BRIDGES AMMANN 573 



has begun to regain the prominent position which it occupied in 

 the earl}'^ part of the nineteenth century. A large number of 

 bridges of this type, particuhirly for highway and combined high- 

 way and rapid transit rail traffic, have been built in the last 10 or 

 15 years, even with moderate spans, and its greatest length of span 

 has been continuously leaping to new records. 



The Manhattan Bridge in New York with a span of 1,470 feet 

 was the most outstanding modern suspension bridge only 10 years 

 ago. Since then there followed in rapid succession the Bear Moun- 

 tain Bridge across the Hudson with a 1,630-foot span, the Delaware 

 River Bridge in Philadelphia with 1,750 feet, the Detroit River 

 Bridge with 1,850 feet and now the George Washington Bridge near- 

 ing comjDletion with 3,500 feet. And a start has been made on the 

 Golden Gate Bridge in San Francisco with a span of 4,200 feet. 



A factor wdiich, I believe, has very materially contributed to the 

 revival of the suspension bridge, is the changed conception regarding 

 the proportioning of the so-called stiffening system of this type. As 

 a result of the insufficient rigidity of many of the early light and 

 short suspension bridges it became a general practice here and abroad 

 to proportion suspension systems as rigid systems, such as the truss 

 or the upright arch. This theory leads to enormous waste of material 

 in long-span suspension bridges, more particularly those bridges 

 carrying highway or mixed highway and rail traffic, because it does 

 not take into consideration the stiffening effect of the large suspended 

 mass, compared to the relatively much smaller load units which 

 cause the span to sag or oscillate. Conspicuous stiffening systems 

 also give an unsightly, clumsy appearance to such bridges and destroy 

 the gracefulness of the cables hanging in their natural catenary. 



To-day the justification of a flexible, more economical, and more 

 graceful stiffening system in long and heavy suspension bridges is 

 generally recognized. Studies made in connection with the George 

 Washington Bridge indicated that such a long span of 3,500 feet, with 

 comparatively short side spans, designed to carry vehicular and rapid 

 transit traffic, required practically no stiffening of the freely sus- 

 pended cables. Accordingly, the bridge was designed and is being 

 built without any stiffening wdiatsoever in its initial stage, in which 

 only the upper deck for highway traffic will be in place. When the 

 lower deck for rapid transit rail traffic is added, it will have com- 

 paratively flexible, very light stiffening trusses between the two decks. 



When it is considered that in such a long span every pound of 

 steel unnecessarily applied for stiffening is merely ballast, and that 

 this pound of useless material requires the use of another pound of 

 material in the cables, towers, and anchorages it may be realized that 

 such wasteful proportioning involves millions of dollars. 



