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Bottom plate fixings to slab-on-ground floors general fixing requirements

Reproduced from BRANZ Guideline May 2010. There are two separate criteria for the fixing of bottom plates to slab-on-ground floors. For fixing down timber plates as required by NZS 3604 7.5.12. As hold-down fixings for bottom plates for proprietary bracing systems.

Bottom plate fixings to slab-on-ground floors – bottom plates generally

In the first criteria where proprietary fasteners are used, these must be tested to provide minimum capacity loads. These are:

  • Internal walls
    (a) In the plane of the wall 4 kN.
    (b) Out of the plane of the wall 3 kN.
  • External walls
    (a) Horizontal loads in the plane of the wall 5 kN.
    (b) Horizontal loads out of the plane of the wall 4 kN.
    (c) Vertical loads in axial tension of the fastener 8 kN.

The fasteners must be within 150 mm of each end of the plate and at no more than 900 mm centres. It is still possible to use proprietary fasteners that have been tested and shown to have lower capacities than above by closing up the fastener centres.

Bottom plate fixings to slab-on-ground floors – proprietary bracing systems

In the second situation, the fasteners are for holding down both ends of bracing elements.

The need in these situations is for a much higher vertical characteristic tensile strength, which is specified by the proprietor of the bracing system. Typically this is 15 kN for 150 bracing units per metre. Some fasteners are rated at less than 15 kN, e.g. 11 kN for 120 bracing units per metre. Where fasteners have been additionally tested, they can be used also for the horizontal loads of NZS 3604 as above.

For high-strength proprietary bracing systems where hold-downs are specified, fasteners that are only tested to NZS 3604 7.5.12 cannot be used as bracing system hold-downs.

Proprietary fasteners for either situation must be tested for the particular slab edge detail, i.e. formed concrete or concrete masonry header block.

In NZS 3604 Sea Spray Zone where there are 90 mm wide bottom plates, there is insufficient concrete cover to fasteners unless they are stainless steel. The options in Sea Spray Zone are therefore 140 mm timber framing, a formed concrete foundation or stainless steel fasteners.

Bottom plate fixings for internal walls

Proprietary fasteners require a minimum concrete slab thickness. Most fastener suppliers require the hole to be drilled 10 mm deeper than the fastener embedment length. BRANZ recommends an additional 10 mm minimum of concrete below the hole. This means that a fastener with up to 80 mm embedment length can be used in a standard 100 mm slab-on-ground provided there is a DPM under the slab. Where fasteners have a greater embedment length, there must be a slab thickening. Drill depth guide stops must be used when drilling holes in slabs.