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Marlborough District Council maintains more than 340 bridges around Marlborough - $69M worth of vital structural assets belonging to our region.
Marlborough Roads continually checks the capacity of every bridge to maintain its safety. The average lifespan of a bridge is about 100 years so, on average, we expect to replace three bridges each year.
As bridges deteriorate they may be assessed as able to carry only a limited Heavy Commercial Vehicle loading. When a bridge is no longer able to carry Class I loadings (the maximum permissible legal load for a truck to operate without an over-weight permit) Council will then “post” a reduced weight limit on a sign at both ends of the bridge. This might either limit axle loadings or require a reduced speed limit. Marlborough Roads advertised the list of “posted” bridges each year.
Sometimes, very detailed structural analysis of a bridge will be done, which will enable a higher weight limit to be set. For example, the High Street Bridge over the Taylor River was previously weight-restricted but concrete strength-testing and strain gauge analysis in the early 1990s has confirmed this bridge is capable of carrying all Class I vehicles without restriction. Similarly, very recent analysis of some bridges on State Highway 6 has provided reassurance that these bridges don’t need strengthening to take High Productivity Motor Vehicles - which can weigh up to 62 tonnes.
The risk of a serious bridge failure or serious bridge damage through overloading or speeding on a “posted” bridge is very high, with potentially catastrophic results.
Marlborough Roads Manager Frank Porter says heavy vehicle operators are generally good at obeying the weight limits but he is concerned that some drivers don’t observe the speed limits. Speeds higher than the posted limit will effectively increase the axle loadings by up to 30%, which then places these bridges in a very highly stressed state.
Over the next three years, Council will be strengthening or renewing eight bridges on Northbank Road to cope with the forest industry’s heavy vehicles. Council is also seeking funding to renew Storey’s and Weaver’s bridges (Kaituna Tuamarina Track), and Stafford Creek Bridge (Redwood Pass Road) over the next two or three years.
Anyone with concerns about bridges, including damage or maintenance needs, please contact Marlborough Roads, Ph: 03 520 8330.