Twelve months ago the world witnessed the biggest structural collapse ever, when the World Trade Center's twin towers came down after being struck by two large passenger aircraft. This week NCE assesses the lessons that engineers are starting to take from the tragedy, and examines how the events of 11 September 2001 could impact on high rise buildings in the future.
Ever since the unthinkable happened one year ago, engineers have been grappling with the implications for high rise structures says Dave Parker.
Most would agree that the terrorist attacks on New York's World Trade Center and Washington's Pentagon finally made the world aware of what every engineer already knew - there are extreme events that no building could be designed to resist. A large enough aircraft, a big enough bomb, could bring down any building, however robust.
So, argued some, this meant there was no point in changing codes or spending money on research into how buildings might better resist extreme events. In any case, proposals to amend codes implied current codes were inadequate. Lawyers might seek to prove someone liable for these inadequacies.
But as twin towers structural engineer Leslie Robertson says (below), it may be far better to direct the search for solutions towards the area of politics and diplomacy. Practical measures should be focused on improving security, or remote overrides for straying airliners.
But this is little comfort to office workers at their desks in Canary Wharf, for example.
They know the IRA has already targeted their buildings. They can see large aircraft flying by every few minutes.
Their anxieties are not unreasonable. No security procedure is ever perfect; every political settlement leaves angry, disappointed activists unwilling to abandon their campaign of violence. And airliners fall out of the skies by accident far more often than by intent.
Engineers have to be able to respond to such perceptions of vulnerability. They have to clarify the risks and the physical realities, and spell out what is possible and what is not.
The most encouraging lesson to emerge since September 11 is that engineers do have answers to reassure clients and tenants alike. The skills and the technology are available to produce safer, more robust buildings without significant initial cost penalties.
Computer programs can now model a building's response to extreme trauma or simulate human behaviour in mass evacuations more realistically than ever before. Building owners can get a realistic assessment of the risks to their property and its occupants and what can be done to minimise these risks, whether from impact, explosion, fire or chemical and biological agents.
Designers can offer clients a range of safety measures, from more ductile structures to larger escape stairs, and quantify the cost/benefit relationship with confidence.
There are still problems to be resolved, however. Those who believe in the superiority of British and European design codes have no real basis for complacency. Lessons from Ronan Point and the three bloody decades of the IRA's bombing
Frank Lombardi, chief engineer at the Port Authority of New Jersey and New York, worked all his life in the Twin Towers, originally built and owned by his employer.
'My desk was on the south west faþade on the 72nd floor of Tower1 - the first plane hit over 20 storeys above on the north. I didn't hear anything. The building swayed however and I thought perhaps we were having an earthquake. I didn't know that it was a definitive terrorist strike. If we had known we were under attack that could have been deadly, but we remained nice and calm.
'Then a piece of the aircraft like a large beach ball went through the building above. With a rain of correspondence and mail coming from south side I realised the building facade had been breached. There was no panic, more a heightened awareness that something was happening. We decided to clear the floor without panic.
'When I reached the plaza, that was the worst scene, I cannot even think about it now. Body parts in the plaza, blown from the aircraft or their offices, not one body in one piece. That was the saddest part of the whole thing.
'I met a colleague on the street, and we decided to meet with other engineers in the Marriott Hotel which was also the meeting place after the 1993 bomb attack to see what we could do together.
'Around 11 of us at the lobby decided to move closer to a bar where there were tables and chairs. That was when Tower 2 began falling on top of us - we had no idea what was happening.
'We happened to be under a girder that we had ordered to be placed in 1993 to refurbish the hotel. All 11 of us with some firemen walked out without a scratch on us because that 7 foot (2.1m) girder protected us as if it was an umbrella. But we were lucky - 75 colleagues and our chief executive were not.'
campaign have been incorporated into codes and Building Regulations. Buildings designed to the latest codes are unlikely to suffer progressive and/or disproportionate collapse, whatever the event.
But too little is still known about the behaviour of buildings in real fires to be certain that current codes will provide the fire performance needed.
