The rods are here arranged in pairs, one above the other, in a vertical plane. In girders, the bar in the tension side is straight, while the other one of the pair is horizontal for a short distance along the center of the span, the ends being inclined upward near the ends of the beam. The two bars are connected by bent straps or U-bars so that the steel reinforcement may be compared to a queen post truss within the concrete. This system has been used in the construction of bridges, both arch and girder, floors, roofs, stairways, etc., but it is in beams and girders that its distinguishing characteristics are best displayed.
572. A beautiful arch on this system is the bridge over the river Vienne at Chatellerault, France, consisting of three spans, the central one of which is 164 feet long, with rise of 15 feet, 8 inches. Four arch ribs 20 inches deep support the roadway, 25 feet wide, by posts forming a skeleton spandrel.
In this system, which is somewhat similar to the Hennebique, the distinguishing feature is the care taken to provide against shear, or against that combination of tension and shear which tends to cause failure in a beam by cracks that extend diagonally upward toward the center of span from near the points of support. The steel plates forming the tension members are sheared longitudinally at intervals, and short ends are bent up at an angle of forty-five degrees with the horizontal. These ends, which may be compared to the tension diagonals of a truss, are thus a part of the main steel member, and the stress is transferred directly to the latter without dependence on the concrete.
The advantages are the great resistance offered by the bar to being pulled out of the concrete and the thorough manner in which all tension stresses may be provided against. The main disadvantages would seem to be the necessity of detailed shop work for each size of girder, the inconvenience of shipping the steel in its complete form and the difficulty of thoroughly tamping the concrete around the diagonals.
One of the earliest patents to be issued in this country for a method of using concrete and iron in combination was that issued to Mr. E. L. Ransome in 1884. The valuable and distinctive feature of this system is the use of a square bar that has been twisted cold. This twisting not only insures a good bond between the concrete and iron, but actually somewhat increases the strength of the bar.
In building beams with twisted bars as tension members, the latter are given a slight inclination from the center upward toward the ends. For use in buildings, as in floors and columns, and for covers to areaways, and similar uses, this system is largely employed.
As its name implies, wire forms the main feature of this system, and in a general way it resembles the Monier. Its application thus far is found principally in floor construction, two distinct methods being used. In the arched floor a wire netting, stiffened by round steel rods woven through it is sprung between the lower flanges of the main I-beams of the floor. This netting, further stiffened and held in place by iron rods running parallel to the axis of the arch, forms a permanent center for the placing of the concrete, which fills all of the space to the level of the top of the I-beams. A level ceiling below is obtained by a similar netting laid flat against the under side of the I-beam and fastened thereto. This acts as a wire lath to receive a coat of plaster. If the level ceiling is not necessary, the plaster may be applied to the under side of the arch netting, in which case the lower flange of the I-beam should be encased in concrete to protect it from corrosion and fire.
576. For lighter loads, flat bars are placed at suitable intervals above and below the I-beams and clamped to the flanges. To these bars the wire netting is attached, a thin layer of concrete laid on the upper wire incasing the bars, and plaster applied to the lower netting forming the ceiling. Cinder concrete is usually employed with this system.
The use of what is commonly known as expanded metal lath has been extended to concrete-steel construction. As in the Monier and Roebling systems, the strength and stiffness of the structure are increased by the use of steel rods in connection with the expanded metal, the chief duty of the latter, where great strength is required, being that of a distributing member. Expanded metal is made from sheet steel by shearing short slits parallel to the grain, and extending the sheet at right angles to the slits, resulting in a network of diamond shaped openings. The metal used is of all weights up to one-quarter inch thick with meshes six inches long.
578. The steel bars used in connection with expanded metal by the St. Louis Expanded Metal Fireproofing Co. are square, with frequent corrugations surrounding the bar. These corrugations serve only to prevent the slipping of the bars in the concrete without adding to the strength.