Bridge Load Rating: Evaluating Existing Structures for Posting and Permit Loads
Every highway bridge in service carries a rating, a specific answer to the question of how much load it can safely support, and that rating is recalculated periodically over the structure's life, not fixed at the design stage and forgotten. Load rating is where the theoretical design capacity of a bridge meets the reality of decades of traffic, deterioration, and trucks that are frequently heavier than anything the original designer imagined.
Two Rating Levels Answer Two Different Questions
The AASHTO Manual for Bridge Evaluation, the governing reference for load rating practice in the US, defines two distinct rating levels that serve different purposes. The inventory rating is the more conservative figure, representing the load level a bridge can carry indefinitely under normal traffic without accelerating deterioration, and it's the number typically used for planning and general traffic decisions. The operating rating is a higher, less conservative figure representing the maximum load the bridge can carry safely for occasional passage, and it's the number invoked when a permit load, an oversized truck or crane that exceeds standard legal limits, needs to cross a specific bridge on a specific route. Confusing the two, or using an inventory rating where an operating rating would be the technically correct and less restrictive answer, needlessly closes routes to legitimate permit traffic that the bridge could actually carry.
Rating factor, the core output of the calculation, compares the bridge's available capacity after dead load is subtracted against the demand from a specific rating vehicle, and a rating factor below 1.0 means the bridge cannot safely carry that vehicle at that load level, triggering either a posted weight restriction or a load path analysis to see if an alternate route through the structure carries the excess. This calculation draws on the same section capacity concepts used in original design, similar to how plate girder design checks flexural and shear capacity against demand, but rating works backward from an existing, often deteriorated section rather than forward from a clean design, and the section properties used have to reflect actual field-measured condition, not nominal as-built dimensions.
Section loss from corrosion is the detail that most often separates a rating calculation from a simple recheck of the original design: a steel girder's bottom flange, exposed to deck joint leakage and de-icing salt runoff for decades, can lose enough net section at a localized point to control the rating even when the girder's mid-span condition away from the leak looks essentially undamaged, which is why field inspection measurements, not drawings, drive the rating input.
Load Testing Can Recover Capacity a Calculation Can't Prove
Analytical load rating is inherently conservative because it has to assume a specific, often simplified, load distribution among girders and account for uncertainty in material properties and section condition with safety factors stacked on top of each other. Diagnostic load testing, instrumenting the actual bridge with strain gauges and running known truck loads across it, can reveal that the real structure distributes load among girders, and picks up unintended composite action or continuity, far more favorably than the conservative analytical model assumed, sometimes recovering enough rating capacity to avoid a posting that the paper calculation alone would have required. This isn't a substitute for good analysis, but a supplement used specifically on borderline structures where the calculated rating factor sits just below 1.0 and the cost of a load test is smaller than the cost of unnecessarily posting or replacing a structurally adequate bridge.
Fatigue life is evaluated separately from strength rating and matters most for older steel bridges with fatigue-prone details, since a bridge can have adequate strength rating for today's loads while still accumulating fatigue damage from decades of cyclic truck traffic at connection details that predate modern fatigue-resistant detailing practice, a distinction closely related to the detail-category approach covered in fatigue design in steel structures. State departments of transportation maintain and periodically update load ratings for their bridge inventories as part of the National Bridge Inspection Standards program, with the underlying rating methodology and manual published by the Federal Highway Administration in coordination with AASHTO.
Rating also has to account for the actual traffic pattern a specific structure sees, not just the standard design vehicle families. A rural bridge on a state route carrying occasional agricultural equipment or logging trucks needs its rating checked against those specific axle configurations, since a legal-load rating built around a generic design truck can miss an axle spacing that concentrates load differently on a short-span structure than the standard rating vehicle does. Bridge owners increasingly pair periodic inspection with rating updates rather than treating the two as separate programs, because a rating calculated from an inspection report that's several cycles old can understate deterioration that's occurred since, particularly at joints and bearings where moisture intrusion accelerates section loss faster than a simple visual survey might suggest between inspection cycles.