572 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1931 



most important factors in the construction of large bridges, and is 

 a question which has often led to animated discussions and differences 

 of opinions in the profession. It is a question which depends upon 

 many different factors and not in the least upon the personal con- 

 ceptions of the designer. 



At the beginning of this century and up to as late as the World 

 War, two types of bridges appeared to be particularly favored, 

 although they are generally the least satisfactory from the esthetic 

 point of view : the cantilever bridge for the longer spans and the 

 simple span truss type for the lesser span lengths of up to 700 feet. 

 The most outstanding example of the former type is the Quebec 

 Bridge across the St. Lawrence River with a main span of 1,800 feet, 

 which until recently has been the longest span in existence. The 

 longest simple span is that of the bridge across the Mississippi at 

 Metropolis with a length of 720 feet (pi. 5, fig. 2). 



Many bridges of the cantilever type have been built across the 

 Ohio, Mississippi, and other wide streams. To the most recent and 

 typical examples belong the two bridges built by the Port of New 

 York Authority across the Arthur Kill in New York. The type 

 is particularly suitable where foundation conditions make other 

 types expensive or subject to the effect of possible settlements. 

 But its merits, particularly the alleged advantage of its being stati- 

 cally determinate, have been overrated. 



For a long period there existed a very general prejudice against 

 the so-called continuous truss, so much so that, in spite of its eco- 

 nomic advantages, it was practically excluded from consideration in 

 favor of the simple truss or the cantilever. Its application in 1916 

 in the bridge across the Ohio at Sciotoville with two spans of 775 

 feet each marked a revival of that meritorious type and it has since 

 been employed in quite a number of bridges (pi. 4, fig. 1). 



Less frequently, and only under favorable circumstances, such 

 as the presence of rocky abutments to resist its thrust, has the arch 

 type been used. Until the present the famous Hell Gate Bridge, 

 completed in 1917, across the East River in New York with a span 

 of nearly 1,000 feet has been the most outstanding example, but it 

 is now being outranked by two bridges, both now nearing comple- 

 tion ; namely, that across the entrance to Sydney Harbor in Australia 

 with a span of 1,650 feet, and that across the Kill van Kull in New 

 York with a span of 1,675 feet. 



The suspension bridge is the type eminently suited for long 

 spans and is now recognized as the only one to be considered for 

 very long spans. The true nature of this naturally graceful type 

 has long been misunderstood and it is only very recently that it 



