The effect of fatigue damage in rating bridges under overload conditions

In recent years there has been an increasing trend in the frequencies of overloads on highway bridges. An overload consists of a vehicle with a gross weight exceeding that of the legal maximum permitted on highway bridges. Most overloads on bridges are by special permits issued by the respective states' transportation agencies. The procedure via which an overload permit is issued is primarily based on the AASHTO guidelines on load rating of bridges. The rating is used as an index to determine whether it is safe for the bridge to be used by the overloaded vehicle. The current rating procedures, in most part, do not specifically include the significance of the frequency of application of overloads. This study shows that in certain cases the stress range resulting from overloads can be as much as 60% more than those resulting from standard legal loads.;This study presents the results of comprehensive study on (i) the effect of overloads on bridges; and (ii) the significance of the frequency of overloads on bridge load rating procedure. As part of this study, the statistics of bridge overloads were compiled and used in conjunction with a simulation study to establish the distribution of stress ranges at fatigue critical components of steel girder bridges. An index for fatigue life expended (FLE) was used to determine the significance of overloads in causing damage to bridge structural components. The FLE was then used as a measure to control the number of overload permits that can be issued for a given bridge. A modified load rating procedure was then developed with considerations for the fatigue strength, the applied vehicle load, and the FLE. The use of the load and resistance factor design (LRFD) approach in deriving a load rating procedure that is more consistent with bridge structural condition and various levels of uncertainties involved in the rating process is also presented. Illustrative examples are provided to demonstrate the applicability of the load rating procedure.