560. A much more picturesque beginning of the concrete-steel industry is furnished in the story, quite true, that about 1876, a French gardener, Jean Monier, used a wire netting as the nucleus about which to construct his pots for flowers and shrubs, and seeing that the practice might be extended, he called to his aid engineers and capitalists who developed the Monier system of construction.
This system consists of imbedding in the concrete two sets of parallel rods at right angles to each other, the rods of the two sets being tied together with wire at all intersections.
1 Proc. Am. Soc. Mech. Engrs., Vol. iv, p. 388.
The larger wires run in the direction of the greater tensile stresses and are usually spaced two to four inches apart. The rods at right angles to these main tension members are to assist in distributing the stress to the main members and may be of smaller diameter.
The iron rods in this system are designed primarily to resist the tension only, and the form of the bars is not such as will stiffen the structure while the concrete is fresh. In an arch, two systems of netting are used, one near the intrados and one near the extrados.
561. The main advantages which this system has over some of its competitors are the simple shapes required, that is, round rods, which may always be obtained without difficulty, and the fact that these may be so readily put together by ordinary workmen under supervision. This system is especially adapted to vertical walls, whether curved or straight, and found its first extensive use in the construction of tanks and reservoirs. It has been extended, however, to the construction of sewers, floors, roofs, and arch bridges.
One of the practical disadvantages of the system is that the nets are somewhat difficult to handle and keep in position, and in thin sections it has not been found practical to imbed the nets in concrete containing broken stone of the ordinary size. The use of cement mortar, usually one part cement to three sand, has been found necessary in order to get a perfect connection between the wires and the body of the work. This, of course, increases the Cost. Another objection has been urged against it, namely, that the transverse rods do not in general have any duty to perform, and are simply a waste of material so far as the final strength of the structure is concerned. While this may be so in certain forms of construction, it may be met by the statement that these cross-rods may be made as small as desired if they are to act merely as spacers for the main rods. In slabs, walls, etc., however, these cross-rods have a purpose, and in some other systems members are supplied to fulfill this necessary function.
562. Some very bold arches have been built on the Monier system, including three bridges in Switzerland having 128 foot span, 11 foot rise, and a thickness of but 6 2/3 inches at the crown and 10 inches at the abutments.
A Monier arch of 32.8 foot span, rise one-tenth of span, width 13.2 feet, in which the mortar at the crown was six inches thick and eight inches at the abutments, was tested in Austria in 1890. It held a fifty-three ton locomotive on half the arch, and finally failed at the abutments under a load of 1,700 pounds per square foot over half the span.
563. Pipes are now made by this system at yards and transported to the place of use. It has also been used as a substitute for iron in cylinders for bridge piers. A novel use of this system consists in making a pipe covering for piles exposed to marine borers. The pipe, which is long enough to reach from above the water surface to below the bed of the waterway, is slipped over the head of the pile and settled a short distance into the mud or silt of the bottom with the aid of a water jet. A question, however, has been raised as to the action of concrete and iron in combination in sea water on account of the possible setting up of galvanic action.