Timber joints include different ways of transferring compression, tension and lateral effects between timber elements. The text presents compression and tension joints, tenons, notches, straps, glued joints and gusset plates.
1. Compression joint
If the end of one element touches the end of another element and the joint is secured against lateral displacement, or if the end of one element touches a longitudinal element, it is sufficient to install dowels to prevent lateral displacement at the joint surfaces, or to nail straps on the outside. If small tenons are used, the holes into which they enter should be deducted from the compressed surfaces (Figs. 1 and 2). Therefore, the vertical posts of trussed structures—especially if they are narrower than the chords—are recessed with their full surface by about 2 cm into the chord and secured by nailing on pieces of timber with a triangular cross-section (Fig. 3).
Figures 1–3. Compression joints between timber elements.
If the permissible compression perpendicular to the grain is exceeded, pieces of hardwood or steel C-sections are inserted (Fig. 4). The compressed surface of individual columns can likewise be enlarged by inserting a foot (Fig. 5). Figure 6 shows the connection of a vertical member with solid crown segments, whose continuation is located above it. To achieve a certain rigidity of the connection, T-shaped steel straps must be fitted. If posts made of round or sawn timber are extended and bending or buckling stresses occur at their joint, these stresses must be taken up by timber or steel straps placed at the sides, using nails or bolts.
Figures 4–6. Reinforcements and extensions of compression joints.
It is often convenient to cover the joint with a tubular sleeve. To prevent the fibres from pressing into one another at the joint, a layer of semi-moist, well-compacted cement mortar is placed before the upper part of the post is fitted. If the elements run horizontally or meet at an angle, they should be secured against displacement with straps or flat-steel strips, and against twisting with fitted angular timber blocks (Figs. 8 and 9). Double timber side plates fastened with bolts are used to connect scaffold sills. Such a structure can also take bending moments (Fig. 10). A straight or oblique scarf is used when the joint is supported and there is not enough space to support the individual elements separately (for example, the connection of purlins above roof ties) (Figs. 11 and 12). To transfer greater tensile forces, shear connectors are sometimes placed between the contact surfaces.
Figures 7–11. Joints, straps and scarfs.
2. Tenon
A tenon serves solely to secure the relative position of two timber elements. During assembly, it must hold the elements in their position. Figure 13 shows the connection of vertical and diagonal members with doubled scaffold chords. Washers measuring 8/8 to 10/10 cm and 8–10 mm thick are placed beneath the bolt head and nut. The bolts should be tightened until their tensile resistance at the shank is utilised, so that the strongest possible frictional forces are transferred to the contact surfaces of the elements by the action of high pressure. Rigid nodes are created in this way.
Figures 12–15. Scarfs, tenons and rafter joints.
If roof rafters do not rest on a ridge purlin, they are connected with a fork or a full-slot tenon (Fig. 14). In some cases, tenons are exposed to bending perpendicular to their direction, for example as a central tenon on cross members of intermediate floor structures (Fig. 15). The walls around the mortise must be left with a suitable thickness to avoid splitting the timber.
3. Notch
A notch is used to make a firm connection capable of taking the compression of members that meet obliquely. The usual construction is an end notch, whose body a-b bisects the external angle, while angle bac is usually greater than 90° (Fig. 16). This produces the most favourable stress conditions for the diagonal member and the sill. Values for a:
a ≤ 50°, notch depth t = 0.25h
a ≥ 60°, notch depth t = 0.17h
Linear interpolation is used between angles of 50° and 60°. If a is less than 50°, use of the member cross-section will be better if a double notch is applied (Fig. 17). Distrust of this type of connection is justified because the required accuracy of workmanship is not often achieved.
Figure 16. Geometry of an end notch.
