A connection consists of two or more timber members joined with one or more mechanical fasteners. Connections are important in timber structures because they provide continuity to timber members, as well as strength and stability within structure.
Connections are a critical factor in the design: the strength of the connectors used to form connections will determine the strength of the overall structure. And connection design plays a significant role in determining the dimension of timber members.
The inherent variability of timber, plus its dimensional changes and movement due to both moisture and temperature, must also be considered when designing connections. Physical restrictions may limit timber dimensions for transport, necessitating connecting multiple sections to meet design requirements. Structural design for timber connections follows Eurocode 5.
As well as their structural importance, connections also provide visual appeal. In modern timber structures, connections are often exposed, making connection design important not only from a structural aspect but from an aesthetic one too.
Metal Connections
In modern timber construction, connections with metal fasteners are the most common. There are a wide range of metal connection methods for timber, divided into two distinct categories depending on behaviour and how load is transferred between the connected members:
Dowel-type fasteners
Demountable connections hold together the glulam frame at the Brent Cross Visitor Centre (designed by Moxon architects, with timber engineering by Ramboll and Hybrid Structures)
Dowel-type fasteners transmit either lateral or withdrawal loads. Lateral loads are transmitted by bearing stresses developed between the fastener and the members of the connection. Withdrawal loads are axial loads parallel to the fastener axis transmitted through friction or bearing to the connecting materials.
Metal connector plates are a special case of dowel-type fasteners. They combine a reliance on the lateral load actions of dowel fasteners (most commonly nails), or the pressed teeth and strength properties of metal plates to connect them to timber members.
Dowel-type fasteners include nails, screws, bolts, and dowels, while the most common metal plate connectors encountered are two-dimensional nailing plates (e.g. nail plates and punched metal plates) and three-dimensional nailing plates (e.g. joist hangers, truss clips, angle brackets, framing anchors, straps, pressed metal webs and wall ties).
Bearing-type connections
Bearing-type connections only transmit lateral loads. Bearing-type fastenings such as shear plates and split ring connectors transmit shear forces through bearing on the connected materials.
Special connectors
Special connectors – or simply ‘specials’ – are designed and manufactured for unique conditions and specific purposes. Outside a manufacturer’s standard product range, these connectors are engineered and fabricated to order.
Such bespoke special connectors are designed and manufactured to meet specific requirements and are only suitable for a specific application, although where certain applications or conditions may become more commonplace, certain special connectors may become more widely available as a standard product, joining a manufacturer’s standard product range.
Adhesive connections
Adhesive joints connect structural elements or manufactured structural elements made from engineered timber products, such as glulam, laminated veneer lumber (LVL), and cross laminated timber (CLT). There is a range of structural adhesives suitable for timber connections, each with different characteristics which influence their specification, e.g. cost, curing time and method, resistance to moisture, and gap-filling properties.
Different adhesive types may be more suitable for different applications, depending for example on what material the adhesive is bonding, or the moisture content of the members being connected.
A demountable timber connection used for New Wave House (Thomas McBrien architects)
When compared to mechanical connections, adhesive connections benefit from greater stiffness, requiring smaller timber cross sections, as well as being resistant to corrosive environments, and in some cases seeming more aesthetically pleasing. However, some key disadvantages of adhesive compared to mechanical connections include:
- Need for strict quality control to ensure the structural bond is strong and durable.
- Limited suitability in conditions with changes to moisture content where dissimilar materials are bonded, or if the angle of grain is different on the bonded faces.
- Inability to perform where a significant load perpendicular to the plane of adhesion is applied.
Connections can be made without dowel-type fasteners or metal plate connectors by using adhesives independently.
However, as adhesives are affected by moisture content and temperature during the curing process, they are often better suited to fabricating connections in controlled factory environments to ensure effective, strong, and durable structural bonds.
As such, gluing structural connections on site is usually avoided for anything other than remedial details in accordance with specific procedures, and sometimes in conjunction with other mechanical fixing methods. All adhesive types for bonding timber can be separated in to three distinct categories:
- Polyurethane adhesives (PUR)
- Resin adhesives such as Phenol-resorcinol formaldehyde (PRF)
- Formaldehyde adhesives such as Melamine urea formaldehyde (MUF).
