What is the difference between a bridge condition rating and a bridge load rating?
A bridge load rating is an analysis of the bridge’s safe live-load capacity, while a bridge condition rating is not an analysis at all: it is an inspector’s coding of the defects present in a component on one day. The National Bridge Inspection Standards at 23 CFR part 650 subpart C define load rating as "The analysis to determine the safe vehicular live load carrying capacity of a bridge using bridge plans and supplemented by measurements and other information gathered from an inspection."
Deterioration feeds the load rating analysis, since the National Bridge Inspection Standards require a re-rating when a change warrants one, but the bridge condition rating and the bridge load rating remain separate items produced by separate work.
What do the bridge component condition rating codes actually mean?
Each component condition code in Table 20 of the FHWA Specifications for the National Bridge Inventory (SNBI, FHWA-HIF-22-017, March 2022) describes the defects in a bridge component, and on that scale a closed bridge is described only at the two lowest codes: code 1, IMMINENT FAILURE, and code 0, FAILED. Code 5 is FAIR, "Some moderate defects; strength and performance of the component are not affected." Code 4 is POOR, "Widespread moderate or isolated major defects; strength and/or performance of the component is affected." Code 3 is SERIOUS, major defects where the condition "typically necessitates more frequent monitoring, load restrictions, and/or corrective actions."
How does a bridge get classified as Poor?
A bridge is classified as Poor when the lowest of its deck, superstructure, substructure and culvert condition codes is 4 or below. Item B.C.12, Bridge Condition Classification, in the FHWA Specifications for the National Bridge Inventory (SNBI) is calculated by FHWA rather than reported by the bridge owner, and takes the lowest of those four component codes: Good covers 7, 8 or 9; Fair covers 5 or 6; Poor covers 4 and below.
Because the SNBI Bridge Condition Classification is a minimum, one component governs the whole classification. The SNBI’s own example of a Poor bridge is a steel box girder with a deck coded 4, a superstructure coded 6 and a substructure coded 7.
Does a load posting affect a bridge’s condition rating?
Not by itself: under the FHWA Specifications for the National Bridge Inventory (SNBI), load posting alone on an existing bridge designed for less than current legal loads does not affect the condition rating code. The SNBI is explicit on a point that surprises people reading a bridge file for the first time: "Load posting alone, for an existing bridge designed for less than current legal loads, is not considered a defect and does not affect the condition rating code."
A posted bridge coded in good condition is therefore a coherent record, and describes a structure whose load posting reflects its design live load rather than any observed defect.
Which bridge load rating triggers a posting?
Under the National Bridge Inspection Standards at 23 CFR 650.313(l), bridge posting is required when legal or State routine permit loads exceed what the operating rating, legal load rating or permit analysis allows, and the posting must follow within 30 days. An inventory rating factor below 1.0 is not by itself a posting trigger. Under 23 CFR 650.313(m), bridge closure is required when gross live load capacity is under 3 tons.
The FHWA Specifications for the National Bridge Inventory (SNBI) carry three separate load rating factors: Inventory (B.LR.05), Operating (B.LR.06) and Controlling Legal (B.LR.07). The National Bridge Inspection Standards at 23 CFR part 650 subpart C define operating rating as "The maximum permissible live load to which the structure may be subjected for the load configuration used in the load rating," adding that "Allowing unlimited numbers of vehicles to use the bridge at operating level may shorten the life of the bridge."
What does a bridge load rating depend on?
A bridge load rating is only as good as the plans and section properties behind it, because the load rating inherits their assumptions. Item B.LR.01, Design Load, in the FHWA Specifications for the National Bridge Inventory (SNBI) provides a code U for cases where "the design plans are not available and the likely design load cannot be inferred from design characteristics of the bridge or agency policy at the time the bridge was built."
Bridge load ratings must follow Sections 6 and 8 of the AASHTO Manual for Bridge Evaluation, Third Edition, 2018, with its 2019 and 2020 interim revisions and excluding the third paragraph of Article 6B.7.1, incorporated by reference at 23 CFR 650.317. Under 23 CFR 650.309(d), bridge load ratings must be performed by, or under the direct supervision of, a registered professional engineer.
Does every bridge still have to be inspected every two years?
No. The biennial bridge inspection is no longer the universal rule, and under 23 CFR 650.311 bridge inspection intervals follow risk rather than a two-year clock. Under Method 1 of 23 CFR 650.311, the routine inspection interval is not to exceed 24 months, drops to 12 months when a deck, superstructure, substructure or culvert is coded 3 or less or scour condition is coded 3 or less, and permits 48 months only where seven criteria are all met, among them an inventory load rating factor of 1.0 or greater and no Category E or E' fatigue details.
Method 2 of 23 CFR 650.311 allows 12, 24, 48 or 72 month bridge inspection intervals under a Risk Assessment Panel policy approved by FHWA. The bridge inspection interval itself carries information.
Can a bridge condition rating or load rating predict collapse?
No. Neither a bridge condition rating nor a bridge load rating is a remaining-life estimate, and neither predicts collapse. A bridge condition rating describes defects observed on one day by one team. A bridge load rating describes capacity under specified load configurations, using distribution factors, section properties and material strengths an investigation has to reopen rather than adopt.
Neither a bridge condition rating nor a bridge load rating speaks to scour below the waterline, to crack growth at one fatigue-sensitive detail, or to a load that crossed outside its permitted route.
Which part of a bridge file actually carries the evidence?
The calculation package behind a bridge load rating factor, not the rating factor itself, carries the evidence in a bridge file: the rating factor is a summary. A record review should pull the load rating computations and their assumptions, the rating date and method (items B.LR.03 and B.LR.04 in the FHWA Specifications for the National Bridge Inventory (SNBI)), the inspection reports and element-level data, the posting status and its change date (SNBI items B.PS.01 and B.PS.02), and the critical findings records.
Under 23 CFR 650.313(k), an updated bridge load rating is due no later than three months after the initial inspection and whenever a change warrants a re-rating, so gaps in that sequence of load ratings are themselves findings.
This article is general technical orientation, not a failure analysis, an engineering opinion, or advice on any specific matter. Determining the cause of a particular incident requires hands-on examination by a credentialed expert.