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.