Traditional Timber Connections
Traditional timber connections, used in timber frames since at least the medieval period, are still employed in restoration and reproduction work, as well as modern construction that utilises traditional forms without adhesives or metal connections. In terms of timber structures, “traditional connections” refers to timber-to-timber connections linked together by geometries that provide interlocking friction, transferring loads between timber members while maintaining rigidity. It is only in relatively recent times that timber construction has utilised metal fasteners and glued joints. Traditional timber connections are grouped into three categories:
- Mortice and tenon joints
- Lap joints
- Scarf joints
Timber connections have limited tension and moment capacity. Therefore, most connections are configured to transfer only compression and shear forces. This means there are limitations to their use in structures, as they cannot achieve a full moment connection.
Mortice and Tenon Joints
Mortice and tenon joints consist of a tongue secured into a slot. The joint must be tight fitting to ensure maximum strength, but also to achieve a neat, flush appearance. Mortice and tenon joints connect principal structural elements such as timber frame beams and columns. There are multiple variations, which can include the use of pegs to keep the two timber members in tight contact, stopping movement at the joint.
Stub tenon
Also known as the blind tenon, a stub tenon has a mortice that is not cut all the way through the timber member, offering a neater profile with a concealed tenon end. The joint can be strengthened with dowels or wedges.
Tusk tenon
Used in flooring construction, in a tusk tenon joint the tenon passes through the mortice hole to project through the opposite side, with a tapered key or dowel driven into a hole cut through the tenon to keep it securely in place.
Wedged tenon
A wedged tenon joint has the tenon mechanically wedged into the mortice through hammered wedges inserted into the joint from the other side. It has more tensile resistance than a simple mortice and tenon, giving significant bending and tension resistance. Wedged tenon joints are used in making traditional doors, and the principle is how hammers and axe heads have been held onto their handles for centuries.
Dovetail tenon
A dovetail tenon joint is often used to connect a beam to a column. A series of pins cut to extend from the end of one timber member interlock with a series of “dovetails” cut into the end of another member. The pins and tails have a trapezoidal shape and have high tensile strength (meaning they are hard to pull apart).
Stub tenon joint
A stub tenon joint is used to connect a column (post) to a sill plate (the bottom horizontal member). This joint is not fixed but ensures that the column stays in the correct place. A short stub tenon is inserted into the shallow mortice in the top of the sill plate and the weight of the structure holds it in place.
Lap joint
Lap joint refers to timber-to-timber connection types in which the members overlap. A lap joint may be a full lap or half lap:
- In full lap, no material is removed from either of the joined timber members, resulting in a joint equal to the combined thickness of the two members.
- In a half lap joint or halving joint, material is removed from both of the members so that the resulting joint is equal to the thickness of the thickest member. Most commonly, the members are the same thickness and thus half the thickness of each is removed.
Lap joints have a significant ability to resist shear forces.
Scarf joints
Scarf joints connect two timber members end-to-end to increase the length of their span. They are primarily used to connect two timber beams together, typically to increase their length. The joint is formed by cutting opposing tapered ends on each timber member, which are then fitted together. The tapered ends can end in a blunt“nib” to engage with a matching shoulder in the partner piece, or can be feathered to a fine point, obscuring the joint in the finished work.
Non-Structural Traditional Timber Joints
The following connections are often used in non-structural designs and furniture:
Tongue and groove joint
Tongue and groove joints are used to connect similar timber members edge-to-edge. These are often seen in flooring, paneling, and parquetry as they allow two flat pieces of timber to be joined together to form a single flat surface. Tongue and groove joints have a slot (or groove) cut all along on one edge and a thin deep tongue on the opposite edge, with the tongue of a lower depth than the groove). The joint is not glued. Several tongue and groove joints can be used, one above the other, when joining thicker materials.
Butt joint
The most basic timber connection, a butt joint is a very simple and weak joint formed by butting two members together. The joint can be held together with glue, nails, screws, or dowels, or a combination of methods, to increase its strength. A butt joint’s strength can be increased by adding a rectangular or triangular block of wood in the corner to increase surface area for adhesive application.
Mitre joint
A mitre joint is a 90° angle corner joint, created by fixing two 45° angle ends of two timber members. Common applications include picture frames and mouldings.
