0:00:36 > 0:00:40This rather sad-looking railway viaduct behind me
0:00:40 > 0:00:46means a lot to me, you know, because right from being a very small boy,
0:00:46 > 0:00:49I used to go climbing in the iron girders
0:00:49 > 0:00:52when I was eight years old. When a train came,
0:00:52 > 0:00:57with a load of coal wagons on, the whole lot used to shake about.
0:00:57 > 0:01:01In foggy days... There's a little ledge up there
0:01:01 > 0:01:05that just looked like a sentry box.
0:01:05 > 0:01:10This guy sat there all night with a coke brazier and the fog signals.
0:01:10 > 0:01:15You'd stick them on the track. Each time the engines come - bang! bang!
0:01:15 > 0:01:18It were really exciting times in a way.
0:01:18 > 0:01:20Before this road appeared,
0:01:20 > 0:01:23the valley were about 50 or 60 foot deeper.
0:01:23 > 0:01:28The River Croal which is still down there somewhere under the road
0:01:28 > 0:01:31used to flow along the bottom.
0:01:32 > 0:01:37From the earliest times, man's had the problem of going across rivers.
0:01:38 > 0:01:41The first bridges were very simple affairs.
0:01:41 > 0:01:45By the Middle Ages, they were more ambitious.
0:01:45 > 0:01:51Even then, they were limited by the length of the arch they could build.
0:01:51 > 0:01:57In 1741, Europe's first wrought-iron suspension bridge was built over the River Tees.
0:01:59 > 0:02:04The basic principle of suspending a path or roadway from cables
0:02:04 > 0:02:09rather than building one on arches meant wider spaces could be crossed.
0:02:11 > 0:02:14The idea was taken up rapidly.
0:02:14 > 0:02:16But it was not until the 1820s
0:02:16 > 0:02:21that advances in the design and manufacture of wrought-iron chains
0:02:21 > 0:02:26made it possible for Thomas Telford to build his two suspension bridges
0:02:26 > 0:02:28in North Wales.
0:02:28 > 0:02:30When it was opened in 1826,
0:02:30 > 0:02:35the Menai Bridge was the longest suspension bridge in the world.
0:02:35 > 0:02:38Telford's original wrought-iron chains
0:02:38 > 0:02:40have now been replaced by steel.
0:02:40 > 0:02:43So I went to have a look at the Conwy Bridge
0:02:43 > 0:02:47where all the original wrought iron is still in place.
0:02:47 > 0:02:51Telford surveyed quite a few places round Conwy
0:02:51 > 0:02:53for his bridge.
0:02:53 > 0:02:57But he selected this place near the castle,
0:02:57 > 0:03:02because the rock for the anchors - the anchor chambers - was superior.
0:03:02 > 0:03:04There were plenty of it.
0:03:04 > 0:03:08It started in 1822, when the first stones were laid.
0:03:08 > 0:03:14Then he got the chains across in an odd way. They built a rope bridge
0:03:14 > 0:03:18and started from each end, advancing towards the centre.
0:03:18 > 0:03:23He must have been nervy with that tonnage resting on ordinary ropes.
0:03:23 > 0:03:25Then the middle pin went in
0:03:25 > 0:03:30and, once they'd got the chains across, it were quite a simple job
0:03:30 > 0:03:34putting the vertical bolts or bars down to the road surface
0:03:34 > 0:03:37and building a road on it.
0:03:37 > 0:03:41In all, it took a little more than four years to construct.
0:03:41 > 0:03:45All the iron work was made in a workshop in Shrewsbury.
0:03:45 > 0:03:49Basically, each chain consists of five bars,
0:03:49 > 0:03:51about ten feet long by about four,
0:03:51 > 0:03:53by an inch and a quarter thick,
0:03:53 > 0:03:56with an eye forged on each end.
0:03:56 > 0:03:59They're all held together by...
0:03:59 > 0:04:02fishplates that are spaced in-between them
0:04:02 > 0:04:07and then two bolts slammed through the lot, three inches in diameter.
0:04:07 > 0:04:10It's certainly a good bit of drilling and fixing.
0:04:10 > 0:04:13It's stood the test of time.
0:04:15 > 0:04:19Thomas Telford was one of our greatest civil engineers.
0:04:19 > 0:04:21He built roads, bridges and canals.
