This floor, which was for a time known as the "Manhattan" system, and is protected by letters patent, is constructed as follows : Cables, each composed of two galvanized wires (usually of No. 12 gauge) twisted together, are suspended between the top of I-beams, as shown in Fig. 190, and spaced from 1 inch to 1½ inches apart, according to the load which is to be carried. The ends of the cables are secured to the beams by means of hooks 3 inches long made of ¼-inch square iron, which grasp the upper flange. A length of gas pipe is laid over the cable midway between the beams to give them a uniform sag. Forms or centres are then placed under the cables, and a composition consisting of 5 parts, by weight, of plaster of Paris and 1 part of wood shavings, mixed with sufficient water to make a thin paste, is poured on.
As plaster of Paris sets very quickly the resulting floor is sufficiently strong to be used at once under loads, with a surface uniform and level above the top of the beams.
Where a paneled ceiling can be used wire netting is stretched over the beams and the same composition poured around them, fireproof-ing the beam, as shown at B, Fig. 191. Where a flush ceiling is required flat bars are placed on the bottom flanges of the beams and wire netting stretched over them. Forms are then placed underneath and the same composition as in the floor plate poured on, forming a plate about 1½ inches thick and extending 1 inch below the bottom of the beams, as shown at A, Fig. 191.
The usual thickness of the floor plate is 4 inches, with beam spacings of from 4 to 6 feet. It will be seen that in principle this floor closely resembles the Ransome tension bar system, as the cables take Tip the tension and the concrete resists the compressive stress. This combination of steel (in its strongest shape) with concrete is theoretically one of the most perfect forms of fireproof construction, and although defects may be discovered in the details of construction, the system itself seems destined to become of wide application.
No tie-rods between the beams are required in this system, as the floor plate is practically a beam, and transmits only a vertical pressure to the I-beams.
The tests that have been made of this floor construction seem to prove that it is thoroughly fireproof and heat-resisting, and that its ultimate strength for floor plates 4 inches thick and 6 feet span is about 1,500 pounds per square foot, while loads as high as 2,000 pounds have been supported by it.
The remarkably light weight of this floor is one of its chief advantages, the average weight of the floor plate being about 18 pounds per square foot, and the weight of the ceiling plate, without the plastering, 6 pounds. A floor constructed by this method with I-beams 6 feet apart would therefore weigh, when all complete and ceiling plastered, less than half as much as the old style dense tile systems.
The greatest objection thus far brought against this floor is the great amount of water used in its construction and the time required for the wood shavings to dry out.
310. Mr. J. Hollis Wells, C. E., in reviewing some tests of fireproof floors made at Trenton, N. J., in 1894, makes the following comparison between the concrete and wire and hollow tile floors: "The method of suspending a fireproof material on wires of proper strength from beam to beam makes a strong homogeneous floor, absolutely fireproof, and each bay or section independent of those adjoining. The hollow tile arch, creating a thrust on the floor beams, depends on tie-rods to counteract it. Tie-rods seldom set in proper place, oftentimes are not screwed up tight, and the construction is weakened. In the suspended floor tie-rods are not used at all; beam is tied to beam from upper flange to upper flange, and a rigid base extends clear across the floor from wall to wall." *
* Engineering Record, December 32, 1895.
Various styles of floors have been constructed on the principle of the Metropolitan floor, although nearly all use Portland cement concrete instead of the plaster composition. Wire lathing, expanded metal, and various shaped bars are used for the tension members.
The principal advantage sought in these floors over the terra cotta tile arches is a reduction in the weight of the floor, thereby causing a saving in the steel construction. The floors themselves are also, as a rule, a little cheaper than the tile floors.
The strains in floors of this kind are the same as in those of a beam, the effect of the load being to pull the tension members apart at the bottom and to crush the concrete on top. When the concrete is of the proper thickness, and of good quality, the strength of the floor will be determined by the strength of the tension members.
Several tests of beams made of Portland cement, concrete and wire netting made by the New Jersey Wire Cloth Company, appear to show that only about one-half the strength of the tension members (when of wire cloth) can be developed. In all floors constructed of concrete, plaster or tile, with steel tension members, it is of the first importance that the two materials shall be so closely united that the tension members will not be drawn through, or slip in the concrete, for the minute this occurs the strength of the floor, as a beam, is destroyed. To secure this perfect adhesion, it is necessary that the materials and work shall be of the best quality and not slighted in any way.
