Researchers test how much force ageing bridges can stand

For three years, parts of the Manawatū's 84-year-old Whirokono Bridge have been have been bashed, beaten and bent as part of a university project testing what the country's ageing bridges can — and can't — stand.

"We went, 'OK, let's just push as far as we can, and just see how does it break," the University of Auckland project's lead researcher Lucas Hogan said.

Using hydraulics — and up to 60 tonnes of force — researchers have been pushing parts of the bridge to its limits.

"A lot of the bridges in New Zealand were constructed by the Ministry of Works in the '30s, in the '50s, long before the world knew how to design structures for earthquakes", Hogan said. "And what we found with these older structures is they have sufficient strength and they do have lots of deformation capacity."

Basically, that means they can bend a lot without breaking.

The research has been funded by research centre QuakeCoRE and Toka Tū Ake EQC, also known as the Earthquake Commission.

"The findings were a bit surprising really," EQC's head of research Natalie Balfour said. "It's great to see that these bridges are performing well."

The Whirokono Bridge used to be part of State Highway 1 south of Foxton.

The project started on-site, as it was being taken apart.

Since then the research team has been testing different parts of the bridge and the New Zealand Transport Agency Waka Kotahi is keeping a close eye on the project.

"A lot of our bridges are actually not far off that 80 to 100 year age", the NZTA's Moustafa Al-Ani said. "The more we understand about our existing structures the more we can kind of use that to inform the decision making we take forward in terms of where we put effort and money."

And earthquakes aren't the only threat, made more apparent by events like Cyclone Gabrielle earlier this year.

The research team is now investigating how they can make old bridges even stronger.

"What we're looking at with this one, can we do some of these lower cost retrofits which can get implemented cheaply which means they can happen on more bridges," Hogan said.