0:04:23 > 0:04:27But one of his most dramatic engineering feats
0:04:27 > 0:04:31is this aqueduct that carries the Shropshire Union Canal
0:04:31 > 0:04:35across a valley high above the River Dee near Llangollen
0:04:35 > 0:04:37in North Wales.
0:04:37 > 0:04:4119 arches, each with a span of 45ft
0:04:41 > 0:04:45carry the waterway over in a cast-iron trough.
0:04:46 > 0:04:48We're now about to go over
0:04:48 > 0:04:51Mr Telford's famous aqueduct.
0:04:53 > 0:04:56I've read about it and seen it on postcards.
0:04:56 > 0:04:59The sides are thin, made of cast iron.
0:04:59 > 0:05:01Number one,
0:05:01 > 0:05:04it would be better if I got it lined up right.
0:05:04 > 0:05:10- How much space is on each side? - When the boat's on it, three inches.
0:05:10 > 0:05:12We're going to bump into the side.
0:05:12 > 0:05:14Here we go.
0:05:14 > 0:05:17When exactly were it completed?
0:05:17 > 0:05:19When were it built?
0:05:19 > 0:05:21They started it in 1795.
0:05:21 > 0:05:24It was completed in 1805.
0:05:25 > 0:05:28And it must have been a wonderful feeling
0:05:28 > 0:05:33when they first did it, when they first filled it up with water.
0:05:33 > 0:05:36They left it, didn't they, for quite some time
0:05:36 > 0:05:41to see if it leaked or anything terrible happened?
0:05:41 > 0:05:43How far up is it?
0:05:43 > 0:05:45It's 126ft
0:05:45 > 0:05:47at the highest point over the Dee.
0:05:47 > 0:05:51They're all sandstone pillars coming up.
0:05:51 > 0:05:54The arches are cast iron as well.
0:05:54 > 0:05:57- Cast-iron arches. - Yeah, the rails, the arches.
0:05:57 > 0:06:01Under the towpath is all cast iron.
0:06:01 > 0:06:05- Do they ever empty it? - Yes, to do maintenance.
0:06:05 > 0:06:11In a hard winter, they have to break the ice, else it would push the sides out.
0:06:11 > 0:06:16- If it did freeze, it wouldn't do it any good.- To empty the aqueduct,
0:06:16 > 0:06:18it's about three hours,
0:06:18 > 0:06:20just the aqueduct.
0:06:20 > 0:06:22How big's the bung hole?
0:06:22 > 0:06:26It's probably about a couple of feet, if that.
0:06:26 > 0:06:28A foot and a half, two foot square.
0:06:28 > 0:06:32- It's fairly big, then.- Big waterfall down into the River Dee.
0:06:32 > 0:06:36I bet it's a rare sight when it's gushing out.
0:06:36 > 0:06:42- Does it ever go over the edge? - No, because there's a weir at the far end.
0:06:42 > 0:06:47So if there's any excess water, it runs over the weir into the river.
0:06:47 > 0:06:50But it's never run over the top.
0:06:50 > 0:06:53I wonder what these other holes were for.
0:06:53 > 0:06:57I don't know. They've never had a rail on as far as we know.
0:06:57 > 0:06:59They might be for a rail.
0:06:59 > 0:07:03Unless it was part of the casting when they casted it.
0:07:03 > 0:07:06Now, this aqueduct has got a strange name
0:07:06 > 0:07:09that I can't pronounce. I'll let you do it.
0:07:09 > 0:07:12It's called the Pontcysyllte Aqueduct.
0:07:12 > 0:07:15The Ponty-suckley Aqueduct?
0:07:15 > 0:07:18- Yes.- I knew I'd get it right. - A bit of practice.
0:07:20 > 0:07:23- There's another boat coming. - He's coming this way.
0:07:23 > 0:07:26He's waiting for us to come off.
0:07:38 > 0:07:40MUSIC OBSCURES CONVERSATION
0:07:48 > 0:07:51The building of Britain's canal network
0:07:51 > 0:07:53in the 18th and early 19th centuries
0:07:53 > 0:07:57left a legacy of great engineering projects.
0:07:57 > 0:08:02As well as aqueducts to get across valleys, they also had the problem
0:08:02 > 0:08:06of getting over hills. If they came to a small hill,
0:08:06 > 0:08:10they'd dig a cutting. If they came to a longer hill
0:08:10 > 0:08:15with a plateau, they'd build locks up one side and down the other.
0:08:15 > 0:08:17If they came to a big hill,
0:08:17 > 0:08:19they'd build a tunnel through it.
