Carrying the Load

New Luxury High-Rise Straddles Subway

by Adam Macy

900 Eighth Avenue, the Moinian Group's new 42-story residential tower, will span a full block on the east side of Eighth Avenue from 53rd to 54th streets.

At the edge of the theater district, near restaurant row and the reinvigorated Columbus Circle where the new AOL Time Warner headquarters is rising, the tower will feature 393 luxury rental apartments, a 550-car garage (replacing the city-owned garage that had occupied the space) and flying buttresses top and bottom.

The street level will feature 10,000 sq. ft. of commercial space. The building, which is scheduled for completion in September 2004, will comprise about 430,000 sq. ft. overall. Hard construction costs - brick and mortar expenses - are expected to run about $63 million.

From the Ground Down
In creating the concrete tower, workers first had to deal with the ground underneath because a subway train rumbles under the lot roughly every minute.

Two subway tunnels - one for the IRT's Nos. 2 and 3 trains and the other for the IND's B and D trains - bisect under the lot, running from the southeast corner at Eighth Avenue and 53rd Street to the northwest corner at 54th Street. The two tunnels lie at different levels below the site. One tunnel is cored into the bedrock, while the rock was cut out for the other tunnel.

"No load is permitted above the subway walls or roof and we could not allow the load to spread through the rock," said Jacob Grossman, structural engineer on the job and president of Rosenwasser-Grossman Consulting Engineers PC.

The solution was to build a series of major transfer beams to shift the load toward the outer edge of the building. While the transfer beams could have been made of steel or concrete, concrete better served the complex needs of the project, which included not only shifting the load to the outer edge, but also dealing with the varying column layouts required for the garage and residential portions.

"Concrete allows for more flexibility, and on this job the concrete transfer beams are shaped to match the changing height and space," Grossman explained.

Ninety caissons were drilled more than 50 ft. down and a foundation and superstructure girder system of heavily reinforced concrete was built over the tunnels to support the structure above. Heavy 12- to 16-in. structural arches across the subway's easement take the load of the garage and its four hydraulic elevators. The building's 1,520-sq.-ft. cellar level bridges the B and D line.

In addition, the tower's elevator shaft is cantilevered over the subway tunnels because of the preferred location for the residential levels, which forced the transfer of large gravity and wind loads over several building levels.

"It's one of the most complex jobs we've done and we've done close to a thousand concrete buildings in the city," Grossman added.

Tony Rafaniello, the onsite superintendent for HRH Construction LLC, said the building is "not just slab on grade, slab over tunnels."

He stressed the importance of the reinforced concrete girder/caisson system and added that while slab on grade has no structural value, the arches - which could be on grade or higher up - have reinforcement that make them self-supportive.

"This project is atypical of most concrete jobs," said Frank Ross Jr., project executive for HRH Construction. "To span the subway, build a 42-story building on top and have no load imposed on the tunnels is a pretty amazing feat."

From the Ground Up
The garage will be in the five-story, naturally ventilated pedestal upon which the tower will extend. It will be clad in architectural precast, poured-in-place concrete designed to look like limestone.

The 37-story residential tower will feature floor-to-ceiling windows and light-colored brick panels, culminating in a 50 ft. bulkhead. Other aesthetic niceties include the flying buttresses, at the seventh floor and at the top, made of precast concrete beams stretching up to 20 ft. from one corner to another.

In the planning since 1997, 900 Eighth Avenue was redesigned three times, said Moinian's project manager, Maria Rosenfeld. Initial designs called for a taller, 52-floor building. "That was too expensive to build. This is a simplified version," she said.

The project was financed under the "80/20" program, through which a low-interest mortgage is guaranteed by the New York State Housing Finance Agency. It also comes with some long-term tax abatements. In return, Moinian agreed to set aside one-fifth of the units for low- and moderate-income tenants.

Market rate rents will be between $40 to $50 per sq. ft., Rosenfeld said. Studio apartments will be about 500 sq. ft., and one-bedrooms between 650 and 700 sq. ft. Two-bedroom units could be as large as 1,200 sq. ft.

At the end of March, workers were completing the second-floor slab and the third-floor frame, about three weeks away from a hoped-for "two-day cycle."

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