Bridge rating is the process of evaluating the structural capacity and the safety of a bridge. It is used to determine the maximum loads that a bridge can safely carry, including both live loads (such as vehicles and passengers) and dead loads (such as the weight of the bridge itself).
The purpose of bridge rating is to ensure that bridges are maintained in good condition and are able to safely accommodate the demands placed on them. This information is then used to prioritize maintenance and rehabilitation needs, and to make decisions about the continued use or replacement of the bridge. The results of a bridge rating assessment are usually expressed as a rating index or number.
Lets take an example of HS-20 vehicle from above image.
HS-20 is the truck live loadings of the AASHTO specification, where H stands for highway, S stands for semi-trailer, 20 stands for 20-ton weight of the tractor (first two axles). Each axle will carry the loads as follow, the first axle carries 8,000 pounds, the second axle, 14 feet away carries 32,000 pounds and a single-axle semitrailer 14-30 ft away from the second axle carries 32,000 pounds.
After carrying out Analysis in Bridges using BrR software. We obtain the Rating Factor as 1.267. This rating factor signifies that the bridge can sustain the 1.267 times the load of the truck without any damage to bridge.