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>> Telford designed his bridges before the railroad age.

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The Menai was not a railroad bridge.

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And railroad introduces new challenges.

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We have heavier loads due to the locomotive.

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And those locomotives, which are traveling very fast, also create impact loads.

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To study the railroad bridges of Great Britain we need

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to now introduce Isambard Kingdom Brunel.

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Just some brief background on Brunel.

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In 1824, he went to work with his father on the boring of a tunnel under the Thames River.

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During which time he was seriously injured when part of the tunnel collapsed.

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So his family sent him to Clifton to recuperate.

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Shortly after arriving, there was a bridge competition in Clifton.

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Brunel had no experience designing bridges but he submitted 4 suspension bridge designs

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that spanned from 890 feet to 916 feet.

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Now remember that the Menai only expand 580 feet,

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to give you a sense of context.

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The bridge commission felt uncertain about judging the 22 entries.

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So they asked Telford, who at that time was 72 years old, to be the judge.

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I am going to show you some entries to that competition,

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to show you the state of the art at the time.

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None of these, however, are Brunel's entries.

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One example is an underbelly type truss.

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So it's a truss that gets deeper towards the mid-span.

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Another shows a classical design that is essentially unbuildable.

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Or at least very expensive to build.

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This one is an ambiguous form.

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It's an arch and a cable.

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The designer was, we're guessing, worried about wind.

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And therefore he is using the arch to stabilize the cable.

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Telford who was the best bridge designer at the time thought that all the designs were bad.

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So he made one of his own,

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shown here with the large gothic towers.

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It's a bit of a strange design,

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putting huge gothic-like towers there, down near the water.

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He doesn't want to build longer spans than the Menai,

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because, remember, he is noticing that Menai is having trouble with the wind.

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The idea of going from cliff edge to cliff edge

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with the towers would make the span too long for Telford.

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Brunel objects to Telford's design in a letter to the commission.

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He says that those 2 huge towers are not necessary,

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and the bridge should be able to span cliff to cliff.

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So the commission essentially discards that competition and holds another one in 1831.

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And in this one Brunel enters and wins with a span of 702 feet.

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Work for the Clifton Bridge began in 1831 but it was suspended when political riots

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in Bristol made it impossible to raise funds.

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In 1831, there were revolutions in Western Europe and the British had

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to stop a lot of the building process.

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It wasn't until 1843 that both towers had been built.

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But the bridge wasn't complete until 1864,

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which is 5 years after Brunel died.

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The Clifton Bridge still stands today.

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And as you look at it up close you'll see that the cables are made

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up of 3 independent rod iron chains.

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Let's examine 2 other bridges by Isambard Kingdom Brunel:

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the Maidenhead Bridge completed in 1835 and the Saltash Bridge completed in 1859.

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Both were part of the Great Western Railway Project.

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With bridge construction at a halt, Brunel turns to the railroad.

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And between 1833 and 1841, he directed the design, construction and operation

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of the longest rail line in the world,

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the great western railway that went between London and Bristol.

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This line contained the world's longest standing brick arch bridge

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at Maidenhead expanding 128 feet.

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Later in 1959, he designed the Saltash Bridge as an extension of this rail line.

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And I will come to that in a moment.

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At the London end of this rail line is Paddington Station.

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If you look up Paddington Station, you see it's formed by a series of iron arches.

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Brunel designed the Paddington Station as well.

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The Saltash Bridge of 1859 is an extension of this great Western rail line beyond Bristol.

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So it's built near Plymouth.

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And if you look at it, the form is what we call a lenticular truss.

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It is a combination of an arch and a cable.

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And it forms the shape of a lens hence the name lenticular truss.

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It is an ambiguous form because it's not clear how the loads are being carried.

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By tension through the cable?

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Or by compression through the arch?

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At the tower, the horizontal components of the arch and the cable essentially cancel out so

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that the tower carries vertical load.

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An image of this bridge after the construction shows

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that the lenticular truss was lifted into place.

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At that time, Brunel had a rival and his name is Robert Stephenson.

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They were rivals but also friends,

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because Stephenson was on site with Brunel during the construction, assisting him

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with the construction.

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Stephenson is famous for the design of the Britannia Bridge,

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which is a railroad bridge also over the straits of Menai.

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He constructed tubes through which trains went.

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And it was constructed on shore and floated out and lifted into place.

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Brunel was on site helping Stephenson during the construction of the Britannia,

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just like Stephenson was on site with Brunel helping with the construction of Saltash.

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It is a little bit of a strange looking bridge,

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because it was supposed to be a suspension bridge.

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But the suspenders were too flexible for the railroad.

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Therefore they made the deck so stiff -- that hollow tube deck —

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that they realized they didn't need the suspension chains.

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So the towers were built to contain cables but in the end, those cables were unnecessary.

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At that time, economy was less crucial than safety.

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Because unfortunately bridge failures were not uncommon.

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And it was a society that had grown wealthy.

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Unfortunately, this Britannia Bridge as it was originally is no longer there.

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It was burnt down and something else was there put in its place.

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The towers are still the same.

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But it's no longer a tubed section.

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It's now an arch.

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Let's do one of our comparative,

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critical analyses by comparing the Britannia and the Saltash.

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From an efficiency point of view, the Britannia is a hollow box.

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Whereas the Saltash is a lenticular form.

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So these are different form for bridges.

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The span is essentially the same, 460 feet versus 455 feet.

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If we look at how much they weigh, the Britannia weighs 7,000 pounds per foot.

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Whereas the Saltash weighs 4,700 pounds per foot.

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From an economy point of view, the Britannia cost 198 pounds per foot.

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Whereas the Saltash 102.

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So the Britannia is more expensive and it's also heavier.

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But remember that the Britannia was designed to be a suspension bridge.

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And in the end ended up being a different form.

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And from the elegance point of view, the Britannia is a closed form.

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It's unexpressive.

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It's not really expressive of the structure.

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Whereas the Saltash is opposite, in the sense it's an open form.

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But it is ambiguous as I mentioned earlier.

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It's not clear how the roads are being carried.

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Both the Menai bridge by Telford and the Britannia Bridge are next

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to each other crossing the Menai straits,

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Telford's bridge carrying carriage loads,

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and the Britannia Bridge carrying railroad loads.