Gained Joint. The joints which have so far been described are applicable only where the members are subjected to direct compres sion, 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 modifi cation 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 corre sponding 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 trans versely, 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 neces sary 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 em ployed. The available bearing area here is furnished by the sur faces 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.
The operation of cutting such a tenon and mortise is known as "gaining," and one piece is said to be "gained" into the other.
Tenon-and-Tusk Joint. A joint in very common use in such situations as those which have just been mentioned is a develop ment of the gained joint which is called the "tenon-and-tusk" or the "tusk-tenon" joint. This joint is shown in Fig. 45. The char acteristic feature of this joint is to be found in the peculiar shape of the tenon which is cut in the end of one of the pieces to be joined, as shown in the figure. It may be seen that there is a small square tenon B cut in the treme end of the piece, and that in addition to this there are other cuts C which constitute the "tusk." The bearing area is furnished partly by the under side of the tenon and partly by the under side of the tusk.
This joint makes a very good connection, and the cutting of the mortise does not weaken the piece of timber so much as does the mortise for a gained joint. It is especially applicable when it is desired to have the two pieces flush on top, although it may also be used in other positions. When the top of the tenoned piece must project above the top of the mortised piece, the tenon may be cut as shown in Fig. 46.
There are several ways of securing the tenon in place. The simplest is that shown in Fig. 47, where the pin B is passed through the tenon A and the mortised piece so as to hold the tenon securely in place. Another scheme is to cut the square tenon a little longer, as shown in Fig. 48, so as to pass clear through the mortised piece, and to fasten it with a peg B on the other side. The peg may be cut slightly tapering, as shown, so that when it is driven in place it will draw the pieces together. Still another plan is shown in Fig. 49. Here a small hole is cut in the header some distance back from the tenon and a nut C is placed in it, while a bolt B is passed through a hole bored lengthwise in the header to receive it. The bolt passes through the nut, which may be screwed up tight, thus drawing the pieces closely together and making the joint secure. In tightening this up, it is the bolt which must be turned, while the nut is held stationary inside of the square hole in which it is inserted and which is just large enough to receive the nut and a wrench.