Bridge Rating for Vehicles with Grandfather Provisions

Abstract

To ensure bridge safety under vehicular traffic, the owners must systematically verify the adequacy of bridge live load-carrying capacity. Bridges are evaluated for design, legal, and permit loading. The legal load rating uses AASHTO legal trucks, which meet the Federal Bridge Formula requirements. However, each state has a federal law exemption for truck weight and size. The state-specific grandfather provisions allow the operation of legally overloaded vehicles. Thus, the objective of this study is to determine whether the current live load factors provide adequate safety for exempted vehicles because there are no provisions for overloaded vehicles under grandfather provisions. This study presents a reliability-based analysis to determine state-specific live load factor for bridge rating on an example of Montana’s 20% overload allowance for agricultural vehicles. This approach does not require a reanalysis of the state bridge inventory. Linear scaling of existing rating results from bridge records can be used to determine ratings for agricultural vehicles. The study provides a calibration method to maintain the required minimum safety level with the minimum cost and effort.