Christchurch shifts from concrete to steel in post-earthquake rebuild

A UB engineering professor has co-authored a report with potential significant impacts on how modern cities may be reconstructed following earthquakes.

“Reconstructing Christchurch: A Seismic Shift in Building Structural System(41.6 MB)” is a 170-page report that details the reconstruction of Christchurch, the largest city in the South Island of New Zealand, following the 2010-11 earthquake series that shut down the city’s central business district for years.

The report examines the types of structural systems used during the reconstruction of the city, and some of the technical, sociological and political choices associated with those decisions.

Co-authors Michel Bruneau, professor in UB’s Department of Civil, Structural and Environmental Engineering, and Greg MacRae, professor at the University of Canterbury in Christchurch, developed the report to provide a resource for other urban areas that experience natural disasters.

Bruneau and MacRae published their findings through the New Zealand Quake Centre, a partnership between the New Zealand government, University of Canterbury and several industry groups. MacRae is a board member of the Quake Centre.

“It is becoming a more widely held belief that preventing loss of life as a seismic performance objective is not sufficient for a good modern structure,” says Bruneau.

Currently, the minimum design code regulations are intended to prevent loss of life. However, “what emerged through this study is that many building owners and tenants — and the engineers who design these structures — want to prevent the loss of buildings and minimize damage caused by earthquakes,” he says.

Bruneau has made four trips to Christchurch since 2010 to examine the damage caused by the 2010-11 earthquakes. Prior to that, he spent three months in New Zealand to share research experiences with MacRae, a longtime professional colleague.

When Bruneau visited the area again in 2016, most buildings in the central business district had been demolished, and rebuilding was underway.

“The breadth of structural systems being used as part of the Christchurch reconstruction, and the significant differences with what used to be the norm before the earthquake, could not be missed. However, the explanations for these substantial changes in structural engineering practice were anecdotal at best,” Bruneau says. “We thought, ‘Why not document the changes?’ Christchurch used to be a concrete city, but this certainly wasn’t the look of the new Christchurch based on sidewalk surveys.”

Reinforced concrete structures are “difficult to inspect, repair and reinstate after a major event. For that reason, reinforced concrete moment frames as lateral-force-resisting systems… are practically non-existent in the central business district rebuild,” according to the report.

The researchers wanted to determine why this change occurred, and when the trend toward steel structures started.

MacRae and Bruneau interviewed different stakeholders. They spoke with structural engineers from the Christchurch firms that designed more than 60 percent of the post-earthquake buildings in the city’s central business district, engineers in New Zealand’s two largest cities (Wellington and Auckland), an architect, a project manager and a developer. The report includes data on 74 buildings.

Following the earthquakes, the number of buildings with steel structures increased substantially. The authors attribute the shift to a number of factors, including the belief that steel is easier to repair than reinforced concrete.

Two factors that played a major role in determining which types of structural systems to use as part of the Cristchurch reconstruction were tenant and owner expectations, and the structural engineers’ professional opinions, according to the report.

The larger impacts of the report could affect seismic-design practices in other modern cities in seismic regions, Bruneau says. New Zealand’s building codes and seismic-design requirements are similar to those in North America, and Christchurch’s mix and vintage of construction types before the earthquakes were comparable to some major North American cities.

“It’s difficult to predict what the report’s impact might be,” he notes. “But I suspect it will reach many engineers, as well as producers and trade groups representing structural engineering materials and devices. It may also affect the way they design for earthquakes and plan for post-earthquake repair and reconstruction. The steel industry, in particular, will likely be keenly interested in the findings of this study.”

The research was funded by the Quake Centre.