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>> Now we're going to look at the social context of the Industrial Revolution and Great Britain.

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We see industrialization happening in Great Britain in particular, and we're going to look

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at three reasons why this is happening.

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One is we have Queen Elizabeth who outlaws wood cutting.

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The forests in Great Britain are getting diluted.

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So the Queen outlawed wood cutting.

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Wood was used for fuel and construction, and they needed to control this.

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They looked underground and found coal.

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Now they found a new material and need it for building and for fuel.

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Next, British Democracy encourages free enterprise.

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Britain wasn't really run from London.

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The transportation system in Great Britain was pretty bad.

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There was tendency for local freedom in what was done.

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London was far from the west midlands and so they left them alone.

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And that kind of democracy encouraged free enterprise.

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All didn't come from London.

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And finally, island isolation provides political stability.

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Great Britain avoided wars on Continental Europe

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and they hadn't been invaded for about 700 years.

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Therefore, there was a sense of security.

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So very briefly, that is the social context surrounding Great Britain

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and what allowed structural art to flourish in particular in Great Britain at that time.

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And the famous structural engineer of this era was Thomas Telford; born 1757, died 1834.

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Telford was brought up in poverty.

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He worked since he was eight years old, and began his career as a stonemason.

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In 1782 at 25 years old, he left for London where he worked

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as a draftsman in an architect's office.

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And in 1787, he worked as a county surveyor.

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He designed his first bridge, a 3-stone arch span, completed in 1792.

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And at that time, he began to become recognized.

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In 1795 there was a big flood over the Severn River where the Iron Bridge is.

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This flood took out all bridges except for the Iron Bridge.

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The stone bridges essentially acted as dams.

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The water pushed them over, and the wood bridges were lifted up by the water.

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The Iron Bridge was light enough so that water could go through it and it was anchored down.

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When Telford saw this, he was impressed and he turned his attention from masonry to iron.

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There was an opportunity in Great Britain to build bridges and canals,

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because this was the infrastructure for the Industrial Revolution.

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The Buildwas was the first bridge designed of iron.

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It wasn't a great work of structural art because it has two arches, and you don't know by looking

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at it which arch is carrying the load.

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So this is what we call an ambiguous form, meaning there is ambiguity in the way

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that we see the bridge because we don't understand how the loads are being carried.

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It was copied after wood arches.

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So it's essentially half engineered, half craftsman design.

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And it's also built over the Severn River.

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If you look at Thomas Telford's early works,

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we're going to study three of them in this lecture.

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One is the Buildwas Bridge, 130-foot arch that I just spoke about completed in 1795.

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Next we're going to look at the Pontcysyllte Aqueduct

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which has short-span arches completed in 1805.

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And finally, the Bonar, a 150 foot arch completed in 1810.

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To be clear, Telford wasn't the only one building iron bridges at this time,

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nor were they the longest spanning ones.

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Telford's only bridge design rival, John Rennie, designed iron bridges

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on the order 200 feet for example.

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But as David Billington writes in the Tower and the Bridge, quote,

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"What set Telford apart is his distinct personal style.

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His iron arches are more visually attractive and they are also technically superior.

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A compilation of cast- iron bridges built between 1779 and 1871 lists the bridges

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in order of their technical quality.

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Of the top 9 listed, 8 are Telford's.

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Of those 8, 5 are still standing today," end quote.

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One of the bridges that no longer stands is the Bonar Bridge.

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It was taken down after 90 years because it was hard to maintain, not because of a defect.

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It's a cast-iron bridge spanning 150 feet.

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His design criteria for this bridge were essentially efficiency, economy, and elegance.

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He didn't use those words exactly, but he did use these words

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to describe the design criteria for Bonar Bridge.

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Quote, "To improve the principles of constructing iron bridges,

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also their external appearance, and to save a very considerable portion

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of iron and consequently weight."

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So if you take apart that sentence, we see he writes, "To save a very considerable portion

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of iron and consequently weight."

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There he's talking about efficiency.

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When he speaks of, "To improve the principles of constructing iron bridges," there he is talking

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about economy, because economy is strongly linked to construction.

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And finally when he speaks about the external appearance, he is talking about elegance.

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So in essence, the design criteria for the Bonar Bridge encompasses efficiency,

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economy, and elegance.

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The Pontcysyllte Aqueduct carries the Llangollen Canal over the Valley

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of the River Dee in Northeast Wales.

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Completed in 1805, it's the longest and highest aqueduct in Britain.

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Viaducts were important to connect cities in Great Britain.

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The Industrial Revolution required transportation.

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A pre-Industrial Revolution map of 1760 shows

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that there's not many rivers connecting the major cities.

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For example, Birmingham and Manchester were great industrial cities, but they were isolated.

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You couldn't go North or South via water, for example.

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Therefore, there's lots of canal building to connect these rivers.

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And a map just 30 years later in 1790 shows tremendous progress

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in connecting these cities via waterways.

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The Barton Aqueduct of 1760 is an example

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of what was done prior to the Industrial Revolution.

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Everything has wind, human, and animal power.

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For example, you see the boat being pulled by horses.

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The Barton is an arch form, a pre-Industrial Revolution stone bridge with Roman form.

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In contrast with the Barton Aqueduct,

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the Pontcysyllte Aqueduct is iron and it's much higher.

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The columns are also hollow.

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So Telford is beginning to think about minimum materials, efficiency.

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Not only are the columns higher, but they're much more slender than those of Barton.

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And if you see on the top this image, little tiny dots,

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those are people just to give you a sense of scale.

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This bridge is very tall and very large.

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If we take a closer look at the structure, we see exceptionally slender arches.

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And this is a different aesthetic from the stone arches of the past.

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In the front there's a weathering plate,

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but the actual structure are the verticals and the arches.

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In 1799 there's a huge competition for a London Bridge across the Thames River,

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and Telford proposes a single 600-foot span to allow shipping to pass beneath unobstructed.

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This is way beyond what had been done before in any material.

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Nothing of the span had been done, not even close.

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Telford's design impressed the committee the most.

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So the competition committee consulted many iron users, including university professors,

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to see if such a design was even feasible.

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Although the consensus was that the design could be built,

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Parliament never acted upon it and it was never built.

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I have a question for you.

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How do you critique bridges?

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How do you measure the success or failure of a bridge?