Bridges

At 1.76km long, the Rakaia River Bridge is the longest bridge in New Zealand. When combined with the Rangitata River Bridges (identical structural form), these structures comprise 223 spans and over 2.7km of reinforced concrete bridge on State Highway 1S in South Canterbury. The bridges were all constructed circa 1939 and in recent years have become a significant impediment to freight movements in the South Island. To avoid costly bridge strengthening, the New Zealand Transport Agency embarked on an ambitious programme to develop a cost effective, real-time Structural Response Monitoring system.

The Structural Response Monitoring systems employed to date include a range of state-of-the-art technology using accelerometers, displacement transducers, vehicle weigh-in-motion testing, radar interferometry, wireless technology, and low demand solar powered systems. By understanding how the bridge is behaving under live load, analysis models could be calibrated to better understand the true capacity of the bridge and its likely failure mechanisms. This allowed monitoring systems to be focussed on key locations, allowing early intervention to be undertaken to avoid significant structural damage.

The system has been in place for over a year now, and has provided a significant improvement in our understanding of the behaviour of these common reinforced concrete T-beam structures. This includes the low likelihood of first degree resonance of the beams, calculation of a bridge specific impact factor, considerable continuity of the previously assumed simply supported spans, the influence of vehicle positioning on load effects, the degree of overloading that the bridge is sustaining, and the influence of temperature on the performance of the structure. The real-time monitoring system has provided confidence that the increase in loading is not adversely affecting the bridges, and has helped ensure that the South Island freight routes remain well connected.