Since the collapse of the World Trade Center towers on Sept. 11, 2001, there has been a movement in New York City to build cast-in-place reinforced concrete-framed office buildings.
In recent weeks, the federal government, in a way, is supporting this movement with the recent public disclosure of findings by the Gaithersburg, MD-based National Institute of Standards and Technology (NIST) regarding the collapse of the World Trade Center’s twin towers and a not-so-public 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 said 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 said.
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 West 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 also was 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 Sept. 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 fireproofing 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 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 in wider egress stairs, provides a stronger, safer building that is terrorist-resistant, soundproof and fireproof and has the ability to reduce the feeling of motion caused by wind, Gerosa explained.
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 said. 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 noted.
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 28-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 terrorist-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.
This office tower also features a 12-in.-thick concrete shear wall that completely encases the fire stair above the building’s mezzanine floor. The shear wall is located at the rear of the property to maximize rentable space. Any eccentricity in the lateral load resistance system is handled by the reinforced concrete slab-frame system.
The owner is going even one step further to ensure safety by having an 8-in. thick reinforced concrete curb at the first floor as a barrier against vehicle intrusion.
In addition to safety, the mass of a concrete structure such as this makes it a significant thermal reservoir with the ability to store larger amounts of energy, Gerosa said. In the months in which heat is required, concrete walls and floors absorb the interior heat during the day and then radiate warmth back into the space at night. The same holds true for cooling. This thermal inertia allows concrete to help maintain a relatively steady interior temperature, Gerosa added, noting that by storing and releasing energy needed for heating and cooling, concrete delivers year round energy benefits.
Gerosa said using a reinforced concrete frame to construct a Class A office building provides owners and developers with flexibility if the market changes or if the building is sold, permitting easy conversion for residential use. And, a cast-in-place reinforced concrete frame provides a lower floor-to-floor height that increases rentable space and lower construction costs in the form of fewer electrical wiring ducts, less piping and less building skin required compared with a steel-framed office building.
Owners and developers can save even more money on the cost of insurance, according to Gerosa. Due to its high-strength and lower thermal conductivity rating, a cast-in-place reinforced concrete framed office tower results in less liability on the part of insurance companies. Therefore, there is less liability on the part of the owner. As a result, the annual cost of property insurance for cast-in-place reinforced concrete-framed office buildings, according to three major insurance companies from whom rates were gathered in research conducted by the Concrete Alliance, were 20 percent to 25 percent per year less than had the same office building been constructed using a steel frame.
In addition, the Concrete Alliance said insurance underwriters are looking to incorporate cast-in-place reinforced concrete frames for the construction of office buildings into their underwriting criteria, making it less costly for a cast-in-place reinforced concrete Class A office building to be built and maintained.
“It seems even the federal government has determined that cast-in-place reinforced concrete–framed office buildings are indeed safer and stronger than those constructed using a steel frame,” Gerosa said.
Gary Higbee, director of industry development for the Steel Institute of New York. Strongly disagrees.
“While I can understand the concrete industry wanting to expand its share of the office tower market — a market in which steel has traditionally and continuously been the material of choice — I find their assertion, which they proclaim as a public service that the NIST World Trade Center findings found reinforced concrete stronger and the concrete industry’s claim that insurance companies offer 25 percent less on the cost of property insurance using reinforced concrete over steel frames, a gross distortion of the facts.” CEG