An unusual trapezoidal site in Manhattan is the setting for Astor Place Tower, a 21-story residential condominium building at Fourth Avenue and Astor Place. Construction on the 140,000-square-foot building, completed in 2006, made it a showcase for the versatility of reinforced concrete.

This $50-million “rotational, asymmetrical structure” includes mixed-use commercial space and a residential condominium tower. The complex contains 39 luxury residential loft units, 13,000 square feet of retail space, a below-grade parking garage for residents’ cars, and penthouse gallery space for Cooper Union, the owner of the site.

The building's design consists of three basic elements: a limestone and glass retail podium; a serpentine glass and aluminum tower; and a butt glazed, zinc column, spandrel frame, tower hat. The most notable feature of this building is its curved perimeter layout. The design’s three setback levels help conjure a resemblance to a grand piano. The exterior of the building also features a 5,000-square-foot urban plaza connecting Cooper Square and Lafayette Street.

The building features a striking and complicated curved design by Charles Gwathmey, principal with Gwathmey Siegel & Associates. Adding to the challenge for DeSimone Consulting Engineers, the structural engineer, was the presence of two different street gridlines, says Michael Gabbay, Senior Vice President of Plaza Construction, general contractor on the project.

"The configuration of the building is such that the curves at the concrete slabs had to be laid out carefully," Gabbay says. "We had to maintain the integrity of the design by preserving the curvature of the slab. Concrete was the ideal material to make the curves for the shape of the building."

According to the architect, the design draws on the iconic Flatiron Building farther uptown for inspiration. Astor Place also pays tribute to ancient masters of architecture. "The second, more historical precedent we looked to was the obelisks of ancient Egypt," Gwathmey says. "By its sculptural clarity, the obelisk is the acknowledged object that defines that place. We wanted to introduce a distinct object as opposed to making an extended-wall building." Gwathmey says that using steel would have taken more time. "Concrete is more natural and appropriate," he adds. "It is efficient and incredibly fast in terms of structurally framing the building, which did not have the same floor plate on each floor. It is asymmetrical as it spirals to the top."

Concrete also allowed for thicker slabs at transfer floors to support the building's irregular shape, with slabs 24 and 28 inches thick at the 17th floor and 14 inches thick on the transfer slab at the 21st floor. DeSimone's engineering plan called for using 8,000-psi concrete with silica fume to strengthen shear walls and transfer floors.

A reinforced concrete flat plate system with shear walls was chosen for Astor Place, as it provides the most economical structural system for the project’s intricate architectural and programmatic layout. Maintaining the integrity of the architect’s design, concrete is easily shaped to any form and does not require extra material or “skins” to buttress the system. Placement of embedded reinforcement at the perimeter was curved to follow the façade’s strict geometrical requirements, and allowed for the curtain wall to be easily attached to the structural slabs by means of embedded anchors.

The trapezoid-shaped site is near a subway and historic pre-war buildings. In order not to disturb these sensitive neighbors, drilled H-shaped piles and concrete filled steel caissons were used.

The building employs a reinforced concrete frame, featuring flat slab construction and exposed concrete columns. The amoeba-like floor slabs do not always line up with the floors above or below. Slabs were formed on site using custom-made wood forms. Typical tower floors consist of 8-inch-thick slabs supported by a system of round perimeter columns. Floors utilized a maximum of 5 interior columns. Shear walls provide the lateral support against wind and seismic loads.

The building is like a multi-tiered layer cake, with setbacks that accommodate roof terraces, but further complicate the design. The first setback occurs at the 3rd floor, just above the podium levels, and required curved concrete beams to make up for the elevation difference between the terrace and tower layouts. An integral cantilever concrete beam system was used in conjunction with the curved perimeter beams to transfer a perimeter column at the building’s southwest corner. Steel reinforcement for this transfer system was extensively detailed to enable the main bars, stirrups, and hanger reinforcement to work effortlessly in conjunction with each other.

The second setback level occurs at the 17th floor. At this transfer mat slab, concrete columns are discontinued from below and transferred to accommodate the shift and change in programmatic layouts from the floors above. This level utilizes a 24-inch-thick mat slab. The building again steps back at the 21st floor, where columns are discontinued and transferred via a 20-inch-thick mat slab. This transfer created loads that induced stresses not often found in conventional residential slab situations. Here transfer slabs were reinforced using mid-height bars, stirrup cages around columns, and Decon studrails.

Astor Place possesses other unique structural characteristics. The basement level, containing services and parking, is accessed via a ramp that also serves as the bracing system for the structure’s southern wall. The southern portion of the ground floor slab is designed to accommodate additional planting and tree loads. As well, the building’s southern wall extends 15 feet, unbraced, above ground to accommodate a signage/party wall. To help maintain the glass curtain wall, a revolving window-washing tower beam was installed on the upper roof slab, and a 22-inch-thick transfer mat was designed to help support the resultant downward and uplift loads imposed.

A key advantage of conventional flat plate systems is the reduction of floor to floor heights, which significantly reduces the cost of formwork and building frame. The flat plate lends itself to the use of conventional plywood construction, while lower floor to floor heights allow for the use of conventional stick shoring. Building each floor on a two-day cycle is facilitated by the simplified formwork. A trend set in the late sixties, the aggressive two-day cycle is the preferred method of construction in New York City, made possible by the moderate spans and lower floor to floor heights common in residential hi-rise flat plate construction. Unprecedented in office building construction, the two-day cycle will reduce the floor completion schedule by 50% compared to an equivalent structural steel floor system.

The lateral force resisting system efficiently incorporates the flat plate with a 16-inch-thick concrete shear wall that completely encases the fire stair from the ground floor. Located at the rear of the property, and in keeping with post 9-11 concerns, all means of egress are protected with 16-inch impact resistant concrete shear walls. These vertical structural elements also maximize the rentable floor space and provide economical drift control of the structure for occupant comfort.

Higher strength concrete (12000 psi) was chosen by the design team to reduce the size of the tower columns and increase rentable space. In addition, cast-in-place concrete construction provided superior acoustic properties, fireproofing at no additional cost, and enhanced robustness in the event of terrorist attacks.

Case Studies:

Cultural Buildings
Educational Institutions
Green Buildings
Healthcare
Hospitality
ICF Buildings
Luxury Residential
Mixed Use
Office Buildings
Religious Structures
Tilt-Up Buildings

Owner: Related Companies, New York, N.Y. Architect: Gwathmey Siegel & Associates, New York, N.Y. Structural Engineer: DESIMONE Consulting Engineers, Inc., New York, N.Y.

 
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