0:08:19 > 0:08:22This is the Dudley Tunnel which was opened
0:08:22 > 0:08:25in 1792
0:08:25 > 0:08:28to connect Dudley with the Birmingham canal.
0:08:28 > 0:08:31Modern canal boats have got engines.
0:08:31 > 0:08:33But, of course,
0:08:33 > 0:08:36in the olden days, they had horses.
0:08:36 > 0:08:41- What did they do with the horse when they came to a tunnel?- Simple.
0:08:41 > 0:08:45They'd let the horse wander over the top of the hill itself.
0:08:45 > 0:08:50Or one of the boat crew would lead it over, one of the kids, maybe.
0:08:50 > 0:08:52And then they'd have to
0:08:52 > 0:08:55use manpower to get the boats through.
0:08:55 > 0:08:5770 ton of tackle.
0:08:57 > 0:09:02There'd be about 30 ton of goods in the boat which weighed 15 tons.
0:09:02 > 0:09:07The one method was to use a boat shaft and push on the roof.
0:09:07 > 0:09:10But that used to wear the bricks away.
0:09:10 > 0:09:14So the canal company preferred them to use legging.
0:09:14 > 0:09:18Things have changed. People got paid to do the legging.
0:09:18 > 0:09:22- Now people pay us to let them do the legging. Want a go?- Aye.
0:09:22 > 0:09:25We've got a legging board here.
0:09:28 > 0:09:31Put the board across the middle of the boat.
0:09:31 > 0:09:34This is where we get friendly.
0:09:36 > 0:09:39How's that?
0:09:41 > 0:09:43I'm going to enjoy this.
0:09:43 > 0:09:45OK? Get down flat.
0:09:45 > 0:09:47Oh, like that! Right.
0:09:47 > 0:09:51- Which way are we going? - Push towards the stern,
0:09:51 > 0:09:53towards the cabin.
0:09:53 > 0:09:55The wall's going further away!
0:09:58 > 0:10:01We're doing a fair rate of knots.
0:10:01 > 0:10:03I don't fancy it for two mile!
0:10:03 > 0:10:06- You don't want to do it for a living?- No.
0:10:06 > 0:10:09I'd sooner be a traction engine driver.
0:10:09 > 0:10:11They used to cheat a bit.
0:10:11 > 0:10:14They'd sometimes tie three boats together.
0:10:14 > 0:10:18Once they got them going, they'd keep them going
0:10:18 > 0:10:22- and earn three times the money. - My cap's falling off!
0:10:24 > 0:10:29In lots of ways, it's a lot harder to build a canal than a motorway,
0:10:29 > 0:10:33when you think of getting water across valleys and all of that.
0:10:33 > 0:10:36The credit goes to those who built all this.
0:10:36 > 0:10:44On the main canals, workers were named navvies - navigators - because they were building a waterway.
0:10:44 > 0:10:47I suspect when it came to the tunnels,
0:10:47 > 0:10:50they got more skilled labour in to do that.
0:10:50 > 0:10:54Here we are, light at the end of the tunnel.
0:10:56 > 0:11:00Keep running in the air, you keep it going!
0:11:00 > 0:11:05The name stayed with the navvies. In the 1800s they had more work
0:11:05 > 0:11:08building the railways.
0:11:08 > 0:11:11It was an age of massive engineering projects.
0:11:11 > 0:11:17Isambard Kingdom Brunel tunnelled under the Thames at Rotherhithe,
0:11:17 > 0:11:22the first tunnel under a vast expanse of water.
0:11:25 > 0:11:29When he was building his Great Western Railway in the 1830s,
0:11:29 > 0:11:35he crossed the river at Maidenhead on the longest and flattest arches ever built,
0:11:35 > 0:11:38a record that still stands today.
0:11:39 > 0:11:44The coming of the railways pushed forward the development of bridges.
0:11:44 > 0:11:47As the 19th century progressed,
0:11:47 > 0:11:52bridge-building became daring and dramatic, but not without mishaps.
0:11:52 > 0:11:55The Tay Bridge was opened in 1878.
0:11:55 > 0:11:58But one year later, it collapsed in a storm
0:11:58 > 0:12:01just as a train was crossing over.
0:12:04 > 0:12:09The engine and all its carriages plunged into the river.
0:12:09 > 0:12:11All 75 people on board lost their lives.
