Note: Dec 23 2024 - this article is in Draft form and is Incomplete:
Severe Cyclone Tracy was considered to be the most damaging cyclone in Australia's history.
Although extremely small in area the wind speeds were substantial - the anemometer at Darwin Airport recorded a gust of 217 km/h before being destroyed - with sustained speeds of 189 km/h as it hit Darwin. An accepted peak speed is reported as 236 km/h.
66 people were killed either on land or lost at sea and the city was flattened. Most people were unprepared as an earlier cyclone passed by without causing havoc.
Cyclones have different "Categories" from 1 ("Damaging winds. Negligible house damage. Damage to some crops, trees and caravans. Craft may drag moorings.") to 5 ("Extremely dangerous with widespread destruction of buildings and vegetation. Maximum mean wind speed greater than 200 km/h. Typical strongest gust greater than 279 km/h.") Tracy was a Category 4 when it was over Darwin.
See article in Wikipedia (consider contributing a few dollars as I do)
Refer to an ABC Article with short videos of the damage, interviews with survivours and the support by other Australians.
Some Reflections from an Engineering Perspective
In particular about 80 percent of houses were destroyed - many losing their roofs and the walls then collapsing like a deck of cards. Significant changes happened following Tracy in that the roof elements needed to be tied in with walls and down to the foundations in cyclonic regions.
It was also at a time when structural design was commonly done using the "working stress design" (WSD) approach and when each State in Australia had its own Building Regulations. A decade later the first Building Code of Australia (BCA) was published in 1988.
Much later the BCA evolved into the National Construction Code (NCC) which is updated approximately every three years by the Australian Building Codes Board ABCB which was established in 1994. One of the first tasks of the ABCB was to convert the BCA into a more fully performance-based document - more in line with the Limit States Design approach.
The mid 1970s was also the period where the philosophy of design started to adopt the "limit states design" (LSD) approach. Many of the Standards (we use to call them Codes until the BCA was published) were still in imperial units or a direct conversion from imperial units.
The Australian Standards Series AS 1170 (1973 edition, pre Tracy) replaced the earlier Imperial Standard SAA Int 350 (pre 1971) covering the design for wind. It is reported that the Design Pressures for some similar locations and building types increased by over 40% whilst if the Wind Loadings based on AS 1170.2:1989 were almost doubled!
Over the years the Wind Loads given in AS 1170.2 or the way of calculating them has changed from the 1973, 1989, 2002 and 2011 Editions.
Tensile Fabric Structures Under Wind Loads
One of the major problems with designing a tensile fabric structure to resist wind loads is the determination of the pressure coefficients to use since the geometric forms of membrane structures are unique. It is vital that the fabric is adequately prestressed so that as the structure deforms to resist the wind loads, areas do not go slack. Following the passing of a wind gust, slack areas - or areas that have wrinkles - can suddenly snap tight and tear. Once a tear happens it is a very large stress concentration.
