Where a flat ceiling is not essential, and for warehouses, factories, breweries, etc., the segmental arch gives the strongest, best and cheapest (considering the saving in ironwork) fireproof floor that can be built of tile. Segmental arches can be used for spans up to 20 feet, thus dispensing entirely with the usual floor beams; they also effect a considerable saving in the dead weight of the floor, thereby enabling the columns and girders to be made lighter.
There are at present two distinct systems of segmental arches in vogue in this country.
Hollow Tile Segmental Arches. - The most common form of segmental arch is that shown in Fig. 184, which is made of hollow blocks, usually 4, 5, 6 or 8 inches square and 12 inches long, the tile being laid so as to break joint longitudinally of the arch. Nearly all manufacturers of hollow tiling make one or more shapes for segmental arches, and also different styles of skewbacks to use with them. Hollow tiles for segmental arches are also made both of dense and porous tiling. The latter is generally considered as the best material for this purpose. Segmental arches should have a rise of not less than 1 inch per foot of span, and 1½ inches wherever practicable.
With this type of arch it is better to use a very heavy or solid skewback without the flange projection, as the thrust on the skew-back is very great where the arch is of wide span. The bottom flange of the beam should be covered with heavy, stiffened wire lath before the skewbacks are set. When plastered the ceiling has the appearance shown in Fig. 184.
If the span of the arch is not more than 8 feet, hollow brick, with raised skewbacks, may be used, as shown in Fig. 185. This makes a very light and strong floor.
The tie-rods for segmental arches should be placed just above the bottom flange of the beam, as shown in Fig. 184, and should be protected either by special tiling, made so as to form a paneled effect in the ceiling, or by wire lathing and plaster.
Arches 4 inches thick, 20 pounds per square foot; safe span, 8 feet. Arches 6 inches thick, 30 pounds per square foot; safe span, 16 feet. Arches 8 inches thick, 40 pounds per square foot; safe span, 20 feet.
The weight of the concrete should be figured for each special case, allowing 120 pounds per cubic foot of concrete. Plastering should be taken at 8 pounds per square foot.
The spans for different thicknesses should not exceed those given above, except that for spans of 20 feet about 7 feet of the centre portion may be built of 6-inch tile.
The segmental form of arch is undoubtedly the strongest that can be built, whether of brick, hollow tile or concrete.
In the celebrated Austrian tests* a common brick arch 5½ inches thick and 8 feet span, with a rise of 9.85 inches, carried an eccentric load of 885 pounds per square foot before failing. The failure was then caused by buckling and not by crushing. A porous tile arch of 15 feet 4 inches span, with a rise of 16 inches, built with 6-inch hollow blocks for a distance of 7 feet 8 inches across the centre and with 8-inch blocks for the balance, was tested by loading one side with a pile of bricks measuring 4 feet 6 inches lengthways of the arch and 7 feet 6 inches widthways. When the weight reached 42,000 pounds (1,235 pounds per square foot) the unloaded side commenced to buckle, and in 30 minutes collapsed.*
* Architecture and Building, January 4, 1896.
Segmental arches, with spans not exceeding those given above, built with a rise of 1 inch per foot of span and laid in good cement mortar, may be safely relied upon to carry as much as the beams, when uniformly loaded.
Setting. - Segmental arches are set in the same way as flat tile arches, except that the centres are arched to the desired curve and are suspended at the sides from the beams or girders by hooks passing over the beams. The bottoms of the hooks are made round, and have a thread and wing nut for bringing the centre into its proper place and for lowering it after the arch has set.
Holes are left where the hooks pass through the arch, and after the centres are removed these are substantially plugged with mortar and tile.