Joinery

Traditional joinery connections (also known as carpentry connections) are typically created by cutting notches, holes and tongues in connected members so they interlock—such as with mortice/tenon and scarf joints. In these types of connections, forces are in principle transferred in compression/bearing. Interlocked connections in tension require wood or metal pins or keys to prevent separation. Tension connections are often kept to a minimum and in some cases metal straps and bolts are provided to achieve better connections, mainly for long-span roof trusses in historic structures. In a properly designed joinery connection, there is sufficient interlocking, tightening of connections and friction due to the self-weight and supplementary support from walls, buttresses and adjacent buildings to design for both gravity and lateral forces associated with wind or seismic events.

While fairly common in single-story homes, commercial and recreational structures, joinery connections are seldom used for modern, multi-story heavy timber buildings. One reason is the fact that these connection systems demand highly developed skills usually associated only with experienced carpenters. Such connections are also labor intensive which makes them uneconomical for industrial production.

Although joinery parts can be produced with great speed and precision using Computer Numeric Controlled (CNC) technology, this approach is not as widely used in North America as it is in Japan and Western Europe. Here, mechanical connections dominate primarily because they can be made without expensive equipment and assembled with relative ease on site.

Avoiding Potential Problems

Notching

Although notching is an essential technique in traditional joinery, and may be required in the field for a number of reasons, any field notches or holes that are not properly made, or that deviate from approved drawings, may significantly reduce the capacity of even a properly designed member. The NDS provides guidance on notching, and the wood industry has produced extensive technical notes with recommendations related to the size of notches appropriate for different materials. With glulam, for example, end notching is limited to 1/10th the depth of the member or 3 in., whichever is less. When a notch is used, designers should consider driving a lag screw or self-tapping screw in 6-8 in. from the bearing going up past the neutral axis. If a crack were to develop, the screw will keep it from progressing down the length of the beam and acts as a transverse reinforcement.

Moisture Effects

Wood expands and contracts as a result of changes in its equilibrium moisture content (EMC). Expansion in the direction parallel to grain in a wood member is minimal; however, change in the direction perpendicular to grain can be considerable and must be accounted for in connection design and detailing. The common rule-of-thumb is that wood will experience a 1% change in the radial or tangential direction for every 4% change in moisture content. A nominal 2x12, for example, can decrease in depth through shrinkage by just under 1/4 in. as it changes from 18% to 10% EMC. In designing connections, it is important to design and detail the connection such that the member's shrinkage is not restrained. Otherwise, shrinkage of the wood member can cause excessive tension perpendicular-to-grain stresses and splitting may occur. Specific issues related to shrinkage include:

Beam-to-column connections – Continuous full-depth side plates can cause problems because they restrain wood shrinkage and may cause splitting. Solution: Smaller discontinuous side plates transmit forces and allow wood movement.

Beam-to-beam connections – For beam hangers, fasteners placed near the top of the supported beam can restrain wood shrinkage and may cause splitting. Solution: Fasteners placed near the bottom of the supported beam can minimize the effects of shrinkage while top tabs provide lateral restraint.

Beam-to-wall connections – If bolts are placed high on the beam, or high and low, tension perpendicular to grain stresses may cause splitting. Solution: Bolts near the bottom of the beam, preferably with slotted holes, allow for wood shrinkage. (A variety of pre-engineered connections are available with pre-drilled slotted holes.)

Another issue related to moisture is exposed end grain. This can result in checking and possible decay, even in low rainfall climates. Solutions: Redirect water flow around the connection. Use preservative treated wood products. Use end caps and flashing. Allow for air flow and drying—e.g., by detailing drain holes or slots in box-type connectors or maintaining a gap of at least 1/2 in. between wood and concrete or masonry construction.

Selection and Installation

Ensuring that structural connectors perform as intended involves a number of considerations for both the designer and installer. In the ASD/LRFD Manual, AWC highlights them as follows:

Wood Members

Specific details related to the wood members being connected will have an impact on the capacity of the connection. The following are important considerations regarding the wood members themselves:

▶ The species of wood must be the same as that for which the connector was rated by the manufacturer. Manufacturers test and publish allowable design values only for certain species of wood. For other species, consult with the connector manufacturer.

▶ The wood must not split when the fastener is installed. A fastener that splits the wood will not provide enough capacity for the design load. If wood tends to split, consider pre-boring holes using a diameter not exceeding 3/4 of the nail diameter. Pre-boring requirements for screws and bolts are provided in the NDS.

▶ Regarding shrinkage, most connectors are manufactured to fit common dry lumber dimensions. Other dimensions may be available from the manufacturer.

▶ Where built-up members (multiple members) are installed in a connector, the members must be fastened together prior to installation of the connector so they act as a single unit.

▶ The dimensions of the supporting member must be sufficient to receive the specified fasteners. Most connectors are rated based on full penetration of all specified fasteners. Refer to the connector manufacturer for other situations.

▶ If a connection is designed to transfer load by bearing, the bearing capacity of the wood members should be evaluated.

▶ Local stresses in connections using multiple fasteners should be checked to ensure adequacy. Where a fastener group is composed of closely spaced fasteners loaded parallel to grain, the capacity of the fastener group may be limited by wood failure at the net section or tear-out around the fasteners caused by local stresses.

Connectors

The condition of the connector is critical to how it will perform. The following are important items regarding the connector:

▶ Connectors may not be modified in the field unless noted by the manufacturer. Bending steel in the field may cause fractures at the bend line, and fractured steel will not carry the rated load.

▶ Modified connectors may be available from the manufacturer. Not all modifications are tested by all manufacturers. Contact the manufacturer to verify loads.

▶ In general, all holes in connectors should be filled with the nails specified by the manufacturer. Contact the manufacturer regarding optional nail holes and optional loads.

▶ Different environments can cause corrosion of steel connectors. Always evaluate the environment where the connector will be installed. Connectors are available with different corrosion resistances. Contact the manufacturer for availability.

Fasteners

Most wood connections rely on fasteners to transfer the load from one member to the other. Therefore, the choice and installation of the fasteners is critical to the performance of the connection. The following are important considerations:

▶ All fasteners specified by the manufacturer must be installed in accordance with the manufacturer's installation recommendations to achieve the published value.

▶ The size of fastener specified by the manufacturer must be installed.

▶ The fastener must have at least the same corrosion resistance as the connector.

▶ Bolts must generally be structural quality bolts, equal to or better than ANSI/ASME Standard B18.2.1.

▶ Bolt holes must be a minimum of 1/32 in. and a maximum of 1/16 in. larger than the bolt diameter.

▶ Fasteners must be installed prior to loading the connection.

▶ Power-driven fasteners may deflect and injure the operator or others. Nail tools may be used to install connectors, provided the correct quantity and type of nails are properly installed in the manufacturer's nail holes. Nail tools with nail hole-locating mechanisms should be used. Follow the nail tool manufacturer's instructions and use the appropriate safety equipment.