Foundation and Sub-Floor Bracing Analysis: the 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 sample of 80 dwellings’ foundations was assessed against the current “Light Timber Framed Construction Standard” NZS3604:1999. The NZS3604 standard was introduced in 1978 and has been subsequently tested by many New Zealand earthquakes, most significantly being the Edgecumbe earthquake in 1987. The observed damage to dwellings built to the then current NZS3604:1984, showed only negligible damage due to foundation inadequacies and as a result, the standard required only minor amendments. The most current 1999 edition of NZS3604 is therefore considered to have seismically appropriate detailing and provisions to withstand design earthquakes; so for the purposes of this study, NZS3604:1999 is assumed to be the residential benchmark for seismic adequacy. The results from the study suggest that 39% of the sample had inadequate sub-floor bracing. Overall, 16% of the sample relied solely on the strength of ordinary piles, while 11% relied entirely on large concrete anchors. 76% of dwellings had some form of fixing deficiency, ranging from degradation to incorrect or non-existent fixings. The overall condition of the sample dwellings was compared with the House Condition Survey 2005. The results of this study showed that inadequacies identified in the House Condition Survey 2005, were also prevalent in the majority of sampled dwellings in the study, including non compliance with minimum height and sub-floor ventilation requirements. However, the House Condition Survey produced by BRANZ does not assess any rented accommodations and so the condition results may be underestimated. The study sample, however includes a proportion of rented dwellings, but may still be unrepresentative of the actual average dwelling, in terms of condition and range. After identifying the common deficiencies both in the sample and also from similar studies, remedial measures were costed and applied to different foundation types based on the required strength and suitability to the existing foundation system. The remedies, to upgrade bracing, fixings and the general condition, including labour, ranged between $15 per m² and $60 per m². These costs were then projected to all Wellington City foundations, which totalled over $250 Million. It was assumed that each dwelling should be remedied to comply with the standards in NZS3604:1999 and the remedies were applied based on the average condition of the sample. To understand the anticipated losses and therefore benefits of upgrading, the estimated damage cost to residential dwellings was calculated using an Earthquake Loss Modeller, which was 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, around Kaiwharawhara. In order to formulate a cost saving, or economic benefit from upgrading foundations, the cost of specific damage and collapse to residential dwellings was calculated to be $2.1 Billion, assuming no remedial measures had been applied. The Mean Damage Ratio for each foundation type was then modified, based on similar earthquake damage projections based on the same Wellington earthquake scenario. Dwellings that had either significant configuration issues or were located in an area likely to experience higher earthquake shaking, were still anticipated to collapse despite applying sub-floor remedies. The cost of damage to dwellings following remedial measures was calculated at just over $1.1 Billion. Therefore, the total savings were anticipated to be around $950 Million. These results were considered as a ratio of cost over benefit which is used to understand whether the associated economic benefit is greater than the anticipated cost of remedy. The cost / benefit ratio for dwellings likely to collapse is less than 10% , while extensively damaged dwellings have a higher cost / benefit ratio of around 25%. The highest benefit was seen in Piled dwellings, where savings upwards of $500 Million were projected. The economic saving due to the application of remedial measures has the potential to reduce pressure on the public sector including emergency management systems, hospitals and organisations involved with evacuations and erection of temporary shelters. In addition, there will also be a saving for both the public and private insurers, which will facilitate the quicker reconstruction of the postearthquake society to pre-earthquake levels. For the results of this study to be beneficial to New Zealanders, the information must be disseminated and implemented using proactive initiatives. These must be targeted at the homeowner in an easily understandable format, which is focussed on better performance and savings, rather than on the worst case scenario which has been shown to increase ambivalence and fatalistic mindsets within society.