0:12:15 > 0:12:20The bridge's designer, Thomas Bouch, had already made plans for a bridge
0:12:20 > 0:12:22across the Firth of Forth.
0:12:22 > 0:12:27After the disaster, they were dropped. A new design was sought.
0:12:27 > 0:12:29The bridge that was constructed
0:12:29 > 0:12:33was the greatest engineering wonder of the Victorian age.
0:12:33 > 0:12:38The design of the Forth Bridge had two major innovations
0:12:38 > 0:12:41the use of steel and the cantilever principle.
0:12:41 > 0:12:44Three great piers were built.
0:12:44 > 0:12:46On these, they erected steel towers.
0:12:46 > 0:12:49From the towers,
0:12:49 > 0:12:53the six cantilevered arms were built out on both sides.
0:12:55 > 0:12:58By the time it was completed in 1890,
0:12:58 > 0:13:00it was the wonder of its age.
0:13:01 > 0:13:04I would have loved to have seen it
0:13:04 > 0:13:07when steam trains came thundering across,
0:13:07 > 0:13:12and to have been able to go up on the girders with the painting gangs.
0:13:14 > 0:13:16Now the bridge carries
0:13:16 > 0:13:20150 trains a day, but most are just little diesels.
0:13:23 > 0:13:27It's now 110 years old and a major refurbishment is under way.
0:13:27 > 0:13:30It gave me the chance to have a good look.
0:13:33 > 0:13:36When you climb on the Forth Bridge,
0:13:36 > 0:13:39it's amazing how the great cantilevers
0:13:39 > 0:13:43are not mechanically connected at all.
0:13:43 > 0:13:46To allow for contraction and expansion,
0:13:46 > 0:13:49they are just linked up together like a chain.
0:13:49 > 0:13:52And it's because of this, of course,
0:13:52 > 0:13:55that when you stand on the very top of it,
0:13:55 > 0:13:57350ft up in the sky,
0:13:57 > 0:14:00and a locomotive comes onto the bridge
0:14:00 > 0:14:02under the cantilevers,
0:14:02 > 0:14:05you can feel the whole thing rock.
0:14:08 > 0:14:13It was a great feeling up there and a credit to the men who built it -
0:14:13 > 0:14:16and all that based on the cantilever principle.
0:14:16 > 0:14:21So let's have a practical demonstration of how it works.
0:14:21 > 0:14:24This is the principle of the cantilever bridge,
0:14:24 > 0:14:27very similar to the Forth Bridge.
0:14:27 > 0:14:32Whether Mr Fowler did anything like this or not, I don't know.
0:14:32 > 0:14:38As you can see, it's supporting the whole weight of my wife here,
0:14:38 > 0:14:40with not too much effort.
0:14:40 > 0:14:43If I were replaced by a girder,
0:14:43 > 0:14:46one up and one down with middle supporting struts,
0:14:46 > 0:14:49it would be quite successful.
0:14:50 > 0:14:54It's creaking a bit but it's holding her weight.
0:14:54 > 0:14:58This bit here in my sort of...left hand
0:14:58 > 0:15:00is the cantilever.
0:15:00 > 0:15:03This other bit is the counterbalance
0:15:03 > 0:15:06that stops the thing from falling over.
0:15:11 > 0:15:16I've come to London to see another Victorian engineering feat -
0:15:16 > 0:15:20Tower Bridge, built at the end of the 19th century.
0:15:20 > 0:15:23Inside that great castle-like exterior,
0:15:23 > 0:15:25there's a great big steel frame
0:15:25 > 0:15:30that was constructed by the same men who built the Forth Bridge.
0:15:30 > 0:15:37Before the Victorian age, there had never been a bridge downstream of London Bridge.
0:15:37 > 0:15:42But a massive growth in the population made a new one essential.
0:15:42 > 0:15:45The problem was, this stretch of the river
0:15:45 > 0:15:48had some of London's busiest docks.
0:15:48 > 0:15:53Any bridge would have to give up to a 140ft clearance for tall ships.
0:15:55 > 0:15:59The solution came from Horace Jones, the city architect,
0:15:59 > 0:16:02with his design for Tower Bridge.
0:16:02 > 0:16:04It took eight years to build
0:16:04 > 0:16:09and five major contracting companies,
0:16:09 > 0:16:11and the relentless labour of 500 men.
0:16:11 > 0:16:13There's about 11,000 tons
0:16:13 > 0:16:17of steel in the towers and walkways and roadways.
