BRIDGE 



921 



BRIDGE 



Arch Bridges. A century and a half ago 

 the only bridge materials other than wood 

 were stone and brick. They were employed 

 in the arch, which had been a favorite form 



from very ancient times. Figs. 5 and 6 illus- 

 trate the points of weakness and strength in 

 this type of bridge. The weight of the key- 

 stone (the central stone crowning the arch) 

 exerts a downward pull represented in Fig. 5 

 by the arrow a. But in effect this pull is re- 

 solved into thrusts against each of the next 

 stones, called voussoirs (see COMPOSITION OF 

 FORCES). The voussoirs in turn thrust against 

 their neighbors until in the skewback, the low- 

 est stone, the force is exerted directly down- 

 ward (see ARCH). It is a law of physics that 

 when bodies are in equilibrium there is an 

 equal and opposite force for every force ex- 

 erted by them or upon them. Thus when an 

 arch is properly built the downward element 

 of the thrust b is exactly equaled -by the up- 

 ward thrust d of the voussoir, and the outward 

 thrust of a voussoir is counterbalanced by the 

 inward thrust e of the pier. If an arch is too 

 broad, its top too heavy or the piers too light, 



it will break, as shown in Fig. 6a, "because of 

 the excessive outward thrust. If, on the other 

 hand, an arch is too light at the top, or too 

 heavy at the sides, the upward thrust will be 

 too powerful, and the result will be as illus- 

 trated in the Fig. 66. 



The Romans did not build arches of more 

 than one hundred feet span, but modern engi- 

 neers have learned to construct them nearly 

 three times as long with stones, and even 

 longer with concrete. A stone arch bridge at 

 Plauen, Germany, has a span of 295.3 feet, 

 and a concrete bridge in Rome, Italy, a span 

 of 328 feet. The principles of the arch bridge 

 combined with the truss are seen in some iron 



bridges, instances of which are the cast-steel 

 Eads Bridge (which see) over the Mississippi 

 at Saint Louis, Mo. (520 feet span), the beau- 

 tiful wrought-steel bridge over the upper 

 Niagara between the United States and Can- 

 ada (840 feet span), and the new Hell Gate 

 Bridge at New York, with the remarkable span 

 of 1,016 feet (see HELL GATE, for illustration). 

 Perhaps the most usual reason for the choice 

 of the arch form of construction is its beauty, 

 which is well illustrated in the accompanying 

 picture of the Tunkhannock viaduct, and in 



TUNKHANNOCK VIADUCT 

 A great concrete and steel structure, on the 

 Lackawanna Railroad, in New York. It is a half 

 mile in length, is 240 feet high, and cost 

 $12,000,000. This vast expenditure was under- 

 taken to shorten the running distance between 

 New York City and Buffalo three and one-half 

 miles and reduce heavy grades. 



the picture of the Los Angeles viaduct which 

 will be found with the article CONCRETE. 



Suspension Bridges. A short time before 

 the year 1800 a form of bridge hung on chains 

 or wire cables was found to be strong enough 

 for long spans. The suspension bridge has the 

 merit of being less expensive than other long 

 bridges, and can be made an object of great 

 beauty, as the picture of the Manhattan Bridge 

 shows. But its great fault is its flexibility; it 

 sways with every wind and an army marching 

 over it with uniform step would cause it to 

 swing so violently that it would collapse. The 

 prominent steel work at the roadway of the 

 Manhattan Bridge is merely stiffening and 

 wind-bracing; the whole support of the struc- 

 ture comes from the four steel cables that 

 pass over the high towers. These cables were 

 spun by drawing a steel wire back and forth 

 across the river, over the towers; each cable 

 has a diameter of about 21% inches, is com- 

 posed of 2,368 wires, and is fastened to anchor- 

 ages at both ends. The longest suspension 

 bridge is the Williamsburgh Bridge which 

 crosses the East River of New York with a 

 span of 1,600 feet. See also, BROOKLYN BRIDGE. 



