A bridge qualifies for a four-year gap between routine inspections only if its superstructure is one of the shapes the federal government wrote down. The list, set out at 23 CFR 650.311(a)(1)(iii), covers certain arches, box girders and beams, frames, girders and beams, slabs and culverts, in concrete or steel, identified by the span material and span type codes the owner reports. Trusses and cable-supported spans are not among the categories named. Neither is anything with a Category E or E prime fatigue detail, or a roadway vertical clearance under 14 feet, or an inventory rating below the standard AASHTO HS-20 or HL-93 loading.
That list is the most revealing artifact of the 2022 rewrite of the National Bridge Inspection Standards, because it shows what the interval is actually keyed to. For half a century the answer to how often a bridge gets looked at was a duration. It is now the output of a documented risk assessment, and the duration is the last thing computed.
Method 1 sorts bridges into three boxes
The final rule published at 87 FR 27396 on 6 May 2022, effective a month later, gives owners two routes to an interval. Method 1 is the simplified assessment: it classifies each bridge into one of three categories with a routine interval of 12, 24 or 48 months. Twenty-four months remains the default. Twelve is compulsory where one or more of the deck, superstructure, substructure or culvert components is rated in serious or worse condition, coded three or less, or where observed scour condition is rated serious or worse. Forty-eight requires every one of the seven extended-interval criteria at once, including satisfactory or better condition ratings on all components and on the channel, plus stability against potential scour.
Further down the same section sits a provision that does more work in practice than any of those durations. A bridge carrying a routine interval longer than 48 months must receive a service inspection during the month midway between routine inspections, so the longest arrangements are not left unattended. A separate rule governs new work. Any new, rehabilitated or structurally modified bridge has to receive an initial inspection, sit in service for 24 months and receive its next routine inspection before it becomes eligible for anything longer than 24 months. New work does not inherit its predecessor’s schedule.
Method 2 puts four people in a room and writes it down
The second route is a more rigorous assessment producing four categories, with intervals not to exceed 12, 24, 48 or 72 months. It costs an institution. The assessment must come from a Risk Assessment Panel of not less than four people, at least two of them professional engineers, with collective knowledge in bridge design, evaluation, inspection, maintenance, materials and construction, and including the NBIS program manager of the state transportation department, federal agency or tribal government operating the panel. The policy and criteria go to FHWA for approval. The panel must define deterioration modes and attributes and score them, and the regulation names the modes it will accept: section loss, fatigue and fracture for steel members, flexural cracking, shear cracking and reinforcing and prestressing steel corrosion for concrete, settlement, rotation, overload and vehicle or vessel impact for superstructures, and settlement, rotation and scour for substructures.
The panel’s discretion has a floor written into the screening criteria it must adopt. A bridge classified as in poor condition cannot have an inspection interval greater than 24 months, and one classified as in fair condition cannot exceed 48. Whatever the risk model concludes, condition caps the answer.
Underwater work and nonredundant steel tension members run on their own clocks. Underwater intervals under Method 1 are 60 months as a rule, 24 where the underwater portions or the channel are in serious or worse condition, and 72 under a documented extended-interval policy. NSTM intervals run 12, 24 or 48 months, and the 48-month option requires among other things that the bridge was constructed after 1978 and fabricated in accordance with a fracture control plan, and that no NSTM includes a pin and hanger assembly. The 2022 rule renamed fracture critical members as nonredundant steel tension members, which is a change in vocabulary rather than in physics.
What a rating of 4 commits an owner to
Condition ratings run 0 to 9 and the regulation reveals its own vocabulary through use. Six is satisfactory. Four is poor. Three is serious. Two is critical. The full descriptive scale sits in the Specifications for the National Bridge Inventory, the March 2022 document that replaced the 1995 Recording and Coding Guide and is incorporated by reference at 23 CFR 650.317.
