Forth Road Bridge: Acoustic sensors monitor internal wire fractures as corrosion halts
Engineers have successfully slowed corrosion in the Forth Road Bridge’s main cables by pumping dry air through them, reducing internal humidity to below 40%. However, acoustic sensors continue to detect individual steel wire fractures, with 93 breaks logged by 2015 and a spike of 24 breaks in a single cable shortly after. While the bridge’s strength loss is estimated at 8-10%, the drying process has prevented further degradation, allowing the structure to remain open for non-motorized traffic.
Key points
- The Forth Road Bridge, opened in 1964, features two main cables containing 11,618 steel wires each, which suffered 8-10% strength loss due to internal corrosion.
- In 2004, inspectors discovered rust inside the cables, leading to a dehumidification project costing approximately £10.3 million to pump dry air through the cables.
- Acoustic monitoring systems, costing nearly £1 million, were installed to detect individual wire fractures, logging 93 breaks by early 2015 and 24 additional breaks in one cable within months.
- The dehumidification effort reduced internal humidity to below 40%, halting further corrosion and averting the need for a £122 million cable replacement.
- Traffic was shifted to the Queensferry Crossing in 2017, and the Forth Road Bridge now carries buses, taxis, cyclists, and pedestrians, with strengthening work commissioned for 2025-2026.
Background
The Forth Road Bridge, a 1,006-meter suspension bridge, was constructed using standard 1960s practices that included red lead paste and galvanised wire sheathing, which proved insufficient against moisture. The 2004 inspection was part of a broader European response to similar issues found in US bridges, prompting the development of new inspection guidelines. The bridge’s current status as a pedestrian and cyclist route follows the 2017 opening of the Queensferry Crossing, which took over motor vehicle traffic.
Why it matters
The Forth Road Bridge case highlights the critical importance of proactive infrastructure maintenance and the limitations of visual inspections in detecting internal structural degradation. The successful implementation of dehumidification and acoustic monitoring demonstrates how innovative engineering solutions can extend the lifespan of aging infrastructure, potentially saving hundreds of millions in replacement costs. This approach offers a model for other suspension bridges worldwide that face similar corrosion risks.
What to watch
Transport Scotland has commissioned additional strengthening work for 2025 and 2026, focusing on tower bearings, deck concrete, and viaduct girders. Continuous acoustic monitoring will remain in place to track any new wire fractures, while periodic inspections will assess the long-term effectiveness of the dehumidification system in preventing further corrosion.
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