Retrofitting can be a low-cost option to strengthen at-risk older buildings

By Geoff Cumming

Upgrading a building to earthquake standards can be cheaper than imagined, say some experts. Photo / Doug Herring
Upgrading a building to earthquake standards can be cheaper than imagined, say some experts. Photo / Doug Herring

What frightens owners most about upgrading buildings to earthquake standards is the unknown.

Until buildings are stripped back, it's often hard to tell what needs to be done. But the costs may be surprisingly small, argue experts promoting the "retrofitting" of older buildings.

The Government-funded project promotes wide-ranging options for both reinforced concrete buildings prevalent from the 1940s and earlier unreinforced masonry buildings which are most vulnerable to shaking. Auckland University's Jason Ingham and colleagues Mick Pender and Charles Clifton were in Christchurch on February 22 to deliver a seminar on the options. They took the chance to absorb the lessons.

(See graphic included as a link with this story for more details.)

Fixing parapets to roof supports and awnings to internal framing are basic options. "It was surprising how many parapets had not been restrained and how many restraints had failed," says Ingham.

But buildings which had been earthquake-strengthened generally stood up well.

With URM buildings, the main principle is to tie walls, floors and roofs together with mechanical fixings including steel rods and dowels. This way the building behaves more like a rigid box.

Brick walls can be made more resilient using post-tensioning, inserting steel rods - vertically in walls or attached to the outside - which are then pulled tight.

Thin membranes and coatings, such as fibre-reinforced polymers, can be applied to internal walls so they hold together, and mortar can be strengthened. But there are still lessons to be learned: Ingham and colleagues noticed anchor plates and dowels had pulled from the walls under the intensity of shaking.

Retrofitting is also recommended for a more recent class of building found wanting in Christchurch, reinforced concrete structures built from the 1940s to 1976, when more flexible earthquake design techniques became law. Canterbury University's Stefano Pampanin likens the pre-76 buildings to the human body: the concrete columns are like legs - they resist gravity but sideways force can bring them down. Extra support at the ankles (where the column joins the floor) and knees (column and beam joints) helps resist sideways movement, he says. Techniques include "jacketing" columns with lightweight membranes, bolting walls to columns and steel connections between column and beam.

"These are not expensive solutions ... the work can be undertaken during office refurbishment, or staggered."

Published research by PhD student Temitope Egbelakin of Auckland University's civil engineering department highlights the reluctance of most owners to undertake earthquake "retrofitting" and the perceptions which threaten progress. The 2 year study supervised by Associate Professor Suzanne Wilkinson found most owners had little understanding of earthquake risks, retrofitting options and costs while there was confusion about the threshold levels. The study found a fatalistic attitude towards earthquakes and scepticism that buildings could be satisfactorily upgraded for a reasonable price.

Ninety per cent of owners in low risk cities were unwilling to take mitigation steps within five years. The paper, published in Building Research and Information, calls for agencies to undertake awareness programmes and look at incentives to encourage retrofitting.

- NZ Herald

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