House Upgrades in the Wind

This article first appeared in the Autumn 2015 edition of Fire Australia magazine. By Nathan Maddock.

How can we ensure our houses can withstand the rigours of severe winds?

Cyclone Tracy, which destroyed Darwin overnight on Christmas Eve 1974, is one of Australia’s most well known disasters. The Northern Territory capital today bears little resemblance to the town it was the night Tracy hit. New research by the Bushfire and Natural Hazards CRC is investigating the most practical ways of retrofitting older homes to withstand severe wind, and has its origins in Tracy’s aftermath.

Typical Australian houses built before the mid-1980s do not offer the same level of performance and protection during windstorms as houses constructed to contemporary building standards. Given that these houses will represent the bulk of the housing stock for many decades, practical structural upgrading solutions based on the latest research will make a significant improvement to housing performance and to the economic and social well-being of the community.

“The key to this research is producing practical and economical upgrade solutions for these legacy Australian homes,” said researcher Dr Daniel Smith from James Cook University’s Cyclone Testing Station (CTS).

Dr Smith, who completed his PhD studying tropical cyclone loads on housing at the University of Florida, USA, is a recent addition to both the CRC and the CTS, joining Associate Professor John Ginger, Dr David Henderson, Adjunct Professor John Holmes and Adjunct Professor Geoff Boughton, alongside Geoscience Australia’s Mark Edwards and Martin Wehner, on the Improving the Resilience of Existing Housing to Severe Wind Events project in the second half of 2014. He brings to Australia his research experiences from the cyclone-prone state of Florida, where wind-resistance issues for legacy housing are similar and have been the subject of research for many years.

The need for better standards

Cyclone Tracy in 1974 was the catalyst for change in Australia, Dr Smith explained.

“In the aftermath of Cyclone Tracy, there was a clear performance gap between infrastructure (i.e. industry, critical function, type of buildings), that seemed to withstand severe wind loads, and most residential structures that incurred significant damages,” he said.

“It became clear that engineered structures perform better in cyclonic loading scenarios.”

Research from that point forward became more focused on how to design houses to resist the loads experienced during Tracy.

Both the Northern Territory and Queensland changed their building codes in the early-to-mid 1980s to incorporate the lessons learned from Tracy. Other states also gradually began to modify their building codes too.

“This means that homes built before these code upgrades are not designed to the same performance criteria as those built to the current building code standards,” Dr Smith said.

When considered across the country, that is a substantial proportion of houses.

“One of the project objectives will be to determine the proportion these at-risk homes represent, using multiple survey sources including the Geoscience Australia National Exposure Information System database,” Dr Smith said.

But not just houses in cyclone-prone areas across northern Australia are at risk. Houses further south can experience storms with severe winds too.

“Our aim is to develop practical solutions to upgrade, or retrofit, these older Australian homes. We are looking for cost-effective solutions and as an added benefit, to quantify potential insurance benefits for the homeowner too,” Dr Smith said.

The current guideline

Currently, Standards Australia Handbook HB 132 provides guidelines for structural upgrading of older homes in non-cyclonic (HB 132.1) and cyclonic areas (HB 132.2).

To gauge the effectiveness of HB 132, the researchers have undertaken a nationwide survey of the building industry to evaluate usage of the guidelines and how effective they are at providing retrofit construction information. “So far [late January] the survey has been completed by 245 participants from a range of building industry groups across Australia. Over 91% have responded that they are unfamiliar with HB 132,” Dr Smith said.

“Of the 9% who are familiar with HB 132, the majority of respondents said the handbook was either too expensive, and therefore they would not purchase it, or that solutions in the standard were not practical or architecturally pleasing.”

What older houses are out there?

There is a lot of incentive Australia-wide for the research. To help achieve a nationwide outcome, the research team also includes Geoscience Australia, with access to the National Exposure Information System (NEXIS) database. NEXIS aims to collect, collate and maintain nationally consistent and best available exposure information at the level of individual buildings. Geoscience Australia has assisted in undertaking a physical survey of ten houses in Canberra. These houses were selected as they provided a reasonable representation of housing types across southern Australia.

“Older homes (pre-mid 1980s) were not built to any one standard, so there is a degree of variability in how they were assembled. It was not standardised construction,” Dr Smith said.

“The Canberra survey allowed us to record physical dimensions and observe a range of building components, to estimate the range of features in these older houses.

“We will be supplementing this information with additional surveys in areas where our current physical information may be lacking.”

Brisbane severe storm, November 2014

In addition to investigating older homes in Canberra, the research team seized the opportunity presented by Brisbane’s 27 November 2014 severe storm, and were on the ground within days.

“Most of the damage was hail related, although there were some cases of wind damage. We wanted to know if homes were subjected to the 141 km/h wind gust speed measured at Archerfield airport. Our general feeling was that the housing stock likely experienced wind speeds lower than this due to the localised nature of the storm, and the typical reduction in wind speed that occurs near the built-up topography of residential neighbourhoods,” Dr Smith said.

“It is important that homeowners are provided realistic wind speed information. If the media presents only the strongest wind speed recorded, people assume their home was exposed to that speed. If little or no damage occurred at their property, there is a false sense of confidence and often they are less likely to properly prepare or upgrade their home before the next wind event.

“Shaping how people look at this storm is very important. While most of the damage was hail-impact related, those structures that were damaged by wind were generally subjected to speeds much lower than 141 km/h. The range of wind speeds one storm can generate is quite large, and just because one speed is measured in an area does not mean the same speed was experienced elsewhere.”

Making a difference

While it is early days in the research, the CRC’s industry partners believe the project will deliver significant benefits.

“It is certainly important applied research aimed at long-term improvements in community resilience,” said Geoscience Australia’s Leesa Carson.

“The strategies under development will not only benefit homeowners and builders, but policy-makers too.

“It is hoped the case studies the team has undertaken, as well as future studies, will illustrate effectiveness in risk reduction, encouraging homeowners to implement changes.

“As part of Geoscience Australia’s ongoing work program in natural hazard mitigation, our involvement with the CRC is providing an opportunity to collaborate with leading researchers—ensuring that our capability includes the current thinking in the field of vulnerability to severe wind.”