In the concluding part of this article, Sean Brady explores in more detail the technical cause of the Quebec Bridge collapse and the human factors that lay behind it.
In Part 1, published last month, we examined the tragic series of events that culminated in the collapse of the Quebec Bridge. One of the first significant technical issues manifested itself early in construction, when it was discovered that bridge members arriving on-site were heavier than expected.
The cause of this discrepancy was a self-weight miscalculation by the bridge’s designer, the Phoenix Bridge Company, with the actual bridge being 10 per cent heavier than initially calculated.
Despite the error, which resulted in a seven per cent increase in bridge stresses, Theodore Cooper, the prestigious bridge engineer engaged on the project as a technical consultant, allowed construction to proceed.
However, by mid-1907 evidence began to mount that something serious was amiss — significant bowing was observed in a number of the bridge’s key compression members. Phoenix — along with Cooper — would initially conclude that this bowing was unlikely to be related to in-service stresses, but was either pre-existing or due to the members sustaining impacts during construction.
Norman McLure, Cooper’s site inspector, disagreed: he was convinced that the bowing had occurred post-installation and found no evidence of impact damage. On August 29, 1907, with bowing in one member having increased from 19mm to 57mm over the previous two weeks, Cooper finally accepted an issue existed and directed that work cease pending an investigation. It was too late — the collapse occurred later that evening with the loss of 75 workers.
Technical cause of failure
The Royal Commission into the collapse would identify that the buckling of one of the lower chords in the anchor arm, located near the main pier (member A9L), initiated the progressive collapse of the structure(1).
It was this very member that exhibited the significant bowing in the days leading up to the failure. The commission would conclude that the failure of the member was not related to detailing, fabrication or material quality. It was due to defective design.
Firstly, the self-weight miscalculation played a role, with the commission identifying it as a 'grave error' which should have been sufficient to require condemnation of the bridge(1).
However, what makes Cooper’s decision to continue after the error came to light quite extraordinary was that the working stresses adopted for the bridge’s design were already very high — in fact, considerably higher than the norm at the time of construction.
Cooper allowed working stresses of 145MN for normal loading and 165MN for extreme loading conditions(1). Compared to modern American Institute of Steel Construction (AISC) allowable stresses, Cooper’s stresses were up to nine per cent higher.
So Cooper was utilising higher working stresses than AISC, but was doing so more than 100 years ago, at a time when steel was still a relatively new construction material. While concerns regarding this approach were raised by a bridge engineer from the Department of Railways and Canals, they were dismissed by Cooper. Cooper’s reputation ensured his view prevailed(1).
The issue was further complicated by a lack of industry knowledge regarding the behaviour and capacity of latticed compression members. The Commission found that an engineer engaged in the design of such members “finds little or nothing in scientific text books or periodicals to assist his judgement”(1).
Amazingly, despite Cooper’s willingness to accept uncharacteristically high working stresses, with little theoretical guidance and no practical precedent in the design of such members, no member testing was undertaken. Certainly, obtaining funds for testing was a significant issue. (Indeed, funding issues plagued almost every aspect of the project: “virtually every conflict between safety and economy was resolved in favour of economy”(1).)
[caption id="attachment_50313" align="alignright" width="300"]
Figure 3: Collapse of second Quebec Bridge.[/caption]
However, it also transpired that when the Phoenix Bridge Company did secure funding for testing, Cooper turned down the offer, citing “time constraints”(1). It appears that Cooper was simply confident that the actual performance of the bridge could be determined by theoretical means alone, a view shared by Phoenix, despite the bridge’s record-breaking length.
Following the failure, testing was undertaken and showed that one-third scale copies of the latticed members “failed explosively” under loading, confirming the Commission’s view that the latticing was insufficient for the member to act as a complete unit(1). (It is a sad fact that funding for testing is almost always available after a failure.) So Cooper and Phoenix were relying on a theoretical basis that was simply inconsistent with actual performance. Cooper's approach was “unsafe practice”(1).
Human factors
So why did an experienced engineer like Cooper take such an approach? Given that the bridge was to be the crowning achievement of his career, was his ego driving him beyond the boundaries of engineering knowledge and practice?
Certainly, ego played a role, and perhaps one way of attempting to understand Cooper’s state of mind as he approached the design and construction of the Quebec Bridge is to go back to the comments he made following the completion of the Forth Bridge in Scotland in 1890(2).
