Bridge inspection: getting close to the places nobody can see

Bridge deterioration falls into three families: concrete deterioration (cracking, spalling and exposed rebar, efflorescence, chloride attack, alkali–silica reaction, freeze–thaw), steel deterioration (corrosion, coating breakdown, fatigue cracks, loose or missing bolts), and problems at bearings, expansion joints and drainage. All of them concentrate where water, salt and repeated loads get in.

In Japan, after the 2012 Sasago Tunnel ceiling collapse, periodic inspection of road bridges (and tunnels) every five years, based on close-up visual inspection, became mandatory in July 2014. Inspectors get within arm’s reach and tap or touch where needed. Each bridge is rated in four categories from I (sound) to IV (urgent action).

The means of getting close become the cost and the risk. Under-bridge inspection vehicles (“snoopers”) need lane closures and cannot reach every span. Tall piers and river spans need scaffolding, rope access or even a boat. In the United States routine bridge inspections are required at intervals of up to 24 months (12 months for bridges in poor condition), and owners face the same problem.

The other pain is people. Japan has roughly 720,000 road bridges (end of FY2024), many of them owned by municipalities, and many small towns have no bridge engineer at all. Bridges more than 50 years old rose from about 130,000 at the end of FY2018 to about 230,000 at the end of FY2024. The number of ageing bridges grows while the number of inspectors does not. That is why drone inspection is becoming a necessity, not a convenience.

The drone replaces the *means of access*. Without closing lanes or building scaffolds, it carries a camera right up to the member. There are three main ways to use it.

The basic pattern is a grid: hold a constant stand-off from the surface and overlap images in both directions (60–80% overlap is common). The images can then be stitched into orthomosaics or 3D models, so cracks can be mapped onto drawings with position and length.

Under a bridge is a hostile place for drones. The deck blocks GNSS, so position drifts; steel girders disturb the magnetic field, so the compass lies. Over rivers wind funnels through the valley, and water reflections can fool downward range sensors.

• GNSS-denied: choose aircraft that localise from cameras or LiDAR (visual-inertial odometry, SLAM). GNSS-denied environments
• Magnetic disturbance: check compass health before take-off and be ready to fly in modes that do not rely on magnetic heading. Magnetic disturbance
• Wind: air wraps around piers. Keep an escape path away from the structure and extra stand-off margin. Wind & turbulence
• Darkness: box girders and girder ends are pitch black. Use onboard lighting and fixed exposure to keep image quality consistent. Darkness

Taking pictures does not finish an inspection. From thousands of images you must find defects, measure them, locate them, compare with last time, and rate the bridge. AI helps with finding, measuring and comparing; rating remains the responsible engineer’s job.

Japan’s periodic inspection guidelines were revised in February 2019 so that, while close visual inspection remains the basis, inspection-support technologies such as drones can be used to inspect more efficiently. The guideline is generally described as allowing methods the inspector judges to give a diagnosis equivalent to close visual inspection. MLIT publishes a performance catalogue of inspection-support technologies so owners can compare them (the 2019 draft started with 16 technologies).

Flying is regulated separately. In Japan, flights over densely inhabited districts or within 30 m of people or property are “specified flights” that require permission or approval depending on conditions, plus a flight-plan notification and a flight log. Bridges cross roads, railways and rivers, so coordination with road, river and rail authorities is essential. Compare countries in world regulations.

• Hammer sounding & touch: delamination is traditionally found by tapping. Drone-mounted sounding devices are being tested but are not yet a general substitute.
• Corrosion under coatings, internal flaws: photos show surfaces only. Thickness and internal cracks need ultrasonic, eddy-current or other NDT.
• Areas hidden by mud or nests: clean first. What you cannot see, you cannot photograph.
• Rating and accountability: interpreting defects and deciding actions remains a human job.

At the foot of a town bridge sits a small drone port. Before dawn, a drone threads the girders on a fixed route and photographs each member from the same pose under the same conditions as last time. By noon, only the members that changed appear on the engineer’s screen. The five-yearly “big inspection” has become yearly watching.

Nobody needs to climb the bridge. People interpret the changes, decide the order of repairs and explain it to residents. Inspection shifts from a job of going to dangerous places to a job of caring for the whole life of the bridge.