Inadequate foundation bracing
Many bungalows do not comply with modern standards for earthquake bracing of foundations.
Section 5 of NZS 3604 requires foundations to be adequately braced. Unless they have been since the late 1980s, bungalows are unlikely comply with this requirement.
Where new work is proposed, particularly an alteration that involves removing load-bearing walls or adding an extra floor, the existing piles must be checked by an engineer for their adequacy to provide the required lateral support and bracing.
Bracing can be provided by a combination of:
- anchor piles
- cantilevered driven piles
- perimeter footings
- timber bracing between piles.
With most suspended floor construction, it is possible to provide diagonal bracing between adjacent piles or between piles and bearers/joists. Use 12 kN pile fixings or make the connections with M12 hot-dip galvanised or stainless steel bolts.
Alternatively, embed a number of piles more deeply into the ground – typically 900 mm for an anchor pile in accordance with NZS 3604. The number of anchor piles depends on the size of the house, its location, and the earthquake and wind zones for the region.
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.