The single word on NCE's 13 September 2001 issue, pasted over an image of the fi rst World Trade Center tower in freefall, still rings true. Just hours earlier, it had been genuinely inconceivable that structures of such magnitude could succumb to this fate.
Progressive collapse was by no means a new concept in 2001, yet on that bright autumn morning in Manhattan the world witnessed two of the most comprehensive examples of the phenomenon ever seen.
It is obvious to state that the impact and result of deliberately crashing a fully fuelled airliner into each tower was off the scale of the predictable. But there can have been few structural engineers who were not a little surprised to see two of the world's tallest buildings reduced to rubble less than an hour later.
This is not how structures are supposed to react.
Four years on we know pretty much all we can about how and why the WTC towers collapsed.
Yet the report and recommendations this week by the US National Institute of Standards & Technology following its exhaustive investigation into the events of 9/11 must be compulsory reading for all design professionals. The changes it urges will without question have a massive impact on building designs across the world.
Many in the UK will doubtless argue that NIST has adopted an overly prescriptive approach which will rule out the 'new' science of fi e engineering in favour of expensive over-engineered solutions.
Perhaps. But it is absolutely right that at the heart of NIST's recommendation is the need for designers of future tall building really to get to grips with disproportionate collapse and ensure that no matter how damaged, buildings will never fall down.
For British engineers, the Ronan Point collapse changed building design philosophy 40 years ago. And the Oklahoma City bombing 10 years ago prompted the US to rethink its federal building codes. Progressive and disproportionate collapse mechanisms are so well understood today it is crazy not to insist that all buildings meet minimum standards of structural redundancy.
And it is absolutely right for NIST to insist that designers also get to grips with the design of routes, strategies and facilities to get occupants out of a burning or damaged buildings quickly. We have the technology, the modelling techniques and the materials to provide proper escape paths and routes in for firefi hters. And there is no reason why we cannot make fi -protected and structurally hardened escape routes in tall buildings mainstream.
Major shifts in design thinking are usually prompted by major disasters. When infrastructure fails, engineers learn.
So while terrorists brought down the World Trade Center towers, we cannot ignore the lessons. As always, the crucial step is being bold enough to turn the learning into action to ensure that the risk of repeat is controlled.
The world is certainly a different place since 9/11 redefined the concept of terrorist threat.
NIST's recommendations this week show how far the structural engineering profession must now go in response.
The recommendations are tough and will no doubt be considered by many in the profession, at times, as a step too far.
But we need to heed them to ensure the world's concept of what is 'unthinkable' is never again experienced.
Antony Oliver is editor of NCE