Slender, Robust, and Very Tall
Cast-in-place reinforced concrete rises to the occasion.
By Sara Hart
 
   

Click some images for a closer look


Photo © Jeff Goldberg/Esto

Across from the United Nations Plaza, Trump World Tower soars 860 feet into the Manhattan skyline.


Photo courtesy Peri GMBH; Drawings courtesy
Costas Kondylis & Partners

Cast-in-place reinforced concrete provides stiffness against wind loads, reduced floor-to-floor height, elimination of ceiling finishes, better soundproofing, and a two-day construction cycle.

505 Fifth Ave. images courtsey Kohn Pedersen Fox

505 Fifth Avenue
505 Fifth Avenue by Kohn Pedersen Fox is the best of two worlds— a fortified concrete body clad in an exquisite transparent skin.

Above: The columns on the upper floors of 505 Fifth Avenue are recessed 15 feet to allow for a curtain wall that is uninterrupted, thus optimizing daylight and views. Below: Thin concrete floor plates made it possible to get an additional two floors in the envelope. The floors are cantilevered 15 feet from the columns, which required careful detailing of the curtain wall to accommodate deflection and shrinkage.


Photo © Jeff Goldberg/Esto

Carnegie Tower is a slender slab of cast-in-place reinforced concrete.

Think of the Empire State Building, the most famous skyscraper in the U.S. and the tallest building in the world from 1930 until the 1970s. Riveting archival film of the steel erection shows ironworkers casually eating lunch perched on steel beams hundreds of feet above the ground, working without a net, the steel flying up at a rate of four-and-a-half stories per week. The speed and heroics are just as vital to the mystique and admiration as the architecture of Shreve, Lamb, & Harmon. It remains the most prominent architectural icon on the Manhattan skyline and the most notable symbol of steel’s domination in New York high-rise construction for most of the 20th century.

In 2004, all sorts of innovations in methods and new materials are attracting attention in the high-rise building industry. And yet it’s the world’s oldest building material—concrete, in its modern cast-in-place reinforced incarnation—that is being touted as the material for the future. While it’s true that other kinds of innovations propelled steel ahead of concrete, including the acceptance of new forms of fireproofing, such as intumescent paints, cementitious spray-on coatings, and sprinkler systems, the concrete industry is blitzing developers, builders, and architects, arguing that concrete is the safest and most reliable material for skyscrapers. (Due to legitimate concerns regarding the safety of tall buildings after September 11, even most laypeople these days know what constitutes a “hardened core.” This is not lost on the advocates of concrete construction, even as they try not to belabor the point.)

Concrete construction offers other advantages in addition to better fire and blast protection—slenderness, thinner floor plates, and fewer internal columns. Modernism is associated with narrow towers rising elegantly, in the best cases, into the skyline. Cast-in-place reinforced concrete provides the rigidity such slenderness demands. Degree of slenderness is described by the aspect ratio, or the relationship between a building’s height to its width. For example, Jacob Grossman, principal of Rosenwasser/Grossman Consulting Engineers in New York, has engineered many slender towers, including the Carnegie Tower designed by Cesar Pelli in 1991, which remains the tallest commercial concrete skyscraper in Manhattan. It has a “double-tube” concrete structure with wind-load resistance designed into the spandrel beams, which made possible an aspect ratio of 15:1 and 10:1 for the two vertical elements.

Donald Trump’s Manhattan real estate conquests are as legendary as his architectural contributions to the urban fabric are notorious. At least 10 skyscrapers bearing his name inhabit Midtown. Given that great height is an economic reality in New York, altitude alone does not usually arouse harsh architectural scrutiny. Tall is the accepted rule until a cunning developer slips outside the designated high-rise zone. Trump did just that by choosing a site across from the United Nations to erect Trump World Tower—the tallest residential tower in the world (since surpassed by Tower Palace Three in Seoul). Opposition was loud and fierce and much reported, but Trump prevailed and his $400 million monument to luxury living was completed in 2001.

It’s unfortunate that, at the time, the controversial real-estate coup overshadowed the engineering and design finesse of the project’s architect, Costas Kondylis and Partners, and structural engineer, Cantor Seinuk. Trump World Tower is an unadorned rectangle that rises 860 feet without setbacks or visible bracing. Most observers would assume that its height alone would require a steel frame, even though most residential buildings are concrete. In fact, concrete is the only practical material that can achieve such slenderness.

The owner wanted a spacious loftlike feel to the apartments, which would also be reminiscent of those sprawling pre-war apartments for which New York is famous. Ceiling heights in the tower’s apartments range from 10 to 16 feet. To balance such volumetric generosity, it was necessary to create as many floors as possible. Because concrete floor plates are thin, a cast-in-place concrete system gains an additional floor for every 10 stories when compared to steel framing.

The engineering strategy is ingenious in its concrete management. According to the Concrete Industry Board, the building claims the first use of 12,000-psi concrete in New York. This provided the stiffness in the shear walls to allow the architect to achieve an aspect ratio of 11:1, which gives the tower its elegant slenderness. In addition, 10,000-psi concrete was used to minimize column sizes while increasing their load-bearing capacity. Although the slabs required no more than 4,000 psi, the concrete was upgraded to comply with the load-transfer requirement at the slab-column joint.

