173. When flat ceilings are not required, as in warehouses, etc., tile arches of the forms shown in Figs. 83 and 84 are generally used, the plaster being applied directly to the under side of the blocks. The bottom flanges of the beams are protected either by plaster laid on wire lathing or by properly shaped skewbacks. This kind of arch makes very strong floors, and is much cheaper than flat tile arches. Segmental arches have been built in spans up to 20 feet, thus saving considerable weight in the columns and beams, as in such cases many of the latter may be omitted which would otherwise be required for short spans.

174. The ordinary segmental tile arch consists of blocks from 4 inches to 8 inches deep, and about 1 foot long, the rows being laid with joints broken longitudinally. The rise of such arches should be about one-eighth of the span, if possible, and, in any case, not less than one-twelfth. Fig. 83 shows a common segmental tile arch, a indicating the skew-backs, b the hollow inside blocks, and c the tie-rod taking up the thrust of the arch. This rod should be placed near the lower flanges of the beams, and be well protected by special tiling, or by metal lathing and plaster. At e is shown the concrete filling; at d, the nailing strips; and at f, the flooring.

Segmental Tile Arches 210

Fig. 83.

Fig. 84 shows a hollow-brick arch, suitable for spans up to about 8 feet; it is similar in most respects to that represented in Fig. 83, but the skewbacks a have flanges to protect the I beams, and the tile blocks b are only ribbed one way; d, e, and f indicate the concrete, nailing strips, and flooring, respectively.

Segmental Tile Arches 211

Fig. 84.

175. Weight

Weight. The weight per square foot of segmental tile arches, exclusive of concrete and plastering, is about as follows:

Depth.

Safe Span.

Weight.

4

inches.

8

feet.

20

pounds.

6

inches.

16

feet.

30

pounds.

8

inches.

20

feet.

40

pounds.

The tile may be had of either dense or porous tiling; the latter is lighter and equally as strong as the former, and, consequently, is preferable to the dense kind.

The weight of the concrete depends upon its composition, and varies from 120 to 140 pounds. The plastering may be estimated at from 6 to 8 pounds per square foot.

176. Strength

Strength. The strength of segmental tile arches, properly constructed, is practically only limited by the safe load for the beams. An arch of porous tile, having a 15-foot span and a 16-inch rise, the central 7 feet 8 inches being 6-inch blocks, and the remainder 8-inch, was loaded on one side by placing on it a pile of bricks; when the weight reached 1,235 pounds per square foot, the unloaded portion buckled and collapsed. A common brick arch of 8-foot span, 10-inch rise, and 5 1/2 inches deep, failed by buckling under an eccentric load of 885 pounds per square foot.

177. Setting

Setting. Segmental arches are laid in the same way as flat arches, with the exception that the centers are suspended on hooks from the beams, and can be raised or lowered by nuts threaded on the lower end of the hook rods.