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Concrete In Australia : June 2008
PERSPECTIVE Lessons from the Boston Big Dig collapse by Geoff Fletcher I was pleased to read in the previous edition of this magazine in March (Vol 34 Issue1) an analysis of the Boston Big Dig report issued by the US National Transportation Safety Board (NTSB). It correctly noted the focus of the report which many other commentators and anchor system suppliers have missed – that the prime cause of the anchor failure was not defective product but defective engineering by many of the parties involved, including the product supplier. It may seem a small point but the fi rst reference cited in Geoff Fletcher. the article incorrectly calls up the ACI (ref 1) when it should have been the NTSB (ref 3) which clearly highlights failures in the engineering processes and not at all with the quality of the product used in relation to its own material specification. A hypothetical analogy I fi nd helpful in understanding this is one where, for example, a perfectly code-compliant 20MPa concrete might be used in an application where 60MPa strength was actually required. Would the concrete fail? Probably – but would the concrete be defective? Not by its own standard – it just should not have been specifi ed or allowed to be there in the first place. Back to Boston – far from suggesting that creep susceptibility for types of chemical anchor is a disaster waiting to happen, it was actually an unintended feature which allowed much warning that something was not right while still supporting load – akin to the use of structural timber in mine shoring. Excessive displacements in loaded anchors were first noted up to seven years before the failure occurred yet no one competently identifi ed, assessed and responded to these irregular observations. Given that creep behaviour is not a foreign concept to civil engineers (concrete and timber can creep, as well as some polymers – we all should have studied it) this might seem inexplicable. Worse still, in-situ load testing was carried out satisfactorily in response to the creep but using a test method which was completely unrelated to creep behaviour. It made as much sense as checking the lighting circuits in response to a hypothetical drainage problem. And, having established that the lights tested OK, continuing to rely on the lighting test as proof of no problem while observing the rising inundation! This sounds ridiculous but it parallels what happened in Boston. It’s all in the NTSB report. There are more sad but instructive lessons to be gained from this incident and it is the subject of a paper I am to present at the ASEC2008 conference in June. As the failings were in applied engineering practice generally, the lessons are universal for all engineers. And this is relevant not just to the few who focus on concrete anchorage. I no longer work directly in the concrete anchorage sector yet the message of the NTSB report – the risks of poorly applied engineering (and conversely, the value of correctly applied engineering) – is just as relevant to the wider structural precast industry in which I now operate. Concerning standards, it is worth noting that while the article rightly states there is no Australian Standard for post- installed anchor systems we do have something better than the minimal reference in AS3600 cited by others. AS3850 (Tilt- up concrete construction) attempts to address these systems insofar as they relate to precast concrete applications. In the case of Boston the ACI codes were ineffective to prevent the prime cause of the path to failure (incorrect specification, unmanaged construction, badly applied engineering) and they contain no provision for in-situ proof-testing in relation to creep behaviour, assuming the published acceptance criteria refl ect the ACI requirements. I imagine the US system might have some merits but it seemed inappropriate for others to go claiming that this system “works”. Extolling its benefi ts for manufacturers, designers and users is doubtful, as the issue at hand is a failure and a fatality where many parties, supposedly working under the Code provisions, share accountability, as the multi-million dollar settlements might suggest. Even more parties, including the ACI, have been cited in the NTSB report as having some responsibility “to make (design and construction agencies) aware” of what they need to know. This implies that the current state of affairs in the US is not adequate and clearly these agencies are not well enough informed at present. I think it is to our CIA’s credit that this issue has been tabled here. It is best-practice applied engineering which completes successful projects and which delivers value with managed risk, not unthinking and unmanaged use of standards. Geoff Fletcher Post accident scene with the crushed passenger car barely visible under the wreckage. The open area in the ceiling is the original location of the concrete panels. 16 Concrete in Australia Vol 34 No 2 PHOTO: MASSACHUSETTS STATE POLICE Business Manager Westkon Precast Sunshine, Victoria