|Excessive weight and a growing number of overloaded vehicles can cause an accelerated consumption of bridge service life. Therefore, there is a need to control the operation of heavy traffic. This study considered the following issues: a growing number of issued permit vehicles, outdated permit fee regulations, lack of uniform provisions throughout the United States regarding notional permit vehicles, and no provisions for bridge rating under legally overloaded grandfather vehicles.
This dissertation is divided into three parts (papers), with a common topic being the impact of overloaded vehicles on bridge structures. It consider issues with bridge design and evaluation procedures under heavy traffic, and assessment of bridge damage under overloaded permit vehicles.
The objective of the first paper is to assess bridge life consumption by the permit vehicles. The economic lifetime of a bridge is considered in terms of the number of load cycles to failure so that the structure has to be replaced. Each truck's passage causes one or more load cycles and reduces or consumes a part of the remaining fatigue life of the bridge. The consumption by a permit vehicle can be considered as a superposition of two parts. The first one is caused by the vehicle loaded up to the legal limit, which would not require any permit, and the other part is the additional consumption caused by the weight exceeding the legal limit. Thus, a formula is derived for the incremental consumption to quantify the damage caused by the permit overload. Furthermore, the fatigue damage is converted to dollar damage based on the bridge construction cost, traffic volume, and state-specific bridge parameters. This approach allows state agencies to establish a rational and fair permit fee structure. The methodology is transparent and can be applied by other states using the available state-specific database.
The objective of the second paper is to propose a reliability-based methodology to determine a notional permit vehicle(s) for bridge design and evaluation analysis. Notional vehicle is developed to represent the existing traffic and assure the minimum required safety along with specific live load factors. Over 300,000 notional trucks with different axle weight and spacing combinations were generated to develop a candidate for the notional permit vehicle. The analysis led to the selection of a notional permit vehicle with eight axles and gross vehicle weight (GVW) of 94.8 tons (209 kips) and a length of 33.83 m (111 ft).
The selected notional permit vehicle can serve as a basis for verification of the design provisions for the Strength Limit State II and rating under permit vehicles. It can also simplify the permit issuance procedure and help to control permit vehicle operations.
The third paper aims to determine if the current live load factors provide adequate safety for exempted legally overloaded vehicles under grandfather provisions. Presently, there are no guidelines to evaluate bridges for grandfather vehicles. A reliability-based analysis procedure is developed to determine the adequacy of the state-specific live load factor for bridge rating under exempted vehicles. The procedure is demonstrated in the example of grandfather agricultural vehicles in Montana. The developed approach does not require any re-analysis of the state bridge inventory. Linear scaling of existing ratings from bridge records can be used to determine ratings for legally overloaded agricultural vehicles. The study provides a calibration method to maintain the required safety level with the minimum cost and effort.
The research presented in this dissertation advances the state of the art and practice in understanding and assessment of the impact of overloaded vehicles on bridge structures. The proposed approach helps to improve and enhance control of overloaded vehicles and assure safety of bridges.