This section is from the book "Cyclopedia Of Architecture, Carpentry, And Building", by James C. et al. Also available from Amazon: Cyclopedia Of Architecture, Carpentry And Building.
There are several common forms of splices for resisting direct tension. These differ from each other mainly in the amount of labor involved in making them. The simplest of them is shown in Fig. 59, and it will be seen that it is only a slight modification of the halved splice used for resisting compression. It is evident that the pieces can not pull apart in the direction of their length until the timber crushes along the face marked A-B, or shears along the dotted line A-C. By varying the dimensions of the splice it may be made suitable for any situation. The parts are held closely together by the light fish plate shown in the figure, which also incidentally adds something to the strength of the splice.
Instead of cutting the ends of the beams square, as shown in Fig. 59, they frequently are cut on a bevel as shown in Fig. 60, and a further modification may be introduced by inserting a small "key" of hard wood between for the pieces to pull against, Fig. 61. This key is usually made of oak and may be in two parts, as shown in Fig. 62, each part in the shape of a wedge, so that when they are driven into place a tight joint may be obtained. The two wedge-shaped pieces may be driven in from opposite sides, the hole being a little smaller than the key. If the key is made much too large for the hole, however, a so-called "initial" stress is brought into the timbers, which uses up some of their strength even before any load is applied. This should be avoided.
Fig. 59. Squared Splice for Tension.
Fig. 60. Beveled Splice for Tension.
Fig. 61. Beveled and Keyed Splice for Tension.
If it is desired, two or more keys may be employed in a splice, the only limiting condition being that they must be placed far enough apart so the wood will not shear out along the dotted line shown in Fig. 61. Another feature of the splice here shown is the way in which the pieces are cut with two bevels on the end instead of one. One bevel starts at the edge of the key and is very gradual, the other starts at the extreme end of the piece and is rather steep and sharp. These bevels can be used only in joints which resist tension alone. If such a splice were subjected to compression, the beveled ends would slide on each other and push by each other very easily, except as they are prevented from so doing by the fish plates, if these are used.