Grounded Structure

Brooklyn Condominium Building Breaks the Mold in Seismic Design

A new mixed-use complex over the Hoyt-Schermerhorn Street
subway station in downtown Brooklyn is pushing the design envelope for structures capable of handling seismic tremors.

by Katherine S. Robertson

An otherwise conventional mixed-use Brooklyn building is breaking ground in seismic design protocols in large part because its developer stuck to a plan to construct it over a New York City Transit subway station even after the Sept. 11 attacks changed the rules of structural design.

State Renaissance Court, an eight-story, 196,000-sq.-ft., residential, retail, and commercial development, is rising over less than an acre of the Hoyt-Schermerhorn Urban Renewal area - and right over the Hoyt Street-Schermerhorn Street subway station in downtown Brooklyn serving the A and G lines. While the structure relies on girders built into the original subway station as a support for its gravity load, it uses an innovative system of steel springs and augured piles to transfer virtually all lateral loading from seismic activity or heavy winds into a separate foundation.

What had started out as a routine development shifted midstream into a complex research and development program, said John Frezza, president of Strategic Development & Construction Group, a Brooklyn firm that is one of the joint-venture partners developing the $38 million project.

"We had no idea the project would have these design and technical requirements," said the Brooklyn-born and reared Frezza.

The project has involved extensive coordination with the Metropolitan Transportation Authority - the city transit agency's parent organization - to develop criteria for catastrophic seismic event protocols. A major contributor to that effort was Manhattan-based Wexler & Associates, an engineering firm basically hired to define a new nut and then crack it.

"Even though I didn't know what the answers would be, I knew we were going to find them," said Neil Wexler, president and chief structural engineer at the firm.

Scheduled for completion early next year, State Renaissance Court is one of two projects in the district that Frezza's firm is developing in a partnership with Brooklyn-based IBEC Building in assignments the team won through RFPs administered by the Empire State Development Corp. The second project, a market-rate housing development, is expected to break ground next spring on an adjacent parcel.

The current project, which sits on a site that had housed a parking lot over the Hoyt-Schermerhorn station, will have 158 market-rate and affordable housing units, 20,000 sq. ft. of ground-floor retail space, and an indoor parking garage.

Frezza said that the development team originally won rights to the site in 2001. The team had planned a structural design with seismic controls in the form of moment frames, a rigid, welded steel network of large beams and columns that carries each floor's lateral load.

But before construction could begin, the Sept. 11 attacks later that year caused the MTA and other agencies to re-examine seismic requirements and criteria for buildings near subway structures. The agency told the developers they would have to meet new and stricter criteria.

Frezza said that pulling back was not an option for the development team.

"We had closed the bond financing and were personally at risk," he added. "There was no alternative. We had to make it work."

Under Wexler's guidance, the team strove to meet the MTA's stricter seismic requirements, which resulted in a building using technology mostly found in Japan and California that allows it to withstand the force of a 500-year earthquake.

A central element of the new design was introducing a new structural system of concentric braces and hollow-core, precast plank floors.

"The combination of steel braces and plank floors was rigid and didn't deflect a lot," Wexler said.

The new superstructure also reduced the amount of steel required by half, shaving 10 to 15 percent off of the construction budget, Frezza said.

The main engineering challenge, however, was designing a building that would not damage the subway station or tunnel in an earthquake or as the result of other significant lateral forces.

The solution incorporates old and new elements. The building stands above 26 concrete-encased steel girders, or load points, which had been incorporated into the station during its original construction in the 1920s with the intention of supporting above-ground development.

The building does not stand on the girders directly. Instead, beams for the 440-ft.-long and 90-ft.-wide first floor sit on custom-designed steel spring isolators fabricated by Vibration Mountings & Control of Bloomingdale, N.J. The isolators stand on Teflon pads.

The pads, in turn, rest on concrete pedestals that the team built to serve as an an interface on top of the subway structure's girders.

Those connections only support the gravity load, however. If the building were to move due to an external force, the lateral load would almost completely bypass the subway structure's girders.

The key element is a parallel foundation that ties into the building's first-floor girders, which act as a truss supporting the whole structure. The first-floor girders tie into seismic restraint devices shaped like large steel "dumbbells" that are welded to strong pile foundations - all dug completely apart from the subway structure.

During an earthquake, the building would effectively "slide" on the steel spring system and shift its lateral load away from the subway structure, said John Giuliano, president of VMC.

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But it wouldn't go too far, thanks to the heavy-duty piling system. It consists of two concrete thrust blocks, each 12.5-ft.-wide, 9-ft.-deep, and more than 50 ft. in length. The blocks encase 140 separately augured minipiles, battered at angles and punched as deep as 100 ft. into the ground.

An added benefit to the system is that the building's residents won't feel vibrations from subway trains rumbling below the structure.

Of the 1,600 tons of steel used on the project, 650 tons are in the support systems on the below-grade levels and first floor, said Michael Senneway, vice president and general manager of Ocean Steel & Construction of Conklin, N.Y., the project's steel contractor. That setup includes beams, the heaviest weighing more than 18 tons, that are spaced every 18 to 20 ft. along the width of the first floor, braced horizontally, and strengthened with stiffeners and thickeners, per MTA requirements.

Even a 1,000-lin.-ft. skirt wall, which follows the profile of Schermerhorn Street, hangs from the building in order to have no contact with the subway structure, said Richard Steffens, president of Steffens Corp. of Breezy Point, N.Y., the project's concrete contractor.

"Nothing on this job actually touches the subway structure," he added. "Everything is isolated."

The building's aesthetic design may be the only element that directly ties into the surroundings. It aims to create "an interface between downtown Brooklyn and brownstone Brooklyn to the north," said James McCullar, principal of New York-based James McCullar & Associates, the project's architect. The building will be clad with a lightweight composite stone and brick-face paneling system.

But even visual features of the building, which stretches along commercial Schermerhorn Street and residential State Street, exist to accommodate the subway structure. One example is how it thins at the eastern end to snake around a vent shaft.

"The building envelope is a response to the transit structure," McCullar said. "When you look at the building, it looks normal, and you may not understand that it is guided by the transit authority."

The end result is a building shaped by its obstacles, Frezza said.

"We did not know if this was physically possible, but through teamwork and synergy, we were able to confront things for the first time and come up with workable solutions," he added.

Key Players

Developer-Builder: Strategic Development & Construction Group, Brooklyn; IBEC Building Corp., Brooklyn

Architect: James McCullar & Associates Architects, New York

Engineer of Record: Gleit Engineering, New York

Structural Engineer: Wexler & Associates, New York

Vibration-Noise Control: Vibration Mountings & Controls, Bloomingdale, N.J.

Concrete Contractor: Steffens Corp., Breezy Point, N.Y.

Steel Contractor: Ocean Steel & Construction, Conklin, N.Y.