BRIDGE 



922 



BRIDGE 



Iron Beam and Truss Bridges. The iron 

 beam bridge is of course an evolution of the 

 simple log bridge. Fig. 7, a, shows what takes 

 place when a wooden beam bends beneath 

 weight. The upper fibers are crushed together, 

 the lower ones pulled apart ; that is, the beam 

 is subject to both compression and tension. 

 The intensity of the forces is least near the 



F, 8 .7 



MANHATTAN BRIDGE, NEW YORK CITY 



One of the newer of the suspension bridges connecting Manhattan Island with the Borough of 

 Brooklyn and New York suburbs on Long Island. 



Bridge (Fig. 8) is a simple truss of 640 feet 

 length. 



Cantilever Bridges. A bracket fastened on 

 the wall to hold a shelf is a cantilever. In 

 the forms of bridge already described it is 

 plain that if any section were removed the 

 structure would collapse. If in a cantilever 

 bridge, however, the central section were re- 

 moved, as shown in Fig. 8, the remainder of 

 the bridge would be unharmed. At each of 

 the two piers of the Quebec Bridge an inde- 

 pendent truss structure was erected, which is 

 in perfect balance at all times, though the 

 cantilever arm (a) extends outward 580 feet 

 from the pier and the anchor arm (b) 515 feet. 

 In most cantilever bridges the central part (c) 

 is built from the cantilever arms, and to offset 

 its weight, which might tip the whole struc- 

 ture inward at the center, the anchor arms are 

 fastened to the anchor piers (d). For the 

 Quebec Bridge, however, the engineers planned 

 to lift the 5,000-ton center section into place 

 from six scows on which it had been floated 

 to the bridge site. This was attempted on 

 September 11, 1916, but through defects in 

 the hoisting gear the heavy load fell into the 

 river. The Quebec Bridge has the longest span 

 (1,800 feet) of any bridge in the world. An 

 a b 



center and greatest at the edges, hence the 

 usual steel beam, shown in Fig. 7, b, is made 

 stronger at top and bottom. From its shape it 

 is called an I-beam. Steel beam bridges are 

 suitable for railway spans of not more than 

 twenty-five feet, and for roadway spans of less 

 than forty feet. 



The iron truss bridge is constructed on the 

 same principles as the wooden truss, but with 

 much greater variety of design to suit its par- 

 ticular purpose. In the Saint Louis Municipal 

 Bridge a simple truss has a span of 668 feet, 

 and the central section of the new Quebec 



THE GREAT CANTILEVER BRIDGE AT QUEBEC 

 It was the central portion of the 1,800-foot span, itself 640 feet in length, which collapsed in 1916. 





