Bracing foundations

This section looks at various techniques for strengthening residential foundations.

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Bracing perimeter foundations

A residential property with a concrete perimeter foundation is probably already sufficiently braced. Corner foundations also usually provide sufficient bracing, unless the corner walls are taller than they are long, which makes them susceptible to toppling under lateral seismic load . In this case, either sheet bracing or diagonal timber braces should be installed.

Diagonal timber brace.

Added diagonal timber brace to timber piles.

In corner wall construction, the floor joists may not be adequately connected to the foundation wall. In this case, diagonal timber braces should be installed between the joist and the concrete wall. This must be done at both ends of the building to evenly distribute the earthquake loads.

Corner wall-to-joist remedial connection.

Corner wall-to-joist remedial connection.

Bracing open pile foundations

Pre-NZS 3604-type foundations with no perimeter foundation walls are sometimes enclosed with spaced horizontal boards fastened to the perimeter piles. These boards provide no bracing and should be replaced with either sheet bracing or diagonal bracing.

Sheet bracing

Sheet bracing involves fixing a series of flat panels such as fibre-cement or plywood sheets between the piles and the floor.

The top of the sheet should be fixed to the boundary joist or the bearer. An additional framing member should be installed between adjacent piles to support the bottom edge of the sheet material.

Sheet bracing may be used with open concrete and timber pile foundations where there is more than 600 mm between the ground and the underside of the joists. It may also be possible to use sheet bracing with shorter pile foundations, but engineering advice is required.

When applying sheet bracing, ensure adequate ventilation is provided. NZS 3604:2011 states that ventilation openings should be at least 3,500 mm² per m² of floor area and evenly distributed around the foundation perimeter. See NZS 3604:2011 clause 6.14 Prevention of dampness for details.

Sheet bracing.

An open pile foundation may be braced using sheet material around the perimeter of the building.

Sheet bracing on a stepped subfloor.

Sheet bracing on a stepped subfloor.

Diagonal bracing

Diagonal bracing involves fixing a timber bracing element between the bottom of piles and the floor frame.

Diagonal braces are suitable for timber piles and may be placed between the pile and bearer or between the pile and joist. The latter connection provides a more direct fixing to the floor plane.

To brace a pile foundation using diagonal bracing, at least two braces are required. They should be installed at an angle of no more than 45° on the outside row of piles and on every second row in both directions.

Bracing jack-stud frames

Some residential properties built on sloping sites have timber jack studs between the piles or foundation walls and the floor framing. The main function of a jack stud is to transfer the vertical load to the foundations, but their height can make them vulnerable to lateral loads during an earthquake.

If the existing sheet cladding is missing or damaged or has become brittle, it should be replaced with new bracing. If sheet bracing is used, a series of fibre-cement sheets should be fixed between the timber top and bottom framing members and to the jack studs around the perimeter of the building. Alternatively, diagonal bracing may be installed between the top and bottom framing members.

Bracing a jack-stud frame.

Options for bracing a jack-stud frame above a concrete perimeter wall.

Engineering advice is required to brace jack-stud framing over shallow piles.