From above: "What design vehicle is used to analyze existing bridges? An H12 (20 kip axle) seems highly inappropriate as its gross weight would not jibe with the heavier legal trucks on the road?"
A rating merely a method of determining the actual capacity of a given structure. The rating vehicles are legal loads, (vary by state law) and also sometimes widely used design vehicles (like AASHTO). The result of a rating is the rating factor which must be greater than zero if it has any live load capacity. if the Rating factor is more than 1.0, then the capacity is greater than the legal load or the design load. the factor multiplied by the total weight of the rating vehicle gives an equivalent truck weight that can be converted to a short hand like (H-20, HS-20 or H-12)
The basic formula for ratings is
Strength-Dead Load effects
-------------------------- = Rating Factor
live load effects
Rating factor * Truck weight = Rating in Tons
Bridges are rated for Moment, Shear (any other appropriate loadings) and Rating factors are computed for each location (end support, interior support, section change, midspan) and load type (moment, shear, etc.). A large bridge could have many ratings, the lowest factor controls the overall rating for a given truck type. Virginia records ratings for 1 design vehicle (AASHTO HS type), and 4 state legal vehicles. 2 of the vehicles represent Permit type vehicles (legal only in special cases). in addition to all of the above, ratings are performed at different stress levels (inventory and operating)
Inventory represents a long term safe stress level. Operating indicates a level that is safe but will cause deterioration over time, so is generally only allowed with a permit or for short term before repair or when weights can be closely monitored.
Signs indicating weight limits are based on the rating results.
Ratings are independent of the original design code or truck. Ratings indicate what the current capicity is based on current engineering knowledge and conditions.
Hope this helps.
