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>> Now let's take a look at Thomas Telford's later works.

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We started to look at the Craigellachie Bridge completed in 1814 spanning 150 feet.

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Next we're going to look at the Mythe Bridge completed in 1824

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at about the same span, 170 foot span.

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And then finally the Menai Bridge completed in 1826.

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This is not an arch.

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This is a 580 foot suspension bridge.

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With the design of the Craigellachie, Telford noticed that he made,

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what he would call, essentially a mistake.

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And he corrected this with the Mythe Bridge.

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If we look at these two bridges we see that the landscape is different

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so you might notice they are different bridges by the landscape.

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But if you look at just the bridge itself, can you notice the difference

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between the two bridges? Because they are very similar to one another.

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The difference between the Craigellachie Bridge and the Mythe Bridge is in the spandrels.

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It's in those diagonal members that connect the deck to the arch.

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If you look closely at those diagonal members you'll see that they're oriented differently

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in the Craigellachie versus the Mythe.

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In the Craigellachie if you take the bisector of those diagonal pieces you'll see

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that that bisector is normal to the arch.

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It's coming perpendicular to the arch.

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Whereas in the Mythe Bridge the bisector of those diagonals is vertical.

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It's completely straight up and down.

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In the Mythe Bridge this is a more efficient way of carrying the loads.

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The loads in those diagonals are more efficiently or more evenly distributed

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between those diagonal members, whereas in the Craigellachie Bridge

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in particular those diagonals that are leaning or more horizontal are carrying much less load

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than the diagonals that are more vertical.

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Now we come to the Menai Bridge completed in 1826,

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a 580 foot span suspension bridge designed by Thomas Telford.

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Now there was a need to design this bridge and the need arose from the active union of 1800

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which merged the Kingdom of Ireland with the Kingdom of Great Britain.

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They needed to connect London to Dublin and to do that you had to go through the Island

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of Anglesey all the way to the tip of Holyhead.

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And to get to Holyhead and even Anglesey you had to cross the Menai Straits.

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If you look at the side spans of this bridge we see that it has both arches and suspenders.

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And this again is ambiguous.

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It doesn't tell you clearly how those loads are being carried.

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But Telford did this because he was concerned about wind.

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He wanted to make sure the back stands were heavy and anchored.

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Just before the bridge opened, Telford's resident engineer noticed undulations

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from gusting winds so Telford added bracing, which cut down the movement.

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Ten years later, about two years after Telford's death,

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the bridge keeper reported large oscillations and unfortunately no action was taken

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and in 1839 a gale tore part of the roadway loose.

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Telford's writings in 1820s and his resident engineer's field observations showed how

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horizontal wind can cause extensive vertical motion in a suspension bridge.

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Unfortunately this lesson in history was lost in the bridge designs to come, as we will see.

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In the Menai Bridge, although the towers look heavy, they're actually hollow,

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like in the Pontcysyllte Aqueduct Bridge.

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So Telford again is thinking about efficiency in his designs.

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Let's use the Menai Bridge to define some terms for you as related to suspension bridges.

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The first term that we have to understand is span.

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When we talk about a span of a bridge we're talking about the longest unsupported length

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and for suspension bridges that distance is from one tower to the next.

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Next let's look at the cable.

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The cable goes from anchor to tower, to the next tower, to the next anchor, and it is in tension.

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And it's in tension due to the uniform loads imposed by the hanging suspenders.

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The suspenders are the vertical elements that suspend, or support, the deck.

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The form of that cable is parabolic.

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I'm going to do a brief demonstration for you to show you the shape that these cables take

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when loaded and it will give you a better sense for why the form

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of a suspension bridge cable is parabolic.

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In this demonstration this chain represents the cable of a suspension bridge.

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We know that the chain can only take tinsel forces.

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It can't take any compression forces.

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It could be stretched but it can't be squeezed.

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So let's look at how the shape of this chain changes when we add loads.

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So if I add one load right in the center we see the V shape that this chain takes.

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But if I— Now, I'm going to add continuously load along this whole chain,

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we're going to see it start to take the form of the cable

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of a suspension bridge, which is a parabolic form.

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Now you start to see the shape change a little bit more…

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and more. So these weights represent essentially the load that's transferred from the suspender,

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which is the vertical elements of a suspension bridge, to the cable.

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And it's the load represented by the weight of the deck.

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And we see that as I add more and more of these loads along the chain, we're starting to see

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that parabolic form take shape, which is the shape of the cable in a suspension bridge.

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Telford designed his bridges for carriage loads but the railroad age was approaching

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and that is where we move to next.