By Herbert Margrill
The concrete core of the new One World Trade Center Tower has now ascended above the 66th floor, and its steel columns stretch several floors higher.
For New Yorkers and, indeed, for all Americans and civilized people around the world, it is a potent symbol of freedom’s triumph over terrorism. The building may no longer be called “Freedom Tower,” but that name will surely embody its enduring legacy.
After 9/11, reinforced concrete’s superior safety, and robustness has been the focus of impassioned public-awareness campaigns and appeals for wider use of cast-in-place reinforced concrete for security sensitive structures by the Concrete Alliance, Inc., and its member organizations, including nine building-trades unions.
>>> See more images of the construction site.
One of the most prominent structural engineering companies, WSP Cantor Seinuk (WSPCS), by using reinforced concrete core at One World trade Center Tower, effectively has authenticated that position.
“Our message helped raise support for the choice of reinforced concrete for rebuilding Ground Zero,” said Concrete Alliance President Robert A. Ledwith. “It started with the 7 World Trade Center building. The architects at Skidmore, Owens & Merrill and structural engineers at designed a tower with a core of cast-in-place, reinforced concrete–an extra-strong backbone running the full height of the building. And for One World Trade Center,” he adds, “Skidmore’s and WSPCS’s design provides even better protection for stairwells and elevators.”
The developer of 7 World Trade Center, Larry Silverstein, president of Silverstein Properties, Inc., said he was determined to build “the world’s safest skyscraper.” That’s the chief reason its stairwells are surrounded by cast-in-place, reinforced-concrete.
The stairwell walls in One World Trade Center are even more massive. On the floors already poured, the core walls measure three feet thick or more above ground level – and up to twice that below grade. Higher up, the “cast-in-reinforced” concrete will slim down to two-feet thick concrete walls around the tower core.
Tishman Construction Corporation completed the 52-story 7 World Trade CEnter in 2006, and the company is also managing construction of the 105 story at One World Trade Center. For both buildings, WSP Cantor Seinuk took on the structural engineering task of assuring that Skidmore’s architectural concepts would perform as intended when in the real world.
New York’s first office high-rise with a concrete backbone, at 919 Third Avenue, was a collaborative effort between Cantor Seinuk and Skidmore. That 47-floor building was completed in 1971, before Britain’s WSP Group PLC acquired Cantor Seinuk Group Inc. in 2000. The concrete-and-steel structure helped solve an unusual engineering challenge: The office tower had to be constructed around P.J. Clark’s bar and restaurant, because the owner wouldn’t sell the property.
The concrete core of 919 Third Avenue launched a revolution in skyscraper engineering. It was the first building cited in the lifetime achievement award presented in October to Ysrael A. Seinuk by the Chicago based Council on Tall Buildings & Urban Habitat. Cantor Seinuk’s latter concrete-core effort was the 58-story Trump Tower, finished in 1983.
“Our initial goal in developing the “hybrid” concrete core and steel approach was simple economics, while addressing the structural performance” recalls Silvian Marcus, Chief Executive Officer of WSP Cantor Seinuk. “In terms of speed of construction, it’s very economical. The hybrid usage coupled with large span floors eliminates the need for interior columns thus increasing the interior layout flexibility.
Despite the benefits demonstrated by those two pioneering concrete-core projects, the concept was slow to catch on in New York City, said Marcus. “The high-rise commercial was primarily viewed in the past, as all steel-framed buildings,” he explains. “In many other cities we have been using hybrid construction for commercial buildings for past decades.”
Now, Marcus says, concrete cores are finally becoming more prevalent in the City. For recent high-profile examples, he points to 11 Times Square, One Bryant Park, also known as the Bank of America Tower, Three World Trade Center and Four World Trade Center.
The Trump World Tower, the tallest all-residential building in the world when completed in 2001 and the tallest in New York until the 76-story Beekman Tower, also engineered by WSP Cantor Seinuk, brought another technological marvel to New York: “super” cement. The super or high-strength concrete is produced by blending fly ash, slag cement and silica fume with concrete. Super high-strength concrete cannot be produced with only concrete.
Typically, the high-strength concrete used in skyscraper cores (in the 1990’s) would have a compressive strength of 8,000 to 10,000 pounds per square inch (psi). Because Trump World Tower is a rather slender high-rise, WSP Cantor Seinuk specified concrete compressive strength of 12,000psi for the first time in New York City.
“On One World Trade Center WSP Cantor Seinuk’s engineers worked on specifying the compressive strength and modulus of elasticity,” says Marcus. “But we needed even higher compressive strength—14,000 psi—for the taller One World Trade building.”
Dr. Ahmad Rahimian, President of WSP Cantor Seinuk said, “For One World Trade Center we took an unconventional approach. From past experience we have learned that in high strength concrete the value determined using ACI expression for modulus of elasticity can be exceeded. Accordingly, we specified dual requirements for the shear walls concrete: A modulus of elasticity of 7,000ksi and corresponding concrete compressive strength of 12,000psi and 14,000psi. This contributed to the stiffness of the tower without the premium of calling for even higher strength concrete.
