Bridges

The Kawarau River Bridge in New Zealand has presented some challenges to address design shortcomings identified after more than 40 years of service.

The two lane steel bridge spans the Kawarau River Gorge with an 85m arch and associated land spans for a total length of 120m. Bridge inspections have revealed cracking in transom members outside the lines of the steel arch ribs. Subsequent assessment indicated that eccentric live loading generated differential displacement of the arch ribs that was not allowed for in the original detailing of the deck, and very high strains were being induced in the transom cantilevers as a result.

The cracking had progressed through half the transom section in some mid span locations, creating the potential for the live load and seismic capacity to be compromised. This led to the need for a repair strategy that recognised the threedimensional deformation characteristics of the arch ribs and deck system under live loading.

The actual live load behaviour of the structure tested under static and dynamic conditions showed some differences from the analysis, but enabled the fatigue analysis and repair technique to be validated for construction. The effectiveness of the repair was established through a programme of instrumentation and live load testing.

This project revealed further shortcomings in the original detailing that has led to additional inspections and non destructive testing of steel deterioration mechanisms being incorporated into the performance monitoring programme for this structure, with the expectation that further enhancements to overcome these shortcomings will be required.