The federal Good, Fair and Poor classification is a separate calculation, defined at 23 CFR 490.409. It uses the minimum condition rating method: the lowest of the ratings for NBI Items 58, 59 and 60, which are deck, superstructure and substructure, or Item 62 for a culvert. Lowest rating of 7, 8 or 9 gives Good. Five or six gives Fair. Four, three, two, one or zero gives Poor. One deteriorated component sets the whole classification, so a bridge with a sound substructure and a spalling deck is Poor without qualification. Every national count of poor bridges quoted anywhere in the bridges and structures section rests on that single-component rule. What the aggregate of those classifications means, and why the federal performance measure counts deck area rather than bridges, is a different question, taken up in the state of the inventory.
Load rating runs on its own clock
Inspection tells an owner what condition the bridge is in. Load rating tells them what it can carry, and 23 CFR 650.313(k) treats it as a distinct obligation with its own deadlines. Each bridge must be rated for safe load capacity under the incorporated sections of the AASHTO Manual for Bridge Evaluation. Ratings must be completed no later than three months after the initial inspection, and again whenever a change is identified that warrants a re-rating, with the regulation listing changes in condition, reconstruction, new construction and changes in dead or live loads as examples rather than as a closed set.
Posting follows from the rating, not from the inspector’s impression. Under 650.313(l) an owner implements load posting or restriction when the maximum unrestricted legal loads or state routine permit loads exceed what the operating rating, legal load rating or permit load analysis allows. The clock is short: posting shall be made as soon as possible but not later than 30 days after a load rating determines a need for it, and missing or illegible posting signs get the same 30 days. Closure has one bright line. Owners must document their own closure criteria, and bridges must be closed when the gross live load capacity is less than 3 tons.
FHWA’s own posting table records how imperfectly that machinery runs. Among the categories in Bridge Condition by Posting Status is one called open with posting recommended but not implemented. The category exists because the condition it describes does.
A critical finding starts a 24-hour obligation
The 2022 rule tightened the definition of a critical finding and attached a reporting duty to it. Under 650.313(q) the list includes full or partial closure of any bridge, an NSTM to be rated in serious or worse condition, a deck, superstructure, substructure or culvert component to be rated in critical or worse condition, a channel or scour condition to be rated critical or worse, and any immediate load restriction, posting or repair work, including shoring, needed for the bridge to remain open. Two duties follow from that list, and they attach to different scopes. For the first two categories on the National Highway System, FHWA must be notified within 24 hours of discovery. Separately, and regardless of NHS status, the State DOT, Federal agency or Tribal government responsible for inspection must provide a written status report, monthly or as requested, for every one of the five categories until the finding is resolved. The final rule moved the component condition threshold in that list from serious, coded three, to critical, coded two, which narrowed the trigger at the same time as the reporting duty around it was created.
The gap the standards do not close
None of this requires anyone to fix anything. That is not an editorial characterisation; it is the National Transportation Safety Board’s finding. In an interim report issued on 3 May 2023 on the collapse of the Forbes Avenue bridge over Fern Hollow in Pittsburgh, which failed on 28 January 2022 with a bus and four passenger vehicles on it, the Board reviewed the NBIS inspection reports for the 17 years before the collapse and found that starting in 2005 each of them documented corrosion damage and deterioration of the bridge legs, including the most recent, in September 2021, four months before the structure fell. Between 2005 and 2021 the bridge received nine routine and five interim inspections. The Board recorded that maintenance actions identified in the yearly inspection reports were not performed during the 11 years leading up to the collapse.
Although the FHWA does not have the authority to require that inspection-identified maintenance is performed, the FHWA can require states and other entities to ensure that their bridges have accurate capacity ratings and are safe for the traveling public.
Read the regime for what it is and the risk-based interval looks less like a relaxation and more like an honest admission. A bridge in poor condition still cannot go more than two years unseen, and the inventory data behind that judgement is collected to a specification detailed enough to fill 154 items. The decision that follows an inspection report, between patching, rehabilitating and starting again, is governed by an entirely different body of federal material, examined in how agencies choose between rehabilitation and replacement. What the standards themselves produce is a description of a structure’s condition and capacity that is accurate, timely and enforceable. Acting on the description belongs to someone else.