158. There are at present three methods of constructing flat tile floors in use in this country. The first is known as the side method, so called because, in each transverse row, the sides of the blocks abut those of the adjacent ones; the second is called the end method, the blocks being set end to end, at right angles to the beams; the third is a combination of the first and second, the length of the skewbacks being parallel to the beams, while the other blocks are set at right angles thereto.

159. Side Method Arches

Side Method Arches. The original hollow-tile arches used in this country were made and set similarly to those shown in (a), Fig. 76, with the exception that the skewbacks did not extend under the beams; the latter, consequently, had no protection from fire, other than that afforded by the plaster of the ceiling. This was found to be an insufficient safeguard, as when beams protected merely by thin plastering were subjected to fire, they became twisted and warped nearly as much as though completely exposed. 2-17

159 Side Method Arches 203

Fig. 76.

An improvement was made by forming the skewbacks as shown at (a), Fig. 76, in which a shows the I beams; b, the skewbacks, extending below the beams; c, the thin pieces of tile - held in place by the skewbacks - which protect the under side of the beams; d, the inside blocks of the arch; e, the wooden strips embedded in the concrete f; and g, the flooring, nailed to the strips. When tile or cement flooring is used, these strips are unnecessary, the floor being then laid directly on the concrete.

After these arches were used for some time, it was found that they were not strong enough to sustain heavy and sudden concentrations of weight, or the rough usage floors receive during the erection of the building. To remedy such defects, the blocks were strengthened by horizontal and vertical webbing, forming a cellular structure, like that represented in (b), Fig. 76. Here a shows the skewbacks with a lower flange b extending to the middle of the I beam, and forming, with the adjacent skewback, a complete protection for it; c indicates an arch block with 6 divisions, separated by webs or partitions from inch to 1 inch thick; d is the center or key block; e, the nailing strips; and f, the concrete. Usually the sides of the blocks are parallel to those of the key, but arches are now made of blocks having radial joints. This is the stronger form, but is more expensive on account of the number of different pieces required. The blocks should break joint lengthwise in order to make a good bond between the sections.

Arches of the type shown in (b), Fig. 76, are sufficiently strong for all ordinary purposes, and, so far as known, have never failed when in actual use in buildings. Some architects, however, consider the end-method arch the stronger.

160. End-Method Arches

End-Method Arches. Arches of this kind are usually formed of rectangular blocks, divided by vertical and horizontal webs into four or more sections, and having beveled end joints. It is not customary to make the blocks in one transverse row break joints with those in the next, as the expense of setting would be thereby increased.

Fig. 77 represents a very common type of the end-method arch; at a is shown the skewback, having a protective flange b; at c, one of the inside blocks; and at d, the key block. This form of construction is economical and simple, as all the blocks in each longitudinal row of the arch are made alike. Porous terra cotta is used almost exclusively in the manufacture of this type of arch.

160 End Method Arches 204

Fig. 77.

161. One objection to this method is the increased weight, due to the thick webs which are necessary to obtain sufficient bearing to properly transmit the load to the beams. Another is that the mortar squeezes out from between the thin abutting surfaces, allowing some of the pieces to sink below the others, and rendering it difficult to secure a flat surface for the ceiling. Furthermore, if any block be broken or removed, the row will fall, there being no longitudinal bond, as in side-method arches. To obtain good bearings, the blocks should bear evenly against one another, being cut, if necessary, to set flat and give a level upper surface, and the webs should also be. in line. It is recommended that solid plates be placed between the ends of the tile blocks, to form a stronger joint.

160 End Method Arches 205

Fig. 78.

Fig. 78 shows a form of block in which greater bearing surface is provided than that represented in Fig. 77. These blocks are made of dense tile, and have braced top and bottom flanges 3/4 inch thick, the webs and central horizontal partition being 1/2 inch thick. A section of the block is shown at a, an end view at b, and a tie-rod at c. The lower part of the I beam is protected by a plate resting on the beveled lower edge of the skewbacks, as shown at d.