Every public bridge in the United States over 6.1 m has been inspected at least every 24 months since the National Bridge Inspection Standards (NBIS) followed the Silver Bridge collapse of 1967; most other countries run comparable programmes on 2- to 6-year cycles. The routine inspection is visual, hands-on where it matters, and produces condition ratings and a list of defects. Special inspections add to it:

• Fracture-critical (NSTM) inspection, hands-on, arm's-length inspection of steel tension members whose failure would collapse the bridge: two-girder systems, truss chords, pin-and-hanger assemblies.
• Underwater inspection, divers or sonar at intervals of up to 60 months, for scour and for the condition of piers below the waterline.
• Non-destructive testing, ultrasonic and magnetic-particle testing of welds, ground-penetrating radar and half-cell potential surveys of decks, acoustic monitoring of tendons.

Drones, lidar and image analysis are making inspection faster and safer; they do not yet replace the inspector's hand on a pin.

The NBI condition rating scores deck, superstructure and substructure on a 0–9 scale; a bridge whose lowest score is 4 or less is poor, formerly "structurally deficient", which means it needs attention, not that it is about to fall. Element-level inspection is finer: each element (a girder, a bearing, a joint) is quantified in four condition states from good to severe, giving owners a database for prioritising money.

1 good2 fair3 poor4 severe

The rating is a management number and a legal record. It drives the inspection interval, the load-rating cycle, and the funding programme; and when a bridge fails, it is the first document anyone reads.

A load rating answers one question: what live load can this bridge carry safely today? The AASHTO Manual for Bridge Evaluation (MBE) computes a rating factor for each member and limit state:

Three levels: the design load rating at inventory (γ_LL = 1.75) and operating (1.35) levels with HL-93; the legal load rating with the state's legal trucks, which decides whether the bridge must be posted with a weight limit; and permit ratings for individual overweight vehicles. A posted bridge is a rating factor below 1.0 at the legal level; a closed bridge is one where the rating falls below a small fraction of legal load. The site's rating tool runs the LRFR arithmetic for a girder.

Bridges age in predictable ways, and the inspector learns the pattern by material:

ASR crackingfatigue crackcorrosionscour

concrete

Cracking (shrinkage, flexural, thermal), delamination and spalling from corroding reinforcement, alkali–silica reaction (map cracking and expansion), freeze–thaw scaling, efflorescence marking where water travels.

prestressed

Corrosion of strand at girder ends and under leaking joints; voided ducts; broken wires detected by acoustic monitoring or magnetic flux.

steel

Section loss from corrosion, especially at girder ends below joints; fatigue cracks at connection plates, cover plates and web gaps; frozen bearings; paint failure.

substructure

Scour, settlement, tilting, cracked pier caps at bearing seats, undermined footings, damaged pile jackets in the splash zone.

appurtenances

Leaking joints, blocked drains, damaged barriers, the cheap problems that cause the expensive ones.

Most bridges are repaired, not replaced. The toolkit is large: deck overlays (latex-modified or silica-fume concrete, polyester polymer) and full deck replacement; cathodic protection and electrochemical chloride extraction for corroding reinforcement; external post-tensioning and bonded CFRP strips to add flexural capacity; steel plates and section replacement on corroded girders; bearing and joint replacement (the commonest job of all); pier jacketing; and scour countermeasures.

jacking for bearing replacementexternal post-tensioning

Two rules from experience. First, understand why the defect happened before fixing it, or the fix fails the same way. Second, strengthening changes the load path, adding stiffness attracts load, so a strengthened member must be analysed as part of the new system, not patched in isolation. The economics are set by traffic: a lane closure on an urban bridge costs more per day than the repair.

Every rule in this chapter exists because a bridge fell. Silver Bridge (1967): a single eyebar cracked, and the NBIS was born. Mianus River (1983): a pin-and-hanger assembly corroded, and fracture-critical inspection followed. Schoharie Creek (1987): scour under a footing, and the scour programme. I-35W (2007): undersized gusset plates that thirty years of inspection could not have caught, and the rule that gusset plates be load-rated. Morandi (2018): corrosion in encased stays that could not be inspected; FIU (2018): a construction-stage failure; Baltimore's Key Bridge (2024): a ship far larger than the pier protection was designed for.

The site's failure library holds 300 cases classified by mechanism. The pattern that repeats: scour, fatigue and fracture in non-redundant steel, vessel and vehicle impact, construction stages, and hidden corrosion. Read the cases in your own bridge's family before you inspect it.