- 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.