A structural assessment of the effects of Cyclone Vance on houses in Exmouth WA.

Cyclone Vance hit the small North Western Australian township of Exmouth in March 1999. The Bureau of Meteorology designated Vance a category 5 cyclone, the highest rating. At the airport at Learmonth, some 35 km south of Exmouth, the Bureau measured gusting up to 267 km/h (74 m/s), setting a new record for the highest surface wind speed ever recorded on mainland Australia. The cyclone did not pass directly over Exmouth but was to the east as it headed down Exmouth Gulf. This meant that the maximum winds at Exmouth were not in the category 5 range. The wind speeds hitting the buildings are estimated as ranging from 200 km/h (55 m/s) to 250 km/h (69 m/s) in different parts of town. A survey was conducted of the structural damage to 460 houses, about half of the total in the town. It classified about 70% as having only minor damage, with damage to the remainder varying from impact damage to loss of walls. Of the different house types, virtually no damage was observed to the block houses with their concrete roofs. The DHA houses performed nearly as well, but some had damage to soffit linings which allowed rain water into the roof space and damaged ceilings. Nearly 30% of the houses surveyed had been built for the Housing Commission. About two thirds of these had virtually no damage while the rest were classified in the range up to loss of roofing. Just over 10% of the houses were transportable, having been brought up to Exmouth from the south. Nearly half of them had serious damage, even newer ones. The newer conventional houses generally performed better than the average, although they had the advantage of being in the less exposed part of town. This better performance also reflects the advances made in cyclone resistant construction in recent years. Engineered structures had virtually no damage. This was to be expected as the estimated maximum gust wind speeds were less than the design wind speed specified for Exmouth in the wind loading code. Although efforts were made to secure caravans and on-site cabins, they were generally not successful. A universal hold down system needs to be developed. Water entry through vents or broken external linings caused a problem. Ceilings collapsed and contents became saturated. This problem needs to be addressed as the Bureau did not consider the cyclone to be an extremely wet one