This form of cutting is shown in Fig. 51. It gives comparatively narrow boards, is expensive, and generally used for expensive woods, and for expensive work, as for fine furniture.

The main or ground floor of the machine shop being intended to sustain moderately heavy weights, both of machines and materials - as well as the hard usage in moving them from place to place, and the shocks of heavy work - is now usually made of concrete, and laid as shown in Fig. 41.

In some cases those portions of floor included in the side wings are constructed of wood. This form is objectionable on account of the obstruction formed by the joining of the concrete floor and the planks; for at this point the former is apt to be cracked and broken, and the latter dented, split, and defaced.

This is particularly so if the planks are a trifle higher than the concrete, as is likely to be the case when newly laid down. Then, too, where such a floor is of wood it is necessary to excavate a foot or so below the floor timbers, to provide an air space for preventing the decay of the materials.

To lay a concrete floor for this purpose the earth should be excavated to the depth of from 18 to 24 inches, according to the weights which the floor is to carry.

For ordinary purposes of machine shop work 22 inches is desirable and sufficient. If the ground is sufficiently firm at this level, no further preparation need be made. If soft and yielding, the excavation should be carried down to solid ground, and then filled up with solid earth, or still better with gravel, the excavation being flooded with water and the filling material thoroughly puddled as it is put in.

On top of this bed should be placed a layer of coarsely broken stone, from 8 to 12 inches deep; and upon this a layer of crushed stones, none of which should exceed 2 inches in dimension. This layer should be from 4 to 6 inches thick. On this is spread a layer 2 to 4 inches thick of concrete composed of one part Portland cement, two parts clean, sharp sand, two parts clean gravel, and three parts fine crushed stone - all taken by measure, and not by weight.

These ingredients should be mixed rather wet so as to settle well down into the spaces between the stones of the previous course. The concrete should be rammed hard and made perfectly level.

Then comes a coating of from « to 1 inch thick, consisting of a mixture of one part Portland cement and two parts clean, sharp sand, which should be laid before the former course is dry, in order that the two courses may firmly unite. This last course is laid quite wet, to facilitate "floating" - that is, the leveling off and smoothing.

Sometimes the intermediate course of concrete work is made up of shingle (coarse gravel, stones, or pebbles), mixed with hot coal tar or Portland cement; but this has the objection that, whatever be the medium used for cementing the mass, it will not adhere to the rounded surfaces of the pebbles as effectively as it does to the more porous surfaces of crushed stone.

Therefore, where subjected to hard usage, this shingle is more likely to disintegrate and break up than where crushed stone is used.

The gallery floors of the machine shop are supported on built-up girders 20 inches deep, placed at each of the columns dividing the wings from the central part of the building. Carried upon angle bars, riveted to the girders at a proper height, are the ends of 3 x 16-inch floor joists, placed 20 inches from center to center, their upper edges coming 2« inches above the top of the girders, which space is occupied by a spiking piece. On these joists is laid a floor of 2« × 6 inch planks, planed on both sides and matched with tongue and groove.

This construction is shown in vertical section in Fig. 52. The girders here shown may, of course, be solid I-beams, with angle bars riveted on them for supporting the ends of the joists.

Where a lighter construction may be safely resorted to, on account of less weight to sustain, the form shown in Fig. 44 is proper. In this case an I-beam is used, say 10 to 15 inches deep, and the ends of the joists rest upon the lower flange of the beam. They should be of such depth as to project a couple of inches above the top of the beam, as shown, to provide a space for a spiking piece.

In either case the ends of the joists should be beveled as shown, so that they may drop out clear in case of fire, without displacing or warping the I-beams.

Wood joists may be dispensed with altogether, if safety from fire is of more consideration than first cost.

In this case I-beams of proper strength are laid upon the girders, or with their ends resting upon the lower flanges thereof - say 4 to 8 feet from center to center - and upon them are laid planed and matched floor planks from 3×6 inches to 5 x 8 inches, according to the distance between supports and the load to be sustained. These are bolted to the upper flange of the I-beam. This arrangement is shown in Fig. 52.

Floor Supported Entirely by Steel Beams.

Fig. 52. Floor Supported Entirely by Steel Beams.

The vertical space occupied by these methods of construction varies considerably, as is shown in the engravings, and must be taken into account in designing the building.

Where it is desired to support floors by wooden beams, the form shown in Fig. 46 is proper. The dimensions of the beams must be sufficient to carry the load, taking into consideration also the distance between supports. The ends of the beams resting in the brick wall should be upon a "header course" of bricks, as shown, and the ends of the beam beveled off the same as if used in connection with an iron girder or I-beam, so that in case of fire the beam will fall freely out of the wall without injuring it.