0:16:17 > 0:16:21On the completion of the steelwork,
0:16:21 > 0:16:24it was clad in Cornish granite and Portland stone,
0:16:24 > 0:16:30both to protect the iron work and give it that beautiful appearance.
0:16:30 > 0:16:32When you come inside one of the towers,
0:16:32 > 0:16:37you can see its great steel skeleton all riveted together.
0:16:37 > 0:16:41The whole thing would stand up without the fancy stonework
0:16:41 > 0:16:44or beautification on the outside.
0:16:44 > 0:16:47It's a wonderful bit of iron work.
0:16:47 > 0:16:49Let's do some riveting.
0:16:51 > 0:16:55The bridge is hydraulically operated.
0:16:55 > 0:16:58The original machinery is in the engine rooms,
0:16:58 > 0:17:03including these beautiful steam engines that once powered
0:17:03 > 0:17:06the hydraulic pumps.
0:17:06 > 0:17:11The energy that was created was stored in six massive accumulators,
0:17:11 > 0:17:13like this.
0:17:13 > 0:17:17So as soon as the power was needed to lift the bridge, it was there.
0:17:19 > 0:17:23Great engineering didn't end with the Victorians.
0:17:23 > 0:17:26Just downstream from Tower Bridge,
0:17:26 > 0:17:30there's an example of a modern engineering feat.
0:17:30 > 0:17:32Over the last 20 years,
0:17:32 > 0:17:36there's been some impressive engineering achievements,
0:17:36 > 0:17:41two of which have been the Channel Tunnel and the Thames Barrier here.
0:17:41 > 0:17:46This unique piece of engineering spans the Woolwich Reach
0:17:46 > 0:17:49and it's 520 metres from this side to the other.
0:17:49 > 0:17:52Basically, it consists of ten large gates
0:17:52 > 0:17:57that are supported on great concrete piers and abutments
0:17:57 > 0:18:00which the gates actually swivel round.
0:18:00 > 0:18:03The piers also contain all the machinery
0:18:03 > 0:18:07for activating the thing in case of emergencies.
0:18:07 > 0:18:09Each of the four main gates
0:18:09 > 0:18:12weighs 3,700 tons.
0:18:12 > 0:18:16When open, they stand as high as a five-storey building
0:18:16 > 0:18:18and as wide as Tower Bridge.
0:18:20 > 0:18:23It took 4,000 men and women
0:18:23 > 0:18:27from all over Britain, to construct it. It took eight years.
0:18:27 > 0:18:30It cost nearly £500 million.
0:18:37 > 0:18:40Most people won't know where we are, Martin.
0:18:40 > 0:18:46- You don't expect tunnels like this under lock gates.- That's right.
0:18:46 > 0:18:48This is the concrete sill.
0:18:48 > 0:18:52The main reason for the tunnels is to give us access to the piers.
0:18:52 > 0:18:54But the sill's there
0:18:54 > 0:18:57like a door frame for the gate,
0:18:57 > 0:19:01so the gate can open and close in and out of this concrete sill.
0:19:01 > 0:19:03There's two of these, isn't there,
0:19:03 > 0:19:09one on each side of the hollow bit the gate sits in when it's open.
0:19:09 > 0:19:13That's right. The two tunnels give us duplicate services.
0:19:13 > 0:19:16They both carry power and water.
0:19:16 > 0:19:20So if one flooded, we could still close the barrier using the other.
0:19:20 > 0:19:23This is the machinery room.
0:19:23 > 0:19:25Here are the big hydraulic rams
0:19:25 > 0:19:28which we use to close and open the gates.
0:19:28 > 0:19:34When we pump the hydraulic oil in, they pull on the link to rotate the gate.
0:19:36 > 0:19:38Blooming 'eck.
0:19:38 > 0:19:40This is an interesting bit.
0:19:40 > 0:19:44It's the great crank that makes it go round.
0:19:44 > 0:19:48This is connected to the other end of the hydraulic cylinders.
0:19:48 > 0:19:53As the hydraulic cylinders push and pull, it rotates this rocking beam
0:19:53 > 0:19:58that's connected to a gate arm that rotates the gate out of the sill.
0:19:58 > 0:20:02The 3,600 tons comes rising up to close out the tide.
0:20:02 > 0:20:07If there's a spring tide, a strong easterly wind and a low pressure,
0:20:07 > 0:20:09they combine to create a high tide
0:20:09 > 0:20:13and that's when we need to go into closure mode.
