Floor Construction. The size and spacing of the floor beams will depend upon the style of floor construction which may be adopted. The original fireproof floor consisted in spacing beams five or six feet apart, and turning a brick arch from flange to flange of the beams, as in Fig. 213. The space above the arch is filled with concrete up to about are inch above the hearers, completely enclosing the steel; and screeds running parallel to the beams, bed dPcl in the concrete, give a sufficient nailing for the floor boards. fhe ceiling underneath may be finished by simply plastering on the underside of the brick arch, or a level ceiling may be hung to the rough floor. As an arch of this shape will exert considerable thrust, this must be taken up by the use of tie rods ~-inch or 1-inch in diameter, spaced along below the center line of beams at intervals of about six feet. This style of flooring will weigh about seventy pounds to the foot, and has been practically superseded by the lighter constructions employing terra-cotta arches or concrete.
Terra=Cotta Floor Arches. PIollow terra-cotta blocks, moulded in the form of a flat arch, are used to a large extent for fireproof floors, and are to be obtained in a variety of patterns and devices. These various patterns may be divided into two principal classes, the side method and the end method.
Side Method. The side method arch, where the blocks are laid with the webs parallel to the beams, Fig. 214, is the original form of terra-cotta floor arch, while the end method, where the blocks are laid end to end at right angles to the beams, is a later improve ment, designed to present the full end section of the material to resist the great thrust of the arches. These side method arches are usually made of dense terra-cotta and may be obtained of various depth from six to fifteen inches, and they should be set with close joints, and be thoroughly cemented together. Specially moulded blocks, called "skewbacks," are made to fit the lower flanges of the beams and project about two inches below the beam, which is covered by a thin strip of tile. The space above the blocks is filled with a cinder con crete, in which bevelled wood strips are embedded for a. nailing for the wooden floors.
Side method arches are made to break joints endways, so as to give a bond; and they are usually strong enough for all ordinary floor Loads. The joints in the blocks are generally made parallel to the sides of the key block, as this gives a uniform pattern, and so is less expensive than a radial jointing, though the latter would make a stronger arch, but on account of the expense of the different patterns, to make and adjust, it is little used.
End Method. In this Method, the blocks are usually made of porous terra-cotta and are set end to end, giving greater resistance to the thrust by forming a series of continuous webs from beam to beam. (Fig. 215.) In this system, the blocks are usually set in continuous lines, not breaking joints. The jointing of these tiles must be done with great care, as the open ends do not give so good a surface for cementing as in the side method, and, the bearing on the beams being given by the thin webs, it is necessary that they should fit perfectly.
For this reason a combination method is often used, shown in Fig. 216. This is done to gain the extra strength of the end blocks, and the better bearing of the flat skewbaeks, so that the skewbacks are made with many webs and of small sections.
Setting. Floor tiles of either pattern must be set upon plant: centers which are hung from the beams, and should be crowned one-quarter of an inch in an arch of six feet. All joints must be close, and made with cement. The centering should be left in place until the cement is thoroughly set, which will require from twelve to thirty-six hours, according to the weather and the nature of the cement, and care must be taken that the freshly laid tiles are not too heavily loaded with materials until they are hard. If the arches are to be plastered upon for a ceiling, they must be kept clean, or bad stains are likely to appear. All Boles or irregularities on the under side must be filled with cement mortar to give a proper surface for plastering. Where it is not required to have a flat ceiling, but strength is the main factor to be observed, segmental tile arches are often used. (Fig. 217.) These may be used, with a rise of an inch to the foot, up to twenty feet of span, and are employed to great extent in warehouse construction.
While the nature of floor tiles will not permit of a fine joint being made, they should be laid as closely as possible, especially the key blocks. Joints its great as 0 half-inch should not be per mitted, and the tiles should be set in place by being "shoved" to gether as in brick laying. Unless the building is closed in, floor tiles cannot be laid in cold climates when there is danger of freezing and thawing, as the joints are liable to be affected to the extent of causing deflection if not more serious trouble.