Firefighters at the World Trade Center - 12 September 2001

World Trade Center - rebuilding a national icon

It's been ten years since the twin towers of the World Trade Center were destroyed in a terrorist attack. Regeneration of the iconic site was always going to be difficult, but after much soul-searching a new, bigger, stronger and safer tower is rising from Ground Zero.

There are numerous events that are deemed to have changed the world, but when two Boeing 767 aircraft were flown into the twin towers of the World Trade Center on 11 September 2001, it was a pivotal event in modern history. Since then the United States of America and its allies have been engaged in a 'war on terror' that shows no signs of reaching a satisfactory conclusion; security has become the over-riding concern in everyday life; and a cloud of trepidation hangs over long-distance travel.

By far the greatest impact was felt by the citizens of New York City. The collapse of the twin towers claimed the lives of 2,752 people and left a vast tract of Lower Manhattan devastated. The healing for those who lost family and friends in the terrorist attack will be lengthy, but for the city itself, life and regeneration goes on.

On 30 May 2002, an official ceremony at Ground Zero marked the culmination of the clean-up effort; it was time for a renaissance. But redeveloping the site with emotional scars still raw was bound to be shrouded in controversy, and so it proved. A multitude of stakeholders including Silverstein Properties, owners of the original buildings, The New York Port Authority, who owned the land, New York Governor George Pataki, New York Mayor Michael Bloomberg and families of the victims all had strong opinions. These ranged from leaving the entire site as a memorial to those who lost their lives to rebuilding a replica of the fallen towers.

A company, Lower Manhattan Development Corporation (LMDC) was created to channel the views of the stakeholders, and architects from around the globe were invited to submit their visions for the site. Submissions were received from architectural luminaries such as Norman Foster, Richard Meier and Daniel Libeskind, but the task of designing a new icon fell to Skidmore, Owings & Merrill (SOM). Its brief was the design of 'building seven' (on the site of the less iconic building to collapse that day) and the showpiece One World Trade Centre, originally dubbed 'The Freedom Tower'.

Because it sat outside the LMDC master plan, Seven WTC moved on at pace and was completed in 2006. However, the designs for One World Trade Center were not without their problems. Although their design had won the competition it faced some criticism from law enforcement agencies because of the amount of glass at its base and the proximity to the main thoroughfare.

Back to basics

'We needed to make a much more robust design and so abandoned the initial design and went to a much more simple and elegant solution for the building,' Ken Lewis, SOM director and project manager of One WTC says. 'This took us back to our basic ideas, which were using the same building footprint as the memorial size. The height of the building is also the same height as the old towers, 1,368ft, but the new requirement in the master plan is to reach a height of 1,776ft, which would be achieved by the digital antenna.

'The idea is really simple and is based on the iconic nature of 'the icon in the sky'. There will be the memorial that you see at ground level where you have the emotion, but the building is 'the hole in the sky', the icon that you will talk about and think of immediately when you think of New York City.

'These are designs that mark the millennium, with purity of shape. They are very elegant and simple, but in a way very high-tech. The level of technology that is embedded into these buildings goes far beyond that which you would see in any traditional buildings.'

The sight of the twin towers of the WTC crashing to the ground left an indelible memory on all who witnessed it, but for the architectural community it added a new dimension to their work; how to make buildings safe from even the most unforeseeable attacks. 'I couldn't believe it when the building went down,' Lewis says. 'We have had a lot of discussions since then with several structural engineers and everybody, including myself, has said it did greater damage than anyone could have ever imagined.

'Remember, this is a building that has a structural steel frame on the outside and a full quarter of that had been literally ripped out, completely sliced off. The building still started to shed the load and to the greater or lesser extent anybody above the impact spot didn't survive but those below either did, or at least had a chance of surviving.'

The original WTC was built when downtown New York City itself was not a healthy or a safe place and any who had visited the buildings before their demise will recall how at street level it was dark and enclosed. The construction began in a period when Wall Street itself was dying and the executive's headquarters were all moving to Midtown. But the Rockefellers were building their bank at the time and they felt they had to save downtown as the financial centre.

