This section is from the book "Spons' Mechanics' Own Book: A Manual For Handicraftsmen And Amateurs", by Edward Spon. Also available from Amazon: Spons' Mechanics' Own Book.
One of the first requirements in the use of timber for constructive purposes is the connection of 2 or more beams to obtain a greater length. Fig. 453 shows the method of lengthening a beam by lapping another to it, the 2 being held together by straps and prevented from sliding by the insertion of keys. Fig. 454 shows a similar joint, through-bolts being used instead of straps, and wrought-iron plates instead of oak keys. This makes a neater joint than the former, but they are both unsightly, and whenever adopted the beams should be arranged in 3 or 5 pieces, in order that the supports at each end may be level, and the beams horizontal. This joint is more suitable for a cross strain than for tension and compression. Fig. 455 shows the common form of a fished beam adapted for compression. If required to resist tensile strain, keys should he inserted in the top and bottom joints between the bolts. Fig. 456 shows a fished joint adapted for a cross strain, the whole sectional area of the original beam taking the compressive portion of the cross strain, and the fishing-piece taking the tensile portion. Fig. 457 shows a fished beam for the same purpose, in which a wrought-iron plate turned up at the ends takes the tensile strain.
Tabling consists of bedding portions of one beam into the other longitudinally. Occasionally the fishing-pieces are tabled at the ends into the beams to resist the tendency to slip under strain, but this office is better performed by keys, and in practice tabling is not much used. The distinction between fished beams and scarfed beams is that in the former the original length is not reduced, the pieces being butted against each other, while in the latter the beams themselves are cut in a special manner and lapped partly over each other; in both cases, additional pieces of wood or iron are attached to strengthen the joint. Fig. 458 shows a form of scarf adapted to short posts. Here the scarf is cut square and parallel to the sides, so that the full sectional area is utilized for resisting the compressive strain. When the post is longer and liable to a bending strain, the scarf should be inclined, as in Fig. 459, to allow of greater thickness being retained at the shoulder of each piece, the shoulder being kept square. In this joint a considerable strain may be thrown on the bolts from the sliding tendency of the scarf, if the shoulders should happen to be badly fitted, as any slipping would virtually increase the thickness of the timber where the bolts pass through.
The width of each shoulder should be not less than 1/4 the total thickness. Joints in posts are mostly required when it is desired to lengthen piles already driven, to support a superstructure in the manner of columns. Another form of scarf for a post put together without bolts is shown in Fig. 4G0, the parts being tabled and tongued, and held together by wedges. This is not a satisfactory joint, and is, moreover, expensive, because of its requiring extra care in fitting; but it may be a suitable joint in some special cases, in which all the sides are required to be flush. Fig. 461 shows the common form of scarf in a tie-beam. The ends of the scarf are birds'-mouthed, and the joint is tightened up by wedges driven from opposite sides. It is further secured by the wrought-iron plates on the top and bottom, which are attached to the timber by bolts and nuts. In all these joints the friction between the surfaces, due to the bolts being tightly screwed up, plays an important part in the strength of the joint; and as all timber is liable to shrink, it is necessary to examine the bolts occasionally, and to keep them well tightened up. Figs. 4G2 and 463 show good forms of scarfs, which are stronger but not so common as the preceding.
Sometimes the scarf is made vertically instead of horizontally; when this is done, a slight modification is made in the position of the projecting tongue, as will be seen from Fig. 464, which shows the joint in elevation and plan. The only other scarfs to which attention need be called are those shown in Figs. 465 and 466, in which the compression side is made with a square abutment. These are very strong forms, and at the tame time easily made. Many other forms have been designed, and old books on carpentry teem with scarfs of every conceivable pattern; but in this, as in many other cases, the simplest thing is the best, as the whole value depends upon the accuracy of the workmanship, and this is rendered excessively difficult with a multiplicity of parts or abutments.