-world test seismic performance and strength of a common type of New Zealand bridge.
The 90-year-old bridge on State Highway 1 south of Foxton was recently replaced by Waka Kotahi NZ Transport Agency (Waka Kotahi), with a wider $70m structure over the Manawatū River and Moutoa floodplain. Deconstruction of the old bridge is giving Dr Lucas Hogan from the University of Auckland an unprecedented opportunity to do “real life” testing of how a long bridge behaves in an earthquake.
University of Auckland researchers Dr Lucas Hogan, left, and Dr Max Stephens, under the decommissioned bridge where they will conduct seismic testing on the 90-year-old piles.
“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,” says Dr Hogan, whose research is the result of a ground-breaking partnership between a multitude of government, academic and corporate organisations.
The research, funded by Natural Hazards Commission Toka Tū Ake and QuakeCoRE, is carried out in cooperation with Waka Kotahi, as the owner of the bridge, and demolition sub-contractors working alongside lead contractor Brian Perry Civil, a division of Fletcher, who are 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. These types of piles are used in around half of all bridges in New Zealand, and many internationally, so it’s very practical science,” says Dr Hogan.
View from the old bridge to the new bridge across the Manawatū River and Moutoa floodplain near Foxton.
Dr Jo Horrocks, Head of Strategic Research and Resilience at Natural Hazards Commission
Toka Tū Ake, explains that the Commission invests over $17m 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,” says Dr Horrocks.
Waka Kotahi Senior Manager Project Delivery Andrew Thackwray says with more than 4,500 bridges on New Zealand’s roading networks, 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. This research will let us continue that proud tradition and build even more resilient bridges,” he says.
Waka Kotahi is working closely with Brian Perry Civil and deconstruction experts from Jurgens Demolition to support the scientists in carrying out their research, without adding to costs or timeframes. In order to integrate the work effectively into the demolition programme, the research team have become an integral part of the project team.
“Everyone working on this bridge knows how important and unique this research is for New Zealand, as well as for our own work. We are proud to support Dr Hogan and his team to get the maximum results from this once-in-a-generation opportunity,” says Fletcher Wellington Regional Manager, Andy Burgess.
The Whirokino research project has been planned for over two years. Dr 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.
“That first stage includes installing instruments on the bridge to find out how it moves dynamically,” says Dr 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. 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,” says Dr Hogan.