Current Features
 Past Features
 50th Anniversary

Features - May 2003

Concrete: Changing Uses Post 9/11

by Dan Friedman

It was popular in the weeks and months after the terrorist attacks of Sept. 11 to say, "Everything has changed."

Of course, it wasn't quite that simple. Some things, including construction practices, change more slowly than others.

"In New York City, unlike in much of the rest of the country, commercial buildings are done in steel and residential buildings are done in concrete," said Ramon Gilsanz, a partner with Gilsanz Murray Steficek LLP. "I don't know why that division is so inflexible here, but it goes back a long time and I don't see it changing."

However, in the year and a half since the attacks awakened New Yorkers to the vulnerability of their skyline, owners, developers, architects and engineers have taken steps to ensure that new skyscrapers are safer than their predecessors.

Some of those steps involve concrete.

Changed World, Changing Construction
Prior to Sept. 11, 2001, a building's ability to withstand a deliberate, high-intensity, sneak attack was not something architects and engineers thought much about. "Who the hell could ever have envisioned jet planes being flown into buildings or anticipated the need to protect buildings from the temperatures burning jet fuel can generate?" asked Al Gerosa, president of the New York Concrete Construction Institute.

Now, however, architects and engineers do have to anticipate such an event, and the phrase "design threat" has become a part of their vocabulary.

"We've always talked about the wind load," Richard Tomasetti, co-chairman of Manhattan-based Thorton-Tomasetti Group Inc., said in November at a symposium on the use of concrete in high-rise construction organized by the Portland Cement Association and the New York City Concrete Promotional Council.

"We've asked ourselves about the seismic situation. Now, unfortunately, we are using a new word, 'design threat.' The problem is, we don't always know what the design threat is.

"As structural engineers, we've got to push the bar a little bit and contribute to the process of making our buildings more intrinsically safe than they were before."

One of the first moves to make commercial buildings stronger and safer has been to increase the use of concrete in key structural elements. A number of new Class A buildings that have started to rise since that terrible September morning - including 7 World Trade Center and the AOL Time Warner Building on Columbus Circle - have concrete cores, concrete stairwells and stairs and, in the case of 7 WTC, concrete lower floors.

"The use of concrete has definitely increased since Sept. 11, especially in the core; that's the first change we're seeing," said Mike Mota, who works the New York area as a regional field engineer with the Portland Cement Association. "The concern is with safe egress. The core and stairwells are not traditionally a concrete element. The stairwells were sheetrock at the World Trade Center."

Jacob Grossman, president of Rossenwasser/Grossman Consultant Engineers, PC, said stairwells and elevator shafts in concrete are a real plus in terms of fire safety.

"Developers are beginning to insist that the access and egress routes are to be made of concrete," he added. "Concrete is rigid; it doesn't collapse as easily as sheetrock and masonry, and its use supplements the need for stiffness in the structure."

"Concrete is absolutely safer," agreed Curtis Massey, president of Virginia Beach-based Massey Enterprises Inc., the largest disaster planning firm in North America. Massey, a firefighter for a quarter of a century, develops plans with property owners and managers to minimize the loss of life and property during fires, explosions, earthquakes, etc. His firm has contracts in 65 cities in the U.S. and Canada. In New York City he counts among his clients Brookfield Property Corp., Citicorp, The Durst Organization, the Metropolitan Life Insurance Co., Reckson Associates Realty Corp., RFR Realty LLC and Trizec Properties Inc.

"Concrete is always going to perform better under fire conditions than steel," he continued. "Even if steel is coated, there are always points of vulnerability. The fireproofing is often half-assed. Sometimes the steel has been hanging out at the construction site for months and is starting to rust. Fire retardant doesn't hold on rust."

Concrete Examples
Jack Klein, vice president of Silverstein Development, a division of Silverstein Properties Inc., which is building 7 World Trade Center, the first building to rise from the ruins of the World Trade Center, said that the building's concrete core is nothing new.

"They've been doing concrete cores for a long time," he noted. "What we've done is move the stairwells farther from the core. That is unusual. And we've made the stairwells and stairs concrete as well. In addition, we're making the lower floors concrete. What we've done is create additional redundancy in case of a catastrophic event. We've stiffened the inside core and hardened the building in general to make it more structurally sound."

The AOL Time Warner complex is encasing all major steel structural elements in concrete, as a fire-resistance method.

"Walk out and look at AOL Time Warner in Columbus Circle," Israel Seinuk, president and chief executive officer of the Cantor Seinuk Group Inc., said at the symposium in November. "Is that a steel building with some concrete or is that a concrete building with a little bit of steel? We are using the material where the material is most adaptable for that use."

Cantor Seinuk is the structural engineer for both the AOL Time Warner Building and 7 WTC.

Re-evaluating the Code
The safety value of concrete has also been recognized by the city's Department of Buildings. In February, the department's World Trade Center Building Code Task Force, which had been charged with reviewing the current building code in light of the terrorist attacks, issued its report.

Among its 21 recommendations, the task force urged the department to "encourage use of available impact-resistant materials in the construction of stair and elevator shaft enclosures."

This is not to say that concrete is invulnerable. At intensely high temperatures, it explodes.

"Concrete is mostly water and if it gets hot enough it explodes at a ratio of 17 to one," Massey said. "It's happened (in fires in commercial buildings) but it's extremely rare."

Engineers are hard at work attempting to figure out how to minimize the danger. Some have begun to add polypropylene fibers to the concrete.

"Polypropylene melts at a temperature below the vaporizing temperature of water," Rory Rottschalk, vice president with the California-based engineering firm of Cope & Tanner Inc., said at the New York symposium. "The fibers melt in the fire and all the vapor goes into these new little snake-like holes. It's a beautiful answer. It's very simple. It's available."

Groups like the Portland Cement Association and the New York City Concrete Association hope concrete will make further in-roads in New York. At the same time, a number of structural engineers see advantages in concrete beyond safety, including the ability to make last-minute changes with it and the fact that it doesn't take as much energy to produce it as steel.

"Before World War II, the tallest buildings were constructed using steel," Grossman said. "Since then, many tall buildings (outside New York), particularly if the building is slender, are constructed in concrete. … Today the tallest buildings in the world are concrete or a mixture of steel and concrete. For one thing, we've learned how to use concrete material better and for another, the quality and strength of the concrete has immensely improved.

"There is going to be more concrete in the future," he concluded. "There's no question about it."

 Click here for past Features >>




Learn more about our special supplements and special events

© 2015 The McGraw-Hill Companies, Inc.
All Rights Reserved