At the most basic programmatic level, Fulton Center'New York's new $1.4 billion transit hub'is a way for commuters to descend 40 feet from the street to the platforms for subway lines that overlap each other in a mazelike tangle in Lower Manhattan. One of the primary aims of the station, which opened in November and was conceived in the wake of the September 11 attacks, was to simplify access to these platforms. It was also intended to ease the cumbersome transfers among the lines, which were originally built by separate and competing commercial entities, some more than a century ago.

However, its design team set its sights on grander ambitions. 'The aspiration was to create a space with the civic quality of Grand Central'one animated with daylight,' says Vincent Chang, a partner at Grimshaw, the architecture firm responsible for Fulton Center's primary above-ground component'a dramatically skylit glass-and-steel pavilion, approximately 140 feet square and 100 feet tall.

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Execution of Fulton Center's goals'both lofty and pragmatic'was anything but straightforward since the hub comprises several distinct elements: the pavilion; restoration of the Corbin Building, a 125-year-old landmarked structure that is now part of the station complex; reconstruction of about two dozen access points to the station's below-ground areas from the surrounding streets; and creation of a 350-foot-long underground passage that will connect subway riders to the World Trade Center once the Santiago Calatrava'designed station opens later this year for the PATH'a passenger rail system that links Manhattan and New Jersey. The Fulton project was built under nine separate construction contracts and had three architects of record. In addition to Grimshaw, Page Ayres Cowley Architects was charged with the renovation of Corbin, while HDR oversaw the underground network of connecting stations and passageways and many of the street-level access points. The team was led by Arup, which served as the project's prime consultant.

With so much complexity and a large volume of traffic (about 300,000 commuters are expected to travel in and out of Fulton Center daily), the design team needed to thoroughly analyze how people would move through the station. 'With any transit project, there is a lot of competition for a limited amount of space,' says Eric Rivers, a pedestrian planner with Arup. He points to the subway platforms as one problematic area: people use them as corridors as well as for boarding, alighting, and waiting for trains'all at the same time. 'The only way to understand it was with a micro-simulation model,' he says.

But because there were no crowd simulation tools available that modeled 3-D spaces effectively when the client, Metropolitan Transit Authority (MTA), selected Arup 11 years ago, the firm developed its own. The software, named MassMotion and now commercially available, allowed the team to populate the project's digital model with 'agents,' or virtual people, to evaluate how many passengers the facility could handle and predict what route they might take around structural elements or other obstructions, such as columns or walls. In the years since completion of Fulton's design phase, Arup has sufficiently developed the tool so that it can be used to understand how occupants will behave when faced with, for instance, a choice between a crowded escalator and a relatively empty set of steps. It can also help project teams to study the effect of operational changes, such as increasing the frequency of trains arriving at any given platform. 'It helps inform design decisions,' says Rivers.

The station's key orientation device is a $2.1 million oculus created with James Carpenter'the frequent architectural collaborator who works with glass, among other materials, and the phenomenon of light. His scheme, which consisted of a multilayered glass skin for Grimshaw's then-dome-shaped skylight, was selected as the result of a 2003 design competition sponsored by MTA Arts and Design. Carpenter's competition entry was the 'most carefully considered as part of the architectural concept,' explains Andrew Whalley, Grimshaw deputy chairman. Carpenter says he was simply building upon the architects' fundamental goal for a dynamic, light-filled central atrium. 'From the very beginning, our intent was to bring the sky into the space.'

As realized, the oculus is a conical, metal-clad hat that protrudes from the roof of the pavilion and is angled toward the south to harness the maximum amount of sunlight. Within this shell, 952 diamond-shaped perforated aluminum panels are held in place by a net of 112 pairs of ¼-inch-diameter cables that form a skewed hyperbolic paraboloid, or hypar. This doubly curved net, about 50 feet in diameter and 70 feet tall, is tensioned between a compression ring at the top and the second-level floor plate at the bottom. The appearance of the device, called the 'sky-reflector net,' changes in response to the color and character of the sky.

The installation achieves this effect, in part, because the panels are made of a special optical grade of aluminum that reflects approximately 90 percent of the light that hits it, according to Carpenter. (Stainless steel would reflect about 60 percent, he says.) In addition, because the panels are only ⅛-inch thick, and have perforations that range from 20 percent open near the reflector net's top to 48 percent near the bottom, they seem almost gauzy.