0:20:13 > 0:20:18We've actually closed 33 times to protect London from flooding.
0:20:23 > 0:20:27We can actually close the barrier in 15 minutes.
0:20:27 > 0:20:30But it creates a dangerous water hammer effect.
0:20:30 > 0:20:33So we like to take two hours to close.
0:20:37 > 0:20:40The notch level around the pier there,
0:20:40 > 0:20:42that's the height of the walls
0:20:42 > 0:20:44upriver towards London.
0:20:44 > 0:20:47So you know, if it's getting near there,
0:20:47 > 0:20:51- things are getting dodgy? - We need to be closed
0:20:51 > 0:20:54if it's near the notch. The width of the piers
0:20:54 > 0:20:56is the same as Tower Bridge.
0:20:56 > 0:21:00That's this internationally-known design.
0:21:00 > 0:21:03So that anyone who builds a boat
0:21:03 > 0:21:05which is wider than this,
0:21:05 > 0:21:08it won't fit through here or Tower Bridge.
0:21:08 > 0:21:13- It won't fit through the Panama Canal and so on.- I didn't know that.
0:21:13 > 0:21:16The Thames Barrier isn't really a bridge
0:21:16 > 0:21:18or a tunnel.
0:21:18 > 0:21:22But it's a great piece of engineering
0:21:22 > 0:21:24that combines elements of each.
0:21:24 > 0:21:28Building it has been a major challenge but within a year,
0:21:28 > 0:21:33engineers had started to make plans for an equally ambitious project -
0:21:33 > 0:21:35a tunnel under the Channel.
0:21:35 > 0:21:40There have been many attempts to link England and France by tunnel.
0:21:40 > 0:21:42But it wasn't till the 1980s
0:21:42 > 0:21:46that the British and French governments agreed
0:21:46 > 0:21:48to the Channel Tunnel link.
0:21:51 > 0:21:54It opened in 1994.
0:21:54 > 0:21:57It's made up of three tunnels.
0:21:57 > 0:22:00Two are for the trains
0:22:00 > 0:22:03and the third is a central service tunnel.
0:22:08 > 0:22:12Work on the tunnel started in December 1987.
0:22:12 > 0:22:14They cut their way under the Channel
0:22:14 > 0:22:18with these huge tunnel-boring machines.
0:22:18 > 0:22:23It took three years before the British and French tunnelling teams
0:22:23 > 0:22:25achieved their breakthrough
0:22:25 > 0:22:28and another four years before it opened.
0:22:31 > 0:22:36The whole thing is a wonderful piece of civil engineering.
0:22:36 > 0:22:40But, unlike bridges, there's not a great deal of it you can see.
0:22:40 > 0:22:46To see the latest bridge-building design and construction,
0:22:46 > 0:22:49I went back north to the River Humber.
0:22:52 > 0:22:54In the 1960s and 1970s,
0:22:54 > 0:23:00Britain led the world in the design and building of suspension bridges.
0:23:00 > 0:23:03This bridge over the Humber, behind me,
0:23:03 > 0:23:06at the time of its building was the biggest in the world.
0:23:06 > 0:23:11Alas, I'm sad to say, the good old Japanese have built one bigger.
0:23:11 > 0:23:14Work began in 1973.
0:23:15 > 0:23:18It took eight years to complete.
0:23:18 > 0:23:22Thousands of tons of wire and concrete were used
0:23:22 > 0:23:27and over 1,000 people were engaged in its making over various periods.
0:23:33 > 0:23:36The towers are 533ft high.
0:23:36 > 0:23:41I was taken up to the top by Roger Evans, the bridge master.
0:23:46 > 0:23:48Well, Roger,
0:23:48 > 0:23:50here we are
0:23:50 > 0:23:52500ft above the River Humber.
0:23:52 > 0:23:57How the heck did this start with the first wire, sort of thing?
0:23:57 > 0:23:59It starts with a couple of men in a boat
0:23:59 > 0:24:03laying a wire rope on the bottom of the river,
0:24:03 > 0:24:07taking it over the top of the other tower and pulling it tight.
0:24:08 > 0:24:11When you can get several of them across,
0:24:11 > 0:24:15you can put a mesh on top to make a walkway.
0:24:15 > 0:24:18- It must have been a hairy business...- I wasn't here.
0:24:18 > 0:24:22..with dog clips holding them onto the wires
0:24:22 > 0:24:24as they advanced out across.
