This form was devised in order to give the tenon as deep a bearing as possible at the root, without greatly increasing the size of the mortise, and thus weakening the girder.

This object is effected by adding below the tenon (T) the Tush (t) having a Shoulder (s) which penetrates the girder to a depth equal to 1/6 of the depth of the joist; above the tenon is formed the Horn (h) the lower end of which projects to the same extent as the tusk. It will be seen that the strength of the tenon, between h and t is immensely increased as compared with the common form, while the mortise is not made much larger.

The depth or thickness of the tenon is generally about 1/6 of the depth of the beam.

Fig. 175. Tusk Tenon through Narrow Girder.

Fig. 175. Tusk Tenon through Narrow Girder.

1 The neutral axis is coincident with the central line only so long as the limit of elasticity has not been exceeded - that is, so long as the timber can recover its former position when the stress is taken off. Beyond this limit its position changes, as explained in Part IV. 2 Sometimes called shouldered tenon.

It may be carried right through a narrow girder and pinned outside, as shown in Fig. 175.1

In thicker girders it may penetrate a distance equal to twice its own depth, and is pinned through the top of the girder, as in Fig. 176.

Sometimes tenons are formed with a double tusk, but that form is not to be recommended (see p. 57).

The mortise should, for the reasons stated above, be in the neutral axis or central line of the girder, as shown in Fig. 175; practically, however, it is generally placed with its lower edge on the centre line, as in Fig. 176, by which arrangement the tenon is in the compressed portion, and the tusk in the extended portion of the girder.

Tredgold recommends that the tenon should be 1/3 of the depth of the joist above its lower edge. This recommendation cannot always be followed without placing the mortise out of its proper position in the neutral axis, and thus weakening the girder.

For example, when the girder and joist are of equal depth, as in Fig. 175. the tenon must be kept half-way up the joist, as shown, or the mortise would be below the neutral axis - would cut the extended fibres of the girder, and weaken it.

Again, in some cases the relative position of the girder and beam is determined by the space required by other parts of the framing - for instance, in a framed floor (see Fig. 280) more room must be left above for the bridging joists than below for the ceiling joists. This necessitates the tenon being higher, to bring it into the neutral axis of the girder.

In every case it should be considered whether the girder or the joists can best afford to be weakened; if the former has an excess of strength, the tenon may be kept low, so as to strengthen the joist; but if the joist has more strength to spare than the girder, the mortise should be in the neutral axis of the latter, even though the tenon may be high up on the joist.

In practice it more frequently happens that the joists, rather than the girders, have an excess of strength; so it is usual with carpenters to place the mortises with their lower edges on the neutral axis, and to let the position of the tenons on the joists be arranged to suit them.

Double Tenons are often used in joinery (see Fig. 492), but should be avoided in carpentry, as they weaken the timber into which they are framed, and both tenons seldom bear equally, so that a greater strain is thrown upon one of them than it is intended to support.

Fig. 176. Tusk Tenon Joint with Thick Girder.

Fig. 176. Tusk Tenon Joint with Thick Girder.

1 The hole in the tenon is made as shown slightly larger (in the direction of the length of the tenon) than the wedge, so that the latter when driven in may draw the beams tightly together.

Stub Tenon (or joggle) is a very short tenon, used where it is only required to prevent lateral motion - for example, to keep a post in its place upon a sill.

Housing is a term used when the whole of the end of one piece of timber is let for a short distance - or "housed" - into another, thus the end of the rail is housed into the post in Fig. 176a. The " housing " is shown in dotted lines (see Joinery, p. 142).

Dovetail Tenons are those in which one side of the tenon is splayed so as to form half a dovetail, the other side straight. The mortise is also splayed on one side, and is made rather wider than the tenon, which is placed in position, pressed well up against the dovetailed side of the mortise, and then secured by a wedge driven into the interval left on the straight side.

Notched Tenons have one side notched and the other straight; one side of the mortise is also notched to correspond, and the tenon secured by a wedge on the other side.