311. The Roebling Patent Fireproof Floor.* - This also is a concrete construction, but the concrete, instead of being used as a beam, is entirely in compression, the strength of the floor being due to the resistance of the concrete acting as an arch.
The method of forming the floor and ceiling is well illustrated by Fig. 192. The floor construction consists of a wire cloth arch, stiffened by steel rods, which is sprung between the floor beams and abuts into the seat formed by the web and lower flange of the I-beams. On this wire arch Portland cement concrete is deposited and allowed to harden, making a strong monolithic arched slab between the beams. The ceiling construction consists of supporting rods attached to the lower flanges of the floor beams by a patent clamp, which offsets the rods below the I-beams. Under these rods, and securely laced to them, is placed the Roebling standard lathing, with the stiffening rods crossing the supporting rods at right angles. This construction produces a ceiling that is uniformly level over its entire surface, requiring the same amount of plaster over all portions. The ceiling being separate from the floor is not liable to stains, as is frequently the case with tile construction.
* Controlled by the John A. Roebling's Sons Co.
The weight of finished floor and ceiling, including the plastering underneath and two thicknesses of wood flooring, as given by the Roebling Co., varies from 47 to 59 pounds per square foot, according to the span and depth of beams or girders. This is exclusive of the steel beams.
The strength of this floor depends, of course, almost entirely upon the concrete - the quality and proportion of the ingredients and the mixing.
Thus far, where the system has been used, only the best grades of imported Portland cement and the best sharp sand have been used. For dwellings and buildings in which the live load never exceeds 100 pounds per square foot, a concrete made of cement, sand and first-class cinder may be employed, with a saving in weight and cost, and at the same time with ample strength.
Various tests of these floors built by the Roebling Sons' Co., with spans varying from 4½ to 5 feet, have shown a carrying capacity, with no signs of failure, of from 1,000 to 2,400 pounds per square foot. Further evidence of the strength of such floors is also furnished by the celebrated "Austrian" tests on concrete arches.* In these tests a concrete arch only 3 inches thick and span of 4½ feet, without filling above the haunches, sustained 1,638 pounds per square foot over the entire area without failure or cracking, while a similar arch, 3 5/16 inches thick, with span of 8 feet 10 inches and rise of 10½ inches, sustained an eccentric load over one-half of the arch of 1,130 pounds per square foot. The arch then failed by buckling, and not by compression.
* See Architecture and Building, January 4, 1895.
The strength of the Roebling floor, therefore, may be considered ample for any load that may be applied, provided the concrete is of sufficient thickness at the crown and of good quality.
The most economical proportions for this floor, considering also the cost of the steel beams, will generally be obtained by using 10-inch I-beams, spaced as far apart as the loads will permit.
Aside from its strength and fireproof qualities, this construction possesses many practical advantages, a few of which may be briefly mentioned : A perfectly flat ceiling, which may be placed any distance below the beams and which is not liable to discoloration; a continuous air space between floor and ceiling; it is much lighter than many of the tile floors, and can be adapted to any building or to any load. The ceilings may be either flat, paneled or arched.
No special arrangement of floor beams is required, and the spacings need not be uniform.
The floor is not easily damaged; openings of any size may be cut through the concrete to neat dimensions, the wire cloth preventing the concrete from flaking away on the under side.
Where buildings must be erected with great rapidity, or in winter weather, this system is especially desirable. No wood centring is required, and as the arch wire is made to dimensions and bent to the correct curve at the mill, the wire arches can be put in place very quickly and in any kind of weather. Once in place they afford a protection to workmen, as they possess sufficient strength in themselves to sustain a considerable load, or to intercept a person falling from the beams above. The wire arches are generally set so as to keep within two stories of the masons. As Portland cement is used for the concrete, the latter can also be safely mixed in quite cold weather. The floors are safe and available for use two days after the concrete has been applied.
The cost of this system should not exceed that of other systems using Portland cement or tile, and in many instances would probably be less.
312. The "Columbian" System of Fireproof Floors.* -
This system is also one of concrete construction, the shape of the concrete being very much the same as in the Metropolitan floor. In this floor, however, the concrete, instead of being supported by wires or netting, is supported by ribbed steel bars of a special shape, suspended from the steel I-beams and supported on edge by means of steel stirrups, which have the profile of the bar cut in them, as shown in Fig. 193. After the bars are set in place a wooden form is suspended beneath them and a layer of Portland cement concrete is laid on top, flush with the top of the beams and completely surrounding the ribbed steel bars.
