Seismic testing may have major impact on concrete wall construction

Professor Santiago Pujol (left) discusses the wall tests with PhD candidate Charlie Kerby at the Structural Engineering Laboratory at the University of Canterbury.
Professor Santiago Pujol (left) discusses the wall tests with PhD candidate Charlie Kerby at the Structural Engineering Laboratory at the University of Canterbury.

Research into the technique used to connect reinforced concrete walls to their foundations is set to have a significant impact on the New Zealand construction industry.

Researchers at the University of Canterbury are conducting earthquake testing to better understand the performance of a construction technique called staggered lapsed splices, which is used to connect the steel bars in reinforced concrete walls to the steel bars coming out of the foundations.

The practice is no longer used overseas, but is still allowed under the New Zealand Building Code.

“The connections between the walls and the foundation are created by overlapping the steel bars — and they rely on the concrete around the bars to transfer the seismic forces from one bar to the other,” explains lead researcher Professor Santiago Pujol from the Department of Civil Engineering, whose research has been funded by Toka Tu Ake EQC.

“This configuration is economical and easy to build, but does not always provide the toughness for the walls to resist the demands of an earthquake,” Pujol says.

Structures using this configuration have collapsed in previous earthquakes in Turkey, Chile, Japan and Alaska. “When these connection fails, the outcomes are often catastrophic.”

Pujol says New Zealand has fortunately not seen similar catastrophic failures of lapsed spliced walls in the Canterbury and Kaikoura earthquakes, but it’s vital to test the seismic strength of staggered lap splices in a controlled environment.

PhD student Charlie Kerby is carrying out the testing at the Seismic Engineering Laboratory by attaching the walls to hydraulic actuators which mimic the effects of a major earthquake by pushing and pulling the walls until they fail.

“We are not interested in how much force is needed to make the wall fail, but how much the wall can deform before failure occurs,” Kerby says.

“Buildings need to be able to move with the earthquake, and we are looking at how much a lap splice can deform until it fails.”

The research is funded by Toka Tu Ake EQC as part of its contestable Biennial Grants, which supports research in improving the resilience of buildings to New Zealand’s natural hazards in order to better protect people and property.

The organisation invests around $19 million each year into research to better understand New Zealand’s natural hazards.

Kerby explains that engineers have alternative options such as welding the steel bars together or using a mechanical connection to transfer seismic forces, but says tradition and economics dominate most of what happens in the construction industry.

“These lap splices have been used for over a century and, from an economic viewpoint, an extra metre of steel virtually costs nothing compared to a specifically designed connection.”

He says the question as to whether lap splices actually perform well in earthquakes has only been raised fairly recently, so the University of Canterbury research will provide vital new insights to inform engineers and construction standards.

Pujol says his team will not only put the spotlight on a potential problem, but also provide solutions for the industry by designing and testing alternative configurations.

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