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Investigation: Earthquake Impact On Chch Pedestrian Bridges

Investigations Into The Earthquake Impact On Christchurch’s Pedestrian Bridges
November 1, 2012

Many Christchurch pedestrian bridges were damaged in the Canterbury earthquakes and more than 50 percent of the bridges in the city will need to be repaired.

Bridges, such as the Dallington pedestrian bridge which crosses the Avon, were damaged by the lateral spreading of the river banks crushing the bridge deck.

Others, such as the South Brighton Road bridge and the Anzac Drive bridge, were respectively damaged by movement of the abutments (which support each end of the bridge) and also severe bending of the bridge piers which caused concrete cracks and spalling (the cracking/expulsion of the concrete cover which protect bars from corrosion) to occur on the bridge piers.

For a final research project, a University of Canterbury (UC) civil engineering student, Royce Liu, is investigating how to mitigate such damage in future and also to research new technologies which can be used to aid the achievement of less damage to bridges.

``I am especially interested about seismic structural engineering and how that can apply to bridges which are very important for the transport of goods and allowing people to safely cross rivers. Next year I'll work on this project with classmate Matthew Henden under Dr Alessandro Palermo's supervision.

``We will be studying technologies which could reduce damage caused by earthquakes on bridges as well as the effect that the shape of the bridge deck has on its seismic performance. A technology that we will be looking at is the use of rocking bridge piers.

``These piers are different to normal bridge piers because they are made up of concrete segments held together by a pre-stressed steel cable and have replaceable short steel rods which connect each block from one to another. The reasoning for having such a system is that in an earthquake, the blocks which make up the bridge pier can move relative to one another reducing the stresses created in the concrete and stopping large cracks from forming.

``Also the movement of the blocks is resisted by the short steel rods which are deformed as the blocks move past one another and dissipate the energy given to the blocks by the earthquake (this is very similar to bending a paper clip back and forth which creates a resistance to the bending motion and dissipates the energy you put in by deforming and heating up). Finally the pre-stressed steel cable pulls all of the blocks back into line so that the bridge piers are as they were before the earthquake.''

This type of technology was most applicable to large highway bridges, such as the Port Hills highway overbridge in Christchurch, and could reduce damage and cost of repair to bridge piers as well as reducing the down time of the bridge. The technology has been already implemented in buildings and NZTA is now looking with interest in possible application for bridges.

UC’s Dr Alessandro Palermo has just presented two days ago as invited speaker at the first national conference on bridges in Wellington.

Photo: Royce Liu



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