This section is from the book "Modern Buildings, Their Planning, Construction And Equipment Vol1", by G. A. T. Middleton. Also available from Amazon: Modern Buildings.
The simplest way of supporting a beam upon a wall-plate is by allowing the former to rest upon the upper surface of the latter, as at A, Fig. 254. Any tendency to dis-lodgment is prevented by driving spikes of iron through the sides of the joists, thus securing them to the plate.
A rather more secure joint is formed by notching the end of this joist, i.e. by taking out a rectangular portion from the under side of the joist so that it fits over the plate, as shown at B. When a joist is carried over a plate or a cross beam a notched joint is formed, as shown at C. Another form of notched joint, sometimes called & double-notched joint, is shown at D and E, where a portion is notched out of the plate as well as the joists. In floor construction it frequently becomes necessary to make the upper surfaces of the joists and cross beams which support them on the same level. This is often done by means of a double-notched joint, having half the depth of the joist notched out of the joist and an equal amount notched out of the cross beam, as at E. Such a process is called " Halving down."
Housed Joint - Another simple joint for the end of a beam is shown at F and G, the end of the joist being left uncut and simply fitting into a notch in the member upon which it abuts.
The most satisfactory joint between a beam and plate or a joist and cross beam is that known as the Cogged Joint, shown at H and K.
The method of forming this joint when a joist passes over a cross beam is shown at K, a piece the width of the joist being notched out of each edge of the cross beam, leaving a solid portion in the centre of the upper surface over which the joist is notched. When the end of a joist is to have a notched bearing it is more economical in labour to form the cog at the outside edge of the plate at H. When the plans of rooms are not rectangular, skew-cogged joints, such as are shown at L become necessary. The advantage of this joint is that it makes a wooden framework very stiff by anchoring the walls and cross beams together.
When a beam has to be attached to the side of a cross beam some form of tenoned joint is employed. At A, Fig. 255, is shown a ceiling joist with a tenon cut upon it, and a corresponding mortise cut in the cross beam which is to support it. Projecting portions P are usually formed on the ends of ceiling joists to go under the cross beams, to the under side of which they may be spiked for additional bearing strength, or to afford a suitable attachment for the laths of a lath and plaster ceiling.
It sometimes happens that the cross beams or binders of a floor have to be fixed before the joists, or that a decayed joist has to be renewed, and it is obviously impossible to do this when the mortices are made as shown at A.
This difficulty is overcome by cutting a slight bevel upon the upper surface or cheek of the mortise, as shown at B, Fig. 255, which enables the tenon at one end to be inserted when the joist is in an inclined position. This done, the tenon at the other end can be dropped into a Chase mortise, which is cut as shown at B, Fig. 255. The same result may be attained by means of a horizontal chase mortise, and the joist is inserted as shown at C. It should be noted that the portion projecting beneath the cross beam cannot be used in the case of B, its place being taken by a fillet nailed to the under side of the binder.
A very useful bearing joint for framing timbers together is shown at A, Fig. 256. The exact proportions of the various parts of this joint might be theoretically determined, but the relative values determined experimentally for the safe resistances of timber to tension, compression, and shear vary so considerably that the dictates of experience are usually followed. The joint, with its parts proportioned as at A, Fig. 256, is the one most used in practice. The tenon is carried right through the mortised member, and should be made to project at least two-thirds of the depth of the beam, so that a portion of it will not shear out when the wedge is driven home.
It is often undesirable that the tenon of a tusk-tenon joint be carried right through the member into which it is framed, in which case short tenons are used, which are kept in position by means of iron spikes or wooden pegs, as at B. Sometimes the tenon is dispensed with altogether, and its place taken by a hand-rail screw, as at C.
Another joint used for the same purpose as the tusk-tenon joint is the dovetailed housed joint, which is formed as shown in Fig. 257. This joint slightly weakens the notched beam, but it has the advantage of enabling the dovetailed ends of joists to be slipped into position after the beam has been fixed.
The following three forms of joints are used both at the foot and head of posts: -
The simplest form of beaming joint for a beam resting upon the head of a post or for a post resting upon a beam is that known as a Stub-Tenoned Joint, as at A, Fig. 258. The width of the tenon is made about one-third the width of the post. It projects a distance equal to its width, and springs from one plane called the Shoulder. A mortise is cut in the beam slightly deeper than the length of the tenon, so that all the weight is brought upon the shoulder and not upon the tenon.
A very excellent form of joint, much used for heavy timbers, especially in Half-Timbered Construction, is shown at B, Fig. 258, where a tenon is cut at the end of the post inserted in a mortise of corresponding dimensions, and secured by means of oak pins.
A Bridled Joint is formed by cutting away a portion from the centre of the end of the post, and this fits over a corresponding cog formed on one face of the beam, as at C. Sometimes the end of the post is formed as shown at D. This form of joint is preferred by many, as bad workmanship can be detected, while in a mortised and tenoned joint bad workmanship on the tenon is concealed once the joint has been fixed in position.
When the beam is supported on the side of a post, as is the rail of a fence and the straining beam of a queen post roof truss, one of the following joints is used: -
A strong though somewhat clumsy joint is formed by bolting or railing a block of wood - called a Cleat - to the side of the post, and resting the beam on top of it, as at A, B, and C, Fig. 259. Lateral motion in the beam is prevented by the use of dogs, straps, bolts, or stub tenons.
Fig. 259. Cleated Joints.
When a beam has no very great load to carry, but is liable to become dislodged, a tenon and mortise is often used, two opposite cheeks of the mortise being slightly splayed to permit wedges to be driven in on either side of the tenon, thus making the latter secure, as shown at A, Fig. 260, the whole section of the rail is housed into the post to give additional bearing power to the joint.
Another tenoned joint which offers great resistance to drawing is the dove-tailed-tenoned joint, shown at B, Fig. 260, in which
Fig. 260. Dovetail Tenon.
A form of tusk-tenon joint is a suitable means of attachment of a beam to the side of a post where fair bearing power is required. Instead of cutting the horn, tenon, and tusk out of the whole section of the beam, they are cut out of a tenon about one-third the width of the beam, as in Fig. 261. The joint is prevented from drawing by means of an oak pin passing through the cheeks of the mortise and the tenon.
The method of inserting oak pins is as follows: A hole is bored through the tenon at a convenient distance from the shoulder, and at the same distance from the cut face of the post a hole is bored through the cheeks of the mortise. The tenon is now inserted in the mortise, and a Draw pin - a tapered peg of iron - is driven in the bored holes. This draws the shoulder tightly against the face of the post. The draw pin is now removed and replaced by a hard-wood peg.
Fig. 261. Task Tenon.
A joint very commonly used for supporting the rails of fences is formed by cutting a mortise of the shape of the rails right through the post, as shown in Fig. 262. The ends of the rails are splayed and inserted in these mortises, so that they overlap, and are then secured by means of nails or pegs driven through the cheeks of the mortise. This joint gives a maximum bearing capacity to the rails, and enables all the posts of a fence to be set up before the nails are inserted.
Fig. 262. Housed.
Birds-mouthed Joint. - When an inclined timber bears upon a wall-plate the end of the former has an angular portion - called a Bird's-mouth - cut out of it so that it fits over the latter, as at A, Fig. 263. There is danger when this joint is used of the bird's-mouth being cut so deep that the strength of the timber is impaired. Serious accidents have happened, such as the collapse of a grand stand on a football ground, from too deeply cut bird's-mouths. A good way of supporting an inclined timber is by bolting or strapping a cleat on to its under side, as at B.
Fig. 263. Cleated.