Page:Encyclopædia Britannica, Ninth Edition, v. 4.djvu/539

Rh JOINTS.] BUILDING 481 these joints are generally spiked, and it is always required
 * tching. that they be made to fall in or under a pier. Notching is

either square or dovetailed ; it is used in connecting the ends of wall-plates and bond-timber at the angles, in letting joists down on beams or binders, purlines on prin cipal rafters, etc. Nos. 1, 2, 3, 4, and 5, fig. 2, show varieties of notches applied as we have described No. 1 is a simple square notch ; No. 2, a dovetailed notch. No. 3 is the notch most commonly used; it is similar to No. 1, but that the ends are allowed to run on so that the one piece grasps the other, and each forms a cog to the other. No. 4 is an oblique-angled, dovetailed notch ; and No. 5 shows how joists .are notched or let down on beams and binders, and purlines on principal rafters. A notch is cut into the under edge of the joist or purline an inch or an inch and a half in depth, and considerably shorter than the beam, binder, or rafter is in thickness. Notches are also cut down on the upper angles of the bearing pieces, as long as the rider is thick, as deep as the notch before described of the latter is, and so far in as to leave a thick ness on its own edge equal to the length of the notch in the riding joist or purline. In the diagram one joist is indicated in its place let down in the notch, and another indicates the notch in its own edge, and leaves exposed the ( ging. notches in the binder. Cogging, or corking, as it is vulgarly termed, is the last-mentioned species of notch extended on one side, and leaving a narrow tooth or cog alone in the bearing-piece flush with its upper face, No. 1, fig. 3. It is used principally in tailing joists and beams on wall-plates and templets, and the cog is here made narrower, because the end of the joist or rider coming immediately beyond the plate, that part which forms the shoulder of the notch would be liable, on being strained, to be chipped off or torn away, if it were not kept as long as possible; and it is not of so much importance to guard against weakening a wall-plate which is supported along its whole length, as a beam, binder, or principal rafter, which rests on distant points alone. No. 2 of the same figure shows another mode of tailing on joists and beams by a dovetail notch, which, to distinguish it from the flat notches, Nos. 2 and 4, fig. 2, is called corking, or cogging also, though the operation certainly is not cogging. This is a good mode if the timber be so well seasoned as not to be likely to shrink more ; but it would be improved by allowing the rider to take a bearing in a notch like that to No. 1 before the dovetail commenced, as at No. 3, for in the ordinary mode it is weakened in a point of great importance. Whatever notches and cogs for beams and joists are required in wall-plates and templets, should be made before they are set on or in a wall ; for, as they are always bedded in mortar, anything that may break the set must be avoided. A cog-hold is best obtained through the agency of a chair of cast-iron, which should, however, be itself cogged or joggled to a stono templet laid in the wall under it, and be capped or covered by another broad flat stone, as an inverted templet, with a joggle from the chair running up into it. Vtising Tenoning implies mortising also as a matter of course. They are the names of the two operations necessary to one result, that of producing a connection between two pieces by inserting part of the end of one into a hole of similar size cut in the side or face of the other. A tenon is formed by cutting in on each side or edge of a piece of timber, near its end, transversely, to a certain depth, or rather, leaving a certain part of the breadth or depth uncut, and then cutting in longitudinally from the ends as far from each edge as the transverse cuts have been made in depth, thus removing two square prisms and leaving a third undivided. This is the tenon. An excavation in the side of a piece of timber, of a certain depth, in the direction of its thickness, parallel to its edges, and bounded lengthwise by lines at right angles to them, is a mortise. Tenons and mortises are made of exactly corresponding size, and are most frequently at equal distances from one or the other side or edge of the two pieces to be conjoined ; and for the most part, too, every angle formed in the process of tenoning, both internal and external, is a right angle. Tenons are called joggles in some situations, when they are not intended to be borne upon, and their use is merely to keep the piece of timber to which they belong steadily in its place, without being liable to slight accidents from lateral pressure or violence. In combining timbers by means of mortises and tenons, to produce as great a degree of strength as possible, it must be obvious that the object to be kept in view is to maintain the end or tenon of the one as large and efficient as it may be, and weaken the other as little as possible in forming the mortise. For the efficiency of the mortised piece in a horizontal bearing, it is clear that as much of its thickness should be below the mortise as possible, as at a, fig. 4 ; for if it be put low, as at b, the superincumbent weight on the tenon would more readily split or rend it in the direc tion of the grain, as indicated ; but the case is inverted with the tenoned pieces. With the mortise at a the tenon could only have the efficacy of so much of the piece to which it belongs as there is of it above its under surface, which is a very small part of its depth ; whereas with the tenon at b it would command the power of the greatest part of the depth. To guard as much as possible against the danger of too great a mortise and too small a tenon on one side and the other, and to obviate the difficulty arising from the efficiency of one or the other of the two pieces being affected by putting the tenon too high or too low, a com pound, called a tusk tenon, is used for almost all horizontal bearings of any importance, especially to joists and binders, to trimmers, beams, girders, bressummers, &c. The body of the tenon in this is a little above the middle of the end, and it runs out 2, 3, or 4 inches, or more, as the case may require. Below it the tusk protrudes, and above it the shoulder is cut down at an obtuse angle with the horizontal line, giving the strength of the whole depth of the timber above the under tusk to the tenon, and giving it a bearing in a shallow mortise, whilst a greater depth of the mortised piece than the tusk rests on receives the body of the tenon, and so protects its comparatively narrow margin from undue pressure. The diagram No. 1, fig. 5, shows the tusk tenon, with the section of a beam into which it is mortised, and No. 2 indicates perspectively the appearance of the mortise in front. Pinning is the inser- Pinning, tion of nearly cylindrical pieces of wood or iron through a tenon, to detain it in the mortise, or prevent it from being drawn out by any ordinary force. For this purpose the pin is inserted either in the body, or beyond the thickness, of the mortised piece, as indicated at o, fig. 5, or at, fig. G. Wedging (see 6, b, No. 2, fig. 25) is the insertion of Wedging, triangular prisms, whose converging sides are under an extremely acute angle, into or beside the end of a tenon, to make it fill the mortise so completely, or bind it so tightly, that it cannot be easily withdrawn. The wedging of ten ons also assists in restoring to the mortised piece of timber much of the strength it had lost by the excision of so much of its mass, which indeed the tenon itself does if it fit closely in every direction ; but the assistance of the wedge renders the restoration more perfect than the tenon could secure of itself, by compressing the fibres of both, longi tudinally to those of the one, and transversely to those of the other, thus removing the tendency of the mortised piece to yield in any degree in the weakened part, though it cannot make up the loss in its tenacity occasioned by the scission of its fibres. In scarfinir, cogging, and notching, the shoulders are IV 6r.