Fitzwilliam College, Cambridge, 11am, Saturday 15 December 2001. An audience of around 100, all but a handful either undergraduates or recent graduates, is gathered. Session one of a symposium on 'Hi-tech through the ages' organised jointly by the British Group of the International Association for Bridge & Structural Engineering and Cambridge University Engineering Society is about to start - with the most emotion charged presentation anyone there is likely ever to hear.
Over the next two days some of the construction industry's most famous names will be on parade, ranging from former Arup director Sir Jack Zunz to millennium dome engineer Ian Liddell. Speaking on topics as diverse as the Sydney Opera House, advanced composites and disaster relief, they alone would have made this a memorable and inspirational event for the young engineers in the audience.
But it is the 73 year old structural engineer standing diffidently at the side of the stage who is responsible for the electric atmosphere in the crowded conference room. And it is his presentation that those privileged to be there will never forget.
Leslie Robertson had accepted the invitation to speak at the symposium long before the tragic events of 11 September 2001. In early October he was reported to have broken down in tears at a similarly pre-arranged event in New Hampshire after a member of the audience asked him if there was anything he now wished he had done differently when he designed the World Trade Center. Now, three months after the initial trauma, he seemed calm and composed as he took his place at the rostrum, and prepared to tell the unique story of the building that will always be seen as his greatest achievement.
Much of his speech was well rehearsed, a saga Robertson must have recounted scores of times over the last four decades, and one that has been summarised interminably in the media since 11 September.
Delivered with immense dignity and illuminated with flashes of gentle humour, it was never less than enthralling.
On its own, the story of how the WTC's innovative 'pierced tube' structural concept evolved directly from diminutive architect Minoru Yamasaki's agoraphobia - he hated the then fashionable glass clad skyscrapers with their vertiginous views from inside - would have been enough to grip the audience.
This was one of the first major computer-aided design projects, Robertson explained, in the days when computers meant IBM mainframes spewing out punched cards. 'More than two million punched cards were produced during the design process, ' he said. 'And a total of 39 different fabricators from as far away as Japan used them to co-ordinate production.'
But, try as he may, Robertson could not avoid references to the ultimate disaster creeping into the tales of the design development and construction. Explicitly or implicitly he responded to many of the questions and doubts that have been raised since the towers' collapse. No, a concrete structure would not necessarily have resisted a hydrocarbon fire any better, he insisted, showing graphs to support his assertion. The joints between the floor trusses and the perimeter columns were robust, with plenty of reserve capacity. And the different 'survival times' of the two towers could be largely accounted for by the fact that the north tower was struck along the stronger axis of the core columns, while the south tower, which fell first, took the impact along its weaker axis.
Humour returned with Robertson's account of the eerie 1971 topping out ceremony in the clouds, and even illuminated his detailed account of the 1993 bomb attack on the Center. He described how he took photographs of all the crushed cars in the underground carpark whose licence plates were visible under the debris, tracked down their owners, and sent them copies. 'They were remarkably grateful, ' he added.
But as Robertson moved on to the 11 September tragedy the atmosphere in the room heightened. As he dispassionately compared the relative impacts and fuel loads of the B25 bomber which hit the Empire State Building in 1945, the Boeing 767s which brought down the Twin Towers and the potential impacts of a 747 and an Airbus 380 it was hard not to admire his professionalism and selfcontrol. But, as he began to show dramatic photographs of the tragedy, the emotion finally got to him - and many in the audience.
One image in particular sent shivers down the spine; a closeup of the impact area on the north tower taken soon after the attack.
Its first assault on the senses came from the cartoonlike silhouette of the doomed airliner, still obscenely visible on the tower's perimeter. Then it was like staring into an open wound on the body of an old friend.
Fires burnt deep inside, twisted floorplates were revealed in lurid detail. Then, as the audience sat silent and stricken, Robertson directed their attention to the bottom left hand corner of the crater. There, dwarfed by the scale of the devastation above him, stood what appeared to be a solitary man, staring out over Manhattan.
Robertson contemplated the screen, then murmured: 'I often wonder if that really is a man, if he made it out or not.' For a long moment he was unable to continue. It was then perhaps that many of those present began to really understand what this man must have gone through since 11 September.
There were two other emotional highlights to follow. At the conclusion of his formal presentation Robertson showed a haunting image of the Twin Towers at sunset, with the colours clearly visible through the building's glazing. 'Most images give the impression that the towers were massive monolithic structures, ' he commented.
'This shows them for what they really were, lightweight, efficient, translucent.'
The applause that followed was genuine and deafening. But the most moving moment was still to come. Responding to a supplementary question about his relationship with Yamasaki, Robertson began with a goodhumoured and affectionate tale of the ups and downs of working with an allegedly difficult individual, then paused.
'He died of cancer nearly 16 years ago, ' Robertson went on, with obvious difficulty. 'I visited him every day in hospital, and his last words to me were 'look after the Trade Center for me'.'
At that point he could say no more, and sat down again. There were many in the audience who were also struggling to control their emotions. And few of those there will ever forget what they heard and saw that day in December.