The structural engineering profession generally considers structural design to be a rational process, with a good understanding of loads and material properties incorporated into the codes and standards in general use, and failures of most classes of structures are rare events. However, when they have occurred, failures often reveal major gaps in knowledge, particularly on the loading side.
Often there are extensive investigations following failures associated with high economic costs or loss of life. However, these are usually associated with legal actions and the often-valuable information is not made generally available to the profession at large. This ‘cloak’ of legal privilege seems to be a twenty-first century phenomenon, and is in contrast to major failure events of the nineteenth and twentieth centuries which were followed by extensive public enquiries. Examples are the Tay Railway Bridge disaster of 1879 in Scotland and the Tacoma Narrows bridge failure of 1940 in the American state of Washington. In Australia, the collapse of the West Gate Bridge in 1970 invoked a Royal Commission.
Although the details and of individual failure events and identity of the particular structures cannot be revealed, it is worth highlighting some general features of recent local failures with the hope that some lessons can be learnt. There have been two failures of large stadium roofs in Australia in the last ten years, in windstorms with wind speeds well under design values. Although factors, such as undersize bolts, apparently also played a role, in both cases the unbalanced, and non-uniform, loading distributions applied by wind were not properly taken account of. Modern wind-tunnel approaches can routinely generate multiple load cases that can ensure that expected maxima in forces in critical structural members are identified; these approaches have been available for about twenty years but the need for, and the value of, them, is apparently still not appreciated by many structural consultants.
Several large warehouses in the Eastern Creek area, west of Sydney, collapsed under the weight of hail from a storm in April 2015, with over $300 million of damage claims to buildings and contents. Yet hail loading is not mentioned in the Australian loading standards, or in the building code. For parts of the east coast of Australia, roof loading by hail loading appears to be a significant environmental load, for the high return periods associated with ultimate limit states, and should be on the agenda of the relevant standards and code committees.
For many years high-voltage electrical transmission structures have been failing in several states of Australia and in other parts of the world, as a result of winds from small intense storms associated with thunderstorms; these are by no means completely understood by meteorologists or wind engineers. While that industry now understands the significance of these ‘high-intensity’ non-synoptic winds, their effects are yet to be fully incorporated into the relevant codes and standards. However, the economics of transmission line systems inevitably results in structures that are marginally stronger than the expected loads and failures will, no doubt, continue to occur.
The work of the Cyclone Testing Station in Townsville and others, over forty years, has contributed greatly to improvement in building codes in Northern Australia, in relation to cyclonic wind loading of low-rise buildings. This has led to improved resistance of new housing in the tropics. Ironically engineered buildings have often performed worse than houses, in many cases a result of a ‘cavalier’ approach by some designers to the assessment of internal pressures. A recent (2016) amendment to the Wind Actions Standard AS/NZS 1170.2 is intended to change this culture.
Climatologists studying the potential changes due to global warming are predicting more southerly tracks of tropical cyclones. This may put the more heavily populated coastal regions of southern Queensland, and even northern New South Wales, in the tracks of severe tropical cyclones. The committees responsible for the codes and standards which need to look fifty years or more into the future when setting design rules, should monitor research and observations into the effects of climate change at the time of revisions.
While standards such as AS/NZS 1170.2 have been extensively developed over the last forty years, there are some major shortcomings that need to be addressed. One is the need to incorporate the characteristics of non-synoptic winds as discussed earlier. Another is the need to improve the specification of shape factors for industrial structures, particularly those used in the resource industries (i.e. mining and oil and gas). These structures typically consist of many closely-spaced elements, and significant aerodynamic shielding and interference effects. It has been common practice to assess wind drag forces separately on individual elements and sum them; neglect of mutual shielding and interference can lead to gross over-estimates of wind loads. In one recent case, when wind-tunnel tests were undertaken, the individual element approach was found to lead to an over-estimation of wind forces by a factor of about ten. Hopefully many of these shortcomings in the standards can be addressed in time for the next mining boom.
Another area in which the Australian standards currently fall short is the provision of shape factors for renewable energy structures such as wind turbine towers and solar panels. In the latter case the effects of hail, as well as wind, loading should be a consideration.
It is clear, from the examples cited, that many uncertainties still exist on the loading side of structural design, particularly those associated with natural events. These include the ongoing, but uncertain, effects of climate change associated with global warming. The loading uncertainties can lead to over-design, as well as under-design and the possibility of failure.
Notes on contributor
John Holmes is Director of JDH Consulting based in Mentone, Victoria, and represents Engineers Australia on BD006 (General design requirements and loading of structures) and is Chair of BD006-02 (Wind Actions) for Standards Australia.
Director of JDH Consulting
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