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.
93. Fig. 22 shows the simplest form of a scarf joint. The timber a is cut half through its depth, on the line c d, which is perpendicular to the adjoining face of the timber, as is also the line cg. The timber b is similarly cut half its depth on the line cf. The material between these cuts and the end of the timber is then removed to the line de, and the two pieces a and b are then joined in the same manner as the splice joints in Figs. 14 and 15, and are securely bolted. Thus far the joint presents the same mechanical features under tension as would the joint shown in Fig. 21; but when subjected to compression the timbers a and b have a flat bearing on the surfaces in contact at gc d and at ef, which, combined, are equal to the entire bearing surface of the end of the timber, each surface being the full breadth by half the depth. The weakness of the joint under compression arises, therefore, from its tendency to bend and fail, either through the parting of the bolts, or from the breaking of one of the laps near the lines e i or dh. This weakness can be compensated, however, by bolting to the top and bottom of the timber an iron plate or bar Jk, whose length is equal to twice the length of the joint de, with a breadth equal to one-third of the breadth of the beam, and a thickness proportioned according to the strain it is likely to be subjected to. These iron bars are usually turned over on the ends, as shown at /, and let into the top of the timber. The two bolts n merely hold the ends in place, while the bolts m pass through both the bars and the laps of the joint.
The iron bar prevents the timber from bending in the direction of its depth, and the bolts m and the bearing surfaces gc d and ef prevent it from bending in the direction of its breadth under compression, while the bolts m, combined with the bars jk, and the turned-in ends, or lugs /, prevent the joint from parting under tension. This joint is not adaptable to a tensile strain, however, as its strength would depend entirely on the shearing value of the bolts m and /, and the tension of the bar j k. Nor is it fit for a transverse strain, which might bend the iron bar and tear out the bolts.
94. If the timber shrinks after the scarfing is complete and the beam is in place, this joint will work loose and cause trouble unless it is carefully watched and the bolts are tightened up from time to time, until the wood is thoroughly dried out and the shrinkage ceases.
The proper length of the joint on the line de varies with the material and the circumstances, as will be explained further on.
95. In Fig. 23 is shown another form of this joint, where the combined bearing surfaces of the two pieces a and b is still equal to the sectional area of the timber, and the tendency to bend in the direction of the depth is lessened by the form of the scarfed ends of the joint without the use of an iron plate or bar; the joint,however, is not adapted to a transverse strain, as the parts are cut away too much. The timbers a and b are first sawed half through their depth and prepared in the same manner as for the joint in Fig. 22, but from the top of the lap, and at a distance from the end of the timber equal to half the thickness of the lap, the timber is again sawed through, until the line c k, on timber b, is equal to one-quarter the depth of the entire timber, or one-half the depth of the lap. The material between c k and the end of the lap is then removed, and on the shoulders of timbers a and b, half way between the surface and the projecting lap, the timber is sawed through in the direction of its length a distance kd, on timber a, equal to one-half the thickness of the lap. The material between k e and. the face of the shoulder is then removed with a chisel, care being taken to keep the surfaces parallel and true throughout the thickness of the timber. The pieces a and b are then fitted nicely together in the position shown in Fig. 23, and at the middle of the line of the joint cj is marked the position of the rectangular hole lop, the depth I o being equal to about one-sixth the depth of the timber, and the length of the hole of being twice the depth lo. The timbers are then separated and half the hole lop is cut in each piece separately, and the distance from o to c, on timber b, is made 1/16 to 1/8 of an inch greater than the distance Ie on timber a. When the pieces are finally put together, a pair of wedge-shaped keys r s are driven into the hole, thereby forcing all the bearing surfaces into close contact. Holes are then bored through the depth of the timbers and four bolts m are inserted and screwed up tight against the circular washers n.
96. The advantage of this joint over that shown in Fig. 22, is that it is not so seriously affected by shrinkage, and at the same time preserves the most favorable feature of a beam under compression - that of a full bearing surface, equal to the cross-section of the beam. Should the two pieces composing the joint shown in Fig. 23 shrink to any considerable extent, the only result would be the loosening of the bolts m, unless the keys r and s were made of some harder wood than the beams themselves, and, therefore, did not shrink as much in proportion. This would cause the joint to open slightly at the middle part of the line ej, but it could be prevented by making the keys about 1/16 inch less in thickness than the hole at lo. Shrinkage would then not affect the appearance of the joint at all, and the bolts could be tightened from time to time, as in the previous case; the joints being thus maintained close and secure at all times. This joint is better adapted to resist compression than tension, as under tension the entire strain would come upon the bolts m and the keys r, s. Great care should therefore be exercised when this joint is to be submitted to tensile strains, to see that the bolts fit the holes tightly, and that the keys are well driven, otherwise, under alternate compression and tension, the joint is liable to work loose. The projecting ends of the pieces at de and ji serve only to keep the top of the joint flush.