The use of the methods of heavy timbering just described, with floors of matched or splined plank, and with no wood partitions or furrings enclosing hollow spaces, constitutes what is generally termed slow-burning construction. To obtain the best results from this method of construction, wooden girders should always be set flush with the floor timbers on top, as the dropped girder with the space above will permit flames to lap around the timber and it will be more quickly consumed than if it is flush with the rest of the floor beams. Greater protection is obtained by protecting the wood with plaster on metal laths, leaving no air spaces between the plaster and the wood.

Mill Construction, as its name denotes, should properly belong to the particular methods of construction which have been devised for resistance of fire, and the sustaining of the loads and shocks of machinery, to which manufacturing buildings are particularly subjected. This requires primarily the disposition of the timber and plank in solid masses exposing the least number of corners to the action of fire, of separating the floors by fireproof stops, and automatic arrangements of closing hatchways or elevator openings, and of enclosing stairways in incombustible partitions.

The typical construction employed for the mills of New England, and the only form acceptable to the insurance companies of that section, consists of posts at least 10 X 10 inches, spaced about eight feet apart in the length of the mill and twenty-four or twenty-five feet across. Instead of a line of girders running lengthwise over the line of posts, the floor beams are laid across the mill on the tops of the posts. These beams are usually 12 X 14 inches, or two pieces of 6 X 14 inches, bolted together with an air space between. The wall end of these timbers should rest on iron plates and the ends be bevelled off and secured only at the bottom, so that they may fall out easily if burned, and not pull down the wall. (Fig. 183.) These timbers are supported by iron post caps or pintles, as already described. The flooring consists of a layer of three-inch planks, not more than ten inches wide, splined together and blind-nailed; and it should be long enough to span two spaces, breaking joints every four or five feet. All of this construction is usually of Georgia pine.

Fig. 181. Stirrup Iron and Steel Girder.

Fig. 181. Stirrup Iron and Steel Girder.

Fig. 182. Connection of Wooden Joists and Steel Girder.

Fig. 182. Connection of Wooden Joists and Steel Girder.

The upper floor is generally made of hardwood such as maple or birch, and this is laid with square edges over two or three thicknesses of paper, each layer being mopped with tar, asphalt, or similar material; or sometimes a layer of plaster is spread between the upper and under floors.

This construction may be adapted to the use of mercantile or office buildings, but the general requirements of this class of buildings will not admit of the posts occurring so often as every eight feet. This necessitates the use of a girder and intermediate beams. If these beams are spaced four or five feet apart, it will permit the use of two-inch plank which may be tongued instead of splined. With this increased spacing of the posts, it will be necessary to use iron or steel posts for a building four or more stories in height; and if the posts are more than fourteen feet apart, it will be cheaper and better to use steel beams for girders. In this case. all weight-bearing metal must be protected by at least an inch of plaster or other fireproof material.

Partitions

Where slow burning or mill construction is used for the floors of a building, the partitions should be made of solid plank plastered on both sides on metal lathing, or else of light steel framing with metal lath, and plaster.

Fig. 183. Effect of not bevelling ends of Joists.

Fig. 183. Effect of not bevelling ends of Joists.

Roofs

The same methods of framing may be employed for the roofs as for the floors, but lighter timbers can be used, set to the required pitch, with a tar and gravel or metal roof. Steep pitched roofs may also be constructed in the same way, trussing the timbers if the span requires it.

Supervision

The supervision of the framing of floors of brick buildings will call for constant vigilance on the part of the superintendent, as it is not only necessary to follow closely the work of the carpenters, but the fact that the mason-work is being carried along at the same time, by a different set of workmen, will necessitate a great deal of forethought in order to bring the different parts together at the proper time.

The anchoring of floors properly, and at the proper time, will require constant attention, and the setting of plates, the building-in of bolts and hangers, and the leveling-up of floors and walls to their relative positions, will require harmonious action between the mason and the carpenter.

In heavy framing, careful watching will be necessary, to see that proper connections are made between post, girders and floor beams, and that all necessary ties, straps and bolts are set and tightened.

If trusses occur, they should be strained up tightly when built and kept tightly strained as long as there are any workmen remaining at the building, as the shrinkage of the timbers will often loosen the joints and allow sagging to occur.