Was it inexperience, arrogance or just bad luck? Did the involvement of a superstar architect compromise the engineering? Or were the opening day sway problems of the £18M Millennium Bridge over the Thames just another example of the risks innovative engineers take when they push technology and design tools to the limit?
Since the public embarrassment resulting from the 10 June opening, Ove Arup, in public at least, has insisted that its design team complied with all current design guides and codes of practice. The excessive lateral movement that occurred when the bridge was packed with pedestrians was, according to Arup, an extremely rare phenomenon that it could not reasonably be expected to design for.
In fact, says Arup, so little is known about the interaction between large numbers of pedestrians and lightweight footbridges that it had no choice but to commission a Manhattan Project of its own to produce enough reliable data to devise an effective solution to the problem.
Results from the research carried out over the last few months at Sheffield and Southampton Universities and Imperial College, London will eventually be made available to all bridge designers, Arup pledged at its first and only press conference since the bridge was closed.
Since then, little hard news has emerged. Questions about what the solution will be, how long the bridge will be closed, how much fixing the wobble will cost - and who will be paying for it - are now being superseded by others.
Some structural engineers doubt that the 'synchronised marching' identified by Arup is that obscure a phenomenon. A quick trawl through the ICE's Great George Street library yielded at least three other examples of opening day sway problems on large footbridges, in Germany, Japan and, most recently, in Paris (NCE 13 July).
Moreover, the ICE's guide to dynamics, published last year, specifically warns against the problem. It is true, however, that no current code of practice covers this area - and at least one leading expert on structural dynamics believes that even with the results of the Arupsponsored research it will be impossible for a British Standards committee to draw up definitive guidance.
So, how come so few of the recent rash of lightweight high tech footbridges seem to suffer from lateral sway, on opening day or otherwise? Most of their designers will admit this is more down to luck than judgement.
Calculating how much inherent lateral and vertical damping is required is near impossible, designing a specific degree of damping into a structure is way beyond current capabilities. But, if you build in enough internal friction, via timber decks and handrails, the chances are the bridge will behave in a relatively docile fashion.
Even then a bridge may have good resistance to lateral sway, as on the Royal Victoria Dock footbridge, but still bounce too much vertically. The Millennium Bridge has almost no internal friction, ever since its deck was changed from tropical hardwood to aluminium, for very good environmental reasons.
Other design decisions seem to have contributed to the problem. By the standards of conventional suspension bridges the crossing has remarkably long back spans. Structurally this may be fine. Dynamically, it means that the natural frequencies of the main and back spans are very close together, so a wobble on one span can be transmitted right across the structure.
Inexperience again? There have been claims that the Arup team was selected more for its history of successful collaboration with Sir Norman Foster on a succession of building projects than its experience of cutting edge bridge designs. Arup naturally rejects this assertion, pointing to the depth and breadth of its in-house technical resources.
But did the team have too much confidence in these resources?
Is there something in the Arup culture that discourages its engineers from looking for outside specialist help - until after the problem emerges?
Even after bringing in outside experts from all over the world, Arup is rumoured to have had serious difficulties in coming up with an effective fix for the problem. One reason may be uncertainty over what could be considered as a solution - in other words, just how much the bridge can be allowed to sway even under the most extreme conditions. Or, even more likely, there could have been the problem of getting the client to define what he considers to be a solution.
This is where the real dilemma almost certainly hides - public expectation of how bridges should react to pedestrian input.
Northern Ireland's Carrick-arede rope bridge (see box) is seriously scary, yet visitors queue to cross it. They expect it to wobble far more than the Millennium Bridge ever did. Perhaps if Arup had not so confidently predicted that the bridge would be 'as sturdy as nearby Southwark and Blackfriars bridges' there would have been less of an outcry about its opening day behaviour.
Chambers dictionary defines 'hubris' as 'arrogance, such as invites disaster'. Claims on Arup's website that 'the bridge's response to people moving across it, people deliberately trying to excite it and wind have been thoroughly calculated and tested' certainly smack of hubris, in hindsight at least.
Since the bridge was closed Arup has never retreated from its first position - that a costeffective solution would be found which would not involve unsightly modifications to the structure or a long closure.
It could hardly do otherwise in the circumstances. And given that the opening day sway problems on other bridges have been 'solved' by fairly simple means, usually by adding extra dampers, Arup had good reason to be confident. But this is an extreme structure, with extremely complex dynamics.
One leading bridge engineer commented: 'I am torn between respect for the cleverness and courage of the design team in committing themselves to making something which flies in the face of so many basic rules of design actually work and a feeling that it may be very clever but it isn't good engineering.'
Arup's reputation is really on the line and everyone with the interests of the construction industry at heart must hope that its confidence is well founded.