The primary function of a bridge is to cross an obstruction like a river or an intersecting roadway or path to allow for seamless movement of vehicles and pedestrians. With a plethora of bridges already in existence carrying an ever increasing volume of traffic, they need to be evaluated for their existing strength. 

Vehicular bridges designed from the 1950's and onwards were designed with a certain fatigue life based on the Average Annual Daily Traffic (AADT) in mind and a projected volume of traffic based on a polynomial growth. Fast forward to 21st Century, the most recent decade has seen an exponential growth in daily traffic numbers, thereby reducing the fatigue life of bridges. This fact has exacerbated the need to evaluate existing bridges based on their designs and existing conditions. 

Evaluation of Existing bridges is of the utmost necessity to maintain existing infrastructure to extend their usable life. The evaluation is used to determine existing physical conditions, capture deterioration and propose corrective rehabilitations. Not only are bridges subject to cyclical loads, but also are exposed to any and all environmental conditions which may cause further deterioration to the main structural elements. The aforesaid effects are taken into account by performing regular bridge inspections and associated load ratings.

A load rating of a bridge comprises of evaluating the superstructure of the bridge for a set of trucks with varying axle weights and spacing configurations. The variations in axle weights and spacing is evaluated to capture a variety of vehicles plying on the roads. An example of this would be a regular semi-truck vs emergency vehicle. While a semi-truck may weigh more than an emergency vehicle, the axles for an emergency vehicle are closer to each other than a Semi-Truck. Hence, taking these into account, there exist different categories of vehicles.

In the United States, each Department of Transportation has its own set template of vehicles which are used to evaluate the present load capacity of a bridge.

IG & Associates’ principal Ishan Gopalka, PE, has performed over a hundred bridge load ratings for the Indiana Department of Transportation (INDOT) owned bridges and for various counties across the State of Indiana. The associated load rating calculations were performed in the State required software called AASHTOware, a 2D-Finite Element Analysis software.