- 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.
I like Tom's stuff and watch his videos, but I am not sure if every edutainment video that gets posted on youtube really fits to be posted to /r/documentaries (same with wendover productions, veritasium, etc.)
Tl;dw
Summary if you cant watch a video where you are(e.g. work):
Video is 5 mins long.
The cables where, and still are, coated so that they are waterproof. The cabels get affected by the changes in the enviroment around them and humidity increases.
The airpockets between the small wires that make up the thick cable got humid and corrosion could occur. Solution is to pump dehumidified air into the cables and keep the humidity below 40%. This halts the corrosion.
Living 20 minutes away from the bridge, I’ve also been driving across to work a couple of times a week. The entire time I’ve been wondering what they’ve been working on, pretty good to see this pop up randomly on my feed.
I’m currently working on the Hålogalandbridge in Norway, and the solution is implemented for this suspension bridge, which makes me happy.
Not often I get to see anything about Hull on Reddit (without it being something negative)
In my hometown, they’re installing an anti-humidity device on the suspension bridge. I think the only other bridge in the US to have this is the Chesapeake Bay Bridge. ODOT says that if you keep humidity under 40%, steel won’t rust.
Missed a trick by covering the Humber bridge without the Forth Road Bridge too. Would’ve been great to see the contrast of what happens when that flaw isn’t caught and corrected in time.
Good old humber bridge. Nice to see my local landmarks on reddit. If you all get the chance, check out hessle foreshore "beach".
The Humber Bridge is in my City & my partner grandad worked on the building of it.