The tower is supported on bedrock with a bearing capacity of 40 tons per square foot. The lateral force resisting system consists of shear walls and frames interacting for the full height of the building with a perimeter concrete band at midheight and another at the top.

That’s a lot of pouring, reinforcing, and climbing to reach 860 feet. Some of the most important innovations in cast-in-place reinforced concrete construction have been in the erection process. Time is money, and steel has always been faster, or at least that is the perception in the construction industry. PERI, a scaffolding and form work company headquartered in Germany, raised the floors. It formed 22 of the 27 perimeter columns, reserving the remaining column locations for crane access. Twelve climbing platform units on the facade were powered by five hydraulic power packs. A separate power pack was available for each of the three shafts. The PERI scheme called for forming the facade columns and main load-bearing walls and shafts together with the floor slabs. The columns, walls, and shafts were poured first, followed by the floor plate. The workforce was 187 strong during this process. They were able to complete a floor in a three-day cycle. After the 22nd floor was complete, they accelerated to a blistering two-day cycle.

505 Fifth Avenue is one of the last great sites in Midtown Manhattan. On the northeast corner of 5th Avenue and 42nd Street, the site enjoys distinguished company—the New York Public Library, Bryant Park, and Grand Central Terminal. For one developer, the inherent glamour of the location compensated for its small size. Kipp-Stawski develops boutique office buildings for tenants who require less space but more amenities, so it was not in need of the typical 25,000-square-foot floor plates. It was more important to principal Axel Stawski that he get an elegant design for that prominent location.

New York–based Kohn Pedersen Fox (KPF) has built scores of distinguished commercial towers in New York, all with steel structures. Kipp-Stawski offered the firm its first commercial cast-in-place reinforced concrete superstructure. As with the Trump World Tower, concrete offered both sturdiness and thinner floor plates, the latter giving the owner two more floors of rentable space.

KPF exploited the structural system—columns 30-feet on center—to produce flexible floor plans. The flat plate floors are only 11 inches deep, although the depth is 22 inches at the column capitals. To avoid that larger depth at the perimeter in the office tower, the floor plates are cantilevered 15 feet from the interior columns. This move also eliminates the need for perimeter columns, giving the architect freedom to create a delicate facade and to optimize daylight and views. Senior designer Douglas Hocking, AIA, explains how they kept the curtain wall pure and the vision glass from floor to ceiling: “Since we don’t have an 18-inch sill for the base and don’t want a crash bar, the inner lite of the double glazing is laminated. Furthermore, sprinklers run at the perimeter every six feet to provide extra fire protection.”

This strategy is ingenious, but it did create several challenges for the designers. Project manager Christopher Stoddard, AIA, explains, “One of the greatest challenges was designing the curtainwall to accommodate both the interstory live-load deflection at the cantilever and the inherent shrinkage of concrete structures. The amount of combined live-load deflection, shrinkage, and movement due to building sway was just shy of one inch. This potential movement had to be accommodated in the cladding system while maintaining a positive visual aesthetic and a weather-tight enclosure.” Although completion is a year away, 505 Fifth Avenue is already the poster child for the Concrete Alliance, an industry organization that advocates cast-in-place concrete in commercial buildings in New York.

Jacob Grossman, whose firm Rosenwasser/Grossman engineered 505 Fifth Avenue, observes that in the past 20 years, the rest of the world has built the tallest buildings out of cast-in-place reinforced concrete. He predicts that concrete will edge past steel for commercial building in New York in the near future, either as all-concrete or composite (steel frame/concrete core) structural systems. Whereas it’s still true that steel allows for longer spans than concrete, spans greater than 50 feet can be achieved by either rediscovering two-way waffle-joist construction (used extensively before the recession in the 1990s) or by post-tensioning—a method by which a steel cable is threaded through a sheathing and is tensioned after the concrete cures. “If you asked me a while back about post-tensioning, I would have said that it was hard to manipulate, prevent moisture corrosion, and accommodate future penetrations,” explains Grossman. “However, in the past few years, the system has improved a great deal with the introduction of bonded post-tensioning, so my reservations have been somewhat lessened. I’m still hoping that continued development [and a trained labor force] will make it economical, and then my lingering reservations will be gone.” (Bonded means that the cable is grouted to the sheathing, and the sheathing is bonded to the concrete.)

Meanwhile, across the East River in Queens, Local 46 of the Metallic Lathers and Reinforcing Ironworkers Union is responding to the call for highly skilled workers. In the basement of its Learning Center, full-scale mock-ups show two ways of reinforcing cast-in-place concrete. One shows the typical grid of rebars used in most construction. The other installation shows how post-tensioning works with fewer rebars and sloping cables. Determined to stay ahead of the learning curve, the union is teaching those skills in anticipation of a shift to this technology in New York.

Innovation is often buried in the details. The interdependence of architectural form and its underlying structural system is exactly that intersection where art meets science. Trump World Tower can rise solidly 860 feet because of this meeting. For the same reason, 505 Fifth Avenue will win accolades for its taut, transparent skin, seemingly held to the structure by magic.