>> Telford designed his bridges before the railroad age.
The Menai was not a railroad bridge.
And railroad introduces new challenges.
We have heavier loads due to the locomotive.
And those locomotives, which are traveling very fast, also create impact loads.
To study the railroad bridges of Great Britain we need
to now introduce Isambard Kingdom Brunel.
Just some brief background on Brunel.
In 1824, he went to work with his father on the boring of a tunnel under the Thames River.
During which time he was seriously injured when part of the tunnel collapsed.
So his family sent him to Clifton to recuperate.
Shortly after arriving, there was a bridge competition in Clifton.
Brunel had no experience designing bridges but he submitted 4 suspension bridge designs
that spanned from 890 feet to 916 feet.
Now remember that the Menai only expand 580 feet,
to give you a sense of context.
The bridge commission felt uncertain about judging the 22 entries.
So they asked Telford, who at that time was 72 years old, to be the judge.
I am going to show you some entries to that competition,
to show you the state of the art at the time.
None of these, however, are Brunel's entries.
One example is an underbelly type truss.
So it's a truss that gets deeper towards the mid-span.
Another shows a classical design that is essentially unbuildable.
Or at least very expensive to build.
This one is an ambiguous form.
It's an arch and a cable.
The designer was, we're guessing, worried about wind.
And therefore he is using the arch to stabilize the cable.
Telford who was the best bridge designer at the time thought that all the designs were bad.
So he made one of his own,
shown here with the large gothic towers.
It's a bit of a strange design,
putting huge gothic-like towers there, down near the water.
He doesn't want to build longer spans than the Menai,
because, remember, he is noticing that Menai is having trouble with the wind.
The idea of going from cliff edge to cliff edge
with the towers would make the span too long for Telford.
Brunel objects to Telford's design in a letter to the commission.
He says that those 2 huge towers are not necessary,
and the bridge should be able to span cliff to cliff.
So the commission essentially discards that competition and holds another one in 1831.
And in this one Brunel enters and wins with a span of 702 feet.
Work for the Clifton Bridge began in 1831 but it was suspended when political riots
in Bristol made it impossible to raise funds.
In 1831, there were revolutions in Western Europe and the British had
to stop a lot of the building process.
It wasn't until 1843 that both towers had been built.
But the bridge wasn't complete until 1864,
which is 5 years after Brunel died.
The Clifton Bridge still stands today.
And as you look at it up close you'll see that the cables are made
up of 3 independent rod iron chains.
Let's examine 2 other bridges by Isambard Kingdom Brunel:
the Maidenhead Bridge completed in 1835 and the Saltash Bridge completed in 1859.
Both were part of the Great Western Railway Project.
With bridge construction at a halt, Brunel turns to the railroad.
And between 1833 and 1841, he directed the design, construction and operation
of the longest rail line in the world,
the great western railway that went between London and Bristol.
This line contained the world's longest standing brick arch bridge
at Maidenhead expanding 128 feet.
Later in 1959, he designed the Saltash Bridge as an extension of this rail line.
And I will come to that in a moment.
At the London end of this rail line is Paddington Station.
If you look up Paddington Station, you see it's formed by a series of iron arches.
Brunel designed the Paddington Station as well.
The Saltash Bridge of 1859 is an extension of this great Western rail line beyond Bristol.
So it's built near Plymouth.
And if you look at it, the form is what we call a lenticular truss.
It is a combination of an arch and a cable.
And it forms the shape of a lens hence the name lenticular truss.
It is an ambiguous form because it's not clear how the loads are being carried.
By tension through the cable?
Or by compression through the arch?
At the tower, the horizontal components of the arch and the cable essentially cancel out so
that the tower carries vertical load.
An image of this bridge after the construction shows
that the lenticular truss was lifted into place.
At that time, Brunel had a rival and his name is Robert Stephenson.
They were rivals but also friends,
because Stephenson was on site with Brunel during the construction, assisting him
with the construction.
Stephenson is famous for the design of the Britannia Bridge,
which is a railroad bridge also over the straits of Menai.
He constructed tubes through which trains went.
And it was constructed on shore and floated out and lifted into place.
Brunel was on site helping Stephenson during the construction of the Britannia,
just like Stephenson was on site with Brunel helping with the construction of Saltash.
It is a little bit of a strange looking bridge,
because it was supposed to be a suspension bridge.
But the suspenders were too flexible for the railroad.
Therefore they made the deck so stiff -- that hollow tube deck —
that they realized they didn't need the suspension chains.
So the towers were built to contain cables but in the end, those cables were unnecessary.
At that time, economy was less crucial than safety.
Because unfortunately bridge failures were not uncommon.
And it was a society that had grown wealthy.
Unfortunately, this Britannia Bridge as it was originally is no longer there.
It was burnt down and something else was there put in its place.
The towers are still the same.
But it's no longer a tubed section.
It's now an arch.
Let's do one of our comparative,
critical analyses by comparing the Britannia and the Saltash.
From an efficiency point of view, the Britannia is a hollow box.
Whereas the Saltash is a lenticular form.
So these are different form for bridges.
The span is essentially the same, 460 feet versus 455 feet.
If we look at how much they weigh, the Britannia weighs 7,000 pounds per foot.
Whereas the Saltash weighs 4,700 pounds per foot.
From an economy point of view, the Britannia cost 198 pounds per foot.
Whereas the Saltash 102.
So the Britannia is more expensive and it's also heavier.
But remember that the Britannia was designed to be a suspension bridge.
And in the end ended up being a different form.
And from the elegance point of view, the Britannia is a closed form.
It's unexpressive.
It's not really expressive of the structure.
Whereas the Saltash is opposite, in the sense it's an open form.
But it is ambiguous as I mentioned earlier.
It's not clear how the roads are being carried.
Both the Menai bridge by Telford and the Britannia Bridge are next
to each other crossing the Menai straits,
Telford's bridge carrying carriage loads,
and the Britannia Bridge carrying railroad loads.