This section is from the book "Cyclopedia Of Architecture, Carpentry, And Building", by James C. et al. Also available from Amazon: Cyclopedia Of Architecture, Carpentry And Building.
Hollow 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.
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 improvement, 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 concrete, in which bevelled wood strips are embedded for a nailing for the wooden floors.
Fig. 214. Side Method Terra-Cotta Arch.
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.
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.
Fig. 215. End Method Terra-Cotta Arch.
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 skewbacks, so that the skewbacks are made with many webs and of small sections.
Fig. 216. Combination Terra-Cotta Arch.
Floor tiles of either pattern must be set upon plank 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 holes 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 as great as a half-inch should not be permitted, and the tiles should be set in place by being "shoved" together 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.
Fig. 217. Segmental Terra-Cotta Arch.
Floor arches are often tested by applying a heavy roller to the arches after the wood centers have been removed, and by dropping a heavy block of timber upon the arch, a two-inch bed of sand being previously spread over the tiles to prevent mechanical damage.