Shear stresses on surface a-e for softwood should not exceed 900 kPa. The minimum length of the end projection should be 15 cm. If the length is limited, a central notch or rear notch is used. Due to rotation of the members, local crushing and the process of timber shrinkage, it may happen that the force is transferred only to the face of the notch while the joint opens at the rear; the result is eccentric loading of the diagonal member. This occurs to the smallest extent in a joint with a central notch, which acts approximately like a hinge and prevents the creation of large eccentricity when the member rotates. With a rear notch, there is a danger that the tooth may be sheared off (especially in the event of cracks caused by timber shrinkage), allowing the diagonal member to slip.
Figures 17–20. Variants of notches and an oblique tenon.
To avoid excessively deep notches, lateral reinforcements and inserts are added (Fig. 19). In addition, part of the force is transferred beside the notch by straps nailed to both sides; in this way, the node gains a certain rigidity with respect to lateral forces. A triple notch is not recommended because the uniform and simultaneous transfer of force across three end surfaces is impossible.
4. Oblique tenon
Compared with a notch, an oblique tenon (Fig. 20) has the advantage that the connection shows greater rigidity against lateral effects. On the other hand, more difficult workmanship, reduced resistance and increased deformability, especially at large connection angles, do not indicate that its use is advisable.
5. Tension joint
Timber or steel straps are used to cover joint heads exposed to tension; the latter are generally more expensive, but are more suitable because of their better appearance and greater safety in the event of large forces. Figure 21 shows the connection of a double tension member, made with three timber straps and shear connectors in the joints.
Figure 21. Tension joint with timber straps and shear connectors.
Steel straps are fastened using: calculated bolts only; shear connectors with bolts (Fig. 22); shear connectors joined to the steel straps by rivets or welding; or wood screws. Each timber element of the joint is drilled separately according to the corresponding template. The accuracy of manual drilling is not sufficient.
Figures 22–23. Steel straps and oblique scarf joints.
Ordinary straps cannot be used for glued structures. Oblique scarf joints are the primary option here. On the elements being connected, the ratio between the height of the bevel and the length over which the slope extends should be at least 1:5 to 1:8. It is also necessary to adopt the smallest possible dimensions for the cross-sections of the elements being joined.
Figure 24. Wedge-shaped bevels of timber elements.
Figure 23 shows two assembly joints made with an oblique scarf. The pressure required for gluing is produced by bolts. Joints with wedge-shaped bevels (finger joints, Fig. 24) are also used.
Figure 25 shows a connection between a double diagonal member and a likewise double chord (the elements lie in the same planes), with lateral reinforcements and inserts. A greater number of shear connectors is needed for the connection between the additional parts and the chord (load oblique to the grain) than for the connection between them and the diagonal. To accommodate the shear connectors, the width of the added parts must be greater than the width of the web member. Web members of timber structures are normally connected directly to the chords.
6. Intersection
Figures 25–26. Connection of a diagonal member and intersection with a chord.
Tension members (usually double) are connected directly where they intersect a chord. The members are notched sufficiently to transfer their forces perpendicularly or obliquely (by a notch) to the chord (Fig. 26). A solid tension member can likewise be fitted in the space between a double chord. An intersection as a direct connection between elements can be applied only in the case of a small force, for whose transfer the members are generally oversized for structural reasons. Care must be taken that the members have completely sharp edges.
7. Connections of elements with gusset plates
If several members meet at one node, or if the chords of a trussed structure change direction sharply, gusset plates are used, modelled on the gusset plates of steel structures; in terms of economy, they are not always recommended. Steel gusset plates are used with circular steel shear connectors 20 mm thick and 80 mm long for heavily loaded bridges of large spans and small structural depth, which otherwise bend considerably under the influence of progressive drying of the timber. Plywood gusset plates are often used.
In addition to the connections for timber elements listed here, there are many other joints made only by hand by carpenters: laps, housings, “karmom” and others. These “craft” joints show an effort to take tensile forces where possible in order to avoid separation of the elements. If such joints are checked by calculation, it becomes apparent that the forces that can be taken even in the most favourable case are very modest. Carpentry joints, which are often very ingeniously made and require a high degree of craftsmanship, are no longer suitable in many cases today.