"Cogging," "Corking," or "Caulking." - In this joint (see (Fig. 171), the notch on the lower beam is only partly cut out, leaving a piece or "cog" (like that of a cogged wheel) uncut. The upper beam contains a small notch only wide enough to receive the cog.
"Cogging " has the following advantages over notching.
Fig. 171. Cogged Joint.
The cog gives the upper beam a hold on the lower, even when its end does not project beyond the latter.
Joists or binders may thus be cogged on to wall plates: if they project beyond the wall plate, as dotted in Fig. 172, the cog may be made broader, but if not, the cog must be narrow and kept toward the inside, so that there may be sufficient substance of timber (x y) on the joist beyond it to resist the strain.
The above arrangement takes a considerable piece out of the lower beam. When this is supported throughout, as in a wall plate, it is of no consequence, but, if it spans an opening, it is desirable to weaken it as little as possible.
Fig. 172. Joist cogged on, to Wall Plate.
In such a case, for instance, as cogging joists on to binders (see Fig. 281), or purlins on to principal rafters, the notches in the bearer are made very small, only about an inch or so in depth, and extending inwards about the same distance from the sides of the beam. Mortise and Tenon Joints. Common Tenon.- The simplest form of this joint is when a vertical timber A meets a horizontal beam B at right angles.
In Figs. 173, 174, the Tenon (T) is formed by dividing the end of A into three,1 and cutting out rectangular pieces on both sides each equal to the part left in the middle.
The Mortise is a rectangular hole cut to receive the tenon. The sides (C C, Fig. 174) of the mortise are called the Cheeks; the surfaces (C C, Fig. 173) on which the shoulders of the tenon rest are sometimes called the abutment cheeks.
The springing of the tenon from the beam is called its "root" (r, Fig. 174); SS are the shoulders, and p (Fig. 173) the pinhole, which is generally placed at 1/3 the length of the tenon from the shoulder, and is in diameter equal to 1/4 the thickness of the tenon.
If the tenon reached exactly to the bottom of the mortise, it would take its share of the pressure on the post, but it is difficult to make it do so with accuracy, especially as the mortise cut across the grain shrinks more in depth than does the tenon cut along the grain; practice it is therefore generally made a little shorter than the depth of the mortise, so that the shoulders may bear firmly upon the sill, which is more important.
When a horizontal beam is framed into another, and they are subject to a downward stress, as in the case of joists framed into a girder, the position and form of the mortise and tenon will be determined by other considerations.
It has already been stated (page 62), that when a beam is subjected to a transverse stress, the fibres of the upper portion are compressed, and those of the lower portion extended. In the central line dividing these portions from one another there is neither compression nor extension, and it is therefore called the "neutral axis." l
Fig. 173. Mortise and Tenon.
Fig. 174. Mortise ant Tenon Section.
1 The tenon is not necessarily 1/3 the width of the timber, but may be made in any proportion so long as it is thick enough to withstand the stress upon it.
The mortise should be placed in the neutral axis of the girder, where the cutting of the fibres will weaken the girder the least, and where the mortise itself, and the tenon within it, will be free from tension or compression.
With regard to the position of the tenon on the joist, the lower down it is the less likely is it to be broken, because the mutual pressures of the butting surfaces above it protect it from cross strain, and also because there is a greater thickness of timber above it to be bent, or torn off, under a breaking-weight.
The tenon must not, however, be so low down that there is not sufficient thickness of wood left below the mortise to support it. It is evidently desirable for the strength of the tenon that it should be as large as possible, but in the ordinary form, above described, this would necessitate a large mortise, and very much weaken the girder. That form, therefore, is not adapted for joints intended to bear a downward strain, for which the "tush tenon," about to be described, should always be used.