0:24:24 > 0:24:27Would a gale like this
0:24:27 > 0:24:29sort of stop work? I should imagine...
0:24:29 > 0:24:32Yeah, it's getting near that level.
0:24:32 > 0:24:36When they're actually working with the cable spinning,
0:24:36 > 0:24:40the wheel's pulling the wires across that way at 30mph,
0:24:40 > 0:24:44then a cross wind maybe 40mph blowing sideways on.
0:24:44 > 0:24:47So, very hazardous. It took longer as a result.
0:24:47 > 0:24:49How long is it from end to end?
0:24:49 > 0:24:51It's about a mile and a half
0:24:51 > 0:24:54between the two anchorages.
0:24:54 > 0:25:00Think how much wire it is - 15,000 wires going a mile and a half.
0:25:00 > 0:25:02There's enough to go
0:25:02 > 0:25:06- nearly twice round the world. - It looks quite fragile, really.
0:25:06 > 0:25:11I know there's many tons... How much weight is there in the bridge?
0:25:11 > 0:25:17The cables weigh 11,000 tons. Their first job is to support their own weight!
0:25:17 > 0:25:20And then you've got 17,000 tons of road there.
0:25:22 > 0:25:25On a good day, we've got 6,000 tons of traffic.
0:25:25 > 0:25:28So we're talking big figures.
0:25:28 > 0:25:32- So these pillars are really under some compression?- Yeah.
0:25:32 > 0:25:35That's why they can bend so much.
0:25:35 > 0:25:42They have a compressing force in them, so when they bend, you don't get cracks.
0:25:42 > 0:25:44Am I right or am I wrong?
0:25:44 > 0:25:48I felt distinctly that the things were moving when we came out.
0:25:48 > 0:25:53I can feel it here. I'm leaning on this and the tower moves slightly.
0:25:53 > 0:25:56At 500ft, they've got to do, haven't they?
0:25:56 > 0:26:00Where we are, it's designed to move two feet
0:26:00 > 0:26:02either way.
0:26:04 > 0:26:09Them cables that go from one side of the Humber Estuary to the other,
0:26:09 > 0:26:14what most people don't realise is, there are 14,000 pieces of wire
0:26:14 > 0:26:17forming a cable two feet in diameter.
0:26:17 > 0:26:19It has unbelievable tensile strength.
0:26:19 > 0:26:25Ten times what Mr Telford's wrought-iron bars have got.
0:26:27 > 0:26:29It's not 14,000 separate pieces.
0:26:29 > 0:26:34It's one bit that crosses the width of the river from this chamber
0:26:34 > 0:26:37up, over, down, up and over
0:26:37 > 0:26:39and back again 200 times,
0:26:39 > 0:26:42making it 400 passes in all.
0:26:42 > 0:26:45Each of these 400 bundles is about that big.
0:26:45 > 0:26:49And they all fan out in this great chamber,
0:26:49 > 0:26:53this great mass of wires like rays of sunlight
0:26:53 > 0:26:55coming from a funnel near the top.
0:26:58 > 0:27:01How many tons of concrete is it anchored to?
0:27:01 > 0:27:04Well, we're in the middle of about...
0:27:04 > 0:27:06300,000 tons of mass
0:27:07 > 0:27:12and 20,000 tons of pull trying to drag a 30,000-ton lump to the river.
0:27:12 > 0:27:14How many bits of wire is there
0:27:14 > 0:27:16altogether?
0:27:16 > 0:27:19You're looking at 15,000 bits.
0:27:20 > 0:27:2222½ thousand miles.
0:27:22 > 0:27:25And then, of course, these wonderful bolts
0:27:25 > 0:27:27that hold it all down.
0:27:27 > 0:27:30There are, like,
0:27:30 > 0:27:3212 little 'uns.
0:27:32 > 0:27:35They go through this great block.
0:27:35 > 0:27:38Really, you'd think that when you look at that lot,
0:27:38 > 0:27:42you'd think it were almost impossible to compress it
0:27:42 > 0:27:46into a two foot diameter iron rope...well, steel rope.
0:27:46 > 0:27:48I always think that.
0:27:48 > 0:27:51It's from these slender-looking cables
0:27:51 > 0:27:56that the whole weight of the road is suspended.
0:28:16 > 0:28:21Subtitles by Michael Bartlett ITFC - 2000
0:28:21 > 0:28:27E-mail us at subtitling@bbc.co.uk