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>> Hello and welcome back.

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To begin our study of structural art, we need to go to Great Britain and study the works

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of Thomas Telford, and also two other men, named Stephenson and Burnell.

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I'm going to begin each lecture by defining some lecture goals, and in this lecture,

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the goals are: 1, to show how the definition and ideals of structural art began,

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and as I mentioned, they began in Great Britain.

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2, contrast the works of early iron bridges.

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And 3, we're going to do that contrasting by critiquing structures

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through what we call a comparative critical analysis.

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In these analyses, we critique both the technical

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and aesthetic aspects of the bridge design.

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So with these changes, from the scientific point of view, we have a new material that is born

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of the Industrial Revolution-iron.

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From the social point of view, we have a new opportunity, industrialization.

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And from the symbolic point of view, we have a new vision, a new form for structures

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which we define as structural art.

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Let's start by comparing a pre-industrial revolution structure

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to a post-industrial revolution structure.

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One example of a pre-industrial revolution structure is Stonehenge.

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And I use this example of Stonehenge to show essentially how far stone can span.

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In Stonehenge, the unsupported length of that beam,

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that horizontal member, is on the order of 10 feet.

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Stone is not very strong in tension,

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and this beam on the bottom surface is experiencing tension.

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We're going to learn more about beams and tension in later lectures,

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but for now know that, again, stone does not carry very large tensile forces.

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It's not strong in tension.

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In contrast, we have the iron bridge, the first bridge designed of iron.

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And iron is strong in both tension and compression.

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The iron bridge spans about 100 feet.

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Now, 100 feet today is not very long, especially if you compare it to, for example,

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the Golden Gate Bridge at 4,200 feet.

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But back then, it was a very long span.

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It was designed in 1779 by Abraham Darby, the Third.

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It wasn't intended to be designed as a long span bridge, but really it was intended

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to be an advertisement for his company.

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If you go to the bridge, which is still standing today, you'll see a plaque that says,

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"It was intended to be an advertisement for the skill of the Coalbrookdale Ironmakers."

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The Darbys were in business for building pots, pans and weapons, and they used the iron bridge

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as a visible advertisement to show how iron can span 100 feet between supports.

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Let's take a closer look at this new material iron,

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that came following the industrial revolution.

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Iron is stronger than wood and stone.

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For example, in compression, iron is about 10 times stronger than stone.

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And in tension, it is on the order of magnitude 100 times stronger than stone.

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Iron is also more permanent than wood, but not necessarily more permanent

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than stone, because iron will corrode.

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And finally iron permits forms that are lighter than those of stone.

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Because it is stronger, you need less material to build it.

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At closer look of the iron bridge, we see that it is comprised of five iron arches.

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It is, as I said, the first cast iron bridge,

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and is very light compared to others of the time.

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And it is built in a way to make it look like a wooden structure, essentially carpentry in iron.

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We see mortise and tendon connections, for example.

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Next, we're going to look at the social aspect of these British metal forms,

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but before we go there, I have a question for you.

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The density of cast iron is about 450 pounds per cubic foot,

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and the density of stone is about 150 pounds per cubic foot.

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So which of the following is true?

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One, a cast iron bridge will be heavier than a stone bridge?

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Two, a stone bridge will be heavier than the cast iron bridge, or three,

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a stone bridge will be just as heavy as a cast iron bridge?

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The answer to the question is a stone bridge will be heavier than a cast iron bridge.

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Although cast iron is heavier than stone by volume, it's also much stronger than stone.

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Therefore, one can use much less material to build an arch.

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The resulting iron arch is much lighter than a stone arch.

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For example, for the bridge built by Rowland Burdon Esquire over the River Wear at Sunderland,

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an iron arch was estimated to be 15x lighter than a stone arch of similar size.