Anchors, fixings and fasteners

Fixings consist of bolts, welds, anchors, brackets, cleats, gusset plates and so on that connect components and different elements of the restraint system.

All components and associated supports and restraints should be fixed or attached to the supporting structure so that seismic forces are transferred to the structure. They should be attached to the building structure in a way that matches or exceeds the earthquake load demands and relative displacements determined earlier in the design process.

The designer should verify that there is a continuous load path of sufficient strength and stiffness between the component and the supporting structure. Elements of the supporting structure (such as concrete floors and ceilings) must also be designed to withstand the additional locally induced seismic forces from the components as well as normal design loads. Fixings should be designed without considering frictional resistance produced by the effects of gravity.

Where components are fixed to other non-structural elements of the building, such as timber-framed partition walls, the elements must be checked by a seismic specialist to verify they have adequate loadcarrying capacity.

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Screws and bolts

Wood screw and coach screw fixings should be installed into the side grain of dry radiata pine framing timber. Screws should be pre-bored to 0.8 × screw diameter, except for self-drilling screws.

Where bolts are used to connect the ends of steel components, NZS 4219:2009 provides the necessary design strength, minimum edge distance and shear capacities that should be used.

Cast-in anchors

Cast-in anchors should be at least grade 4.6 bolts or threaded rods installed into wet concrete or concrete masonry before grouting. The anchors must be provided with a positive means to prevent rotation while tightening the connecting nut.

For anchors cast into concrete or bolts cast into concrete masonry grout, NZS 4219:2009 provides methods to determine the acceptable design strength, edge distance, embedment depth and minimum spacings between anchors.

For bolts cast into concrete masonry, the minimum edge distance, measured from the centre of the bolt to the outer face of the masonry, should be not less than the embedment depth. Spacing between adjacent bolts should be at least twice the embedment depth.

Proprietary anchors

The design strength of the anchor in tension and shear should be obtained from the manufacturer.

NZS 4219:2009 requires that all post-installed and proprietary anchors used to restrain engineering systems must have passed the seismic qualification test. This is described in American Concrete Institute standard ACI-355.2 Qualification of post-installed mechanical anchors in concrete and commentary.

Suitable post-installed concrete fixings should be selected for both seismic and gravity supports, based on use in either cracked or uncracked concrete as applicable. Selection must be made in accordance with NZS 3101:2006 Concrete structures standard. This refers to American Concrete Institute standard ACI-318-14 Building Code Requirements for Structural Concrete and Commentary Chapter 17 Anchoring to concrete.