This section is from the book "A Treatise On Architecture And Building Construction Vol2: Masonry. Carpentry. Joinery", by The Colliery Engineer Co. Also available from Amazon: A Treatise On Architecture And Building Construction.
Strength. There are extremely few, if any, cases on record of failure of arches in actual use, and under ordinary conditions the arches herein described are amply strong. Experimental results are sufficiently numerous, however, to be some indication of the strength of tile arches. Tests were made at Denver, Col., on two side-method arches, 10 inches deep and 5 feet in span, the blocks having one horizontal web; these gave way under distributed loads of 270 and 425 pounds to the square foot. An end-method arch of porous tile, 10 inches deep, with two horizontal webs, carried 750 pounds per square foot without failing. At Richmond, Va., tests were made of 6 and 12 inch side-method arches, the maximum load on the former being 579 pounds per square foot, and on the latter, 1,057 pounds; the average strength of nine 12-inch arches was 858 pounds per square foot. Record is made of a 15-inch arch, similar to that shown in Fig. 79, having carried 3,287 pounds per square foot before failure.
Setting. To support the blocks during laying, a firm centering is needed. A good form, shown in Fig. 81, is made of 2-inch planks a, dressed and set closely together and resting on 2"x4" or 2"x6" pieces b, which extend parallel to the beams, and midway between them. These are suspended by T-headed bolts c, from similar pieces d, laid across the top of the beams. The tiles forming the I-beam protection are first put in place; if separate pieces, they are laid on the planks directly beneath the beams; or, if the I beams are protected by the skewbacks, as at e, the latter are set. The bolts are then tightened so as to slightly bend the planks, forming the slight camber, which crowning should be about \ inch for a 6-foot span. In setting the blocks, the pieces should be adjusted so that the surfaces abut properly. Only the best cement mortar should be used, and the joints should be thin, but care should be used that the mortar does not become pressed out. The centering should be kept in place until the mortar joints between the blocks have set, which takes from 12 to 36 hours. When it is removed, the arch should have a level surface, showing no open joints or projecting blocks. Should there be any necessity for holes in the floor, they may be punched in the blocks, the holes being afterwards closed with broken tile and mortar; or, if the side-method arch is laid, a block may be omitted temporarily.
The design of the floors often necessitates variations in the placing of the I beams, so that some arches are longer than others. This may be provided for by using blocks of various sizes, in preference to cutting them to fit. Numerous kinds and sizes of skewbacks are also made for different I beams.
167. Tie-rods are necessary in all floor arches, to prevent spreading of the beams. They are placed from 5 to 7 feet apart, and are generally 3/4-inch rods, threaded at the ends, with nuts to take up the thrust of the arch. They should be placed in the web of the beam, as near as possible to the lower flange.
1(58. Like all other masonry work, floor arches should not be laid in cold weather, unless ample protection is provided against injury by rain and snow; otherwise, the mortar is likely to be alternately frozen and thawed, causing the joints to rupture and leading to the displacement of the blocks.
Until the mortar in the joints has set, care should be exercised in placing loads upon the arch. Precautions should also be taken to prevent the bottoms of the tile becoming stained, as the moisture in the mortar, unless thoroughly dried out, carries down the dirt and causes discoloration of the ceiling. It is advisable to apply some waterproof material to the under side of the arches, before laying the plaster on them.