The stiffness of truss bridges has been se cured by adopting stiff bracing' in the lateral systems and sway bracing instead of the light adjustable rods formerly used. At the same time, adjustable counter-ties in the trusses are replaced by stiff ones, while in some cases the counters are omitted and the main diagonals designed to take both tension and compression.
Some of the same influences referred to above have led to much simpler designs for the portal bracings by using a few members of adequate strength and stiffness, conforming in general character to those of the trusses.
The corresponding development of short span highway bridges was long delayed. The conditions under which highway bridges were purchased by township and county commission ers were decidedly unfavorable to material im provements in the character of their design and construction. It is a comparatively rare occur rence that the commissioners employ a bridge engineer to look after the interests of the tax payers by providing suitable specifications, mak ing the design, inspecting the material and ex amining the construction of the bridge to see that it conforms to all the imposed require ments. These provisions are only made in some of the cities, and accordingly one must exam ine the new bridges in cities to learn what progress is making in highway bridge building.
The most important advance in the construc tion of highway bridges outside of cities is due to the organization of State highway depart ments in many of the States, which include the organization of bridge departments for the preparation of suitable specifications, standard plans for different types of bridges, as well as special plans, and which furnish competent in spection during construction. The most note worthy change is due to the extensive adoption of slab and beam bridges of reinforced con crete and of reinforced-concrete arches instead of the flimsy steel truss bridges formerly used. Where steel trusses are employed they are carefully designed for long service and ulti mate economy. This movement is promoted largely on account of the extensive use of automobiles and of auto-trucks.
The longest span of any cantilever bridge in the world is that of the bridge over the Saint Lawrence River near Quebec. Its length
between centres of towers is 1,800 feet. The cantilever arms project 580 feet from each tower and support between them the suspended span 640 feet long. The anchor spans outside of the towers are 515 feet long. The main posts in the towers are 310 feet high. The trusses have a unique "K system" of bracing, which was adopted because it greatly facilitates the method of erection and at the same time gives greater regularity in the deflection of the truss joints under moving loads. In the canti lever span nickel steel is employed except in the floor system and a part of the cantilever arms. Friction brakes with a capacity of 250, 000 pounds are provided to take up the expan sion at both ends of the suspended span. The bridge accommodates a double-track railroad and also highway traffic. It was completed in 1917. The span of this bridge is larger than that of any other class of bridges. The next in size of cantilever spans in America is that of the Queensboro Bridge over the East River at New York, completed in 1909. Its span length is 1,182 feet.
Many bridges with trusses continuous over one or more piers have been built in Europe but exceedingly few in America. Until it was replaced by simple trusses in 1913, the Canadian Pacific Railway Bridge over the Saint Lawrence River at Lachine, near Montreal, had the long est spans, 408 feet. It then seemed as if no other bridge of that type would be built in this country, but this expectation was not re alized. In 1917 a double-track bridge was built across the Ohio River near Sciotoville by the Chesapeake and Ohio Northern Railway hav ing two spans, each 775 feet long. All the connections of the trusses are riveted. As the rock foundations are practically on the same level across the river, the conditions were favorable for a continuous bridge. For other long bridges, see BRIDGE.
One of the most interesting developments relating to the subject under consideration is the construction of a considerable number of metallic arch bridges in recent years and the promise of their still greater use in the future. On account of their form they constitute one of the handsomest classes of bridges.