The adequacy of existing house foundations to resist earthquakes: Cost benefit of upgrading

Abstract

The poor performance of residential foundations in past earthquakes prompted a practical investigation to quantify the adequacy of Wellington timber dwellings’ foundations, including the sub-floor bracing, sub-floor fixings and general condition of the foundation.  The adequacy of a random sample of 80 light timber framed dwellings’ foundations was assessed against the current “Light Timber Frame Construction” NZS3604:1999  which is assumed to be the benchmark for the seismic adequacy of Light Timber Framed buildings. 

After identifying the common deficiencies remedial measures were costed by a registered Quantity Surveyor and applied to different foundation types based on the required strength and suitability to the existing foundation system.  The remedies were sourced from NZS3604:1999 and also the BRANZ document Strengthening Houses against Earthquake: A Handbook of Remedial Measures, written by Russell Cooney (1982). 

To understand the anticipated losses and therefore benefits of upgrading, the estimated damage cost to residential dwellings was calculated using an Earthquake Loss Modeller supplied by the Institute of Geological and Nuclear Sciences.  The cost was calculated by assuming an earthquake of Magnitude 7.5, at a depth of 7.5km centred on the Wellington fault line, near Kaiwharawhara.

Despite the public’s initial reaction to dismiss the risk and likelihood of an earthquake, the fact remains that although the likelihood of earthquake is smaller than other hazards, the consequences are far more severe.  Foundation upgrading is the one area that will receive the highest economic and social benefits for capital expenditure, whether it is made by the homeowner or the Government.  The economic benefits for the homeowner will most commonly result from the cost of evacuations, temporary shelter and the mitigation of injury due to securing the foundation from collapse or extensive damage.  Direct costs of repairing damage are usually borne by the Earthquake Commission and other insurers.  Although the predicted results of upgrading may be clear, the homeowner generally requires more rigorous information regarding the problem, clear incentives for upgrading and proactive initiatives from the authorities.  These initiatives must be targeted at the homeowner in an easily understandable format, which is focused on better performance and savings, rather than on the worst case scenario which has been shown to increase ambivalence and fatalistic mindsets within society.