Whenever the pieces are cut to fit into one another, as they do in the halved and beveled splices, the splice is known as a "scarf" splice, and the operation of cutting and joining the parts is called "scarfing." Scarf splices are used, as we have already seen, both alone and in combination with fish plates. The fished splice is always the stronger, but the splice where scarfing alone is resorted to has the neatest appearance.
Splices for Tension. 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 direc tion 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 suit able 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 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.
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.
Tension Splice with Fish Plates. The splices for tension which have so far been described have all been scarf joints, but there is a fished splice which is very commonly used for tension. This splice is shown in Fig. 63. The fish plates, in this case of wood, are cut into the two pieces to be spliced, so as to hold them firmly together. The pieces can not be pulled apart until one of the plates shears off along the dotted line A-B. The dis tance C-D must also be made large enough so that the piece will not shear. This splice is very often used for the lower chords of the various forms of wood trusses, and it is considered one of the best that has been devised for resisting direct tension.
Splices for Bending. It sometimes happens that a piece which is subjected to a bending stress must be spliced, and in this case the splice must be formed to suit the existing conditions. It is well known that in a timber which is resisting a bending stress the upper part of the piece is in compression, and the tendency is for the fibers to crush, while the lower part of the piece is in tension, and the tendency is for the fibers to pull apart.
To provide for this, a form of splice must be selected which combines the features of the tension and compression splices.
Fig. 64 shows such a splice. The parts are scarfed together, as is the case with other splices de scribed, but in this case the end of the top piece is cut off square to offer the greatest possible resistance to crushing, while the under neath piece is beveled on the end as there is no tendency for the timbers to crush.
We have already seen that in the lower part of the splice, there is a tendency for the parts to be pulled away from each other. In order to prevent this, a fish plate, A, is used, which must be heavy enough to take care of all the tension, since it is evident that the wood can not take any of this. The plate must be securely bolted to both parts of the splice. There is no need of a fish plate on the top of the pieces because there is no tendency for the pieces to pull apart on top, and the bolts shown in the figure are sufficient to pre vent them from being displaced.
In any case where it is not desirable to scarf the pieces in a splice. subjected to bending, the form of butt joint shown in Fig. 65 may be used. The plates, either of wood or iron, are in this case bolted to the sides of the pieces. If wood is used, of course the plates must be made very much heavier than if iron is used. In either case they must be large enough to take care of all the bending stress, and a sufficient number of bolts must be used to fasten them securely to both parts of the splice.