Findings back move towards all concrete office buildings

Real Estate Weekly, April 5, 2006

Since the collapse of the World Trade Center towers on September 11, 2001, there has been a movement in New York City to build cast-in-place reinforced concrete-framed office buildings.

In April 2005, the federal government, in a way, supported this movement with the public disclosure of findings by the Gaithersburg, Maryland-based National Institute of Standards and Technology (NIST) regarding the collapse of the World Trade Center's twin towers and a not-so-pubic -support by the U.S. General Services Administration (GSA) to opt for a cast-in-place reinforced concrete-framed 24-story office building that will be the future home of the U.S. Mission to the United Nations (U.N.) in New York City.

While the new U.S. Mission to the U.N. is a construction project shrouded in secrecy, GSA Project Manager Sek Eng says concrete was chosen because "steel doesn't meet our security needs."

The planned structure has been described as a "concrete bunker designed to withstand car bombs, chemical or biological attacks and other threats," according to industry sources and a report in The Washington Times.

"It's all classified for security reasons," Eng says.

What is known is that the new structure will replace a 12-story office building that housed 300 employees who have been relocated to temporary space a few blocks from the old building's East 45th Street and First Avenue location.

Designed by the New York City-based architectural firm of Gwathmy-Siegel, the new structure is expected to have very thick exterior walls that will be composed of high-strength reinforced concrete. The thickness of the concrete was not disclosed, but it is believed it will be thick enough to withstand an attack.

The strength of the concrete was also not disclosed, however, there are buildings in New York City with 12,000 psi concrete. Concrete producers in New York's five boroughs have gotten yields of up to 16,000 psi and many claim they can produce concrete with even greater strengths.

Not only does it seem that the GSA recognizes the benefits of a cast-in-place reinforced concrete-framed office building, but NIST's findings with regard to the collapse of the World Trade Center's twin towers has borne out what the Concrete Alliance of New York City has been saying since September 11, 2001--that cast-in-place reinforced concrete, due to its strength and lower thermal conductivity rating, is safer than any other building material, according to Alfred G. Gerosa, president of the Concrete Alliance.

Throughout its findings, NIST investigators clearly point out that the extreme heat caused by the ignited jet fuel and other factors, caused the steel frames of the twin towers to buckle, contributing to the collapse of both buildings, Gerosa said.

According to NIST researchers, steel loses its strength and structural integrity quicker than high-strength concrete.

The NIST findings also point out that damaged and dislodged fire proofing from the structural steel frames of the buildings also contributed to the collapse of the two towers.

While spray-on fireproofing is an approved material for use in the construction of New York City high-rise buildings, the Concrete Alliance's inquiries to Underwriters Laboratories (UL) revealed that when UL conducted tests on spray-on fireproofing on steel to ascertain a fire rating for spray-on fireproofing manufacturers, water (in excess) was used to cool down the steel quickly. In these tests, UL testers said they visually noted that spray-on fireproofing "falls off" the steel assemblies after the steel was hit with water to cool it down.

This would not happen if the steel was encased in concrete. However, the use of high-strength reinforced concrete frames for high-rise office towers, with concrete cores and wider egress stairs, provides a stronger, safer building that is terror-resistant, soundproof and fireproof and has the ability to reduce the feeling of motion caused by wind, Gerosa explains.

The primary emphasis has been and continues to be that a cast-in-place reinforced concrete-framed office building provides a safer, stronger structure, Gerosa says. For this reason, owners and developers throughout New York City are now looking at constructing their new high-rise office buildings using concrete cores and reinforced concrete frames, he notes.

One such developer is Kipp-Stawski Management of New York City.

Kipp-Stawski Management is the owner and developer of 505 Fifth Avenue. The company wanted to emphasize safety in this 30-story office building by having it constructed with a concrete core and a flat plate reinforced concrete framing system.

Concrete was chosen over steel for this office building because it is terror-resistant, blast proof, fireproof and soundproof. In addition, the cast-in-place reinforced concrete provides greater lateral stiffness or resistance to horizontal movement caused by high winds, hurricanes, tornadoes and seismic movement. The lateral stiffness means tenant workforces in 505 Fifth Avenue are less able to perceive building motion.