DB: When engineers thought about crossing the Hudson River, this very wide estuary, prior to the 1920s they thought more or less in two ways. First of all, they thought about railroad bridges. That led Lindenthal to propose very heavy suspension bridges. It led others to propose truss-like bridges of various forms. A second way they began to think about already, as the 20th century began, was tunnels. Tunnels were conceived of, first, of course, for railroads. The Pennsylvania Railroad built their tunnels then. And then the question of cars going in tunnels. So these were, in a way, the two competing images of how you get across this wide river, because the span would obviously have to be quite far beyond anything that had ever been done before to get it across in one swoop, because it is a major harbor, it does have to, it does require the passage of boats of great height, and, therefore, it would need to have 150-or-so feet in clearance. So these were the two ideas about bridge design that confronted Ammann, for example, in the 1920s when he began to design. And, of course, for Ammann the tunnel was an anathema. It has none of this emotional appeal that light suspended structure had. And so he was not at all, that did not at all appeal to him. At the same time, the, the heavy suspension bridge for railroads also didn't appeal to him esthetically.

DB: Well if they thought they were carrying railroads, then they had a mixed image. That is to say, there was no single image because everybody knew the problems with railroads and suspension bridges in the 19th century when many of them collapsed and only Roebling survived, and that had to be taken down in the 1890s because the engines got too heavy.

DB: Ammann's design process, the thinking he went through, would be characterized by three components, inseparable components. First would be what was built into him from his Lindenthal experience, would be the component of safety, the component of the bridge's performance in the environment. How would it work? And that's the ground of everything he was thinking about. But inseparably connected to that is that the bridge will never get built, he'll never realize his vision unless it is competitive, unless it is, in this case, reasonably economical. And that meant, of course, thinking hard about performance in the light of reduced materials. In a suspension bridge, perhaps as in no other kind of bridge, the cost is closely related to the materials, not so closely related to labor. Although the labor's included in the materials, it's nevertheless, if you save one pound in the middle of this span, that's a pound you don't have to carry and that means the material can be less and, having been less, then, indeed, the amount of material is then again less. So it is an augmenting factor. So the construction was the second thing in his mind, although, as I said, they're all inseparable. You really can't separate them. And the third was clearly the appearance. So these were the three things that he held together as he was thinking about his design, and he never spoke about the design without referring to all three. And that's the way the best designers think.

DB: Ammann's design choices can be thought about in three ways. First of all, he wanted to reduce the amount of materials and still make the bridge fully safe. Second, he wanted to have it built for as little money as possible, be as economical as it could be, and, third, in addition to that, he wanted it to look elegant. He wanted it to be a beautiful bridge. And these three all come together in his mind. They're inseparable, but we can, as analysts, look at them separately and see what decisions he made. But the basic idea underlying everything is, reduce as much as possible the materials in the main span because any additional weight in the main span has to be carried by additional weight in the main span plus, of course, additional tower and anchorage materials, as well.

DB: When Ammann thought about this reduction in materials, the first thing he thought about was the structure of the deck itself, the horizontal deck. The cable was a very simple issue. That was not something that was in dispute. It was easy to calculate, relatively easy, and to dimension, but the deck was in considerable dispute. And so Ammann's idea was to try to reduce it as much as possible, but he needed a scientific base for that reduction. And there was a new theory, the deflection theory, which said that if you consider the deck and the cable together in your analysis rather than separately, as been, had been the case in the past, if you considered them together, you will find, on the normal suspension bridge at that time, meaning bridges like the Williamsburg Bridge and the Manhattan Bridge and the Delaware Bridge, Memorial Bridge, you will find that you need less material in the deck.

DB: Other engineers knew that. They knew that by changing the basis of their analysis, they could reduce materials. But Ammann carried that one step further, and it was a fundamental step, because he drew not the conclusion that you could just save materials, but he drew the conclusion that the smaller he made the deck, the less materials you needed, and that meant, in the limit, you didn't need a deck at all. That is to say, the less stiff you made the deck, the less the force would be in the deck, the bending would be in the deck due to these difficult live loads coming from the trucks. And so he drew a conclusion that other designers had not drawn. They had simply said, "Two methods of analysis. Here's a bridge, already designed. If we analyze it both ways, we find that, using this more, more advanced method, we can reduce the materials a little bit." Ammann said, "We can change the form." And in changing the form, he found a justification for one of the most extraordinary design decisions in all bridge history, namely that the George Washington Bridge, this longest of all spans, could be built with essentially no stiffness in the deck, and that allowed him to build the bridge with no second deck at this immense span.

DB: What Ammann found when he looked at the results of the deflection theory and studied it carefully, was that the amount of force that would be in the horizontal truss was dependent not upon the loads, as one would normally think, but upon its stiffness. That means that if you reduce the stiffness, you reduce the forces in the truss. On that basis, Ammann realized that by making a much more slender truss for his two-deck bridge, he could save a very large amount of money. Indeed, as he spoke earlier, as he wrote earlier about the bridge, he said he could save on a $30 million bridge, or on a bridge which was to cost $40 million, he could save $10 million, an immense amount, for this one idea. So that was the basis on which he believed he could make an economical design. Then when the issue came up of only a second deck, only a first deck, no second deck, the question was: could you reduce that stiffness essentially to zero, that deck stiffness? This same theory told him, yes, you could do that and, therefore, the bridge was built with no second deck and, hence, practically no deck stiffness.

Now, of course, there's a caveat to the whole thing, obviously.

Ammann's decision to build the bridge with only one deck turned out to be a good decision. The George Washington Bridge, as far as we know, never had any difficulties up until the time that the second deck was built and completed in 1962. So in that sense it was a reasonable decision. On the other hand, it was a bad model to follow because, unlike the George Washington Bridge, all the bridges that followed it, the major bridges in the '30s, were much lighter bridges. The Bronx-Whitestone Bridge, the Deer Island Bridge, the Thousand Island Bridges, and the Golden Gate Bridge, and then ultimately, of course, the poor Tacoma Bridge, were all substantially lighter bridges. So that the stiffness that Ammann counted on, which is called cable stiffness, which means the stiffness arising from the dead load forces in the cable that make it hard to deflect the cable when it's very taught, was very large in the George Washington Bridge and not so large in the others. And the idea of reducing the deck to almost no stiffness, while it was all right for traffic loads, was not all right for wind loads. And it was the wind loads that got the bridges into difficulties in the 1930s, culminating in the Tacoma Narrow Bridge failure.

<< 7 >>