Transportation on Land 287 



by great masses of masonry. The cables are of steel fifteen 

 or more inches in diameter, selected and formed for tension 

 strength. The suspension bridge is peculiarly adapted to long 

 spans. The famous Brooklyn Bridge over the East River, 

 New York, graceful in its lines, and, when built, the largest 

 and strongest in the world, comprises two shore spans of 900 

 feet each, and a central or river span of 1595 feet. Since its 

 completion in 1883, many bridges of the same type have been 

 constructed, exceeding it in size and capacity. The Manhattan 

 Bridge (Fig. 88), a short distance from the Brooklyn Bridge, 

 has a main span of 1470 feet, a width of 120 feet, with a ca- 

 pacity of four electric railways, a roadway, and two footways. 



123. The arch. Quite the reverse of the suspension bridge 

 is the arch. The materials of which the arch is constructed 

 are selected and formed to withstand crushing force, while the 

 suspension materials have to withstand tension. The arch 

 must have very heavy piers against which it may push without 

 spreading. Stone and steel are the usual materials employed. 

 The arch bridge across the East River at Hell Gate, New York 

 (Fig. 89) has a main span between piers of 1017 feet and a rise 

 from the hinges to the crown of the main arch of 220 feet. The 

 capacity of the bridge is for more than four trains of heaviest 

 possible load, on four tracks. Some idea of the weight of the 

 whole structure may be got from the fact that the average 

 weight for each lineal foot is 26 tons. 



SUMMARY 



A simple girder supported at the ends sustains a crushing force 

 in its upper portions and a tensile force in its lower portion. 



The truss is like a girder, having the upper and lower portions sep- 

 arated into distinct members. This makes possible the use of longer 

 spans and gives greater strength than a simple girder. The truss 

 consists of a series of rigid triangles. 



Tubular frame bridge construction is illustrated by the bicycle. 



The suspension bridge, built of materials having great tensile 

 strength, is adapted to very long spans. Notable instances are found 

 in the Brooklyn and Manhattan bridges, New York City. 



