The arch rib may or may not be strengthened bbyy the embedment of steel reinforcement in the concrete. Where used, this reinforcement may consist of a steel arch, built up of rolled steel sections, or single rolled sections, or of reinforcing bars. The modern trend has been toward the use of the reinforcing bar and the elimination of the more complicated forms of reinforcement.
In arches of relatively high rise, the stress under dead load only is at every point com pression. Where the dead load is comparatively large the possible tensile stresses due to the live load and changes in temperature may not be sufficient to cause the resultant stress to be tension at any point. In such arches no rein forcement is needed, except such as may be placed to prevent local shrinkage and tempera ture cracks.
Flat arches, and those where the dead load is not predominant, are subject to tensile stresses at some points under certain combina tions of the loads and changes in temperature, and hence must reinforced with metal, as concrete alone is capable of resisting but a slight amount of tension.
Details of Steel of steel trusses are usually constructed of rolled steel shapes either singly or in combination. Pin connected trusses have at each panel point or joint a steel pin or round bar passing through holes in the chord and web members, and trans ferring the stresses from one member to an other, by means of the shearing and bending stresses generated in it. Riveted trusses have their members joined together by rivets, which either pass directly through the several members at the intersecting joint, or which join each member to a common special connecting plate or plates. Such a joint has great stiffness, and when the truss deflects, this stiffness, since it prevents the members from rotating sepa rately at the joints, causes large additional stresses in these members. Such stresses are known as ((secondary stresses." In the early days of steel bridge con struction, with the comparatively light traffic and slow speed of that time, the pin-connected structure was the more suitable for all but the shortest spans. As the weight of the traffic and its speed increased, the vibration of the pin connected structure of short span became a serious factor, and the maximum span of riveted trusses has steadily increased. The length of span at which it seems desirable to select a, pin-connected truss is dependent upon the relations of dead and live load. Where the dead load is comparatively large, the vibra tions caused by the moving load may not be serious and the pin-connected truss with its low secondary stresses may be preferred. Where the traffic is very heavy the riveted truss may be desirable, and by bending the members be fore erection, so that under the maximum stress from the live load they will assume their normal positions, the maximum primary and secondary stresses can be prevented from occurring simul taneously. Such bending of the members is, however, very costly, while the connecting plates for heavy and long trusses must be enormous, so that pin-connected trusses with heavier members may be the more economical. The longest riveted simple truss in America at present has a span of 425 feet, while one in Europe has a span of 610 feet. The continuous
span shown in Fig. 13, each span being 775 feet, has riveted connections throughout. Such riveted spans are, however, somewhat longer than the average maximum limit for this con struction, but the tendency in the direction of long riveted spans is steadily growing.
For riveted trusses the members are built either of rolled steel angles or channels, or of combinations of angles and plates. Fig. 31 is partial detail drawing of a riveted truss bridge of a rather simple type. In heavier and longer trusses the sections of the members are correspondingly heavier, and composed of a larger number of plates and shapes riveted together in various forms. Pin-connected trusses omit the plates indicated at the joints through a pin, having its bearing in a shoe. The shoe is either of cast steel or built up of plates and angles like the one shown in Fig. 31. Arches have the same general details as trusses. The hinges consist of a pin against is 731T ,F- -sr ka ••• in Fig. 31 and substitute the pin or round bar referred to above, while the members subject to tension only are composed of one or more eye-bars. (Sec Fig. 32). Almost all trusses and some girders transfer the load to their supports which the arch or half-arch has a semi-circular bearing and around which it is free to rotate.
The steel most frequently used for bridge structures at the present time is commercially known as °structural steel.x' Technically, this grade is a rather low steel° and is capable of withstanding a stress of about 60,000 pounds per square inch before failure. There is no reason why a true medium steel having an ultimate strength of about 66,000 pounds per square inch could not be used, except that the steel manufacturers prefer to furnish a softer steel.
In some of the longer structures an alloy steel has been used. A nickel chrome alloy known under the name of Mayari steel which is manufactured from Cuban ores containing both nickel and chrome very nearly in the right proportion to produce bridge material of excellent quality has been found satisfactory. Nickel steel is artificially produced by the addi tion of from 3 to 4 per cent of nickel. Both of these alloy , steels are capable of with standing a stress of more than 100,000 pounds per square inch, but they are somewhat more expensive per pound.
Burr, W. H., (Suspension Bridges' (New York 1913) ; Hool, G. A., (Rein forced Concrete Construction' (Vol. III, New York 1916) ; Ketchum, M. S. (The Design of Highway Bridges> (New York 1908) ; Johnson, Bryan and Turneaure, (Modern Framed Struc tures> (New York 1916) ; Kirkham, J. E., (Structural Engineering' (Chicago 1914) ; Kunz, F. C., (The Design of Steel Bridges' (New York 1915) ; Marburg, F. Edgar, Structures and Girders' (New York 1911); Merriam and Jacoby, (Roofs and Bridges> (Vols. I–IV, New York 1905-17) ; Spofford, C. W., 'Theory of Structures' (New York 1913) ; Tyrrell, H. G. 'History of Bridge Engineering' (Chicago 1911); Usborne, P. O. G., 'The Design of Simple Steel Bridges' (New York 1912) ; Waddell, J. A. L., (Bridge Engineering' (New York 1916).