United States Department of Transportation
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Effects of hauling timber, lignite coal, and coke fuel on Louisiana highways and bridges.
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2005-03-01
[PDF-952.98 KB]
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Abstract:This study included the development of a methodology to assess the economic impact of overweight permitted vehicles hauling timber, lignite coal, and coke fuel on Louisiana highways and bridges. Researchers identified the highway routes and bridges being used to haul these commodities and statistically chosen samples to use in the analysis. Approximately 1,400 control sections on Louisiana highways carry timber, 4 control sections carry lignite coal, and approximately 2,800 bridges are involved in the transport of these commodities. Three different gross vehicle weight (GVW) scenarios were selected for this study including: 80,000 lb., 86,600 lb. or 88,000 lb., and 100,000 lb. The current GVW is 80,000 lb., the 86,600 lb. GVW is the permitted load for log trucks and the 88,000 lb. GVW is permitted for lignite coal and coke haulers. The 100,000 lb. GVW for sugarcane haulers is the highest level currently permitted by the state of Louisiana.
The methodology for analyzing the effect of these loads on pavements was taken from the 1986 AASHTO design guide and involves determining the overlay thickness required to carry traffic from each GVW scenario for the overlay design period. Differences in the life of an overlay were calculated for different GVW scenarios and overlay thickness and costs were determined for a 20-year analysis period. These costs were developed for the sample on all control sections included in the study. These present net worth costs were expanded to represent the cost for all control sections carrying each commodity.
A suggestion from enforcement personnel caused project staff to perform an additional analysis using one load axle at 48,000 lb. (48-kips), which is the maximum permissible tandem axle load. This analysis showed that 48-kip axles produce more pavement damage than the current permitted GVW for timber trucks and causes significant bridge damage at all GVW scenarios included in the study.
The methodology for analyzing the bridge costs was developed by 1) determining the shear, moment and deflection induced on each bridge type and span, and 2) developing a cost to repair fatigue damage for each vehicle passage with maximum tandem load of 48,000 lb.
Results indicate that permit fees paid by timber trucks should increase from the current $10 per year to around $346/year/truck for a GVW of 86,600 lb. when axles are equally loaded and $4,377/year/truck if 48-kip axle load are permitted. The current permit fee for lignite coal should remain at current levels. The legislature should not consider raising the GVW level to 100,000 lb. because the pavement overlay costs double over those at 86,600 lb. GVW and the bridge repair costs become significant. In many cases, the bridge costs per passage of a loaded truck amount to $8.90 meaning that the cost of bridge damage per truck per year can easily exceed $3,560.
The project staff recommends that the legislature eliminate the 48-kip maximum individual axle load and leave GVWs at the current level, but increase the permit fees to sufficiently cover the additional pavement costs produced by the presence of these permitted overweight vehicles.
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