395. The joints in a framing of timber having to resist the strains to which the pieces are exposed, should be formed in such a manner that the bearing parts may have the greatest possible amount of effective surface. For should that part of the joint which receives the strain be narrow and thin, it will either indent itself into the pieces to which it is joined, or become crippled by the strain; producing in either case a change in the form of the framing.
The effect of the shrinkage and expansion of timber should also be considered in the construction of joints. On account of the shrinkage of timber, dovetail joints should seldom be used in carpentry, as the smallest degree of shrinking allows the joint to draw out of its place; they can only be used with success when the shrinkage of the parts counteract each other; a case which seldom happens in carpentry, though common in joinery and cabinet-making.
Joints should also be formed so that the contraction or expansion may not have a tendency to split any part of the framing. The force of contraction or expansion is capable of producing astonishing effects where the pieces are confined, which may sometimes be observed where framing has been wedged too tightly together in improper directions. The powerful effect of expanding timber is well known to quarry-men, as they sometimes employ its force to break up large stones.
396. In forming joints the object to be attained should always be kept in view, as that which is excellent for one purpose may be the worst possible for another. With this consideration the subject will be treated under separate headings, as follows: or by putting keys in the joint, as shown by the lower side of the same figure; but the strength of the beam will be decreased in proportion to the depth of the indents.