They had a two-part plan to do this. The first was to build the WTC and the second was to expand the downtown area by dredging the harbour. The district where the WTC was built was the radio district, but this era was disappearing and the decision was taken to use this area. The designer of the original towers, Minoru Yamasaki, was building a whole city up as a fortress city; the grade was raised 40ft and all of the roads were built so that they were intercepted by one of the buildings in a way that made you feel that 'all roads led to the castle'.

'When this opportunity came along we thought why not look at reintegrating part of the city into this area,' Lewis explains. 'The main thing we are missing and just don't have in downtown is open space; so creating this open space for the memorial is of great benefit.

'It has happened again and again in cities where memorials have become great public parks, and we think this will happen over time. This is a protected area so it will always be there.'

Safety concerns

The legacy left behind by the twin towers means that there is naturally a greater scrutiny on the safety in high-rise buildings, particularly those that will be perceived as a target for terrorism. The various reports into the rescue attempts ten years ago have highlighted several deficiencies in the original building that SOM have addressed in One WTC.

The building incorporates advanced life-safety systems that exceed the requirements of the New York City Building Code and are expected to lead the way in developing new high-rise building standards. In addition to structural redundancy and cementitious fireproofing that is dense and highly adhesive; the building includes biological and chemical filters in the air supply system.

One of the major problems in the rescue attempts has been highlighted as the problems that civilians had in exiting the building while rescue workers were attempting to enter the building by the same stairwells. To allow for optimum egress and fire-fighting capacity, extra-wide pressurised stairs, multiple backup systems for emergency lighting, and concrete protection for all sprinklers and emergency risers are being provided, in addition to interconnected redundant exits, additional stair exit locations at all adjacent streets, and direct exits to the street from tower stairs. All of the building's life-safety systems – egress stairs, communication antennas, exhaust and ventilation shafts, electrical risers, standpipes, and elevators – are encased in a three-foot-thick concrete core.

This building is being designed to facilitate emergency response with enhanced communication systems, together with a dedicated stair for use by fire-fighters. These are used in conjunction with enhanced elevators, housed in a protected central building core, that serve every floor of the building.

SOM had the opportunity to pioneer various security measures in Seven WTC before adopting them on One WTC. 'You need to make it an environment that works as an office space, but you put in systems and use the knowledge of what has happened before to anticipate what may happen in the future,' Lewis explains. 'At Seven WTC, where it all really started, we began looking at these considerations for Tower One, the street-born threat is everywhere in the world and in this scenario you need to protect people in the lobby. You begin looking at duct tiles systems and systems that give, but don't fail. You would look at the whole structural system so that there is not progressive collapse like we saw on 11 September.

'There was a really different attitude that we had to have because the loads and the events that are happening are beyond what anyone would be thinking as a structural engineer. Before you may have thought about what would happen if one column was taken out by a small event, but now you need to be thinking of internal and external attacks that take out two columns on any floor at the same time and have a design that will still have the building maintain its internal integrity.'

At the same time that SOM were designing Seven WTC there were various chemical weapons scares in New York, one of which was anthrax scares at the Post Office and various media organisations. 'If your tenants are media companies then you need to provide them with a level of comfort that the building will, to whatever extent possible, protect people,' Lewis adds. 'We went through a long exercise of meeting with various experts about what types of attacks there are from biological, chemical, radiological and so on. What we all came to the conclusion of was that if you put all of the fresh air intakes up at the top of the building you will knock out a lot of those threats.'

However the risk of chemical attack is not only an external threat but can also come from inside the building, maybe a disgruntled employee such as in the anthrax scares. To combat that each floor in One WTC will be fitted with dual filtration with both charcoal, which is good for chemical and airborne threats, and myrrh that can be used to filter particulates from the air.

'There are major things that have been learnt from the 2001 attacks as far as our job is concerned,' Lewis says. 'Airborne attacks are not for the role of an architect to stop, nor any engineer, but the robustness of the systems has to take that into account. You can't look at is what coming in terms of attacks, but what you can anticipate is what would happen if, for example, two columns were taken out of the building, and what could be done to prevent that progressive collapse.

