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.
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 described, but in this case the end of the top piece is cut off square to offer the greatest possible resistance to crushing, while the underneath piece is beveled on the end as there is no tendency for the timbers to crush.
Fig. 63. Tension Splice with Fish Plates.
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 prevent 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.
Fig. 64. Splice Designed for Bending Strains.
Fig. 65. Butt Joint with Plates.