STEEL WEDS CONCRETE ON HYBRID NEW YORK TOWER 2002

From Concretepumping

Built with a structural steel frame for the first 25 stories of office space, the Random House/Park Imperial tower in New York City gives way to concrete columns on poured-in-place deck slabs for another 25 stories of luxury condominiums. The design was a challenge in wedding the two systems to accommodate load transfers of concrete to the underlying steel.

The 50-story hybrid is the equivalent in height to a typical 70-story highrise, as ceiling heights for some floors reach 18 ft. The structure tops out at 684 ft. The switch to concrete for the residential floors above the office space gives architects greater flexibility in the design of living space, according to the project’s residential architect Ismael Leyva Architects P.C. Lead architect for the overall project is Skidmore, Owings & Merrill. The structural engineer is Thornton Tomasetti, Newark.

Developed and owned by The Related Companies, New York, the building will serve as world headquarters for the giant book publisher Random House. It is located on Broadway between 55th and 56th Sts., where the tower provides panoramic views of Central Park and the Manhattan skyline from the Park Imperial’s 110 condo units that range from 811 to 2,975 sq ft.

Construction manager for the $180 million project is Plaza Construction, New York. Plaza started steel erection in the fall of 2000, following excavation, shoring and underpinning by local contractor Urban Foundations/Engineering. The excavation allows for two subgrade parking and mechanical decks.

A trailer pump project With the project on a fast-track schedule, owners specified that concrete be distributed by pumping the mix at all levels. Concrete contractor Northside Concrete Corp. retained the services of A&B Preferred, Long Island-based concrete pumpers. A&B partner Bob Russo and Northside general superintendent Joe Martinelli Jr. determined that a high-performance trailer pump would best meet the challenging job requirements with limited work space at the crowded urban site.

"We had no street access for truck-mounted boom pumps or ready-mix trucks to unload on Broadway," says Russo. "And cranes were picking steel from the side streets. The conditions also ruled out using a separate placing boom, which requires a tower crane to fly it to the upper decks."

The answer to equipment selection: "We started with a 28-meter Schwing truck pump, but didn’t use the boom. We poured the base building deck slabs below the tower, coming off the back end of the pump by taking off the elbow and adding a reducer to the pipeline," Russo explains. "We positioned the 28 within the building footprint -- right in the (future) lobby -- and ran the line up a steel column to the Q-decks."

For the upper-level pours above the 100-ft-high base building the contractors selected a Schwing BPA-8000, a powerhouse trailer pump capable of producing up to 138 yds/hr, while maintaining pressures at high deck elevations.

The six base building floors each required 20,000 sq ft of coverage by the 28-meter Schwing pump before the structure steps back to start the tower. All base building slabs and tower slabs to the 27th floor – a transition level -- are poured at 5 in. thick on Q-deck forming with wire-mesh reinforcement. Concrete requirements to this level total 10,000 cu yds. For the all-concrete floors above the steel framed levels, the thicker, 8-in. slabs are steel reinforced and require another 12,000 cu yds to top out the 50-story structure.

The pumping setup to start the upper levels uses a horizontal pipeline from the Schwing 8000 pump at grade running 40 ft horizontally to a vertical standpipe tied to a steel column. The line then extends up to the 6th level, where another horizontal run of 60 ft is made to another column. Line sections are added as pours are made to the top. The same setup was used for the 28-meter pump in pouring the base floors.

Tricky load transfer Tower levels 26 and 27 are designed as transfer floors, where steel gives way to concrete framing, presenting a major structural challenge that amounts to erecting one building atop another. Concrete columns beginning at the transition levels do not line up with the steel columns below. Therefore, the concrete loads must be transferred to massive steel girders and distributed among a series of steel trusses that carry gravity and wind loads.

The girders ranged to 87 in. deep and were encased in pumped concrete. "This phase of the job was time consuming," Northside Concrete’s Martinelli recalls. "Clearances were extremely tight, and we went to a higher slump concrete with a superplasticizer to make it more flowable." And, to achieve full coverage in the tight formwork, A&B Preferred reduced the pump’s hose diameter to 4 in. A&B’s Russo describes this intricate phase of the project as "goulash."

While the transition floors are designed for safe wind resistance, a further step is taken to assure a vertigo-free existence for the tower’s condo dwellers. At the 50th floor contractors are pumping mix for what they believe is the first "tuned liquid column damper" in the U.S. Built to control structure movement, the damper consists of a pair of rectangular concrete tanks separated by a series of baffles.

The damper’s tanks hold over 100,000 gal of water that absorb wind energy. "When the wind blows," say Martinelli, "the baffles balance out the water in the tanks." The combined weight of the tanks and water requires a much thicker deck slab, pumped at 14 in. The walls and lids of the two tanks require a combined total of 1,000 cu yds, and are pumped at a beefy 12-in. thickness. And, at this level, designers reverted to a steel frame to avoid high volumes of space-consuming concrete for structural elements necessary to support the tanks and mechanical, electrical and plumbing equipment to be installed.

Consistent concrete production Quadrozzi Concrete Corp., Queens, is supplying all 22,000 yds of ready-mix that has been pumped at a steady 70 cu yds/hr during the duration of the project, according to the contractors. With horizontal hoseline extension reaching 200 ft on the base building floors, and 140 ft to the corners of the tower decks, the pumps generated the pressures needed to keep the work on schedule. Columns and slabs consume virtually all the concrete, as the exterior walls of the base building are stone and glass, and the tower is all glass curtain wall.

"The Schwing 8000 has maintained a steady line pressure of 208 BAR – over 3,000 psi – all the way up," Russo says. "The pump is putting down 230 yds for the high slabs, plus column concrete, in 6-hr pours. That’s about all the masons can handle."

Pump production also is restricted in the critical areas where concrete loads bear on the underlying steel. In addition, the numerous construction trades create complex logistics in planning and coordinating the overall project, according to Martinelli.

Concrete work is nearing completion, and occupancy of the Random House/Park Imperial is scheduled for this summer. The use of a concrete slab and framing system for the tower’s condo levels affords architects the freedom to position columns and shear walls to achieve the efficient and aesthetic use of space in room layout – spatial efficiencies not possible with a steel frame.