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Concrete In Australia : September 2008
PERSPECTIVE Lessons for a young engineer from bridge failure by Rod Johnston On 15 October, 1970, Melbourne’s Westgate Bridge collapsed during construction, killing 35 people. At the time I was a young cadet engineer. Four years later, I found myself in the London offi ce of Flint and Neill, the structural consultants analysing the redesigned bridge for construction stresses. For nearly a year, I wrestled with the plastic behaviour of friction- grip bolt groups, shear lag in webs and fl anges, and catenary cable stresses. Young, idealistic and newly graduated, my colleagues and I pondered and debated the ethics, analysis methods and procedures necessary to ensure a responsible balance between structural safety and economy of such structures – and these lessons are still relevant a third of a century later. The Royal Commission found that the failure could be traced to two main causes – exacerbated by inadequate communication between the principal parties, confusion, poor co-operation, antagonism and site organisational failings. I believe the collapse was caused by: • failure of the designers to give a proper and careful regard to the process of structural design, and failure to adequately check the safety of the erection proposals put forward by the original contractors • the contractor adopting a previously untried method of erection, which would require greater care than the level eventually exercised. I consider myself very fortunate indeed to have had significant infl uence over many years on a number of Australian loading and structural standards. I now represent the ACEA (Association of Consulting Engineers, Australia) on the Building Codes Committee of the ABCB (Australian Building Codes Board). The Building Code of Australia (BCA) makes provision for the complying use of suitable “referenced documents”, including structural design manuals produced by various organisations such as manufacturers and their associations. Such flexibility is a good thing, guaranteed to encourage innovation – but, there is great risk of inconsistency of structural reliability amongst competing products and systems. Structural design engineers must be given assurance that the levels of safety in new and innovative products will be consistent with that of the more commonly used Australian standards. In an effort to foster such consistency, the ACEA has developed a protocol called Structural reliability of BCA-referenced structural design documents, and is now promoting its adoption by the ABCB. While the collapse of a major steel box-girder bridge in Melbourne in the 1970s is a long way from the structural reliability of building products and systems, the lessons learned from the former are still pertinent to the latter. Structural design and construction must reflect: • adherence to clearly defi ned systematic design processes • achievement of consistent measurable structural reliability, which refl ects realistic construction tolerances • close adherence to the construction practices assumed in the design process and specifi ed in the drawings and specifications • respect for the authority of the design engineer in all matters that affect structural safety • responsibility of the design engineer to ensure that construction limitations are practical and correctly observed. The collapse of Westgate Bridge provided a timely wake-up call in the 1970s. We must learn the lessons of history and test new regulations and standards in the context of recorded failures. Rod Johnston is a civil/structural engineer and builder. He is the Principal of Electronic Blueprint, a provider of web-based specifi cation, details, training and software and engineering consultancy Quasar Management Services. Reinforcement Detailing Handbook A complete revision of the original first published in 1975. The 2007 edition takes into account changes to relevant standards, design practice and developments in the choice of available reinforcement types. Available via the Institute’s web site or through Standards Australia/ SAI Global. The basic requirements of good reinforced concrete detailing are clarity and conciseness. Unfortunately, drawings supplied for reinforced concrete over the last twenty years. The net result of poor quality drawings is increased costs in the material supply and construction sectors and unacceptable levels of dispute. The aim of this manual is to guide designers, draftsmen and other professionals toward a uniform method of communicating the design intention to the construction team so that confusion cannot arise from the misinterpretation of the drawings. 20 Concrete in Australia Vol 34 No 3