'Another example is anticipating a penetration to the core of the building and so creating systems to make the core more robust and protect the critical elements such as stairs, elevator and life-safety systems including water. It is important in making sure the water is distributed within the building and not all in one place to make >< sure there is always a water reserve within the building if something happens. You need to do all of this in such a way that it looks like an office building and not a fortress and you don't know that all of those systems are there.'

One of the most vivid memories from the TV coverage ten years ago was of survivors emerging from the building with smoke residue on their faces, which raised the question of ventilation for the emergency stairwells. It was also important to look at the width of the stairs and how people actually move throughout the stairwell. There was a National Institute of Standards and Testing (NIST) report on this that was talking about all of the issues that were raised from the old building.

'For example, firemen in New York City will carry their equipment up into the building because they were finding that despite talk of leaving gear for firemen in these old buildings they would come in and the hose rack wouldn't be functional when they turned on the water,' Lewis says. 'So they would be carrying this gear up the stairs and everyone who was coming the other way had to turn to the side to let them pass, meaning the egress of the building was much, much slower than it needed to be.

'In the case of Tower One, we have created a dedicated fireman/responders stair that is completely dedicated to them and completely independent to the egress of the rest of the building. It could be used in an emergency situation to get people out if that is how the firemen see fit, but it is available so they can plan their attack on any fire in the building and have that access.

'The other issue was smoke getting into the stairwell because suddenly a lot of doors were opened up drawing that smoke into the stairwell. This meant we really pushed for pressurised stairwells so that when a fire alarm was to go off fresh air would be pumped into a shaft so the pressure inside the shaft is greater than spaces outside and in general that will preclude smoke from now leaking into the stairwells.'


Communication was another big issue that hampered the 11 September rescue efforts. There were reports that firemen and policemen's signals were actually stepping on one another making communication difficult; even to the extent that policemen were given the order to abandon the building, but firemen never received this either due to the signal being stepped on or because they were simply unable to get the signals so far up into the building. 'In Seven WTC, and it will also be the case in Tower One, there are repeaters that boost cell signals as well as antennas that run up and down the stairwells inside the building,' Lewis says. 'It allows communications to be able to get to responders inside the tower. As well in the case of Seven, and I think it will happen for Tower One too, a separate antenna for people inside the building to contact people externally, which they couldn't do unless they were on the floors.'

Innovative technology plays an important role in the new WTC, but before making it into the final designs it had to prove that is was efficient, robust and above all cost-efficient. One WTC has searched for technologies to maximise efficiency, minimise waste and pollution, and reduce the impact of its development.

The building will use 30 per cent less water than the New York City Building Code allows for this type of building. A system for collecting 100 per cent of the rain water that falls within the site boundaries is being put in place; this reclaimed water will be used for landscape irrigation and to fill the plaza pool.

A 1.2MW, next-generation fuel-cell plant – one of the largest commercial fuel-cell installations in the country – is integrated into the building's mechanical and electrical systems and contributes to energy use that is 20 per cent less than proposed under the city building code.

One example of a technology that was attempted but did not comply with the requirements was LED lighting. 'We wanted to use it because its compactness could reduce the ceiling sandwich and allow us higher ceilings,' Lewis explains. 'Then we started testing it and the technologies just weren't there yet. They will be in a few years, but not yet.'

One WTC uses a new form of high-performance, low-E glass coating technology to maximise daylight and minimise heat gain. Maximising the amount of natural light used in the building contributes to energy savings by reducing the need for artificial lighting. 'The ultra-low iron glass is almost as clear as plastic glass, it is white; it used to be called water white or crystal white,' Lewis explains. 'The building is a pure glass building. The glass has a low invasivity coating that has a metal coating to them that reflects the heat portion of the light spectrum. The technology is one that you wouldn't tell was there unless you really closely inspected the glass coating and noticed that your reflection had a tiny bit of a purple cast to it.'


Work is continuing apace on the constructions, as at mid-July, the steel infrastructure had been completed on 76 of the tower's 105 floors, with cementing eight floors back and glass exterior just passing the fiftieth floor. Original completion was scheduled to be early 2012, but that was put back to the tail end of 2013. It may no longer be called The Freedom Tower, but when One WTC, along with the adjacent Memorial Gardens, open the $3.1bn project will be a compelling symbol of freedom for people from all over the world. *

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