WEBVTT
Kind: captions
Language: en

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- When it opened in 1981,
the Humber Bridge

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

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More than 1,400 metres of road,

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weighing 17,000 tonnes, held up by cables

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that weigh thousands
of tonnes themselves.

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

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all anchored into concrete
foundations on the shore.

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And those cables, and the
whole structure in turn,

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is held up by these enormous towers.

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The bridge is so wide and so tall that,

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while both towers are vertical,

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

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because of the
curvature of the earth.

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Nearly 40 years after it opened,

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

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and it's time for a checkup.

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

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bridge engineers started to unearth

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severe cable corrosion in
their early structures.

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

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The way that the cables were
designed to be kept watertight

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was, once the wires had been spun

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and formed a cable and compacted,

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the open wires were
coated with lead paste,

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which is lead and linseed oil.

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Wrapping wire was then added

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around the outside of the cable,
trapping the lead paste

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and effectively keeping
the cable watertight,

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and it was thought that
would be sufficient.

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Inspections in the United States

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demonstrated that moisture
was still penetrating

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and gathering in the main cables

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and causing significant corrosion,

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

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so it was affected by
external temperatures,

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day, night, winter, summer.

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- This is a brutal
environment for the cables.

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We're near the ocean, for a start,

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

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

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They're made up of thousands
of individual wires.

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And while they're packed tightly,

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

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and that air gap is the solution.

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- Water-based corrosion of steel or iron

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cannot occur below 40% relative humidity.

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There's approximately 15-20% open space

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in this compacted cable.

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If you can keep that volume below 40%,

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then effectively corrosion is halted.

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Inside the main deck of the bridge,

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there are plant rooms that
produce dehumidified air.

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That air is then pumped
up to the main cable

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and injected into a stainless steel sleeve

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that surrounds the main cable.

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The relative humidity of the
air blown into the cables

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is low enough to have an
affinity to collect moisture

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as it travels through the cable,

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drying the cable and exhausting
the moisture to atmosphere,

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

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essentially for the rest
of the bridge's life.

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

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and then deployed dehumidification
the following year, 2010.

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Here we are, 10 years later in 2019,

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reinspecting the main cable
to ascertain the effectiveness

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of that dehumidification system.

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What we do is, with a sledge hammer,

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

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From each wedge line,
the consultant engineer

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

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and sample that particular wire

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and remove approximately
a five-meter length

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

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

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

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and that wedge line is then finished.

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

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and then that effectively
is the inspection complete.

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With 100-ton hydraulic
jacks in each corner,

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we compress the cable back
to its original diameter.

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- The idea that the wires
are slightly corroded

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

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and it's been noticed
early and dealt with,

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

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

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where infrastructure isn't
maintained, where cutbacks

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mean that this isn't happening?

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Those should give you just
a little cause for concern.

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The Humber Bridge is doing just fine.

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That works?
All right.

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We're going to go for the drone shot.