=31. Floor Construction.=—The floor construction consists of heavy timber girders, no dimension of which may be less than 6 inches, as otherwise it would not comply with the requirements of slow-burning construction. The floor planking consists of 3- or 4-inch tongued-and-grooved spruce, or yellow-pine planking, planed on the under side, and thoroughly spiked to the girder. Planking of the former thickness may be used for clear spans as great as 8 feet, while the latter thickness may be used for up to 10-foot or even 12-foot spans, if the loads are light. The girders are indicated at _d_, and the floor planking at _e_. Usually the girders, in order to obtain the requisite strength, are made of long-leaf yellow pine. On the top of the spruce planking is placed a finished maple floor. This floor is made from either 1-inch maple, which finishes as ⅞ inch, or 1¼-inch maple, which finishes as 1⅛ inches, in thickness. Neponsett sheathing paper, or deadening felt, is placed between the spruce planking and the finished maple flooring for the purpose of preventing dust from percolating through. This sheathing paper or felt is sometimes made waterproof to prevent leakage due to water used for fire-extinguishing purposes. Frequently, the brickwork is corbeled out, as indicated at _f_, in order to form a fire-stop between floors, or at least to prevent an open joint at this place. Where the walls are offsetted, as shown in Fig. 13, there is no need of corbeling out, for the offset in the brickwork can be made to form the fire or dust stop. =32.= Where heavy yellow-pine girders bear on brick walls, it is usual to obtain the requisite bearing area by the use of cast-iron bearing plates, as indicated in Fig. 14 (_a_), (_b_), and (_c_). In (_a_) is shown an ordinary flat plate that has an area figured so that the load on the brickwork will not exceed its ultimate stress, which for brickwork laid in lime-and-cement mortar is about 150 pounds per square inch, while for brickwork laid in cement mortar, it is in the neighborhood of 200 pounds per square inch. This plate is usually cast with a lug on the back, as at _a_, to be built in the brickwork, and dowel-pins, or a lip, as at _b_, over which the girder is fitted, or notched. By this means, a tie to the wall is obtained. There is difficulty, however, in using such a connection, for the carpenters on the job frequently miscut their beams, so that the notchings or borings at _b_ do not come where they should, and to remedy the defect, the notchings, or borings, are cut or gouged out, so that frequently the pin or lip at _b_ is not brought to bear against the timber. [Illustration: (_a_)] [Illustration: (_b_)] [Illustration: (_c_) FIG. 14]