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Bolts, coach bolts, coach screws and dowels

Availability and the engineering design of bolts, coach bolts, coach screws and dowels are covered in this section. Bolts, coach bolts, coach screws and dowels provide high lateral strength transmitted through shear and can act in tension.

The  fasteners  described in this section are:

  • Bolts, which are frequently used in heavy timber construction for timber to timber or timber to steel connections. Most are ordinary machine bolts, used with a washer and nut on the threaded end.
  • Coach bolts, which have a domed head and square projection beneath the head to prevent the bolt turning in the timber.
  • Coach screws, which are large, threaded screws similar to bolts but with a threaded connection to the wood.
  • Dowels, which are smooth rods of steel or plastic, driven into tight fitting holes in the wood, with no nuts or other end fastenings.

Standard bolt

Coach bolt

Coach screw

Illustrations: Courtesy Timber Design Guide, 2007

These fasteners transmit forces through shear at one (or more) interface and develop moderately high lateral strength through an interaction of wood bearing and fastener bending.

At higher loads and deformations, a bolt may act in tension and transmit increased loads through friction and the tensile ‘rope effect’, but only if the tensile forces can be transmitted to the timber members.

This is not the case with dowels.

 

Availability

Bolts, coach bolts, coach screws and dowels are available throughout New Zealand from building supply stores.

There are several New Zealand manufacturers and importers. Custom made bolts and coach screws are also available.

Split-ring connectors, shear plates or tooth plate connectors (bulldog) can increase the strength of a bolted joint, however these connectorsare not readily available in New Zealand.

 

Bolted truss roof structure

 Source: Courtesy Timber Design Guide, 2007