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.
The joints which have so far been described are applicable only where the members are subjected to direct compression, as in the case of posts or braces, or in certain cases where direct tension is the only force acting on the pieces. When bending and shearing are to be expected, as in the case of floor beams connecting to sills or girders, a slightly different sort of joint must be employed.
One of the most common joints for such places is a modification of the mortise-and-tenon joint which is known as the "gained joint." An example of this form of connection is shown in Fig. 44, and it may be seen that the end of one piece is tenoned in a peculiar way. The tenon proper is the part A-B-C and this tenon sets into a corresponding mortise cut in the other piece as shown. It is evident that the tenon can not be held in place by a pin, but it may be secured by nailing.
The reason for this peculiar form of tenon may be explained as follows: A floor beam, or any other timber, which is loaded transversely, has a tendency to fall to the ground, and must be supported at its ends either by resting directly on a wall or sill, or by being mortised into the latter member. Moreover, in order that the end of the piece resting on the support, may not be crushed or broken, a certain amount of bearing surface must be available. This same bearing surface must be provided in every case no matter whether the timber rests directly on the top of the sill or is mortised into it. Of course the simplest connection is obtained by resting the transverse piece directly on top of the sill without cutting either piece; but such a joint is not stiff and strong, and it is often necessary to bring the timbers flush with each other at the top or at the bottom. For this reason a mortised joint is used; and in order to obtain the required amount of bearing surface without cutting the piece too much, the form of tenon shown in Fig. 44 is employed. The available bearing area here is furnished by the surfaces D-A and B-C and it may easily be seen that this area is the same as would be available if the piece rested directly on top of the sill.
Fig. 44. Gained Joint.
The operation of cutting such a tenon and mortise is known as "gaining," and one piece is said to be "gained" into the other.