Quadrozzi Concrete with The Port Authority of NY & NJ formulated several candidate mixes that were further tested in the field. “The engineers in our laboratory fine-tuned what ended up as the final mix,” said Casimir Bognacki , the Port Authority’s chief of materials. “We believe that it sets a new standard for New York City construction.” Prior to the construction of Tower 1, concrete mix proportion contained 1200 to 1300 pounds of cement material to produce 12,000 psi concrete. As a result of work done on Tower 1, this has now been lowered to 900 – 950 pounds; this produces a greener and more economical concrete.
Mr. Yoram Eilon, Vice President of WSP Cantor Seinuk agrees. “This product will probably lead to fairly wide use of 15,000-to-16,000 psi concrete,” he says. “And that will in turn create new structural opportunities for future high-rise buildings and bring higher value to this form of construction.
Creating stronger concrete is, in the words of WSP Cantor Seinuk’s Mr. Eilon “basically a matter of figuring out how to fill the voids.” This involves selecting the right gradation by combining different aggregates that will interlock together tightly–but not too tightly–when combined with different cement materials, and water and other admixtures. The idea is to use local materials in order to meet sustainability requirements and for practical and economical reasons.
It’s a delicate balancing act that requires tight monitoring and quality control. Increasing the mix density also makes it more viscous and harder to pump, especially if pouring involves pumping the concrete through long tubes, as is common in skyscraper construction. 14,000-psi concrete has been continuously pumped distances up to 300 feet at the One World Trade Center site.).
Concrete hardens as the result of a chemical process that produces heat as it cures. “This heat of hydration is a much greater problem for Freedom Tower’s thick core walls, categorized as Mass Concrete, than for the conventional concrete walls outside the core,” said WSP Cantor Seinuk’s Eilon. Excessive heat would expose the concrete to DEF (Delayed Ettringite Formation), produce thermal cracking, reduce the concrete ultimate strength and modulus of elasticity, he explains.
In order to minimize the detrimental affects the heat of hydration can have on concrete, The Port Authority of NY & NJ proposed and insisted on changes to the concrete mix proportion specification. The Concrete Contractor adopted night pours and introduced high levels of ice into the mix to try and control the inherent constraints of the mix design. The Port Authority also monitored the in-place concrete temperatures to make certain specification requirements were being met.
Finding the right proportions of the right ingredients is one part of the battle. “Due to the large quantities of the high strength concrete being used at anyone given time and the very low water cement ratio, consistency became critical to the overall success of delivering and pumping the concrete. Therefore, the collective efforts of QA/QC (Quality Assurance and Quality Control) on all levels from the Ready Mix Producer, Contractor, Engineer and on this project the Port Authority of NY & NJ we’ve been successful thus far which will maintain to the Roof,” says Renzo Collavino, president of Collavino Construction Company, contracted by the Port Authority of NY & NJ for the Concrete Superstructure at One World Trade Center.
So far, Collavino Construction has poured roughly 120,000 cubic yards of the 210,000 cubic yards of concrete that will be needed to cap One World Trade Center’s superstructure. The concrete roof of the topmost “people floor”–the observation deck on the floor numbered 102–will stand just as tall as the former World Trade Center Tower Two, at 1,362 feet. A glass-and-metal parapet will add six feet more, matching the 1,368-foot height of One World Trade Center.
Crowning the summit of the toer will be a ring of television and radio antennas encircling the base of an illuminated mast. The spire’s tip will soar to 1,776 feet – in homage to the year when freedom’s consummate expression, the Declaration of Independence, was signed. One World Trade Center is slated to open in late 2013, according to the Port Authority.
To date, most of the concrete has gone into the tower’s monolithic pedestal. From a footprint of 200-by-200 feet, it rises up for 70 feet. Above ground, this has specially reinforced concrete to defend the building from the blast of a street-level bomb. Below grade, the reinforced-concrete structure is engineered to protect the tower’s structural integrity from a bomb even bigger than the one that exploded in 1993 in the World Trade Center’s parking garage.
“The amount of rebar being installed by union workers from Local 46 of the Metallic Lathers & Reinforcing Ironworkers is truly impressive,” said Ledwith, who was an up-from-the-ranks executive at Local 46 prior to being named president of the Concrete Alliance. “Keeping up with the rate of concrete pouring – typically a floor every seven days – takes about 75 – 85 skilled rebar workers.”
“When construction engineers and executives visit Ground Zero and learn that every cubic yard of pedestal concrete is impregnated with approximately 54,000,000 pounds/27000tons of rebar, they’re invariably surprised. That’s more than what you’d expect for a high-rise of this scale, primarily due to the demanding security requirements, said Collavino of Collavino Construction.
