This section is from the book "A Treatise On Architecture And Building Construction Vol2: Masonry. Carpentry. Joinery", by The Colliery Engineer Co. Also available from Amazon: A Treatise On Architecture And Building Construction.
75. The simplest joint between two pieces of wood is the square butt joint shown at (a), Fig. 10, where the timber b effects a butt joint with the timber a on the line kcd. This joint, called a barefoot joint, is used in balloon-frame construction where the studs rest upon the sills or partition caps, and are secured in position by toenailing, as shown at e. An oblique butt joint is shown at (b), Fig. 10, where Imn and opq are the lines of the joints between the timbers f and j and the timbers g and j. This form of the joint is sometimes used for placing rafters upon the plate, or against the ridge board, and must always be securely toenailed, or a movement in the position of the foot Im or pq would throw the joint out of alinement.
Mortise And Tenon. Where greater strength and security than can be obtained from the butt joint is required, we make use of the mortise-and-tenon joint, shown in Fig. 11. A rectangular hole abcd, called a mortise, is cut into one of the timbers, as w, and on the end of the other timber y is shown the projecting pin or tenon efg, which fits into and exactly fills the mortise.
The proportion of the width da of the mortise to the thickness of timber in which it is cut is one-third, and the mortise is cut into the center of the thickness. Or, if one timber is larger than the other, they are usually so framed that, when brought together, one face of each piece will be flush.
77. These mortise joints are nearly always secured in place by means of a draw-bore pin, which is inserted as follows: After the mortise is cut, and the tenon is accurately fitted to it, a hole is bored in the timber, squarely through both cheeks of the mortise, as shown at x, in Fig. 11. The position of the hole x is accurately marked on the tenon e f g, and a hole is then bored through the tenon but located from 1/16 inch to 1/8 inch nearer the shoulder than the marks made through the hole in the mortise. When the tenon is now inserted in the mortise, the relative position of the holes will be somewhat as shown at (a), in Fig. 12, which is an enlarged section through the girt and corner post on the line xy. A wooden pin is then driven through these holes, and by forcing them into line it brings the shoulders of the tenon p tight up against the cheeks of the mortise q, thus making the joint firm and secure, as well as free from any liability to work itself loose. This wooden pin, usually called a treenail or draw-bore pin, should be cut from a piece of straightgrained, tough, and durable wood, preferably locust or oak, about 1 inch to 1 1/2 inches square on the ends, and about 2 inches more in length than the mortised timber is in thickness.
The corners are planed off, bringing it down to an octagonal shape on the ends, and its sides are then slightly tapered about one-fourth the length, so that the pin will enter the draw bore. If, through carelessness, or" error in measurement, the hole in the tenon is not slightly nearer the shoulder than the hole in the mortise, the joint will not be tight, and may result in what is called a. push bore, which is a term given to this joint when the driving of the pin loosens the pieces instead of tightening them.
78. In braced-frame construction, the mortise and tenon form the joint of nearly every two important timbers that come in contact. In Fig. 12 is shown the corner of a braced-frame building, where b is the corner post mortised into the sill a at the bottom, and into the plate h at the top; the girts, or interties d, on one of which the second tier of beams i rests, are mortised into the corner post, and secured with a draw-bore pin as above described, while the angle braces e are mortised into both the girt and the corner post. The tenons on these beams are cut somewhat differently from those on the other members, owing to the diagonal position of the timbers. At (b) is shown an inside view of the corner post and the joints between it and the lower braces. The top of the brace tenon o is cut so as to enter the corner-post mortise p at right angles to the face of the post; while the bottom of the tenon is in the direction of the plane of the under side of the brace, and the lower side of the mortise is cut on the same plane. The draw-bore hole in the tenon is shown at q, but does not show on the inside of the corner-post, as it is unnecessary to bore it all the way through. This constitutes the system of braced-frame construction, and though the studs are sometimes mortised into both sill and intertie, or into intertie and plate, they are thus joined simply to hold them in their places and not to add strength to the general frame. Double studs adjacent to openings, as shown at k, should always be mortised, but in modern work the small studs are simply spaced 16 inches on centers, and butt jointed to form a filling between the posts. In braced-frame construction, where the building is over 30 feet long, it is usually necessary to provide intermediate posts in the side walls connecting the ends of the building in order to have a mortise joint at the end of each horizontal timber.
79. Mortise-and-tenon joints are used almost exclusively in modern work, for the framing of floorbeams around stair wells, chimney breasts, and other openings, as shown in Fig. 32, where d, d are two heavy floorbeams, called trimmers, spaced each on one end of the chimney breast c, and connected at a by a short piece of joist called a header. The beams b are called tail-beams, and each one is mortised into the header as shown by the ends of the tenons at f.