1 World Trade Center, New York, USA

1 World Trade Center is being built where the twin towers of the World Trade Center (WTC) once stood. As Skidmore, Owings & Merrill (SOM) LLP designs 1 World Trade Center, architect David M Childs, FAIA, constantly reminds his team that their most important objective is to replace a lost piece of the skyline.

1 World Trade Center also marks the rebirth of a major financial and commercial hub, re-establishing New York City as the epicentre of high-rise skyscraper design and serving as a symbol of the revitalisation of lower Manhattan.

1 World Trade Center was originally named "Freedom Tower" to demonstrate US triumph over terrorism. In March 2009, the name was changed to One World Trade Center by the Port Authority of New York and New Jersey for easier identification. The centre's role is similar to that of the old twin towers. The building contains 3.5 million square feet of space, across 69 floors, of which 2.6 million square feet is office space. At its base, street-level lobbies provide access to offices and public areas, as well as to the observation deck and restaurants near the top of the building. Concourses below grade connect directly to 15 subway lines and to transit hubs for Port Authority Trans-Hudson (PATH) commuter train service to New Jersey.

Although many people associate the design of 1 World Trade Center with architect Daniel Libeskind, his only responsibility is for the master plan of the WTC site. He determined the layout of buildings, the location of the memorial and, in collaboration with the stakeholders and their architects, the distribution of programme area. He also worked with SOM on a design that has since gone by the wayside. And he called for 1 World Trade Center to have a spire of 1,776ft (541m). 1776 represents the year that the founding fathers of the US signed the Declaration of Independence, asserting the country's independence from Great Britain.

An awe-inspiring $610m National September 11 memorial and museum, four additional skyscrapers (Towers 2, 3, 4, and 5), the World Trade Center Transportation Hub and a retail complex are also being constructed at 1 World Trade Center.

Change of plan

Beyond that, SOM is the primary firm behind the current 1 World Trade Center design. That was also true of the twisted tower concept that preceded it. In that version, the glass-clad tower's asymmetrical form would have alluded structurally to the nearby Statue of Liberty. In particular, the offset spire rising 400ft above the tower would have mirrored the statue's raised Torch of Freedom.

However, for both cost and safety reasons, that design won't be built. Its glassy base had worried the New York City Police Department. As part of the police department's requirements, the revised design included a concrete protection wall around the lobby, prompting criticisms that the base would resemble a bunker.

The building concept therefore underwent significant revision. The goal was to simplify the design and enhance security while avoiding previous issues.

1 World Trade Center design

The new shape plays with geometry and with perceptions. The base is a square of 200ft x 200ft (61m by 61m), and the first 20 storeys rise straight up, aligned with this square. The design was revised twice in June 2005 and in June 2006 to cover the concrete base with a screen of glass prisms.

Then, from the 20th floor up to a 1,368ft-high parapet, the edges chamfer back, creating eight triangular planes. That is, the square rotates until the 102nd-floor square has turned 45° from the 20th-floor square.

The eight elongated isosceles triangles form a neat configuration on the façade. These alternate between triangles with the apex at the 102nd floor and those with the apex at the 20th floor. The downward-facing triangles slope inward slightly, which is to say that the footprint of each floor shrinks slightly at higher and higher points.

The parapet at the top of the topmost floor clearly marks elevations of both 1,362ft and 1,368ft. These represent the heights of the destroyed twin towers.

Restaurants occupy the 100th and 101st floors. The 102nd floor, the topmost occupied floor, features an observation deck. Visitors can reach these heights via five high-speed elevators (just some of the building's 70 elevators).

Above that level is the communications ring. This contains equipment for electronic news gathering, whip antennae and microwave relay dishes that television stations and businesses will use.

An illuminated 450ft tapered spire crowns the building and the entire WTC site. It also serves as an antenna providing digital broadcasting for the area (a function temporarily served by the Empire State Building). The top of the antenna also brings 1 World Trade Center up to the requisite 1,776ft height.

At the tip of the antenna, a brilliant, slowly rotating beacon will flash the letter 'N' in Morse code, as lightships once did in great harbours of the world. The 'N' stands for 'New York'.

At 1,776ft, 1 World Trade Center will be over 400ft taller than the twin towers but won't be the world's tallest structure. That distinction goes to Burj Dubai, an SOM-designed tower in the United Arab Emirates. That building is still under construction and is projected to be 2,684ft high, including its antenna.

1 World Trade Center materials

1 World Trade Center is to be clad in an ultra-low-iron glass, with stainless steel for the corners. The stainless steel harks back to the twin towers, where the steel corners used to catch the rays of the setting sun and glow at sunset. The new corners should achieve the same effect.

In order that the building is not read as a bunker, the 20-storey base of 1 World Trade Center will be encased in prismatic glass. As demonstrated in mock-ups, the prisms will refract and reflect light, trees and the sky by day, then shimmer at night.

On the west plaza of the tower, illuminated stainless steel steps will serve as a gathering place, providing a gentle transition between the building and the plaza itself. This should improve on the twin towers, which had a much more abrupt transition.

Safe and sound

Given the controversy over the structural soundness of the twin towers, safety has been a key construction issue. 1 World Trade Center incorporates systems that far exceed the requirements of the city's building code.

The building features structural redundancy and extra-strong fireproofing. The elevators are housed in a protected concrete building core. Moreover, the extra-wide, pressurised stairwells lead directly to outside streets, and other stairwells are dedicated for use by firefighters. Further safety features include concrete-protected sprinklers, emergency risers and enhanced emergency communication cabling. The entire World Trade Center site will be supported by a police command centre.

1 World Trade Center construction

The slow progress since the 2004 groundbreaking has provoked complaints and criticism. But SOM explains that they've faced complex challenges because the surrounding areas below grade contain tracks for PATH, a commuter train running from New York to New Jersey. Existing PATH structures must remain in place during construction of 1 World Trade Center, which poses challenges.

SOM notes that because of the high costs of construction in New York City, it's not at all unusual for a foundation to go in while architects complete the design of a building.

Tishman Construction Corporation is the construction manager for the 1 World Trade Center project. The project uses low-energy HVAC solutions, high-performance façades, renewable energy sources, non-ozone-depleting chemicals, recyclable and sustainable materials including rainwater collection systems.

The tower's first entrance, a street-level plaza on the eastern side, took shape in July, 2009 with a concrete pour of about 1,250yd³.

The entrance, which is now above the street level, will form the base of a large fountain on the plaza level and for planting swamp white oak trees. The section of the tower's plaza is complets five storeys underground and core has risen about 100ft above the ground.

The tower's core now rises about 105ft above street level is and descends to 80ft below ground level. A total of about 22,000yd³ of concrete has been poured at the site and nearly 700t of steel has been installed for the tower's substructure.

On 12 August 2009, a 70t perimeter column was installed. The 60ft-high column cast in Luxembourg is the first large column to be lowered at the construction site. A total of 24 such columns will be installed in 2009 to allow the workers to construct the tower's floors.

Transportation infrastructure

The Port Authority of New York and New Jersey is investing $3.2bn for construction of a state-of-the-art railway station at 1 World Trade Center. The World Trade Center Transportation Hub, designed by Spanish architect Santiago Calatrava, will replace the current temporary PATH station. By September 2009, 47 Calatrava arches had been installed and about 8,500yd³ of soil were removed to create an underground passageway between the World Financial Center and the Hub.

A contract to fabricate, furnish and erect the World Trade Center Transportation Hub was awarded to DCM Erectors in March 2009. The $338.8m steel contract includes a transit hall, pedestrian links to the centre's commercial complex and providing permanent support to Greenwich Street and the No 1 subway box.