Chief Jack Parow, Chelmsford (Mass.) Fire Department
Jun 1, 2002 12:00 PM
To most people the cause of the World Trade Center collapse on Sept. 11 seems obvious: Two Boeing 767 aircraft crashed into the towers, toppling them within hours. Yet to fire service professionals, the cause of the collapse is not quite as simple.
Following the attack, the Federal Emergency Management Agency and the Structural Engineering Institute of the American Society of Civil Engineers, in association with New York City and several other federal agencies and professional organizations, deployed a team of civil, structural and fire protection engineers to study the performance of buildings at the site. Results of the investigation can be found at <www.fema.gov>.
The fire service community and others can benefit from these findings and work to prevent future tragedies through improved building construction and fire suppression.
Construction of the WTC towers began on Aug. 5, 1966, and the first tenants started occupying the buildings four years later. Construction on the upper floors was completed in 1972 and the towers were officially opened on April 4, 1973. The architect for the project, Minoru Yamasaki, was faced with the unprecedented task of designing a pair of what would be the tallest buildings in the world, containing more office space than in the entire city of Cincinnati. At that time, conventional construction techniques allowed for buildings to reach a maximum height of 80 floors — 30 floors or 2 million square feet short of what the project manager, Guy Tozzoli, wanted to build. Tozzoli challenged Yamasaki, telling him “If President Kennedy can put a man on the moon, you can design a building 110 stories in height.”
The major obstacle for Yamasaki was the sheer volume of floor space needed for the elevator shafts in a building of this height. A traditional elevator configuration for this kind of building would require the use of half the total floor space on the lower floors just for elevator shafts. Using this amount of space would render construction costs too expensive in relation to the available office space. The solution was to treat each tower as three separate 40-story structures. In this system, passengers would travel up the elevators and change at sky lobbies located on the 44th and 78th floors. This new design cut the number of required elevator shafts in a conventional system by half.
With the elevator problem solved, structural engineers John Skilling and Leslie Robertson needed to design a steel frame structure strong enough to support 110 floors and capable of withstanding severe wind loads averaging 13,000 tons of pressure. The solution was a system composed of a hollow tube exterior, a center core and floor trusses that tied the exterior walls to the core. The hollow tube exterior would consist of a façade 208 feet square with 39-inch columns on the center and would provide wind bracing and support for the floors above. The center core would carry only the gravity load of the building and provide the space for the elevator shafts and stairwells. Completing the system were 33-inch deep trusses, which would span the entire 60 feet from the exterior wall to the core tying the two together and preventing lateral buckling of the exterior walls.
This revolutionary construction system was a major change from the conventional system that used steel i-beams throughout the structure with non-structural exterior curtain walls. Prior to this time, curtain walls were used primarily to keep the elements out and were non-load bearing. All wind loads were transferred through the floor membrane and supported by the core of the structure. This new hollow tube system design resulted in a light and economical structure using only half the amount of steel required in a conventional building. It also provided for the wind bracing to be placed in the exterior walls — the most efficient place. The exterior wall in this system would be load bearing and used to support the structure itself.
Almost 29 years after the towers opened, American Airlines Flight 11 struck Tower 1 from the north at 8:46 a.m. EDT, crashing into floors 93 through 98. United Airlines Flight 175 struck Tower 2 from the south 16 minutes later, crashing into floors 78 through 84. Each 200-ton jet was on a transcontinental flight and carried 15,000 gallons of jet fuel.
Because the Empire State Building had been hit by a low-flying plane in 1945, both buildings were built and designed to sustain a direct hit of a slow-flying Boeing 707, the largest plane at the time. However, the jets that hit the towers on Sept. 11 were much larger Boeing 767s and were traveling at speeds close to 500mph.
It's the opinion of many experts that the WTC towers didn't collapse because of the impact of the crashes, but rather as the result of the catastrophic effects of the burning jet fuel. Prof. Ted Kranthammer of Pennsylvania State University estimates jet fuel temperatures ranged from 1,000-3,000°F. At 1,000°F steel loses up to half of its tensile strength and starts to buckle and deform, and at 1,400°F it retains only 10-20% of its overall strength. These temperatures are much higher than what would have been expected from typical office furnishings and wouldn't have been figured into the original design.
The remaining fuel not immediately consumed in fireballs was believed either to have flowed down through the buildings or to have burned off within a few minutes of the aircraft impact. According to the FEMA report, “The heat produced by this burning jet fuel does not by itself appear to have been sufficient to initiate the structural collapses. However, as the burning jet fuel spread across several floors of the buildings, it ignited much of the building's contents, causing simultaneous fires across several floors of both buildings. The heat output from these fires is estimated to have been comparable to the power produced by a large commercial energy generating station. Over a period of many minutes, this heat induced additional stresses into the damaged structural frames while simultaneously softening and weakening these frames. This additional loading and the resulting damage was sufficient to induce the collapse of both structures.”
Without the protection of the fire-resistive coating, the intense heat from the burning jet fuel was allowed to heat the floor trusses to the point at which they sagged and eventually failed. During a British Broadcasting Corp. interview, structural engineer Chris Wise said, “It was the fire that killed the building. There's nothing on earth that could have survived those temperatures with that amount of fuel burning.” This type of failure produced two fatal structural problems. First, with the failure of the floor trusses, the exterior walls were no longer tied to the core. Without this critical connection, the exterior walls were allowed to buckle from the weight of the structure above them. Once this occurred, the weight of the huge mass above the impact point started to fall, gaining momentum and crushing the structurally intact floors below. The added weight from each floor falling on the floor below it added to the pancaking effect, producing a progressive collapse, which resulted in the catastrophic failure of both structures.
