294. The improvements in fireproof floor construction during the past fifteen years have been many and in rapid succession. Previous to 1880 so-called fireproof floors were constructed of brick arches turned between the lower flanges of wrought iron I-beams. These arches, with the concrete used for leveling, were very heavy, and as the bottoms of the beams were unprotected and the ceiling formed by the arches was very undesirable, brick arches soon gave place to arches of hollow dense tile. The increased demand for fireproof construction, taken in conjunction with the reduction in the prices of steel and fireproofing which occurred about the year 1889, led to many improvements in the designs for hollow tile floor arches, and also to the introduction of various systems of construction based upon the use of concrete and plaster compositions, combined with steel wires, bars and cables, used in different shapes and in different ways, the chief aim of the inventors or designers being to secure the lightest and most economical floor consistent with ample strength and thorough fire protection.
In the following pages the author has endeavored to give an impartial description of the various systems at present approved by the leading architects and engineers.
There are three general schemes of flat tile construction at present in vogue in this country. The first and oldest is known as the side method, in which the tiles lie side by side between the beams, as shown in Figs. 175, 176 and 177. In the second scheme, known as the end method, the blocks run at right angles to the beams, abutting end to end, as shown in Figs. 178 and 180. The third method is a cross between the first and second, the skewback (or abutment) being made as in the side construction, and the "interiors" or keys abutting end to end between the keys, as shown in Fig. 181. This method is known by different names, such as the "Johnson Arch," "Excelsior Arch," "Combination Arch," etc.
The hollow tile floor arches first used in this country were made of dense tile, formed essentially like those shown in Fig. 175, except that no provision was made for protecting the bottom of the beams except by the plaster on the ceiling. It was soon found that the bottom of the beams must be more thoroughly protected from heat, as when unprotected they warped and twisted so badly during a fire as to destroy the building. The skew-backs were, therefore, made so as to drop from ¾ to 1 inch below the bottom of the beams, and either to extend under the beam or else to hold a thin tile dovetailed between them, as shown in the figure. Arches of this type were used for several years, but it was found that they were not strong enough to sustain severe loads and the sudden strains caused by moving heavy safes, or to withstand the rough treatment and heavy weights that floors are subjected to while the building is in course of erection. The blocks were, therefore, strengthened by the introduction of horizontal and vertical webs, resulting in the shapes shown in Figs. 176 and 177, which represent the best types of dense tile arches with ribs parallel to beams made at the present time.
Arches similar to these are also made of porous tiling, but this material is more generally used in the end-method types. Most of the side-method arches have beveled joints, which are parallel to the sides of the key, as shown in Fig. 176, although arches are now made with radius joints, as shown in Fig. 177. Theoretically the latter joint should make the strongest arch, but the increased cost of making so many different shapes of blocks prevents it being much used. The blocks in the side-method arches break joint endways, so as to completely bond the arch, as shown in Fig. 176. Arches of the type shown in Figs. 176 and 177 undoubtedly have ample strength for all ordinary purposes, and the author believes there is no record.
of their failure when in actual use in buildings. The few comparative tests that have been made, however, would appear to prove that for a given weight the side-method arch is not as strong as those built on the end method.