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.
Tusk Tenons. In Fig. 13 (a) is shown the tenon b on the tail-beam y let into the header w at its center, and to secure additional strength a tusk c is cut on the tail-beam, to rest on the shoulder d of the header. The length of the tusk and the thickness of the tenon should each be about one-sixth the depth of the beam.
At (b) the tenon e extends entirely through the beam v, and is held securely in place by a wooden draw-bore pin, or wedge, as at g. This is the form of mortise used to secure the header to the trimmers, while the form shown at (a) is used to secure the tail-beams to the headers.
Where the joints are made with patent beam-hangers or stirrup irons, no mortising is necessary.
81. Stirrup Irons are pieces of wrought iron, generally 3/8 in. X 2 1/2 in., bent to form a hook over the trimmer-beam and a seat in which the header can rest, as shown at e, Fig. 32. The trimmer and header are usually nailed together to keep the joint close, while the stirrup iron carries the weight; for good work, however, a strap or bolt should be used to keep the trimmer and header in close contact.
Splice Joints. When timbers cannot be obtained of sufficient length to form an entire sill, plate, or other horizontal member, it is necessary to resort to some form of a splice joint, as shown in Figs. 14 and 15.
Fig. 14 shows a plain splice joint obtained by cutting away one-half of the thickness and width of each timber, e and d, and then fitting the remaining half of each piece into the space formerly occupied by the half cut away, as indicated by the dotted lines in Fig. 14, where the piece d is shown at right angles to the piece e, which would be its position in the corner of a building, as shown at m, Fig. 12. By turning the piece d so that its edges fg and hi would coincide with the edges of and n q of the piece e, the joint would splice the two pieces into a continuous timber, as shown at Fig. 15, although when such a condition exists, another form of the joint, known as beveled or oblique splicing, is considered preferable.
This form of splice is effected by cutting away a little more than half the thickness of the timber at abcd and lm, Fig. 15, and leaving a little more than half the thickness at the ends f g and jk. The formation of this joint requires great care, lest the timbers be weakened by cutting too deeply at a c and l m, but when properly made, this joint does not pull apart as readily as the previous one.
83. Dovetail-halving is another form of joint used where prevention from sliding is a desirable condition. Fig. 16 shows two forms of this joint formed as follows: The timber m is first halved on the end, in the same manner as the timber d in Fig. 14, and the remaining half thickness is then cut to the wedge shape a bj i, which is called a dovetail. In the timber n a dovetail mortise efhglo is then cut in, half the thickness of the timber m, and the two pieces are then driven together and held in place by spikes or screws.
The form shown at (a) is best adapted to positions where timber m is intended to be placed at right angles to the timber n, and although the form (b) is also used in similar locations, it is best adapted to locations where the timber m makes an acute or an obtuse angle with the timber n, as is shown in Fig. 12, where the timber n forms at the same time a tie and a brace to the two timbers h forming the plate.
84. A straddle, or bridge, joint, Fig. 17, is used when a secure footing is required for the toe of a rafter, or strut. The tie-beam a is notched on top, to receive the foot of the strut b; the side de of the notch is in the direction of and at the same angle as the slant of the strut, while the side d c is at right angles to d e, and equal in length to the depth of the strut at fg. The bridge tenon h i is left standing when the notch c d e is cut, and a mortise jkml is cut into the end of the strut to straddle it, and thus prevent it from slipping.
Another method of treating this joint is to work the tenon h i on the strut timber b, Fig. 18, and sink the mortise jk into the tie-beam a. The tie-beam a is then notched on the line c d, a distance equal to only half of the depth of the strut b, but is still kept at right angles to the direction of the slant of the strut, while the line de has a pitch varying in different cases, according to circumstances.
85. A method of footing the rafters on tie-beams without the mortise and tenon is shown in Fig. 19 (a), where the joint is similar to the one in Fig. 18 in every respect, except that the mortise and tenon are omitted.
Fig. 19 () shows a variation made use of when the notching of the tie eaual to one-third or one-half the depth of the strut would weaken the beam too much. Two notches are then made, each of which should be equal to from one-sixth to one-quarter the depth of the strut, so that the sum of the two bearing surfaces a b and dc, at (b), is equal to the single bearing surface ab, at (a).
86. The various joints thus far considered have been for the purpose of forming a suitable union between two pieces of timber which were in different positions and which served different purposes; and with the exception of the splice joints for sills and plates, shown in Fig. 15, none of these joints could be dispensed with by getting the work out of one piece of timber.
We will now consider such joints as are for the sole purpose of lengthening timbers, in such a manner as will give the most perfect union, with the least amount of loss in strength.