These diaphanous qualities can be appreciated from multiple areas within the station, including one directly below the oculus, on an intermediary floor plate between the street level and the platforms that the architects refer to as the 'lily pad' because of its circular shape. Another prime spot is inside the 'doughnut'—two glass-enclosed levels that wrap the reflector net and are lifted off the ground plane on V-shaped columns. (The doughnut houses some of the complex's 66,000 square feet of commercial space that retail giant Westfield Group is leasing.) From this vantage point, which is likely to be a restaurant, visitors will have views through the veil-like reflector net, of the skylight above and the station's activity below.

To understand the stresses that the delicate panels, the cables, and the node assemblies connecting them would be subject to, the engineers conducted numerous analyses. They even examined, with a computational fluid dynamic (CFD) model, among other types of studies, the effect of variable loads such as those created by thermal gradients and air movement. Since the reflector net was conceived as part of the pavilion's ventilation strategy, these factors could not be ignored: the apparatus behaves like a solar chimney, creating substantial convection and helping pull warm air out through exhaust ducts concealed behind the veil-like panels, explains Zak Kostura, an Arup associate.

The design of the pavilion and its reflector net were technically demanding, but the $84 million renovation of the Corbin Building, an eight-story masonry and iron structure built in 1889, offered its own set of challenges. Even though it was originally slated for demolition to make way for the new station, its designation as a national landmark in 2003 helped make federal funding available for its comprehensive renovation and integration into the station. It now defines the southern edge of the Fulton Center complex, provides both retail and office space for commercial tenants, and offers subway riders another route to the platforms.

Designed by Francis Hatch Kimball, Corbin was deemed historically significant because it is a 'proto skyscraper,' says Page Ayres Cowley. Although extremely skinny in plan (it is just 20 feet wide at its narrowest end and about 150 feet deep), Corbin was, at 142-feet high, briefly the tallest building in Manhattan. In addition, it was one of the first buildings to feature the tile vaulting invented by Rafael Guastavino. The system, which consists of multiple layers of thin terra-cotta tiles, became known for its fire resistance and its ability to accommodate large floor loads. The method would later be used at the Boston Public Library and the Ellis Island Main Hall, among many other buildings.

Corbin was also notable for its 'extraordinary ornament both inside and out,' says Cowley. However, decades of neglect and unsympathetic modifications had taken their toll on the elaborate decorative components, making restoration a painstaking process. For instance, the facade's approximately 5,000 terra cotta pieces were individually mapped for cleaning, patching, or replacement. More than 500 were deemed beyond repair, requiring that they be replicated. But because clay shrinks when fired, molds could not be formed from the existing pieces. Instead, artisans had to sculpt new positives, roughly 10 percent larger than the originals, before forming the molds.

Inside Corbin, one of the many elements that features opulent decoration is the central staircase's copper-plated cast-iron balustrade. Although this stair was in relatively good condition, its handrail height did not meet current code for an egress stair. In order to avoid having to alter an important piece of historic fabric, the Fulton team provided a separate egress stair in an 'interstitial building' that connects Corbin and the pavilion. The narrow and towerlike steel-framed structure, which encloses back-of-house functions and mechanical equipment, also enabled the removal of unsightly external fire escapes that had cluttered Corbin's facade since the 1970s.

The historic building's structure also required retrofit, especially to improve its lateral resistance to wind and seismic loads. This was accomplished by the reinforcement of Corbin's north exterior wall with shotcrete and rebar (now mostly hidden by the interstitial building) and the addition of an internal concrete lateral frame linked to the steel frame of the pavilion. In order to create a structural diaphragm at each floor level, the engineers replaced the original ash used as infill above the tile arches with lightweight cellular concrete.

Arguably, the most dramatic part of the renovated building is a new set of escalators that descend 20 feet below the original foundations and lead to the subway platforms. Contractors underpinned the foundations, an operation that required laborious excavation by hand because the lot was too tight for mechanical excavation equipment. During the process, Corbin was monitored with sensors and strain gauges to ensure it didn't develop unacceptable movements or stresses. The result is a three-story-deep wellway that reveals a series of inverted masonry arches. Passing under these arches, which were designed to distribute the superstructure's loads to the surrounding soil, 'intensifies the feeling of going down into earth,' points out Cowley.

These hefty footings, newly uncovered, offer the perfect counterpoint to the delicacy of the adjacent pavilion's reflector net. One element entices passengers to descend, through history, while the other draws their eyes upward, to a precisely machined device for capturing the sky. If only all New York City subway stations could have such synergies.