The Twin Towers of the World Trade Centre were completed in 1973 following a seven year construction period, dwarfing previous skyscrapers in height and more especially in floor area. I was among a group of Irish graduate civil engineers who worked on other projects in New York in the late 1960s, and we marvelled at the gigantic scale of the evolving steel structures of the Twin Towers reaching skywards.
The towers were designed by architect Minoru Yamasaki and structural engineers Skilling Ward Christiansen Robertson of Seattle for The Port Authority of New York and New Jersey.
Even in a US context, the scale of the project was remarkable: it had a height of 417m (the south tower was 2m lower), floor area of 1.1 million sq m accommodating 50,000 people on 110 floors in each tower, a weight of over one million tons including 400,000 tons of steel, and a glass curtain facade measuring over 200,000 sq m. Ten thousand dampers were installed in each building to control sway from wind forces and a vast basement foundation to a depth of 20 m covered an area of three hectares.
The basement is approximately three times the area of Croke Park. Such was the volume of excavated soil from the basement that it was used to develop new land in the Hudson River to create Battery Park City.
The strength of the towers derived from the 240 steel columns spaced at 1m centres around the exterior of the buildings. The central steel core carried only the gravity loads. Horizontal steel trusses spanned from the exterior wall to the core and they supported the floors. Canary Wharf in London is in many respects a similar design but on a smaller scale. The Twin Towers would have been designed to withstand the severest wind forces and possibly earthquakes, but could not be expected to resist the doomsday scenario of violent impact from an airliner and the subsequent massive explosions and fire.
It has been speculated but not confirmed that the towers were capable of resisting the impact of the smaller Boeing 707, but this most likely did not include a catastrophic fire. A B52 bomber crashed into the 79th floor of the Empire State Building in 1945 killing 14 people but leaving the building relatively unscathed. The Boeing 767 planes which hit the towers on September 11 were almost fully laden with fuel, which on impact triggered explosions and fire.
The failure of the towers appeared to happen in two stages. The initial damage was caused by the airliners cutting through the exterior support columns. Then, as the ensuing fire spread, the mainly steel structure melted progressively under temperatures in excess of 1000
If the airliners had crashed into the towers lower down, the towers might have fallen over on to adjacent buildings, causing much greater loss of life and damage to property up to a distance of one kilometre. The period between the airliner impact and collapse of the towers allowed people to escape. Buildings are generally designed to allow for escape from fire in localised areas but not specifically for the fireball created by impact from airliners.
American engineers and architects have traditionally favoured steel frame construction over reinforced concrete. Concrete gives greater resistance to fire spread, but it is debatable if a concrete frame would have made any positive difference in this case. Scale model studies depicting the terrible events in New York might provide some indication of the comparative performances of steel and concrete skyscrapers under airliner impact and fireball conditions.
Europe's tallest building, the Messeturm in Frankfurt, and the world's tallest structures, the recently completed Petronas Towers in Kuala Lumpur, Malaysia are both constructed of reinforced concrete. The strength of the Petronas Towers comes from both the exterior concrete frame and the core of the building. Steel frame buildings have been favoured because of faster construction programmes, but advances in climbing formwork systems makes reinforced concrete a viable and possibly safer alternative to steel. Following the New York tragedy, it is probably inevitable that more stringent regulations will be imposed on skyscraper design. This might include greater use of high strength concrete over steel, more fireproofing, new maximum permitted heights and floor areas to assist evacuation.
The German authorities allowed significantly less floor space between the core and the outer shell in the Messeturm to that available in the World Trade Centre. The task of reconstruction will commence with the removal of the collapsed towers and surrounding buildings. This is a monumental and sensitive task and may take more than one year. The condition of the concrete basement foundation will be crucial to whether the towers can be built on the same site. It would appear almost impossible that the basement has maintained its structural integrity given the damage inflicted by the impact of the collapsed towers, which have packed into the basement void.
In the grim aftermath of the tragedy it would appear inconceivable that the entombed site would be excavated, and it may be more fitting to have a memorial park in honour of the victims directly on the footprint of the towers and build a new World Trade Centre on an adjacent site. The daring and expertise of architects, engineers and constructors will be required once again to secure the remains of the towers and to devise new and innovative solutions for reconstruction.
Could it be envisaged that the new World Trade Centre would be a global undertaking, funded by the international community? The reconstruction of the Centre will be one of the world's great projects; whether it will be another in the long line of daring record - beating towers beloved of New Yorkers or a series of more human scale skyscrapers like the Chrysler Building, it will always be a memorial to the terrible events of September 11th.
Eugene O'Sullivan, a chartered engineer, is a senior lecturer in construction management at Waterford Institute of Technology
