Concrete slot reduces shake damage

Researchers at Canterbury University have developed an inexpensive detail to improve the way reinforced concrete buildings perform in earthquakes.

Structural engineering PhD candidate Craig Muir said tests at the university on a 60-tonne, two-thirds scale model of the lower two storeys of a seven-storey building stood up incredibly well when subjected to earthquake-like movement.

Tests have found that creating a slot about three-quarters of the way up a concrete column reduces damage to the building and its floors and increases the safety and performance of the building during an earthquake.

Muir said the technique was subtle and would be cheap and simple to incorporate into building designs.

“We don’t want to make these things overly complex because it creates a barrier to designers using it.”

The slotted beam allowed the building to flex and prevented the concrete beams from increasing in length, which happened with traditional building methods, lessening damage to the frame and floors, resulting in a much safer building.

In traditional reinforced concrete beams, cracks form at the beam ends and during an earthquake these cracks increase in size and number, causing the beams to increase in length, damaging the floor. If the shaking was severe enough, it could lead to a building collapse, Muir said.

He has found that if the concrete is used in a different way with different details, then it changed the behaviour of the building.

Students at Canterbury University have spent the better part of 10 years developing safer buildings as part of the Future Building Programme, a project funded by the Foundation for Research, Science and Technology, and undertaken at Canterbury and Auckland universities.

Muir said the original “slotted beam” concept was developed in Japan about 12 years ago, but researchers there could not get the concept to work reliably, so Canterbury University researchers stepped in and developed it further, coming up with positive results.

Muir said he felt proud to be working on a project which would improve the performance and safety of buildings during quakes.

Other methods could strengthen concrete structures and make them perform better, but the techniques were often complicated and expensive, he said.

It was difficult to say what quake magnitude level the structure withstood, because the testing was done in line with internationally accepted methods for assessing structural performance.

The testing was performed by pushing and pulling the structure sideways in each direction individually, and then at the same time, in a realistic and demanding test. The structure was tested up to 3.5 per cent of interstorey drift. In New Zealand, buildings are designed to not exceed 2.5 per cent inter-storey drift, Muir said.

Muir now planned to dismantle the test structure, extract some portions of it and test them to see how much life was left in them. That data would be used to determine the level of retro-fitting that would be needed after a quake.

The portions would then be retrofitted and tested again.Muir said the end goal would be to get the new design included in the New Zealand Concrete Structures Standard so the detail was available to designers around the country.


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