A New Generation of Reinforced Concrete Office Towers in New York City

505_5thAt 505 - 5th Avenue is a modern reinforced concrete office building which conveys elegance in the form of a 30-story tower. Located at the corner of 42nd Street and 5th Avenue in Manhattan, the building gross square footage is 550,000 square feet which includes retail space in the lower floors and typical office space above the seventh floor.

Designed by Kohn Pederson Fox of New York City, the building architectural features offer stunning views across the street to the New York Public Library and Bryant Park. Early on in the design it was decided that the views from the building were the focus of the layout. The building columns were kept away from the perimeter and the main facade was angled to offer spectacular unobstructed views of Fifth Avenue.

Stringent architectural requirements posed structural challenges and required innovative solutions by the structural engineering firm of Rosenwasser/Grossman of New York City. The floor layout required columns to be spaced at 30 inches on center. After many design iterations,  the design team elected an 11-inch deep conventionally reinforced concrete flatslab with 22-inch deep capitals as the most economical floor framing system. This floor system also provided an elegant solution for the large interior spans and eliminated the floor beams otherwise needed to support the slab. Beam supported slabs typically increase the floor to floor height, adds cost and complicates formwork.

505_viewA thickened 15-inch flat slab was used to frame the 15-foot cantilevers at the spandrel to meet the curtain wall design requirements while still allowing an impressive 15-foot dual cantilevers at the southwest corner of the building.  As is common with cantilevered structures, deflection at the building perimeter is of paramount concern. A maximum ¾-inch vertical displacement of the cantilever slab after the installation of the curtain wall was estimated. As a result, a -inch upward camber will be included in the formwork so that after installation of the curtain wall system this camber is eliminated. The perimeter columns supported the cantilevers as well as a portion of the interior span. This structural design solution offered unrivaled architectural flexibility with spans reaching almost 45 feet from the curtain to the core.

A key advantage of conventional flat slab systems is the reduction of floor to floor heights which significantly reduced the cost of formwork and building frame. The flat slab lends itself to the use of conventional plywood construction while the lower floor to floor heights allows for the use of conventional stick shoring. Building each floor on a two-day cycle, is facilitated by selecting this floor system with the simplified formwork it offers. While an aggressive construction schedule, the two-day cycle is the preferred method of construction in New York City. A trend set in the late 1960s and 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 percent compared to an equivalent structural steel floor system.


The lateral force resisting system efficiently incorporated the 11-inch flat slab with a 12-inch thick concrete shear wall which completely encases the fire stair above the mezzanine floor. Located at the rear of the property, the shear wall maximizes the rentable space with views on 42nd Street and 5th Avenue. Along with gravity load support, the flat slab serves as the horizontal diaphragm component of the lateral force resisting system. Managing the eccentricity created by the location of the shear wall elements was easily accommodated without the need for additional structural elements.505_diagram

Higher strength concrete was implemented by the design team to reduce the size of the tower columns and increase rentable space. The team used 12,000-psi concrete up to the seventb floor for the 36-inch round columns that reduce to 28-inch above the 15th floor.

To further enhance the buildings safety, An 8-inch reinforced concrete curb used at the first floor and serves as a barrier against vehicular intrusion. The perimeter columns are encased in a steel shell to maximize their robustness when subject to blast loads. In the event of local overloading and failure of a slab column connection, sufficient steel is provided to arrest progressive collapse.

Cast-in-place concrete construction provided this building with better acoustic properties, fireproofing at no additional cost, and enhanced robustness in the event of terrorist attacks. The 12-inch concrete shear wall completely encasing the fire stairs is a significant safety feature of this structure. The design required two independent fire stairs separated by a 12-inch concrete wall. Floor slab steel reinforcing detailing was modified in some instances and increased in certain areas to provide the added structural integrity mitigating the potential for progressive collapse. With minor additional engineering effort and cost to the owner, concrete structures have always been considered the natural solution to anti-terrorism, force protection, and fire resistance.


Owner Representative: 
Kipp-Stawski Management, New York, New York

Construction Manager: 
Pavarini-McGovern, New York, New York

Kohn Pedersen Fox, Inc., New York, New York

Structural Engineer:
Rosenwasser-Grossman, Inc., New York, New York

Concrete Hi-Rise Contactor: 
Northside Construction

Superstructure Concrete Supplier:
Quadrozzi Corporation, Far Rockaway, New York

Foundation Contractor: 
JCivetta & Sons, Bronx, New York

Foundation Concrete Supplier: 
NYCON Supply Corporation, Long Island City, New York