Inspections of the main cables on the Forth Bridge, followed by inspections on the Severn and the Humber, were undertaken in accordance with guidelines developed by the American authorities. This process involved removing the external wire wrapping between cable clamp positions and driving wedges into the body of the cable to enable inspection of as many wires as practically possible in eight radial positions around the circumference
The first inspection of the Severn cables took place in the summer of 2006, some 40 years after opening and approximately one-third through the original 120 year design life of the bridge. Eight sections between cable clamps were opened up. A number of broken wires were discovered and the condition of the remainder were carefully logged on a scale of 1 to 4 (1 being ‘very little corrosion’ and 4 being ‘widespread corrosion with rust over most of the surface of the wires’). Locations for inspection had been chosen at high, medium and low levels of both the main span and the side spans. It was no surprise that the sections at the lowest level were the worse for wear, because any moisture entering the cable would tend to run down and collect at the bottom.
A statistical analysis of the potential loss of strength was undertaken. This indicated a significant reduction in the factor of safety against failure compared to the original design intent. The only part of the loading that was controllable to any extent was the weight imposed by the traffic, so a regime of traffic monitoring and control was imposed, having particular regard to heavy goods vehicles. The main control measure was to prohibit HGVs from using Lanes 2 of the carriageway; this prohibition is still in force. The monitoring by Weigh-in-Motion (WiM) sensors, assisted by video camera monitoring of flows, is used to confirm that the traffic volumes and mixes remain within the assessment design loadings.
Most of the broken wires that had been found were repaired by cutting out those corroded lengths and inserting new wire that was re-tensioned by the use of specially developed swaged connections, incorporating turn-buckles. The opened sections were re-wrapped and re-painted. In addition a system of acoustic monitoring was installed to record on-going wire breaks. Measures were also put in hand to completely seal the cables in a membrane and, in time, to dry and de-humidify the interior of the cables by forcing dry air through them.
By 2011, when a second intrusive inspection was carried out, it was apparent that the measures imposed were taking effect. The humidity in the voids between individual wires was below 40% relative humidity, at which no corrosion of steel takes place, and the rate of wire breaks had been reduced to a very low level.
The monitoring and control of traffic and the de-humidification and acoustic monitoring is on-going and these measures are likely to be needed for the remaining life of the bridge. At the time of writing, the situation appears to have stabilised and, although the condition of the cables is considered unlikely to deteriorate significantly further, another examination is scheduled for 2016. Should this inspection confirm that a stable state has been reached there is scope for increasing the time between periodic inspections, inspecting only the most vulnerable (lower) panels, and allowing the relative humidity of the injected air to rise somewhat thereby saving some cost of the energy required to dry and warm the air.
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