_____ SYSTEMS OF REINFORCED CONCRETE ARCH CONSTRUCTION.
There are a great variety of methods of reinforcing a concrete arch, most of which are patented. A few of the better known will be briefly described, in chronological order.
Monier Arch. Mr. Jean Monier of Paris, France, in 1875, built the first reinforced concrete arch. He first embedded a wire net near the intrados; but later two nets were used, one near the intrados and one near the extrados. Monier arches have some serious defects, viz.: 1. The wire nets are very flexible, and hence are difficult to place properly. 2. The transverse wires do not aid in supporting the load, and hence add needlessly to the cost. 3. The closeness of the mesh prevents the use of even a moderately coarse aggregate, and hence adds to the cost of the concrete. Notwith standing their defects many Monier arches have been built, partic ularly in Europe, some of which are quite remarkable for their delicate dimensions and surprising strength. The two most notable of these are:* 1. Three arches built in Switzerland in 1891 each having a span of 128 ft., rise 11 ft., thickness at the crown 6.67 in. and at the springing 10 in. 2. An arch built in Germany in 1890 in which the span was 132 ft., the rise 14.7 ft., and thickness at the crown 9.88 in.
Hennebique System. The arch in this system, invented by Hennebique in 1893, is reinforced with solid rolled members parallel to the intrados and the extrados connected at frequent intervals by lattice bars. In this system, small bridges consist of parallel arch ribs built up solid to the level of the crown, which sup port a reinforced concrete slab resting directly upon the ribs; and in larger bridges the floor slab is supported by columns or spandrel arches resting on the extrados of the reinforced ribs.
Thacher System. This system, invented by Edwin Thacher, New York City, in 1899, consists of flat steel bars in pairs, one parallel to the intrados and the other parallel to the extrados. The bars have no connection with each other except through the concrete; but are provided with projections, usually rivet heads, at short intervals to secure mechanical bond between the steel and the con crete.
Oommon System. Recently in America the most common reinforcement for concrete arches consists of a series of plain or deformed bars (Fig. 28, page 236) parallel to and near the extrados and a similar series near the intrados, each series sometimes being connected at intervals by some form of stirrups (Fig. 29, page 238). The chief, if not the sole, purpose of the steel is to resist the bending; and hence the amount of reinforcement parallel to the neutral line should be from 1 to 11 per cent (see equation 10, page 228), but it is frequently considerably greater than this. Not infrequently the concrete alone is able to carry the computed stresses with a fair degree of safety, the reinforcement being added only as an additional element of security.
For examples of what was above called the common system, see Fig. 239-41, page 713-16, and Fig. 243A4, page 717-19.