This system, which was invented in 1884 by Robert Wunsch of Hungary, consists of two iron members of angle irons and plates imbedded in concrete, the lower member being arched and conforming to the outline of the soffit, while the upper one is horizontal and continuous. The two members are riveted together at the crown, and at the abutment are rigidly connected by a vertical member. The several systems of rib bracing thus constructed are connected laterally at the abutment by channel bars running transverse to the arch and riveted to the bottom of each vertical in the abutment. Assuming that the abutments are stable, it is evident that we have here not simply an arch, but also some elements of the cantilever. The spandrels being built up solid of concrete, there is no definite arch ring, and the quantity of material required, especially in long spans, is likely to be much greater than in other systems. On the other hand, the great depth at the springing permits the use of concrete only moderately rich in cement.
565. A bridge of this type, built at Neuhausel, Hungary, consists of six spans of about 56 feet each, rise 3.7 feet, thickness at crown 9.8 inches, and at springing line 54.3 inches.
The total width of the arch was 19.7 feet and contained thirteen systems of arch ribs. Concrete in the abutments below water was made mainly of Roman cement. Above water it was composed of one part Portland to eight or ten parts sand and gravel. Ten to twelve inches of the arch was built of strong Portland concrete rammed in layers at right angles to radial lines of the arch, special care being taken with that part below the bottom arched member. An arch was usually completed in one day, and the centers remained in place thirty to forty days, the greatest settlement on the removal of centers being two-thirds of an inch. This bridge contained 1,346 cubic yards of concrete and 88,180 pounds of iron, and Cost, complete, $13,700.
This system, invented by an Austrian engineer, Joseph Melan, consists of arched ribs between abutments as in bridges, or between beams or girders as in floor construction, the space between the ribs being filled with concrete. Steel I-beams curved to the proper form are usually employed for the reinforcement, though angle iron flanges with lattice connections have been used in some of the large bridges. The steel members extend into the piers or abutments and are there connected by angles or other shapes, and firmly imbedded in the concrete.
567. This system as adapted to bridge construction has probably met with greater favor among American engineers than any other form. Perhaps this is because of the stiffness of the form of iron beam used, and because by assuming a rather high fiber stress for steel the reinforcement may be designed to withstand the entire bending moment without excessive dimensions for the steel members. There is thus a feeling of security in its use that is not felt in the same degree with other systems. The arch dimensions are determined by computing the forces and required thickness of arch ring after assuming certain safe working stresses for the steel and concrete; but if desired, the size of steel members may then be increased slightly where necessary to such dimensions that with unit stresses of, say, one-half the elastic limit, the entire bending moment shall be taken by the steel. Some of the largest bridges built after this system in the United States are the five-span bridge at Topeka, Kan., and the three-span bridge at Paterson, N. J.
A modification of the Melan system is that invented and patented by Mr. Edwin Thacher. Steel bars are used in pairs and imbedded in the concrete near the intrados and extrados of the arch and extending well into the abutments. The bars of each pair may be connected by bolts or stirrups, though Mr. Thacher's original idea seems to have been to have no connection between two bars of a pair except through the concrete. The bars are provided with projections which may be in the form of rivet heads, lugs, or bolts, to increase the resistance of the bars to slipping in the concrete.
569. Mr. Thacher has more recently designed a special form of rolled bar having projections that serve the same purpose as the rivet heads mentioned above. Several bridges have been built on this system, one of the most notable of these being the Goat Island bridge at Niagara Falls, one span of which is 110 feet in length.
570. In the construction of arch bridges many of the other systems are simply modifications of the Melan. The shapes of the steel members may have different forms, and the connections between the pairs of bars forming the arch ribs may vary to suit the idea of the inventors. But though these systems lose their identity in long-span arches, their distinctive features are more apparent in the construction of floors, roofs, columns, etc.