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Subway Switch

New underground passages are being threaded through high rise buildings, and across underground stations to join up New York’s fragmented Subway system.

A little history, geography and economics is needed to explain the Fulton Street interconnection project which involves threading new passageways beneath the streets between the skyscrapers of downtown New York City. The work will improve connections between five Subway lines.

Manhattan island has a relentless longitudinal logic to its shape which is reflected in its transport system. The major avenues run parallel from the north end to the south.

The older Subway lines, built a century ago, run as subsurface tunnels just beneath several of them. "They were deliberately built not to connect,” says Craig Covil a principal and ground specialist at consultant Arup’s New York office, who has been working on the Fulton Street interchange scheme for over four years. "The private companies who built them wanted their route used, not someone else’s," he explains.

The different lines were eventually consolidated within the control of the Metropolitan Transportation Authority (MTA). Over time, the system became more complex as more lines were added in, some running across the others. Critical points in the city became major interchanges for hundreds of thousands of commuters daily.

Fulton Street is one, close to the tip of the island, near the site of the World Trade Center (WTC), now Ground Zero. Here, three north-south lines run above the platforms shared by Lines A and C where these tracks from the upper West Side sweep to the east and across the Hudson River to Brooklyn.

Track for the four lines is shared by serveral services. One is used by Lines 2 and 3, one by the J,M and Z lines and one by Lines 4 and 5.

Around 300,000 commuters use the station daily The lines are connected by a series of cramped underground passageways linked to the surface by narrow stairs (NCE 6/13 January 2005).

It has long been the ambition of the MTA to change all this explains MTA Capital Construction project coordinator Uday Durg. As a result, the Authority is half way into a major upgrade programme which includes the creation of new under street pedestrian connections, station improvements and the reconstruction of the the A,C Lines’ station box and a greatly expanded interchange between this and the other lines.

"This is the central problem," explains Covil. "[At present]the changeover point is at the end of the platform," he says. The resulting congestion at rush hour leads to crowding, delaying trains as passengers struggle to get on and off and move between the platforms and a system of ramps and passageways above it. Part of the solution is to widen the 8m wide box above the platforms by 3m to accommodate a simpler, single level corridor system.

To do this the 10m deep, 8m wide concrete box housing the ramps and passageways must be demolished and opened up to street level. The box rests on the A,C Lines running tunnels which are just below groundwater level and uplift is a risk, so demolition work is being carefully sequenced to prevent the tunnels from heaving.

As part of the programme MTA also has taken over half of a city block at the corner of Fulton Street and the downtown section of Broadway. Here the MTA is to build a new eightstorey glass and steel "occulus" building which will form the main entrance to the station. The building, designed by British architect Grimshaw, is known as the Transit Center and is designed to funnel light into the undergroud areas and to act as an identifiable focal point for the station.

Broad staircases, elevators and escalators will create a new, more spacious surface access point, and the underground areas will be contained within a deep secant piled wall. The transit building will also incorporate an existing building on the site – the historic Corbin skyscraper built in 1888, and the first to feature Otis lifts.

The nine-storey sandstone and terracotta tile clad structure by architect Francis Hatch Kimball is a long slim building which will be reconditioned. To preserve the building the team brought in specialist architect Page Ayres Cowley to research it for restoration as close as possible to the original. The renovated building will be used for retail and office space. New escalators will run through intriguing inverted brick arch footings which were used for the foundations.

Three contracts have been completed on the Fulton Street Transit Center project. In 2007, rehabilitation of the station for lines Lines 2 and 3 was completed, providing customers with better access and easier way finding. There are also new entrances for those nearest Lines 4 and 5 at the southern ends of both platforms. These provide faster, clearer access for customers.

Demolition and preparatory work for the main transit building was also completed in 2007. Most recently, the structural box for the Dey Street pedestrian concourse, an under-ground east-west passageway between Broadway and Church Street that will provide a direct connection between the new Transit Center and the neighbouring WTC PATH station, was completed in November 2008.

Plans to issue bids for the finishes work for the Dey Street concourse are now being finalised. The next phase of the Transit Center work has also begun with the award of the foundations contract to Skanska in December for $69.8M (£46.8M).

The foundations contract will provide a basement for the Transit Center, as well as support the reconstruction of the corridor box above the A, C platform and elevators to the J,M,Z line. This is being advertised this month. with contract award scheduled for mid-2009.

STATIONS AND SCULPTURE
The landmark station also incorporates a net sculpture, by James Carpenter. "All MTA projects must include public artwork and there is special funding for it" says Arup principal Craig Covil. But this one is also functional – its reflectors will help daylight penetrate to the subterranean base of the concourse.

The net is complex. "Nets fall into their own form as gravity acts upon them," explains structural engineer Zak Kostura. This shape has to be "discovered" by running iterative software tools to analyse the nodes and interconnections.

One by one loads are slowly applied, a process widely known as "form finding". Once shapes are derived for the holes in the net the shape of the reflectors can be determined for fabrication. Gravity will ultimately act upon these elements as well as it pulls down on the net, altering its form and the shape of the holes filled by the panels.

Analysis must account for this interplay between force and form. But the net is again affected by position and angle specified for each panel by the architect.

Each adjustment requires another iteration to keep it up to date. The computer programmes for this have been tied together with additional software by Kostura and his colleague Ben Urick.

Software originally used for aerospace design was needed for the complex geometry and a third colleague, Erin Morrow in San Francisco, generated custom routines to adapt it for the unique cable net.

Click here for diagram of Fulton Street: New Links and Layout

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