Rain and wind can make steel cables on the Normandie Bridge vibrate, but engineers use spiral sheaths, needles, and dampers to control this phenomenon.
Rain and wind cause cable vibrations
A moderate wind and ordinary rain can cause steel cables on the Normandie Bridge to oscillate. This happens when two streams of water form on a cable and change its aerodynamic behavior in the face of wind.Spiral sheaths prevent water streams
The spiral-profiled sheaths on the cables prevent water from organizing regularly. This limits the formation of the two streams that disturb the structure's aerodynamics and cause vibrations.Cable design and inspection schedule
Each of the 184 cables on the Normandie Bridge is inspected every four years. However, an early inspection is triggered if a cable emits an unusual sound, indicating potential issues.Cable lengths and sensitivity to wind
The cables vary in length from 95 meters to over 460 meters. Longer and more inclined cables are more sensitive to air movements, making them more prone to vibrations caused by wind and rain.Two streams of water alter aerodynamics
When it rains, water does not flow uniformly around a cable. Instead, it forms two distinct streams on the cable’s surface, altering its shape and creating oscillating forces that lead to vibrations.Rain and wind instability is well-documented
The instability caused by rain and wind has a name in engineering circles and is explicitly cited in French technical recommendations. The Normandie Bridge is a textbook example of this phenomenon.Triggering conditions are surprisingly mild
Wind speeds of around 10 meters per second, which is barely more than a sustained breeze, are enough to set the cables in motion when it rains. The vibrations stop almost instantly when the rain stops.Engineers use multiple countermeasures
To combat the instability, engineers have implemented several solutions. Spiral-profiled sheaths prevent water from forming regular streams, while vertical cables called needles help control movements and tension.Dampers absorb residual energy
At each connection between a cable and the deck, 24 dampers absorb residual energy from vibrations. These dampers act as mechanical brakes when oscillations become too pronounced.Continuous monitoring ensures safety
Despite the apparent robustness of the bridge, engineers remain vigilant. Each cable is inspected every four years, and any unusual sound prompts immediate action to ensure the bridge remains safe and stable.
