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  • - When it opened in 1981, the Humber Bridge

  • was the longest single-span suspension bridge in the world.

  • More than 1,400 metres of road,

  • weighing 17,000 tonnes, held up by cables

  • that weigh thousands of tonnes themselves.

  • Those cables are made up of more than 14,000 individual wires,

  • all anchored into concrete foundations on the shore.

  • And those cables, and the whole structure in turn,

  • is held up by these enormous towers.

  • The bridge is so wide and so tall that,

  • while both towers are vertical,

  • they're just a couple of centimetres further apart at the top

  • because of the curvature of the earth.

  • Nearly 40 years after it opened,

  • this bridge is about a third of the way through its design lifespan,

  • and it's time for a checkup.

  • - Back in the '90s, in the United States,

  • bridge engineers started to unearth

  • severe cable corrosion in their early structures.

  • So, they were building suspension bridges in the '30s.

  • The way that the cables were designed to be kept watertight

  • was, once the wires had been spun

  • and formed a cable and compacted,

  • the open wires were coated with lead paste,

  • which is lead and linseed oil.

  • Wrapping wire was then added

  • around the outside of the cable, trapping the lead paste

  • and effectively keeping the cable watertight,

  • and it was thought that would be sufficient.

  • Inspections in the United States

  • demonstrated that moisture was still penetrating

  • and gathering in the main cables

  • and causing significant corrosion,

  • because, inside here was its own mini climate, in effect,

  • so it was affected by external temperatures,

  • day, night, winter, summer.

  • - This is a brutal environment for the cables.

  • We're near the ocean, for a start,

  • and down by the road there's engine exhaust and spray.

  • Remember, the cables aren't one sold piece of metal.

  • They're made up of thousands of individual wires.

  • And while they're packed tightly,

  • they're cylinders, so there will always be a slight air gap between them,

  • and that air gap is the solution.

  • - Water-based corrosion of steel or iron

  • cannot occur below 40% relative humidity.

  • There's approximately 15-20% open space

  • in this compacted cable.

  • If you can keep that volume below 40%,

  • then effectively corrosion is halted.

  • Inside the main deck of the bridge,

  • there are plant rooms that produce dehumidified air.

  • That air is then pumped up to the main cable

  • and injected into a stainless steel sleeve

  • that surrounds the main cable.

  • The relative humidity of the air blown into the cables

  • is low enough to have an affinity to collect moisture

  • as it travels through the cable,

  • drying the cable and exhausting the moisture to atmosphere,

  • and that is a continuous, 24-hour cycle, 365 days a year,

  • essentially for the rest of the bridge's life.

  • So, the Humber Bridge Board first inspected their main cables in 2009,

  • and then deployed dehumidification the following year, 2010.

  • Here we are, 10 years later in 2019,

  • reinspecting the main cable to ascertain the effectiveness

  • of that dehumidification system.

  • What we do is, with a sledge hammer,

  • we drive the wedges in to form an open wedge in the cable.

  • From each wedge line, the consultant engineer

  • will select a wire, and then our guys will come along

  • and sample that particular wire

  • and remove approximately a five-meter length

  • to then take the data that they need to collect from the cable.

  • We then select a five-meter length from the drum of the new wire

  • and splice that in with ferrules and then remove the wedges,

  • and that wedge line is then finished.

  • That is then repeated seven more times around the circumference of the cable,

  • and then that effectively is the inspection complete.

  • With 100-ton hydraulic jacks in each corner,

  • we compress the cable back to its original diameter.

  • - The idea that the wires are slightly corroded

  • could be worrying, but it's well, well within design tolerances,

  • and it's been noticed early and dealt with,

  • and they're keeping a watch on them just in case.

  • But bridges elsewhere in the world that aren't inspected,

  • where infrastructure isn't maintained, where cutbacks

  • mean that this isn't happening?

  • Those should give you just a little cause for concern.

  • The Humber Bridge is doing just fine.

  • That works? All right.

  • We're going to go for the drone shot.

- When it opened in 1981, the Humber Bridge

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B1 cable corrosion wedge wire main moisture

How To Stop A Colossal Bridge Corroding

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    林宜悉 posted on 2020/04/01
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