Yes, there are computer programs that can predict how fires might develop - but the data that underpins them comes from a pitifully small number of full scale fire tests. Almost the same comments can be applied to blast resistance, or the effects of the traumatic loss of a structural element. Whole building performance can be predicted much better now than in the aftermath of Ronan Point. Is it good enough?
There are signs of real change in the building profession, however. Design is becoming more holistic, with structural and fire engineering beginning to merge into a single discipline.
Security and risk assessments will become as routine a part of the procurement process as environmental impact assessments. And, maybe, tenants will be persuaded to pay more for greater perceived protection.
What still needs to be resolved is the built-in price of greater robustness. Will stronger, more ductile structures be more difficult, expensive or dangerous to demolish once their useful life is over? The impact of September 11 will resonate many decades into the future.
Case for stronger buildings unproven - WTC designer
Should we build stronger buildings as a result of the perceived increased threat of terrorist attack since the Twin Towers? Their designer Leslie Robertson thinks not. Diarmaid Fleming met him.
The scarred earth excavation at Ground Zero is clearly visible from Leslie Robertson's office, 48 storeys up and only a few short streets away. 'I don't feel any differently today to what I did then - nothing has gone away, ' says the twin towers' designer.
'I haven't taken a pill that's cured feelings inside me about the people who died in the project and I don't think I am ever going to make that go away. I'm better able to talk about it: it's not easy today but I am able to carry on a reasonably rational conversation about it without the pain welling up inside.'
Robertson says the disasters have not instilled panic among the tall-building fraternity. 'I don't see developers saying 'let's not build tall because someone might run an aeroplane into my building', or 'let's design it somehow differently'.
What developers would like to say is 'hey, this project is just as good as the WTC in resisting the impact of an aeroplane' - and that's not so difficult to do. It's more of a rental issue than a real concern on the part of the developer. They're rational people, if they can rent it, they'll build it.'
And the threat of disasters doesn't mean an instant change in design approach. Few buildings have been strengthened in New York, he says, adding: 'If you set out in London and start looking at all the buildings built in the last 20 years, you'll find many vulnerable to truck bombs.'
Since last September he has spoken to the developers of the three tallest buildings planned in the world - China's Shanghai building, an 88-storey building in Hong Kong and the 100-storey Kowloon Station building.
'Two of these projects could have been stopped but are going ahead. Developers are very knowledgeable people - they realise that what happened on 9.11 was far from the worst event that could have happened. Imagine the difference between a 767 and a new generation Airbus A380 superjumbo or a 747 freighter, heavier and with more fuel.
No one is going to design for 30 or 40 storeys moving.
'The solution is not to design stronger buildings. The solution is to deal with the Muslim world.
Islam is a beautiful religion, and we need to teach people this in the US and the UK.
'We should start dropping food instead of bombs. There is no real balance against the US in the world. It's not good to have so much unbalanced power in the planet, and if we look critically at ourselves, it is easy to see why people could hate us.'
He says he has received huge support from other engineers, and regularly receives letters from relatives of those who died in the buildings, all supportive. Many ask to meet, which he does often with a stroll in nearby parks. He has had little contact with politicians, but says that should there be an opportunity to take part in future work on the site, he'd gladly do so, but thinks it unlikely.
As to future developments, he says the effect of the tragedy on design will not suppress people's enjoyment of spectacular structural creations, and will not, for example, lead to a move away from glazing for fear of blast damage. 'We have to continue to live, enjoy things and have fun.
'If you were ever on top of the World Trade Center in the late afternoon as the sun was going down, sunset in the west and the shadow of the towers creeping in the east across the city, that was a wonderful sight. And we don't want to take those experiences away from ourselves.'
lNCE's technical editor Dave Parker and Arup Fire International director Peter Bressington are speaking at a special ACE Progress Network seminar 'September 11 one year on' on the evening Wednesday 11 September in London.
Further details from Sean Semple at ACE, email firstname.lastname@example.org lNCE's World Trade Centre coverage over the past year can all be found on a special microsite at nceplus.co.uk/magazine