CONCRETE STEEL MASONRY.
Concrete in the form of blocks made at a factory, and concrete formed in place and reinforced by steel rods and bars of differing shapes is being substituted in many situations for stone and brick masonry. For the construction of bridges and floors it is extensively employed. Several systems are in use, each known by the name of the inventor. Fig. 44 shows the different types which are more or less popular.
The Monier type consists of a mesh work of longitudinal and transverse rods of steel, usually placed near the center line of the arch rib. This type rests on the theory that the steel rods will resist the compressive stresses of the rib, while the concrete acts merely as a stiffener to prevent the steel from buckling, The Melan type consists of steel ribs embedded in the concrete and extending from abutment to abutment. The ribs are in the form of steel I-beams curved to follow the center line of the arch rib. The steel is assumed to be sufficient to resist the bending moments of the arch, while the concrete is relied upon to resist the thrust and to act as a preservative coating for the steel.
The Von Emperger arch is a modification of the Melan arch, the ribs are built up with angles for the flanges and diagonal lacing replaces the web, on the theory that the metal should be concentrated near the extrados of the arch to more effectually resist the bending moments.
The Thacher type is formed by omitting the web and reinforcing the concrete by steel bars in pairs one above the other, one near the extrados and one near the intrados, the steel being relied upon to resist the bending moments while the concrete is expected to resist the thrust of the arch.
In the Hyatt arch that portion of the steel bars or rods which in the Thacher arch is subjected to the greatest compression is omitted.
In the Luter arch the concrete rib is reinforced by tension mem bers passing from one side of the arch rib to the other.
In the Hennebique system an arch barrel or drum, four to six inches in diameter, is supported by ribs of concrete below, the concrete of the drum being reinforced with steel rods placed near the extrados, and that of the ribs by steel rods near the intrados.
Numerous forms of steel shapes are advocated for the reinforce ment of concrete when employed for arches, retaining walls, etc.; twisted bars, corrugated bars, expanded metal and lock woven steel are some of the names applied to the different shapes.
The method employed for constructing concrete walls is in brief as follows: A wooden form is erected, consisting of slotted standards made of 6-inch boards nailed together with spacing blocks between them at their ends, i-inch bolts are used to join the standards on opposite sides of the wall. The standards are for the purpose of holding molding boards in position while the concrete is being de posited'between them. These boards are of dressed pine 11 inches thick. After the lower portions of the concrete has set the boards are removed and used above. Vertical rods of twisted or corrugated steel are built in the wall spaced about 12 inches apart. In some cases level horizontal bars of steel are also embedded in the walls.