If a level ceiling beneath the beams is desired it is constructed independently of the floor by using 1-inch section ribbed bars, resting on the bottom flanges of the I-beams, and filling between and around them with concrete, in the same way as is done for the floors.
The system of floor and ceiling construction is plainly shown by the section drawing, Fig. 194.
Three sizes of bars are used for the floor construction - 2½-inch, 2-inch and 1½-inch, and these are spaced at different distances apart, according to the span and the weight to be supported. The 2½-inch bars are used only in warehouses, heavy storage buildings, etc.; the 2-inch bar for floors in office buildings and where the loads do not exceed 200 pounds per square foot. The 1½-inch bar gives sufficient strength for floors in residences, apartment houses, etc. The shape of the 2-inch and 2½-inch bars is shown by the hole in the stirrup, Fig. 193. The 1½-inch bar has only one rib. The stirrups are made of 2 x 3/16-inch steel. The usual spacing of the bars is about 20 inches.
The concrete recommended by the Columbian Co., and generally used, is composed of 1 part Portland cement, 2 of sand and 5 of crushed furnace slag, although broken brick and certain kinds of rock are also sometimes used.
The most economical spacing of the floor beams for this construction is 6 feet from centre to centre of beams for the double construction shown in Fig. 194 and 7 feet for paneled construction, although either construction can be adapted to spans up to 8 feet. It is also not necessary that the spacing of the beams be uniform, and no special framing is required for this system, as it can be readily adapted to any plan suitable for any of the flat floor constructions described, although with this system the beams can often be made lighter or spaced farther apart, owing to the decreased dead weight of the floor. In most classes of buildings other than offices and dwellings the double construction is not necessary, as the bottom of the floor construction answers for the ceiling, and by enclosing the beams and girders with concrete or tile, a neat paneled effect is produced, and the height of the story increased or the total height of the building decreased, as preferred.
* Patents controlled by the Columbian Fireproofing Co.
Fig. 195 shows two styles of girder casings used in connection with this system, both providing for an air space completely around the steel. The casing shown at A is made of concrete slabs, supported by iron clamps or ties, which are completely imbedded in and insulated by the bottom slab - a very important provision. The casing shown at B is made of hollow tile, thus providing two air spaces on each side of the beam and one underneath. Concealed anchors are also used for this casing.
This floor may be finished on top in the usual way by imbedding nailing strips in cinder filling, or 2½ x 1¼-inch strips (not beveled) may be nailed directly to the concrete floor and the filling omitted. Nailing strips have been applied in this manner in several large buildings, and, it is claimed, with the best results.
The weight of this system of floor construction, exclusive of the I-beams, plastering, nailing strips and flooring, is as follows :
inches of concrete,
pounds per square foot.
inches of concrete,
pounds per square foot.
inches of concrete,
pounds per square foot.
The level ceiling shown in Fig. 194 (2 inches thick) weighs 20 pounds per square foot.
Strength. - The Columbian Co. guarantee that their 3-inch floor, 6 feet span, will support 200 pounds per square foot; the 4-inch floor, 6 feet span, 600 pounds per square foot, and the 2½-inch floor, 5 feet span, 150 pounds per square foot, with factor of safety of four], and the published tests that have been made of this system would appear to sustain the guarantee. This construction appears to be especially strong to resist drop or jarring loads. A ram weighing 238 pounds was dropped from the height of 8 feet on the centre of an 8-foot span several times without perceptible effect on the floor. (The bars in this floor were 2½ inches, spaced 20 inches apart.) It is also claimed that in case of overloading the floor will not fail suddenly, but that the bars will gradually bend, thus giving warning of danger.
The complete fireproof quality of this floor, which is, of course, the same as that of the Roebling and other Portland cement floors, was proved by a severe test of fire and water while the floor was uniformly loaded with 750 pounds per square foot.
Economy. - While this floor is thoroughly fireproof and waterproof and possesses ample strength and remarkable rigidity, it also possesses several advantages of a practical and economical nature.
No tie-rods are required, and no punching of the I-beams is necessary, except where they are framed to the girders or around openings. Lighter beams may be employed than where heavier types of floor construction are used. No channels are required in outside masonry walls.
In buildings having brick partitions and solid masonry walls this floor, with paneled ceiling, is especially economical, as no channels are required, and the beams require no punching, except for anchoring their ends to the walls. This floor can be constructed as rapidly as any and can be carried out without difficulty in winter weather.
Holes may be cut at any place in the floor by plumbers or electricians without injuring the strength of the floor, and the holes may be cut as small or as large as may be necessary.