While Mr. Jackson was experimenting with the Hyatt ties, Mr. E. L. Ransome, a very successful worker of concrete in San Francisco, conceived the idea of using square bars of iron and steel, twisted their entire length, in place of the flat bars and wires used by Mr. Jackson, as shown in Fig. 188. It was found that these bars were held in the concrete equally as well, if not better, than the other, and that they were much less expensive. None of the iron in the ties is wasted, and it has been demonstrated by careful experiments that the process of twisting the bars to the extent desired strengthens the rods instead of weakening them.
Mr. Ransome patented his improvement in 1884, and since that time it has been extensively used in San Francisco.
The Ransome concrete floors are made in two forms - flat (Fig. 188) and recessed, or paneled (Fig. 188 A). These floors have been used for spans up to 34 feet. No floor beams are required, the floor being self-supporting from wall to wall (when the building is not more than 30 feet wide), or from wall to girder. The great strength of these floors has been fully demonstrated by actual use in many heavy warehouses in various portions of California, as well as in many other buildings.
A section of a flat floor in the California Academy of Science, 15x22 feet, was tested in 1890 with a uniform load of 415 pounds per square foot, and the load left in place for one month. The deflection at the centre of the 22-foot span was only \ inch. It was estimated by the architects that the saving by using this construction throughout the building, over the ordinary use of steel beams and hollow tile arches of the same strength, and with similar cement-finished floors on top, amounted to fifty cents per square foot of floor.
Fig. 188 A.
The flat construction shown in Fig. 188 is the best adapted, of the two, for office buildings, hotels, etc., although the paneled floor, shown in Fig. 188A, has much the greater strength for the same amount of material. The latter construction has been used in several warehouses in California without the use of any steel or iron beams or girders, and has supported very heavy loads for several years.
As a fireproof construction this system is undoubtedly equal to any other construction in use. The patents controlling the use of twisted bars in combination with concrete are now owned by the Ransome & Smith Co., of Chicago, from whom more complete information of their system of flooring may be obtained.
Mr. Thomas A. Lee, the originator of the end system of hollow tile arches, about the year 1890 patented a system of floor construction which is the same in principle as the Ransome floor. Instead of using concrete to resist the compressive stress, hollow porous tile blocks with square ends and a rod groove along one side near the base are used, as shown in Fig. 189. The tension member consists of cables made of round, drawn steel rods of about 3/10 of an inch in diameter laid spirally together, usually in two strands. The rods are spaced 8, 10 or 12 inches apart, according to the span and width of tile, and are buried in soft Portland cement placed in the grooves near the bottom of the tile. The cement unites the tiles and cables so as to form a composite beam. The floors extend like a flat plate from wall to wall, or from girder to girder, their thickness being about 5/8 inch for each foot of span.
Fig. 189. - Lee Floor.
Floors and roofs similar to the above have been built in various costly buildings in different portions of this country and in Canada, the spans varying from 10 to 28 feet.
In buildings having solid brick or concrete walls and partitions, these tension member floors may be used to good advantage, but it is doubtful if they ever come into general use in buildings built on the skeleton principle. They require very careful and faithful workmanship and the very best quality of cement to make them safe.