Double Tenon Joint. Fig.
50 shows a form of tenon joint called the "double tenon" joint, which is not very extensively used at the present time but which has some advantages. As may be readily seen, there are two small tenons A and B through which a pin may be passed if desired.
Halved Joint. A form of joint which may be used to connect two pieces which meet at a corner of a building, is shown in Fig. 51.
This is known as the "halved" joint from the fact that both pieces are cut half way through and then placed together. The pieces are held in place by nails or spikes.
If one piece meets the other near the center instead of at the end of the piece, and if there is danger that the two pieces may pull away from each other, a form of joint called the ."dovetail" halved joint is used. This is shown in Fig. 52. Both the tenon and the mortise are cut in the shape of a fan, or dovetail, which prevents the two pieces from being pulled apart. This joint may also be cut as shown in Fig. 53, with the flare on only one side of the tenon, the other side being straight.
Splices. As already explained, a splice is merely a joint between two pieces of timber which extend in the same direction, and is some times necessary because one long piece can not be conveniently or cheaply obtained. The only object in view, then, is to fasten the two pieces of timber together in such a way that the finished piece will be in all respects equivalent to a single unbroken piece, and will satisfy all of the requirements of the unbroken piece. This is really the only measure of the efficiency of a splice.
There are three kinds of forces to which a piece may be sub jected, namely: Compression, tension, and bending. A splice which would be very effective in a timber acted upon by one of these forces might be absolutely worth less in a piece which must resist one of the other forces. We have, therefore, three classes of splices, each designed to resist one of these three forces.
Splices for Compression. The simplest splices are those in tended to resist compression alone, and of these the most simple is that shown in Fig. 54.
This piece is said to be "fished" ; the two parts are merely sawed off square and the ends placed together. A couple of short pieces A-A, called "fish plates," are nailed on opposite sides to keep the parts in line. In the splice shown in Fig. 54, the splicing pieces are of wood, and ordinary nails are used to fasten them in place, but in more important work thin iron plates arc used, the thickness being varied to suit the conditions. They are held in place by means of bolts with washers and nuts.
If for any reason it is desired not to use plates of this kind, four small pieces called dowels may be used, as indicated in Fig. 55. These dowels may be set into the sides of the timbers to be spliced, so that they do not project at all beyond the faces of these pieces and a very neat job may thus be obtained.
It is but a step to pass from this simple splice to the "halved" splice shown in Fig. 56. It will be noticed that it is much like the halved joint described above, the only difference being that the pieces are continuous, instead of being perpendicular to each other. The nature of the splice will be easily understood from the figure without further explanation. A modification of this which is somewhat more effec tive, is shown in Fig. 57. The cuts are here made on a bevel in such a way that the parts fit accurately when placed together, and the splice is called a "beveled" splice.
The halved splice is perhaps the best that can be used to resist direct compression, and when it is combined with fish plates and bolts, as shown in Fig. 58, it may be used in cases where some tension is to be expected. It will be noticed that in Fig. 58 the ends of the timbers are cut with a small additional tongue A, but this does not materially strengthen the splice and it adds considerably to the labor of forming it. In general it may be said that the simplest splice is the most effective.