Building on Tradition

The 46-story Hearst Tower, with its distinctive diagonal structural grid, soars above its six-story predecessor.
August 11, 2010

The 107-year-old Hearst Corporation, one of the world's largest diversified communications companies, quite literally is returning to its roots as it constructs a new headquarters in Manhattan. The unconventional 856,000-sf structure, designed by London-based Foster and Partners, rises 46 stories above the preserved façade of the company's old six-story headquarters at 959 Eighth Avenue, near Columbus Circle.

"The Hearst Tower expresses its own time with distinction, yet respects and strengthens the existing historical structure," says Norman Foster. "The tower is lifted clear of its historic base, linked on the outside only by columns and glazing, which are set back from the edges of the site. The transparent connection floods the spaces below with natural light and encourages the impression of the new floating above the old."

Because a neighboring apartment building eliminated the possibility of views to the west, Hearst Tower's core was placed against a blank wall rather than being centrally located, according to the Foster firm's senior partner, Brandon Haw. The designers then evaluated how to accommodate this offset core with a perimeter moment frame. They ultimately selected a framing system that incorporates diagonal elements. This "diagrid" system not only satisfied structural and architectural requirements, but also capitalized on the inherent superior structural efficiency of a triangle compared to an orthogonal framing system.

Diagrid cuts steel use

Triangulated structures have long been known to be the most efficient means of transferring building loads to the foundation, says Ahmad Rahimian, president of Cantor Seinuk Group, the project's structural engineer. Reflecting this efficiency, Hearst Tower required only 10,480 tons of structural steel—about 20% less than a conventional perimeter frame, a savings of some 2,000 tons.

Rahimian concedes that triangulated systems have seldom been used to construct contemporary buildings. "When all the programming aspects are considered, it may not be the apparent solution," he says. "It is much easier to draw vertical lines."

Once selected, however, the diagrid system allowed designers to provide detailing not found in conventional designs. Each diagonal column spans a distance of four floors. In combination with columns immediately above and below, eight-story elements consisting of two inverted triangles are formed.

These chamfered corners give the building a lantern-like silhouette. The diagrid system also individualizes each floor by making its plan slightly different from the floors immediately above and below.

At the corners, vertical columns have been eliminated and the floor plates have been stepped back to create a distinctive, undulating building profile. "When you have a diagonal structure, the idea of vertical columns at corners is really a convention," Haw says. Zoning regulations and owner requirements pointed to a rectangular, rather than "wedding cake," building profile, with typical floors of 20,000 sf.

A 1.7 million-cubic-foot atrium that begins at the third level and extends to the bottom of the 10th level provides visual separation between the façade of Hearst's former headquarters and the new tower. Diagonal and vertical framing elements are expressed as they rise through the atrium.

Seeking the gold standard

Sustainability is high on the list of project priorities. Hearst expects its new headquarters to become the first New York City commercial office building to earn a Gold designation from the U.S. Green Building Council's LEED certification program.

Hearst Tower has been designed to use 25% less energy than a building that minimally complies with the respective state and city energy codes (based on ASHRAE 90.1-1999), according to Daniel Nall, SVP of the project's mechanical engineer, Flack+Kurtz.

Sustainable features include polyethylene tubing embedded in the topping slab beneath the finished floor of the atrium that will help to control the temperature and humidity of that space. Because water is a more efficient transfer medium than air, the number of air ducts that otherwise would have been required was substantially reduced.

In the summer, cool water will be circulated through the tubing before the building opens. The floor system can also absorb and remove heat generated by sunlight striking the floor. The floor surface will be maintained at 68–70 degrees in summer and about 10 degrees higher in winter. The floor will be paved with limestone, to benefit from limestone's efficient thermal transfer properties, says Michael Wurzel, project architect with Foster and Partners.

Rain collected from the building's 17,900-sf roof will be stored in a 14,000-gallon basement reservoir and reused for cooling tower makeup and to irrigate plantings in the lobby. Electrically actuated faucets in toilet rooms are expected to reduce water use by 25%.

The possibility of using a dual-wall façade system incorporating natural ventilation to increase energy efficiency was evaluated; such systems are common in Europe. However, this was determined not to be feasible because of New York City's high humidity level.

Hearst Tower is being constructed on a 32-month schedule, according to Frank Gramarossa, project manager with construction manager Turner Construction Co. Turner is delivering the project under a guaranteed maximum price contract for $252 million.

A slower steel erection pace

The diagrid structural system added to the complexity of erecting the building's framing and curtain wall. A typical column spans four floors and is 57 feet long. The steel frame was constructed by holding individual columns at a proper angle with guy cables until a connecting beam could be put in place. Bolted, rather than welded, connections were used. Full penetration welds might have taken as long as three days to complete, according to Gramarossa. Welding also would have raised a concern that sparks falling from a welding torch might damage the stainless steel curtain wall being installed on lower floors.

One floor of a conventionally framed building of comparable size typically can be erected in three days. Because the diagrid system was more complicated, the Hearst Tower rate was closer to four days, Gramarossa says. Structural members were fabricated by Cives Steel Co. Roswell, Ga.

Curtain wall installation is taking about six days per floor, compared to two days for a conventional enclosure system. Typical cladding panels, from Perma-steelisa Technologies Ltd. production facility in Italy, are 13½ feet high and 5 feet wide. By mid-March, cladding installation had reached the 26th floor.

The connection of the diagrid structure to the cladding requires "tight, neat joints" that are perfectly aligned, Gramarossa says. "Tolerances on this job were much tighter than we're used to working with."

Steel column sections for a typical high-rise building are lengthened by one-eighth inch to compensate for compression that occurs as framing is subjected to the accumulated weight of the structure as it rises. The performance of the angled columns was monitored to determine whether they would exhibit a greater amount of compression, but Gramarossa says it has been comparable to that of a project with vertical columns.

The foundation for the new tower is anchored by 21 caissons that are 30 feet long and socketed into bedrock. At one corner of the site, bedrock was encountered only a few feet below the original building's basement slab. But the bearing layer lies 20–45 feet lower across the site.

Hearst was able to take advantage of unused air rights acquired for the original building, says Brian Schwagerl, the company's director of real estate. The company also was allowed to increase the building's height by six stories in exchange for providing infrastructure improvements, including three elevators, at the Columbus Circle subway station.

The Hearst Tower Building Team included development manager Tishman Speyer Properties and associate architect Adamson Associates. The building was topped out in February and is scheduled for completion in June 2006. Hearst's 2,000-person New York staff, which is now spread over nine locations, will occupy the entire building.

And what will they see? Schwagerl promises that the building's floor-to-ceiling windows will provide "incredible views" of Central Park, the Manhattan skyline, and the Hudson River.

         
 

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