Increasing Truck Weight Limits: Implications for Bridges

Changes in motor vehicle size and weight regulations have historically been driven by external forces. These limits have changed continually throughout historic development of the highway system worldwide. Issues arising from the legal weight limits on trucks include trade and productivity, climate change, competition with rail freight, pavement wear, and bridge safety concerns. Some European countries are considering the introduction of the European Modular System (EMS) which would allow long distance freight vehicle lengths up to 25.5 metres long, typically with 8 axles. There is some debate in the industry on whether this should be done while upholding existing gross weight limits or if it should be accompanied by a corresponding increase in allowable mass to about 60 tonnes.Many countries are not considering EMS but are seeking other ways to achieve improvements in freight transport productivity while reducing the carbon footprint of the industry. Ireland is considering legislative changes which would promote a shift from 5-axle to 6-axle trucks for long distance transport. This paper reports on a study of the implications of increasing Gross Vehicle Weight limits for road infrastructure, primarily bridge loading. Loading information in the form of Weigh in Motion (WIM) data from a range of sites has been considered for a wide range of bridge lengths and influence lines. Characteristic bending moments and shear forces from this WIM data have been found by extrapolation on probability paper to determine the 1 in 1000 year load. The data is then altered to simulate the effect of increasing the legal GVW limits and the characteristic moments/shear forces compared. A simple analysis on the likely impact of increased GVWs on pavement damage is also considered.