134. Sometimes the walls of concrete buildings are made hollow and strengthened by twisted steel rods, which form what is called the Ransome system of concrete construction. Fig. 69 represents a section of the wall in a structure so built. At a is shown a 6-inch outer concrete wall; at b is a 2-inch inner wall; and at c and d are vertical withes, tying together the two portions. (See "Hollow Walls," in Masonry, §7.) Those shown at c are 1 1/2 inches thick, while at d they are 3 inches, in order that the twisted rods may be better embedded. The 1-inch rods on the outside wall are shown at e, and the 3/4-inch rods in the inner wall at f; these rods are placed from 12 to 15 feet apart.

Hollow Concrete Walls 196

Fig. 69.

At g, h, and k are the tie-rods in the floor construction. In the withes marked d and e are placed 1/4-inch rods, set horizontally and spaced about 12 inches apart vertically; and at each floor level, 3/4-inch bars are embedded in the walls, as shown at g. At all meeting, or crossing, points the rods are securely attached or tied to one another. The spaces in the walls are stopped at each floor, as shown at /, excepting those for ventilation or those used as smoke flues, which are continuous. The interior partitions are also concrete, with twisted rods, and, being monolithic, they further increase the stiffness of the structure.

This form of construction has the advantage of great strength, as the twisted rods tie the walls together in all directions, while the shape of the wall gives stability without waste of material. It is also superior to hollow-brick walls, as practically no moisture will pass through the concrete withes. The average cost per square foot of exterior surface, in a large building so constructed, is about 25 cents.