A New Generation of Post-Tensioned Concrete Office Towers in New York City
New York City's 140 West 42nd Street is the first post-tensioned concrete office building since the early 1970s which conveys elegance and modern architecture in the form of a 24-story tower with a building gross square footage approximately 200,000 square feet which includes retail space at street level and typical office space above. The tower is located between two landmark buildings, the Knickerbocker Building and Bush Tower and is an important addition to the ongoing transformation of the Times Square District of Manhattan.
Designed by Gruzen Samton Architects of New York City, the building architectural features and function required that office space be shared between the existing Bush Tower and the new proposed space. This requirement meant that the new tower’s floor to floor height would have to match the Bush Tower’s floor to floor height of 13 feet 9 inches.
This architectural challenge required innovative solutions by the structural engineering firm of Desimone Consulting Engineers of New York City. Also, the floor layout required interior column free space with all building columns to be located at the perimeter. After many design iterations that included a composite floor system using 36 inch wide by 36 inches deep steel girders as part of the lateral force resisting system, the design team elected to use a 12-inch deep flat plate with post-tensioned bands as the most economical floor framing system. Also, by using the banded beam system, a considerable amount of architectural flexibility was achieved.
The post-tensioned bands were designed to fit within the 12-inch slab profile which eliminated the need for floor beams otherwise needed to support the slab. Beam supported slabs typically increase the floor to floor height, adds cost and slows construction due to formwork complexity. As a result, most of the slab is conventionally reinforced with the post-tensioning limited to these bands which allows for future retrofitting of the floor slab such as inclusion of slab openings for interior staircases without compromising the structural integrity of the post-tensioning system.
This structural design solution offered unrivaled architectural flexibility with spans reaching almost 40 feet from the curtain to the core and allowed the designers to achieve the same floor to floor height to match the Bush Tower’s floor to floor height. The alternative composite floor system added approximately 3 feet to each floor which would have made this architectural requirement impossible.
A key advantage of conventional flat plate systems is the reduction of floor to floor heights which significantly reduced the cost of formwork and building frame. The flat-plate 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 12-inch flat-plate with a 16-inch thick concrete shear wall which completely encases the fire stair from the ground floor. Located at the rear of the property and in keeping with safety and security in post 9-11 construction, all means of egress will be protected with 16-inch impact resistant concrete shear walls for occupant safety. These vertical structural elements also maximize the rentable floor space and provide economical drift control of the structure for occupant comfort.
Higher strength concrete was implemented by the design team to reduce the size of the tower columns and increase rentable space. Column sizes are reduced to the smallest practical size by using 12,000-psi concrete. In addition, cast-in-place concrete construction provided this building better acoustic properties, fireproofing at no additional cost and enhanced robustness in the event of terrorist attacks. 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.
Pavarini-McGovern, New York, New York
Gruzen Samton LLP., New York, New York
Desimone Consulting Engineers, Inc., New York, New York