Connection Options for Wood-Frame and Heavy Timber Buildings

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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 ¾ 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. One method for determining local stresses is provided in Appendix E of the NDS.

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.

 

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Originally published in Engineering News-Record.

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