However, there's some disagreement on what was the major cause of the WTC collapse. New Zealand Heavy Engineering Research Association structural engineer G. Charles Clifton has stated his theory that the jet's impact on both buildings destroyed a significant number of perimeter columns on several floors, severely weakening the structural integrity of the buildings. In a paper posted on the HERA Web site, <www.hera.co.nz>, he maintains damage to a number of perimeter columns, façade beams and the core extending up three or more floors was the main reason for the collapse.
Prof. Abolhassan Astaneh-Asl, pe, of the University of California, Berkley, has been investigating the cause of the collapse by examining the twisted steel salvaged from the WTC. It's his opinion that when the jets exploded during impact, the force of the explosion dislodged and removed most of the sprayed-on fire-resistive coating on the steel of the floors impacted, including the floor trusses. This theory is based on the fact that the WTC towers were designed to withstand the effects of a fire between 120 to 180 minutes before they would collapse. Total failure of the Tower 2 occurred in just 60 minutes.
With a good understanding of why the towers collapsed, we need to ask ourselves what we can do to prevent another tragedy like this one. Due to the high level of security that's been imposed, it's highly unlikely that another plane will crash into a skyscraper again. There are several lessons we can learn about building construction and firefighting techniques.
As a firefighter or officer, one doesn't need to become a structural engineer or intrinsically familiar with all of the different types of building techniques, but we do need to understand construction systems. When dealing with construction systems, we must understand that a system is dependent upon all of its parts. If one or more of those parts are damaged or not functioning as designed, the system and the way we expect the structure to react is severely altered.
For example, many codes allow for unprotected steel trusses in warehouses that are sprinklered. If the sprinkler system isn't operating properly or stored items are piled high, the system won't work as designed. Failure of the building will occur much faster than was planned, without any warning to firefighters. Losing any part of a system can have an adverse effect on the building and those operating there. When using a systems approach we must require enough redundancy to eliminate design failure if we lose a part of the system.
In the case of the WTC, the FEMA report didn't reveal any specific structural features that would be regarded as substandard. In fact, many structural and fire protection features of the design and construction were found to be superior to the minimum code requirements.
However, the failure of the exposed 60-foot long unsupported trusses ultimately caused the WTC towers to come down. The system was designed so the trusses tied the exterior load-bearing walls to the core, which kept them from buckling from the enormous weight above.
Sprayed-on steel fireproofing was a contributing factor in the failure of the steel trusses. Maybe it's time to require steel trusses to be constructed at a size that can withstand the effects of direct heat on its own, for the calculated time before they fail. When will we learn? Truss failure has and will continue to kill firefighters until we stop using them, protect them better, or stop going into buildings with truss construction. Maybe we should label all truss buildings as hazardous just as we label dangerous buildings in Massachusetts with a big red x and fight fires in these buildings from the outside. A trussed building, well involved with fire, is also a dangerous building and should be treated as such.
According to the FEMA report, several design features have been identified that may have played a role in allowing the buildings to collapse in the manner that they did. According to the report, these features shouldn't be regarded as design deficiencies or prohibited in future building codes. Rather, these are features that should be subjected to more detailed evaluation to understand their contribution to the performance of these buildings and future performance in other buildings. These include:
- The type of steel floor truss system present in these buildings and their structural robustness and redundancy when compared to other structural systems;
- The use of impact-resistant enclosures around exit paths;
- Resistance of passive fire protection to blasts and impacts in buildings designed to provide resistance to such hazards; and
- Grouping emergency exit stairways in the central building core, as opposed to dispersing them throughout the structure.
We've all tried, with limited success, to change the building code in an effort to protect firefighters. Ultimately, it's up to us to ensure that firefighters and fire officers are properly educated in building construction, various building types and their related hazards. Firefighters should demand to inspect properties both during construction and after construction to better understand buildings in their districts. We need to thoroughly investigate every building fire and collapse and continue to work with officials to institute changes in an effort to limit future tragedies.
Over the next year many more questions will be asked and much more information will be produced about the collapse of the WTC. Hopefully this will bring about changes that will make these structures safer and provide new and inventive ways that the fire service can deal with and fight fires in these and other buildings.
Jack Parow is fire chief of the Chelmsford (Mass.) Fire Department. His fire service career spans 28 years, the last 11 as chief. A certified hazardous material technician and an EFO graduate, he has an associate's degree in fire science, a bachelor's degree in fire and safety engineering and a master's degree in business and policy studies. Parow is also a professor at Anna Maria College in Paxton, Mass.
- Is hollow tube construction with load-bearing walls still state of the art for high-rise buildings? Perhaps we have to design more sturdy construction at the expense of aesthetics.
- What type of suppression systems should be used or need to be developed for high-rise buildings? We need to progress beyond the current standard of sending firefighters up hundreds of stairs with 80 to 100 pounds of equipment to fight these fires.
- Is sprayed-on fireproofing an adequate way to protect steel members, or do we need to protect steel differently?
- How much longer will we allow trusses to kill firefighters?
- What type of redundancy needs to be built into system-designed buildings?
- Are these systems too dependent on all the components working properly?
- Is it acceptable that when one part of a system fails, the entire system fails?
- Should we continue to set up the command post in the lobby of the affected high-rise building and place operations just a few floors below the fire?
- When will we require dedicated and specialized firefighter elevators stocked with equipment for all high-rises?