Page:Encyclopædia Britannica, Ninth Edition, v. 4.djvu/543

Rh FLOORING.] it is an object to save height in the depth or thickness of this species of floor, the ceiling joists may be tenoned into the binders, instead of being nailed to them; in this case the latter must be chase-mortised on one side, fig. 33, for the convenience of receiving the former when they are them selves set and fixed. A chase is a long wedge-formed groove of the breadth or thickness of F IG - 33. Chaso Mortise, the mortise, of which it is indeed an elongation, so that the tenon at one end of the ceiling joist being inserted in the regular mortise in the binder prepared for it, that at the other end is driven along the chase up to its place in the mortise in the next binder. When ceiling joists are thus chase-mortised, their lower or under faces are allowed to come a little below the under face of the binders, and the space across is firred down by slips not wider than the ceiling joists are thick. No. 2, fig. 8, shows a transverse compart ment, or bay, of a floor in this manner ; but it is not so good a one as the preceding ; for, besides the weakening of the binders, by cutting so many mortises and chases in them, it is almost impossible to give the ceiling floor the degree of firmness and consistency it possesses in the other way, besides requiring the fining down on the binders. The same space would be better gained by cutting the bridging joists so much lower down ; as they may, with the sort of notch indicated above, be let down fully half their depth with out great injury to either bridging joists or binder, for they can always be made to fit tightly or firmly, and very little more labour is involved in notching deeply than slightly. Flooring is said to be framed when girders are used together with binding, bridging, and ceiling joists (see Plate XXIV. fig. 9, Nos. 1 and 2, and PI to XXV. fig. 4). Girders are large beams, in one or more pieces, according to the length required, and the size and strength of which timber can be procured. They are intended for longer bearings than mere binders may be trusted at, and may be strengthened by forming a built beam. The principle of constructing girders of any depth, says Tredgold, in his Carpentry, is the same as that of building beams, and when properly conducted they are as strong as any truss can be made of the same depth. The most simple method consists in bolting;-two pieces together, with keys between to prevent the parts sliding upon each other, the upper one of hard compact wood, the lower of tough straight grained wood. The joints should be at or near the middle of the depth ; the thickness of all the keys added together should be greater than one-third more than the whole depth of the girder; and if they be made of hard wood, the breadth should be about twice the thickness. They may be held together by bolts. Fig. 34 is a good form held by bands, and, if the upper timber be cut so as to be smaller towards the ends, would admit of these hoops being driven on till perfectly Frcs. 34, 35. Girders. tight. In fig. 35 the parts are tabled or indented together instead of being keyed, and a king-bolt is added to tighten the joints. Girders may be further strengthened to almost any extent by trussing ; but to be efficient, the height of the truss must always be greater than the depth of the beam itself, and the strength is increased by extending that height as the space or bearing increases. A truss is indeed a wouden arch, whose lateral thrust will of course be 483 greater the smaller the angle subtended by it, and vice versa. It has been a commonly received opinion, that a truss less than the depth of a girder adds materially to its strength ; but experiments have proved that very little advantage is gained by such a one, even when executed in the best manner, and that, badly executed, the beam or girder is weaker with the truss than without it. In some situations the flooring joists can be raised to a certain height to allow of the head of the truss, which is usually made of iron, being placed at a sufficient height to be truly FIG. 36. Trussed Girder. efficient. Such is the trussed girder shown in fig. 36, intended for a great length. A common mode of strength ening a beam or bressummer, is to cut the timber in hale longitudinally, whereby any defects in the interior can be seen ; then to reverse the pieces, end for end, and bolt them well together. Some additional strength is obtained by putting between the timbers a plate of wrought iron about the depth of the beam and about half an inch in thickness, and then bolting the three together, as in Plate XXIV. fig. 9, No. 3. Binders are made dependent on the girders by means of double tusk tenons, and on and to them the bridging and ceiling joists arc attached as above described. Plate XXIV. fig. 9, No. 1, shows the transverse section of a compart- Plans of ment or bay of a framed floor ; No. 2 the same longitudinally floors, of the girder and of the bridging and ceiling joists, and transversely of the binders. Plate XXV. fig. 1, is the plan of a single floor of joists tailing in on wall-plates with two chains of struts, and trimmed to a fire-place. Fig. 2 is a floor similar to fig. 1, with ceiling joists nailed to deeper flooring joists at intervals, as in Plate XXIV. fig. 7. Fig. 3 is the plan of a double floor ; and fig. 4 is that of a framed floor of joists, bays of which are shown in section in Plate XXIV. figs. 8, 9. It is to be observed, with reference to the diagram fig. 9, No. 1, that binders ought not to be framed into the girders opposite to one another, as they are here shown to be as a matter of convenience, since the girder is unduly weakened by being mortised on both sides at the same place. Cast-iron shoes render mortising the one forming a tenon upon the other almost unnecessary; and in like manner cast-iron shoes laid into a wall upon stone templets give a good and safe bearing to the girders ; but it is not everywhere that cast- iron shoes are attainable, and mortises and tenons may be made anywhere. The .above descriptions of the three sorts of flooring Medieval apply to floors which are to have a ceiling as in house flooring, building, or may be left open, as usual in warehouses. But in house building according to the practice of the mediasval period, these timbers would be left exposed. They would all require to be planed smooth, the girders moulded, the binders partly so, and the joists perhaps only stop-chamfered, which is done by cutting the arris of the timber to an angle along its whole length, but stopping short of the ends by a few inches, when it is returned into the arris by a cant. The underside of the joists in a framed floor may be lined with chamfered boarding or formed into panels and ornamented, a boltel or a set of mouldings forming a frame or cornice all round against the binder. The girders would rest upon stone corbels, either moulded or decorated with foliage or figures, or all three united. Viollet le Due, in his valuable Dictionnaire raisonne de V Architecture, gives several examples of such a floor,