>> Now let's take a look at Thomas Telford's later works.
We started to look at the Craigellachie Bridge completed in 1814 spanning 150 feet.
Next we're going to look at the Mythe Bridge completed in 1824
at about the same span, 170 foot span.
And then finally the Menai Bridge completed in 1826.
This is not an arch.
This is a 580 foot suspension bridge.
With the design of the Craigellachie, Telford noticed that he made,
what he would call, essentially a mistake.
And he corrected this with the Mythe Bridge.
If we look at these two bridges we see that the landscape is different
so you might notice they are different bridges by the landscape.
But if you look at just the bridge itself, can you notice the difference
between the two bridges? Because they are very similar to one another.
The difference between the Craigellachie Bridge and the Mythe Bridge is in the spandrels.
It's in those diagonal members that connect the deck to the arch.
If you look closely at those diagonal members you'll see that they're oriented differently
in the Craigellachie versus the Mythe.
In the Craigellachie if you take the bisector of those diagonal pieces you'll see
that that bisector is normal to the arch.
It's coming perpendicular to the arch.
Whereas in the Mythe Bridge the bisector of those diagonals is vertical.
It's completely straight up and down.
In the Mythe Bridge this is a more efficient way of carrying the loads.
The loads in those diagonals are more efficiently or more evenly distributed
between those diagonal members, whereas in the Craigellachie Bridge
in particular those diagonals that are leaning or more horizontal are carrying much less load
than the diagonals that are more vertical.
Now we come to the Menai Bridge completed in 1826,
a 580 foot span suspension bridge designed by Thomas Telford.
Now there was a need to design this bridge and the need arose from the active union of 1800
which merged the Kingdom of Ireland with the Kingdom of Great Britain.
They needed to connect London to Dublin and to do that you had to go through the Island
of Anglesey all the way to the tip of Holyhead.
And to get to Holyhead and even Anglesey you had to cross the Menai Straits.
If you look at the side spans of this bridge we see that it has both arches and suspenders.
And this again is ambiguous.
It doesn't tell you clearly how those loads are being carried.
But Telford did this because he was concerned about wind.
He wanted to make sure the back stands were heavy and anchored.
Just before the bridge opened, Telford's resident engineer noticed undulations
from gusting winds so Telford added bracing, which cut down the movement.
Ten years later, about two years after Telford's death,
the bridge keeper reported large oscillations and unfortunately no action was taken
and in 1839 a gale tore part of the roadway loose.
Telford's writings in 1820s and his resident engineer's field observations showed how
horizontal wind can cause extensive vertical motion in a suspension bridge.
Unfortunately this lesson in history was lost in the bridge designs to come, as we will see.
In the Menai Bridge, although the towers look heavy, they're actually hollow,
like in the Pontcysyllte Aqueduct Bridge.
So Telford again is thinking about efficiency in his designs.
Let's use the Menai Bridge to define some terms for you as related to suspension bridges.
The first term that we have to understand is span.
When we talk about a span of a bridge we're talking about the longest unsupported length
and for suspension bridges that distance is from one tower to the next.
Next let's look at the cable.
The cable goes from anchor to tower, to the next tower, to the next anchor, and it is in tension.
And it's in tension due to the uniform loads imposed by the hanging suspenders.
The suspenders are the vertical elements that suspend, or support, the deck.
The form of that cable is parabolic.
I'm going to do a brief demonstration for you to show you the shape that these cables take
when loaded and it will give you a better sense for why the form
of a suspension bridge cable is parabolic.
In this demonstration this chain represents the cable of a suspension bridge.
We know that the chain can only take tinsel forces.
It can't take any compression forces.
It could be stretched but it can't be squeezed.
So let's look at how the shape of this chain changes when we add loads.
So if I add one load right in the center we see the V shape that this chain takes.
But if I— Now, I'm going to add continuously load along this whole chain,
we're going to see it start to take the form of the cable
of a suspension bridge, which is a parabolic form.
Now you start to see the shape change a little bit more…
and more. So these weights represent essentially the load that's transferred from the suspender,
which is the vertical elements of a suspension bridge, to the cable.
And it's the load represented by the weight of the deck.
And we see that as I add more and more of these loads along the chain, we're starting to see
that parabolic form take shape, which is the shape of the cable in a suspension bridge.
Telford designed his bridges for carriage loads but the railroad age was approaching
and that is where we move to next.