While there were numerous comments directed at the high level of conservatism in its design — it was, after all, designed in the aftermath of the Tay Bridge disaster of 1879 and there was little appetite for another railway bridge failure — Cooper was particularly scathing in his comments, describing the bridge as: “the clumsiest structure ever designed by man; the most awkward piece of engineering in my opinion that was ever constructed”(2). He then went on to state that an American could have done it for half the cost.
While any examination of how these views shaped Cooper’s approach to the Quebec Bridge is pure speculation, there is evidence to suggest that they affected his technical judgement. Firstly, the Forth Bridge’s span of 521m was longer than the initial design span of 488m for the Quebec Bridge. Cooper would extend this proposed span to 549m to make it the longest cantilever bridge in the world.
While this was not necessarily an issue in itself, and there were indeed reasonable technical grounds for doing so (as discussed in Part 1), the decision contributed to the self-weight miscalculation — Phoenix did not update the span length to reflect the longer and heavier bridge.
Secondly, and perhaps more damaging, was Cooper’s view that the bridge was clumsy and awkward, and that it could have been built for half the cost. This attitude certainly appears to have influenced his decision to allow increased working stresses, thus reducing the cross-sectional area of the bridge’s members.
It was during the design of the replacement bridge that the extent of Cooper’s drive for slender members became evident. Of course, any redesign in the wake of a collapse will be inherently conservative, but in the case of Quebec, the cross-sectional area of some members increased by as much as 150 per cent(2).
[caption id="attachment_50314" align="alignright" width="300"]
Figure 2: ‘Clumsy’ structure of Forth Bridge.[/caption]
The replacement bridge, completed in 1917, was 2.5 times heavier than the original design, and in a final irony, did not look dissimilar in ‘clumsiness’ to the Forth Bridge (Figures 1 and 2), the very thing Cooper was at pains to avoid(1).
And, of course, the story of the Quebec Bridge doesn’t end there. During construction of the replacement bridge, in an operation to lift and install the central span, a lift bearing failed and the 195m long, 4500t central span collapsed into the river 46m below, resulting in a further 13 fatalities (Figure 3).
In all, the story of the Quebec Bridge reminds us of the dangers of innovative projects relying on the judgement of just one individual. Cooper’s immense reputation, far from making the project safer, would, paradoxically, place it at greater risk.
His insistence on a longer span and high working stresses appears to have been guided by concerns of legacy, rather than a sound engineering basis.
His refusal to submit to independent checks, combined with the ‘unquestioning’ environment his reputation created, essentially resulted in his decisions being unchallenged. In all, the Commission would conclude that Cooper’s very presence created “a false feeling of security”(2).
Bowing members
Finally, why were the bowed compression members dismissed as evidence of distress for so long? One issue was the personnel actually managing the issue: Cooper was more than 800km away in New York and never visited the site once construction of the superstructure began; Phoenix’s key engineers were located in Phoenixville, Pennsylvania, with the company's chief engineer later admitting to never actually seeing the critical bowed members; this left McLure, Cooper’s site inspector, in the unenviable position of trying to manage and understand the unfolding situation.
Fundamentally, while the project team looked good ‘on paper’, the practical reality was found wanting, with the Commission concluding that: “it was clear that on that day the greatest bridge in the world was being built without there being a single man within reach who by experience, knowledge and ability was competent to deal with the crisis”(1).
Implicit assumptions
Another issue was one we see time and time again with all catastrophic failures: the role played by implicit assumptions. Implicit assumptions are those that we rely upon, but are unaware we even make, so they typically
escape critical examination.
When faced with evidence of structural distress, both Cooper and the Phoenix Bridge Company made the implicit assumption that the bridge’s design was sound. Once this assumption was made, the evidence of distress was shoehorned to fit alternative theories, none of which considered an error in design.
Only when it was too late, with the opportunity to effectively intervene lost, did the validity of this implicit assumption come under scrutiny. Implicit assumptions, and the very real threat they pose to structural engineering design, will be the subject of a separate article.
This article was first published in November 2014 in ‘The Structural Engineer’, pp 20–21. www.thestructuralengineer.org
Author: Sean Brady is the managing director of Brady Heywood (www.bradyheywood.com.au), based in Brisbane, Australia. The firm provides forensic and investigative structural engineering services and specialises in determining the cause of engineering failure and non-performance.
References
1) Delatte N. J. (2009) Beyond failure: Forensic case studies for civil engineers,
Reston, USA: American Society of Civil Engineers
2) Åkesson B. (2008) Understanding bridge collapses, London, UK: Taylor & Francis