Protected inside the cast in place, reinforced-concrete core walls with a minimum of six-foot walls in certain locations and stairwells – 20% wider than code mandated – to facilitate fast evacuation of Tower One and movement of responders up the stairs in an emergency. The third stairwell is reserved for firefighters and other emergency responders.
Concrete is virtually fireproof, and the stairwells will be pressurized to prevent smoke from hampering evacuation. The building’s ventilation system will even have filters to defend occupants from terrorist attacks with biological agents or chemical weapons.
Behind the pedestal is an expansive lobby with 50-foot ceilings. Above this are several floors crammed with mechanical systems, including tenant generators and air handlers. Six fuel-cells located in a below-grade plant that will provide 1.2 megawatts of continuous pollution-free electricity to satisfy approximately 20% of 1WTC’s basic needs.
Floors 20 to 90 hold 2.6 million s/f of Class A, column-free office space. Just above the top of 7 World Trade Center, at the midpoint of One World Trade Center, will be its sky lobby. Above the uppermost office floors are more mechanical floors that will house additional equipment, such as heating-cooling-ventilating systems, water storage, and motors for some of the building’s elevators.
On the 100th to 102nd floors will be a multi-story observation deck. The tower culminates at 1368 feet, the height of the 1970 north tower, above which stands a 145-foot diameter communication ring holding communication antennae and the 408-foot spire and beacon.
For the below grade construction, the concrete strengths requirement ranged from 6,200 psi to 14,000 psi, coupled with modulus of elasticity of north 7,000 psi. The reinforcing demands were in excess of 16,000,000 pounds or 8,000 tons, which far exceeds any of the amounts ever stipulated in New York City’s building code, which can be contributed to the demanding security requirements.
The 14,000-psi mix flows and handles somewhat differently from normal high-strength concrete, and it requires extra care. But Collavino doesn’t mind. “The Collavino Group may be based in Windsor, Ontario, Canada, but we’re all part of the greater family, the civilized people of the world who were horrified by the tragedies on 9/11. Collectively, we’ll do whatever is necessary to assure that this tower serves as a permanent reminder to terrorists that they can never extinguish the spirit of freedom that unites the world,” he said.
Ledwith, of the Concrete Alliance, echoes those sentiments. “Everyone working on the new One World Trade Center is honored, and humbled, to play a role in this historic project,” he says. “It’s more than a job-it’s a commitment to a saner, safer future.”
“I’m proud that One World Trade Center and 7 World Trade Center are blazing a new trail for skyscraper construction. They are more than just the safest high-rise buildings ever constructed. The innovations pioneered by Skidmore, WSP Cantor Seinuk, and Tishman also provide a shining example of how reinforced concrete can help mitigate climate change. That looming threat,” he emphasizes, “is far more dangerous than terrorism, because almost nobody will remain unaffected,” he added.
7 World Trade Center was one of the first office skyscrapers to earn a Gold rating under the Leadership in Energy & Environmental Design (LEED) program administered by the U.S. Green Building Council. During the same year the Hearst Tower, also engineered by WSP Cantor Seinuk, received Gold rating under LEED program for core and shell as well as interiors. Three years earlier, in 2003, WSP Cantor’s engineering had helped earn a Gold LEED medal for the 27-story Solaire residential building in Battery Park City.
One World Trade Center is following in those ‘green’ footsteps. Besides the fuel-cells, it will feature advanced energy-conservation technology, next generation glass, and high-efficiency equipment to reduce energy usage by an expected 30% water conservation that will capture and reuse rainwater for cooling the building and irrigating the plaza’s trees and plants, and many other environment-friendly attributes.”
Concrete earns more green credits. Cement production is a significant source of carbon dioxide, a harmful greenhouse gas. Producing a ton of cement gives off roughly a ton of carbon-dioxide emissions, with about half coming from the fossil fuels burned to heat the limestone, and half from the calcination of the limestone. Since the concrete at 1-WTC uses 40% less cement, that will save some 33,000 tons of carbon-dioxide gas–not counting the savings that the Port Authority has established by setting the standard for Platinum LEED certification.
“Concrete is what gives commercial structures of any size a head start toward earning LEED certification,” said Ledwith of the Concrete Alliance. Besides being virtually fireproof, concrete also creates excellent thermal and sound barriers. The former helps insulate a building from outside temperature fluctuations, so less energy is needed for cooling and heating. Concrete is always produced locally, whereas steel and other structural materials are typically transported long distances, thus consuming more fossil fuel.
“We have many case histories and research studies that show concrete is usually the best choice for the environment,” said Ledwith. “And when combined with modern techniques like post tensioning and cast-in-place pouring, reinforced concrete can also be the most economic choice for a building’s developer and certainly outperforms other alternatives in operating costs over a building’s lifetime.”
“I’m confident,” he adds, “that One World Trade Center will be the latest proof of concrete’s overall superiority.”
Herbert Margrill is a vice president of H. Margrill & Associates.