Cyclone tracy and the road to improving wind–resistant design

Early on Christmas morning 1974, tropical cyclone Tracy devastated the city of Darwin leaving only 6 per cent of the city's housing habitable and instigating the evacuation of 75 per cent of its population. The systematic failure of so much of Darwin's building stock led to a humanitarian disaster that proved the impetus for an upheaval of building regulatory and construction practices throughout Australia. Indeed, some of the most enduring legacies of Tracy have been the engineering and regulatory steps taken to ensure the extent of damage would not be repeated. This chapter explores these steps and highlights lessons that have led to a national building framework and practice at the fore of wind-resistant design internationally. Tropical cyclone Tracy was a small but intense cyclone, with a landfall radius to maximum winds of 7 km, a forward speed of 7 km/h and central pressure of 950 hPa (Bureau of Meteorology, 1977) (Figure 9.1). Tracy was an Australian Category 4 cyclone with estimated maximum-gust wind speeds on the order of 250 km/h (70 m/s) (Walker, 1975). The recorded gust of 217 km/h (60 m/s) at Darwin Airport before the anemometer failed was, to that time, the highest wind speed measured anywhere on mainland Australia. Tracy's small size minimised the spatial extent of damage, but its slow translational speed meant areas impacted suffered more damage than might otherwise have been the case. Of cyclones that form in Australian waters, one passes within 200 km of Darwin every one to two years. The expected recurrence interval of an event similar to or stronger than Tracy impacting Darwin is greater than 100 years based on historical records