Concrete: Changing Uses Post 9/11
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
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
"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
"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."
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
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
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.
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."