When the girders are near together, and the timbers long enough to span two divisions, a rigid floor may be obtained by "breaking joints" with the floor timbers every five or six feet.
Flush Framing. If the girder is to be framed flush with the timbers, the use of stirrup irons or patent bangers is recommended, as preserving the full strength of the girder. In this case the tim bers should be brought as nearly opposite as possible, and iron clogs long enough to reach over the girder and drive down into opposite timbers should be used. (Fig. 179.) If the girder is deeper than the floor timbers it may be set flush on top, and smaller pieces of hard pine or an angle iron may be bolted to the lower part to receive the floor timbers, as shown in Fig. 180.
Crowning. All floor timbers having a span of more than six teen feet should be crowned, that is, the top of the joist is cut to the shape of an arc of a circle, having a rise of one-quarter inch to every six teen feet of span. This is necessary to allow for the ordinary sag of the timber, so that a level floor may result.
Steel Girders. With the use of steel girders new considerations will arise. If a single beam is set entirely below the floor timbers, it will give a better bearing if the timbers lap and spike to each other. With two or more beams, the timbers may be brought end to end as on the heavy wooden girder. If the steel girder is set flush with the beams, they may be cut so as to run into the trough formed by the flanges of the beam, but should be supported by stirrup-irons or hangers, Fig. 1S1, as the sloping flange of the steel beam does not afford a good bearing. If the steel beam is deeper than the floor timbers, a common method is to bolt a timber to each side of the beam for a bear ing; and of course an angle bar can be used in the same manner. (Fig. 1S2.) In any case, the floor beam should be fastened so that there is no danger of slipping out of the hanger. Most of the patent hangers have a lug, or bolt, to secure the timber in place, and where timbers come opposite, com mon iron dogs turning down into each timber may be used.
Slow Burning and Mill Construction. 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 space:, constitutes what is generally termed slow-burning construction. To obtain the
best results from this method of construction, wooden gird ers should always be set flush with the floor timbers on top, as the dropped gird er 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.
Hill 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 sub jected. 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 auto matic arrangements of closing hatchways or elevator openings, and of enclosing stairways in incombustible partitions.
The typical construction employed for the mills of New Eng land, 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 be tween. 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. 1S3.) These timbers are supported by iron post caps or pintles, as already de scribed. 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.