DB: These great bridges were, clearly, we see them today as symbols of both of their time and of our time. When they were first completed, however, they were accepted in a slightly different way. They were seen, of course, as great technological marvels which were coming one after another at this time. Don't forget that Brooklyn Bridge opened just exactly at the time that Wall Street was being lit by the systems of Thomas Edison. So that there were these marvels that were arriving and people flocked to them.

DB: These bridges are certainly an integral part of the culture of New York City. And when people began to think, after they were completed, about the meaning of the city and, through that, the meaning of our society, the bridges became part of that description, part of that understanding. And in the 1920s, for example, when the country was searching for a usable past, as they called it, critics and poets and painters, writers in New York City focused on, among other things, the great Brooklyn Bridge. And they focused on it because they saw it as prototypically American and as prototypically urban and also as a symbol of industrialization, all of those issues which were providing the culture with so much hope, particularly in the '20s, in the early '20s, and following the end of the "war to end all wars," and also giving these people a sense that a bridge or a work of technology was not just a utilitarian object, but also something that would be part of the more traditional view of culture, namely, a work of art.

DB: Beginning in the early 1820s, the suspension bridge became this major form for long-span bridges. And the reason is very simple, because a bridge, which is a horizontal member that must carry vertical loads, would be, can be very heavy and if it gets very heavy, then all of the material is used just to carry the dead load and nothing is left for the live load. In the case of a suspension bridge, that thin cable stretched between the towers is able to carry these vertical loads by its single tension force. You just imagine you have a string and you put the weights on a string. The cable automatically takes the shape which will carry those loads with a minimum of need for materials. Once you have the minimum amount of materials, that means you have strength left over to carry loads across, which is the whole purpose of the bridge: to carry carriages originally and then trucks and trains and cars. So the suspension bridge became, starting with Thomas Telford's Menai Straits Bridge, which is really the break in bridge design, that's the first major long-span bridge in the world and it's the one to which Roebling always referred himself and Ammann, too. And so, starting with that, these bridges became, therefore, the ideal for engineers for long spans. And so that when one approached these relatively long-span situations, the East River, to begin with, and then the Hudson River, it was natural to think in terms of suspension bridges.

DB: The problem with suspension bridges has always been, and is today, the fact that they are ideal for their dead weight, that is to say they reduce the dead weight greatly, but they are susceptible to various kinds of live loads, various kinds of transient or loads that come on and off. They are perfect, if we hang the weights from it, they give a perfect shape for one set of loads, and usually it's a dead load or a live load uniformly distributed. On the other hand, if you have the live load over only half the bridge or if you suddenly have gusts of winds coming to the bridge, it can be very flexible and susceptible to large movements. And so the problem, starting with the Menai Straits Bridge, was always how do you first get this very light shape to begin with and then how do you prevent that light shape from being transformed into a moving bridge under its various live loads? And Roebling provided one solution with the Brooklyn Bridge and earlier bridges, but most clearly seen in the Brooklyn Bridge, by using these diagonal stays that come down and give that bridge its webbed look. So that that provided a stiffness against these possible oscillations. And Roebling's bridges were, by and large, successful because of that. So the modern bridges had to, had to take up that problem with much greater and greater live loads coming on them.

DB: It was the Industrial Revolution and the development in Britain in the late 18th century of inexpensive iron that allowed the building of bridges in an entirely new way: first, by means of arches which are carrying loads by compression. That means that the vertical loads in the arch are transformed into squeezes, into a force on the arch metal, which squeezes it together. So the load tends to make the arch shorter. The danger with that is that when you take a thin piece of metal and you put a load on the top of it to squeeze it together, long before it will crush, the material will buckle outwards, take a new shape. That means that arch bridges have to be braced carefully or they will buckle. And there have been some dramatic failures of such buckled members. Suspension bridges, on the other hand, do the reverse. Instead of carrying the loads by compression, they carry the loads by tension, which is the same thing as if we have our column now and, instead of squeezing it, we catch it at the top and put a weight on the bottom and we stretch it. So that's pure tension. The idea of the compression bridge, of the suspension bridge is to carry the loads by this pure tension. That does not need any bracing. Therefore, because the steel or the metal will therefore not buckle under pure tension, it is the ideal way to load a metal structure.

DB: The great advantage of a suspension bridge is that you can take very large loads by means of very small amounts of material. And since the material is now in tension, the cable is in tension, you can reach the full strength of the member without any change in its geometry. It just simply stretches. On the other hand, if you have it in compression, when you try to squeeze it together it's very likely to buckle outwards and, therefore, fail long before it reaches its full strength, which means that in compression members such as arches, one needs to have two members braced together or, as we can see, we see in the Bayonne Bridge or the Hell Gate Bridge, a top and a bottom chord with a lot of bracing in between. So that has the advantage of extra stiffness, of course, but it uses a lot more material and, therefore, restricts the span of arches compared to suspension bridges. And one must also say that in the imagination of the engineer, the suspension bridge has always appealed visually, that, starting with Telford and including Roebling and coming up to Ammann and other great designers like Steinman, the appeal of suspension bridge was this flat graceful curve that seemed to be carrying these loads with almost no effort at all, carrying them to the, the strong towers and anchorages. Arches can be very elegant, too, but the long-span arches do need to be braced and, therefore, have an appearance which is quite different, and hasn't had the same effect on engineers as had the suspension bridge.

<< 3 >>