wedges are inserted into square holes, and the pins into augerholes made through the parts connected. As the object of these accessories is simply to bring the parts connected into close contact, they should be carefully driven, in order not to cause a strain that might crush the fibres.

To secure joints subjected to a heavy strain, bolts, straps, and hoops of wrought iron are used. These should be placed in the best direction to counteract the strain and prevent the parts from separating; and wherever the bolts are requisite they should be inserted at those points which will least weaken the joint.

543. Joints of Beams united end to end. When the axes of the beams are in the same right line, the form of the joint will depend upon the direction of the strain. If the strain is one of compression, the ends of the beams may be united by a square joint perpendicular to their axes, the joint being secured (Fig. 91) by four short pieces so placed as to embrace

a

Fig. 91-Represents the manner in which the end joint of two beams a and b is fished or secured by side pieces c and d bolted to them.

the ends of the beams, and being fastened to the beams and to each other by bolts. This arrangement, termed fishing a beam, is used only for rough work. It may also be used when the strain is one of extension; in which case the short pieces (Fig. 92) may be notched upon the beams, or else keys

Fig. 92-Represents a fished joint in which the side pieces c and d are either let into the beams or secured by keys e, e.

of hard wood, inserted into shallow notches made in the beams and short pieces, may be employed to give additional security to the joint.

A joint termed a scarf may be used for either of the foregoing purposes. This joint may be formed either by halving

Fig. 93-Represents a scarf joint secured by iron plates c, c, keys, d, d, and bolts.

the beams on each other near their ends (Fig. 93), and securing the joints by bolts, or straps; or else by so arranging the ends of the two beams that each shall fit into shallow triangular notches cut into the other, the joint being secured by iron hoops. This last method is employed for round timber.

544. When beams united at their ends are subjected to a cross strain, a scarf joint is generally used, the under part of the joint being secured by an iron plate confined to the beams by bolts. The scarf for this purpose may be formed simply by halving the beams near their ends; but a more usual and better form (Fig. 94) is to make

Fig. 94-Represents a scarf joint for a cross strain secured at bottom by a piece of timber c confined to the beams by iron hoops d, d and keys e, e.

the portion of the joint at the top surface of the beams perpendicular to their axes, and about one third of their depth; the bottom portion being oblique to the axis, as well as the portion joining these two.

When the beams are subjected to a cross strain and to one of extension in the direction of their axes, the form of the scarf must be suitably arranged to resist each of these strains. The one shown in Fig. 95 is a suitable and usual form for

Fig. 95-Represents a scarf joint arranged to resist a cross strain and one of extension. The bottom of the joint is secured by an iron plate confined by bolts. The folding wedge key inserted at c serves to bring all the surfaces of the joints to their bearings.

these objects. A folding wedge key of hard wood is inserted into a space left between the parts of the joint which catch when the beams are drawn apart. The key serves to bring the surfaces of the joints to their bearings, and to form an abutting surface to resist the strain of extension. In this

form of scarf the surface of the joint which abuts against the key will be compressed; the portions of the beams just above and below the key will be subjected to extension. These parts should present the same amount of resistance, or have an equality of cross section. The length of the scarf should be regulated by the resistance with which the timber employed resists detrusion compared with its resistance to compression and extension.

545. When the axes of beams form an angle between them, they may be connected at their ends either by halving them on each other, or by cutting a mortise in the centre of one beam at the end, and shaping the end of the other to fit into it. See Fig. 97.

546. Joints for connecting the end of one beam with the face of another. The joints used for this purpose are termed mortise and tenon joints. Their form will depend upon the angle between the axes of the beams.

When the axes are perpendicular to each other, the mortise (Fig. 96) is cut into the face of the beam, and the end of the other beam is shaped into a tenon to fit the mortise.

When the axes of the beams are oblique to each other, a triangular notch (Fig. 97) is usually cut into the face of

one beam, the sides of the notch being perpendicular to each other, and a shallow mortise is cut into the lower surface of the notch; the end of the other beam is suitably shaped to fit the notch and mortise.

The direction of the strain and the effect it may produce upon the joint must in all cases regulate its form. In some cases the circular joint may be more suitable than those forms which are plane surfaces; in others a double tenon may be better than the simple joint.

547. Tie Joints. These joints are used to connect beams which cross, or lie on each other. The simplest and strongest form of tie joint consists in cutting a notch in one or both of the beams to connect them securely. But when the beams do not cross, but the end of one rests upon the other, a notch of a trapezoidal form (Fig. 98) may be cut in the lower beam

B

ос

A

Fig. 98-Represents an ordinary dove-tail joint secured by a pin at c.

to receive the end of the upper, which is suitably shaped to fit the notch. This, from its shape, is termed a dove-tail joint. It is of frequent use in joinery, but is not suitable for heavy frames where the joints are subjected to considerable strains, as it soon becomes loose from the shrinking of the timber.

548. Open built Beams. In framing open built beams, the principal point to be kept in view is to form such a connection between the upper and lower solid beams, that they shall be strained uniformly by the action of a strain at any point between the bearings. This may be effected in various ways, (Fig. 99.) The upper and lower beams may consist

Fig. 99-Represents an open built beam; A and B are the top and bottom rails or strings; a, a, cross pieces, either single or in pairs; d, diagonal braces in pairs; c, single diagonal braces.

either of single beams or of solid built beams; these are connected at regular intervals by pieces at right angles to them, between which diagonal pieces are placed. By this arrange

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ment the relative position of all the parts of the frame will be preserved, and the strain at any point will be brought to bear upon the intermediate points.

549. Framing for intermediate Supports. Beams of ordinary dimensions may be used for wide bearings when intermediate supports can be procured between the extreme points.

The simplest and most obvious method of effecting this is to place upright beams, termed props, or shores, at suitable intervals under the supported beam.

When the props would interfere with some other arrangement, and points of support can be procured at the extremities below those on which the beam rests, inclined struts (Fig. 100) may be used. The struts must have a suitably formed step at the foot, and be connected at top with the beam by a suitable joint.

In some cases the bearing may be diminished by placing

A

Fig. 100-Represents a horizontal beam O supported near the middle by inclined struts A, A.

on the points of support short pieces, termed corbels (Fig. 101), and supporting these near their ends by struts.

Fig. 101-Represents a horizontal beam c supported by vertical posts a, a, with corbel pieces d,d and inclined struts e, e to diminish the bearing.

In other cases a portion of the beam, at the middle, may be strengthened by placing under it a short beam, called

Fig. 102-Represents a horizontal beam C strengthened by а straining beam and inclined struts e, e.

straining beam (Fig. 102), against the ends of which the struts abut.