Inadequate foundation bracing

Many villas do not comply with modern standards for earthquake bracing of foundations.

Bracing requirements

Section 5 of NZS 3604 requires foundations to be adequately braced, but existing foundation designs may not comply with this requirement. It is likely that a significant number of villas will have already been repiled, although there are still some with original foundations. Typically, when a villa was repiled (certainly if done pre-1980s), it was unlikely that any lateral support or bracing was provided to the new piles.

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Adding bracing

Bracing can be provided by a combination of:

  • anchor piles
  • cantilevered piles
  • perimeter footings
  • timber bracing between piles (Figure 1).

With most suspended floor construction, it is possible to provide diagonal braces between adjacent piles or between piles and bearers/joists. Modern 12 kN pile fixings may be able to be used, or the connection can be made with M12 hot-dip galvanised bolts. Another option is to embed a number of piles more deeply into the ground, typically 900 mm for an anchor pile in accordance with NZS 3604 – how many depends on the size of the house and the local earthquake and wind zone.

If a floor is springy because the span is too large for the timber sizes, a new bearer and piles at mid-span can be added to provide more support. For first floors, the installation of support beams can be one option, but this must be integrated with structural changes to the ground floor where the plan of the building is being modified. 

BRANZ has a bulletin outlining how to upgrade piled foundations. BRANZ research has also confirmed the effectiveness of simple retrofit solutions to strengthen foundations of existing houses on sloping hillsides.