By Rebecca Morelle
Freedom Tower is to form the centrepiece of the World Trade Center site in New York, which was devastated by the al-Qaeda attacks five years ago.
Due for completion by 2011, the tower will soar 1,776ft (541m) into the sky and incorporate a number of features intended to ensure maximum safety and security.
THE FREEDOM TOWER
1 High-level air intakes to minimise pollution, and chemical and biological filtering system
2 Central vertical core housing key safety features, including water-resistant lifts, pressurised fire-escape stairs and separate staircase for emergency workers
3 Extra-strong 3ft (1m) concrete casing protecting central core, sprinklers, emergency risers
4 Multi-layered glass curtain wall to protect building from explosions
A parapet at 1,368ft marks the height of the destroyed twin towers
Safety features include:
A vertical core will run through the centre of the building, containing many of the structure's essential safety systems.
It will have 3ft-thick (1m) walls made from extra-strong concrete. In it will be emergency communication cables, exhaust and ventilation shafts, and water pipes, together with stairs and lifts to enter or exit the building.
Inside the core, there will also be a dedicated staircase, specifically for the emergency services to use.
It will be pressurised to keep any smoke out, and will be extra wide to allow firefighters to carry equipment up and down with ease, to avoid a repeat of the congestion in stairwells that occurred on 11 September, 2001.
The core will also contain lifts, made water-resistant to prevent damage from sprinkler systems, and two escape staircases, interconnected so if one staircase becomes blocked with debris, as occurred on 11 September, people can switch to another set of escape stairs.
The stairs also branch off into four street-level exits to avoid any potential congestion in the lobby and allowing a quicker escape.
The force of the impacts as the aircrafts ploughed into the Twin Towers caused the fire-proofing to rip away from the steel frame, allowing fires to rage.
According to a report by the National Institute of Standards and Technology, this was a key contributory factor towards the building's collapse.
For the Freedom Tower, concrete which contains highly adhesive and strengthened fire-proofing will be used throughout the building. The architects say it would survive a very strong impact force.
PREVENTING 'PROGRESSIVE COLLAPSE'
The multiple fires that raged throughout the Twin Towers caused the already impact-damaged structure to further weaken.
The floors began to sag, causing load-bearing columns to fail. Eventually they buckled and the buildings concertinaed to the ground - known as "progressive collapse".
Freedom Tower has been designed, so that for whatever reason, if two columns fail, the building will stay standing, according to the architects.
The steel-frame structure and the beams and columns within the building will be connected so that in the event of the loss of a key column, the loads will be redistributed throughout the building.
Air for the offices in Freedom Tower will be drawn in from the top of the building, to avoid street-level pollution, and also because on 11 September the cloud of debris clogged ventilation shafts that were positioned closer to the ground.
The air will also be twice filtered, by high-efficiency particulate air filters and charcoal-based systems, for clean air in the event of a biological or chemical attack.
At the base of the building will be a curtain wall.
It will consist of three layers of laminated glass, each separated by a vacuum, making it extra-tough, behind which is a reinforced steel structure.
Architects say glass curtain walls can withstand and protect against blasts, and that laminated glass will not shatter into shards.
The building is located at the north-west corner of the 16-acre (0.06sq km) World Trade Center site, between Vesey Street, West Street and Fulton Street.
It is set back from West Street, a main traffic route, by an average of 90ft (28m) to satisfy demands from city officials who were concerned that explosives placed in a vehicle on the street could place the building and its occupants in peril.
Vesey Street and Fulton Street will both have limited access: only authorised vehicles can use these roads.