The vehicle dimension and mass rule change: the effect on state highway bridges

Historically, many studies have been undertaken to assess the effects of increasing heavy vehicle weight limits above the current legal loadings. In April 2010 the Minister for Transport in New Zealand signed an amendment to the changes to the Land Transport Rule: Vehicle Dimensions and Mass 2002, which allowed the introduction of a new high productivity motor vehicle (HPMV). These vehicles will be able to exceed the 44 tonne gross weight limit for divisible loads and will have higher individual axle loads, but will only be permitted on specific routes. The introduction of these vehicles is especially important to New Zealand's economy as the country's demand for freight is forecast to increase by 70 to 75 percent over the next 25 years. Increasing the weights and lengths that trucks can operate under is expected to boost productivity by reducing the growth in the number of trips needed to carry that freight, and therefore cost. However, increasing axle and gross weights will have an impact on New Zealand's national bridge stock - the average span on State Highways being only 14.8m. This paper discusses the technical challenges associated with a further increase in loads and provides an overview of the study that has been undertaken. The paper also provides an overview of the economic model developed as part of the assessment process, and how the bridge improvement costs were developed and linked into the model.