Floor joists are laid upon the beams in the usual manner and spiked to them. Wooden floor joists should be braced by a "bridging" of say 2×3 inch scantling, as shown, placed at intervals of from 6 to 8 feet, according to the dimensions of the joists and the weights they have to support.

In using floor planks of 3 inches or over in thickness it will be found more economical to groove both edges of the planks and insert a separate piece as a tongue, than to cut a groove in one edge and a tongue in the other.

The selection of proper lumber for floors has already been referred to. It is often profitable to consider those things that have failed since it has been well said that "we learn as much by one failure as by two successes." And the failures in shop floors are prolific sources of much annoyance and expense.

A certain machine shop floor was laid upon round chestnut timbers, flattened on top and bedded in gravel laid over "made land," that is, loosely filled in with refuse matter of any sort easy to obtain. The floor proper was of 2-inch spruce planks.

The result was that within a year the chestnut timbers and the under side of the planks began to decay, and since that time about one half of the timbers and nearly all the floor planks have been replaced each year, the patching-up process going on at intervals, and the constant result being an unsightly as well as expensive and annoying affair.

Within a hundred feet of this floor was another of 2-inch planks laid on 3×12 inch joists, supported on 12 × 12 inch timbers resting on piers, raising the floor about two feet above the ground.

Twelve years after this was laid some planks were removed to put in a machine foundation, and the joists and timbers were found looking nearly as fresh and new as when they came from the lumber yard.

Their elevation above the ground and the ventilation of this space by small gratings in the side walls were evidently the cause of their preservation. These cast iron gratings, say 10 × 18 inches, should be inserted at least every fifty feet in the walls of buildings whose ground floors are of wood, and at least a foot of ventilating space should be left between the ground and the floor.

A Floor as Originally Laid.

Fig. 53. A Floor as Originally Laid.

Another example, equally instructive, was a second floor of a machine shop. It was of 2« × 6 inch spruce planks, properly supported. They were grooved on each edge 3-inch wide and strips were inserted as shown in Fig. 53.

The builders evidently thought that planks 2« × 6 inches, with inserted tongues, would make a good and substantial floor. And so they would have, but the unfortunate selection of the planks included many with the grain running in the wrong direction, which caused much warping and distortion.

Fig. 54 is from a sketch taken at the head of a stairway, careful attention having been given to the direction of the grain and the distorted form of the planks. It is, perhaps, needless to say that the tongues were split, and in some cases the planks also.

The Floor as Warped out of Shape.

Fig. 54. The Floor as Warped out of Shape.

Formerly the timbers most used in ordinary construction were of spruce. While this wood is well adapted for floor planks, it has very serious objections when used as supporting timbers. There is great liability to warp, twist, and crack as the seasoning process goes on, while its strength is not as great as some other easily obtained woods.

For instance, hard pine is superior in this respect, while it is about 35 per cent stronger than spruce, and its usual cost is only about 20 per cent greater.

The foundry floor is subjected to a very considerable weight, both in molding sand and in the castings produced, but the rough usage and shocks which the machine shop floor is called upon to withstand are not met with here. Consequently there is no need of such an expensive preparation.

The ground is prepared in the same manner as for the machine shop floor, except that it is only 12 inches below the floor line. This space is first covered with a 4-inch layer of crushed stone, over which is poured a thin mixture of one part Portland cement and two parts sand, mixed rather wet.

Then a concrete is made of the same mixture and finely crushed stone, and laid to a depth of about 3 inches. On top of this is spread a flowing coat of the cement and sand mixture from « to ¾-inch thick, which is properly leveled off. All this having thoroughly set, the remaining portion of about 4 inches is made up of molding sand.

Pits are dug in the central portion of the foundry floor, of such number, area, and depth as the contemplated work renders necessary. The bottom is covered with 6 inches of concrete and laid with two courses of hard bricks. The side walls of the pits are 8 inches thick and are built of hard bricks, all laid in cement mortar.

The top of the wall is level with the final cement coat of the floor. It castings of ten tons or over in weight and with comparatively small bases are to be made in one of these pits it will be necessary to put down a more substantial bottom.

Excavation should be made to solid ground, or "hard pan," and large stones laid in cement mortar built to within about a foot of what is to be the bottom of the pit. Then proceed as above for making ready for the side walls. Care should be exercised in ramming or puddling, or both, to completely fill in around the side walls.