This report examines why firefighters have raised concerns about the growing height of modern skyscrapers. Drawing on the September 11, 2001 collapse of the World Trade Center Twin Towers, the paper analyzes the structural weaknesses of tube construction with truss support, compares these to traditional column-and-beam methods, and surveys global trends in high-rise building development. It also considers the limitations of modern fire-safety protocols—including radio communications failures and the inaccessibility of upper floors to conventional firefighting—in buildings exceeding 100 stories. Despite industry confidence following 9/11, the fire service community remains skeptical that current construction standards adequately protect occupants of the world's tallest structures.
This report examines why firefighters have voiced concerns regarding the ever-increasing size of skyscrapers. When the World Trade Centre Twin Towers—at the time two of the world's tallest buildings—were completely destroyed by a terrorist attack on September 11, 2001, the event should have raised enough doubt and questions about the future of high-rise safety to put an end to the construction of skyscrapers. Yet, after September 11th, the world's demand for newer and taller structures continued to increase dramatically.
It is apparent that the global population likes to live and work hundreds of feet above the ground. The catastrophic events of September 11th did not have a very long-lasting effect compared to the effect created by economic indicators such as historically low interest rates or consumer demand when it comes to the construction of skyscrapers. Designers and construction companies did have to address new safety concerns—such as a building being struck by a larger plane such as a Boeing 767—but the response from the architectural world was that the World Trade Centers' collapse was merely a fluke and therefore could not happen again.
Designers, architects, and builders are confident that the only effect a plane crash would have on newer, taller buildings would be to generate broken glass and limited structural damage only around the point of impact. With this newfound sense of confidence in the design and construction of ever-taller buildings, the world's demand for high-rises is stronger than ever.
From a firefighter's perspective, however, this newfound confidence may not be warranted. The fire service is convinced that newer and taller buildings are no more prepared for a surprise attack than the Twin Towers of New York were (Graham-Rowe, 2003).
Results presented in June 2002 at a meeting in New York organized by NIST were later published in the Fire Safety Journal (vol. 37, p. 707). The team calculated that the south and north towers would collapse after 75 and 115 minutes, respectively. In fact, they fell after 56 and 103 minutes (Graham-Rowe, 2003).
New York's World Trade Center Twin Towers were built in the 1970s. They were designed to withstand normal fires and even hurricane-force winds. The designers had considered what would happen if a Boeing 707 struck either of the buildings, and the consensus was that no such event could bring either tower down. In hindsight, additional testing was obviously needed. The towers were no match for today's faster and bigger planes. But why did the towers fall?
The Twin Towers were constructed by combining lightweight steel and glass, supported by external columns. Stairs and elevators were supported by internal columns that formed the main core of each tower. The Twin Towers differed from previous high-rise construction in several ways, creating a scenario that sped up construction time and also helped builders raise the buildings to new heights.
The primary support for each tower came from an external sheathing, designed to open up the interior of the buildings for additional office space. Another unique concept involved supporting concrete-slab floors to the building frame using steel trusses and special plates that reduced the effects of strong winds.
Upon initial impact of the hijacked planes, the two towers appeared to have withstood the shock and aftermath of the explosions and fire. However, as the jet fuel burned, temperatures inside the towers may have risen to as high as 2,000 degrees Fahrenheit. To make matters worse, the buildings were undergoing increased insulation treatments at the time of the attack, and the existing level of insulation was not thick enough to withstand such extreme heat. "Six months before the attacks, the Port Authority received a copy of a report it commissioned from British consulting engineers Buro Happold to see if there was a more cost-effective alternative to applying thicker insulation" (Graham-Rowe, 2003).
The steel trusses that supported the flooring either melted or buckled, releasing each upper floor downward onto the one below it, eventually causing complete collapse to the ground. As the trusses failed, the concrete-slab floors literally plunged downward. Because the two jets struck near the tops of the towers, the weight of the floors above became a force the buildings could not withstand. Although the towers were raised quickly using these techniques, those same techniques were the eventual cause of the buildings' rapid demise. The Twin Towers' radically different structural design created a situation where the load-bearing walls simply could not support the weight of the floors.
The building industry has undergone significant change. Engineers today use available technology to break barriers once thought of as impossible limits to building size. "The computer has allowed engineers to reduce the mass of a structure by its ability to more accurately determine the load-bearing capability of structural framework. Years ago, before the computer, builders were not sure of a structural element's load-bearing capability, so they overbuilt by using a so-called 'safety factor.' This built-in safety factor could result in a structure twice the required load-bearing strength" (Dunn, 2003).
In other words, buildings will keep getting taller. Height is measured from the sidewalk to the structural top, and those tops are now higher than ever. A "building" is considered to be a structure designed for residential, business, or manufacturing purposes (Infoplease, 2004). Towers may appear in the names of these structures, but they will always be occupied.
Consider that the Empire State Building, once the tallest building in the world when built in 1931, would not even rank in the top ten by the end of 2005. The Empire State Building made a steady decline in height ranking as construction teams throughout the world inched higher and higher with each new edifice.
As of 2004, the new tallest building in the world—Taipei 101 in Taipei, Taiwan—technically had one fewer floor for occupancy than the then-9th-ranked Empire State Building, but the difference in height between the two massive structures was more than four hundred feet. Throughout Asia, several new and ongoing projects would soon easily dwarf Taipei 101. During the construction of Taipei 101, work was stopped because of an earthquake measuring over 5.0 on the Richter scale. Even though several workers died during construction, work did not stop for long. The new construction trends are obvious: buildings will continue to rise into the stratosphere, ignoring hazards or signs of danger. Groups made up of builders, developers, financial professionals, and bureaucrats all concur—up is good.
The same cannot be said for those in the fire service and the academic world of fire science engineering. Safety concerns and the fundamental structural flaws revealed by 9/11 generate considerable concern about newer and taller buildings. The majority of representatives from the fire science, fire engineering, and fire service communities—as well as experts in the collapse of burning buildings—believe that the collapse of the Twin Towers should not have happened and that the builders of new skyscrapers, like their Twin Tower predecessors, will never be able to account for all possibilities to guarantee complete safety.
The confidence of the building community may have some merit insofar as future buildings may be better prepared for an airplane attack, because construction method does make a significant difference. Some buildings are clearly built better than others. The trend in truss construction used for the Twin Towers was noted as a major reason for the World Trade Centers' collapse.
Had the terrorists struck the Empire State Building with one of the planes, some computer animation models have demonstrated that the building might not have collapsed as the Twin Towers did, because the Empire State Building was built with classic column-and-beam construction strengthened by concrete masonry. The computer animations may be correct. The World Trade Center towers utilized what is known as tube construction with truss support, which became popular in the 1970s. In 1945, a B-25 bomber struck and left a huge hole in the Empire State Building, yet it continued to stand.
A plane hitting one of the newer and taller buildings around the world may not be the most alarming concern; high-rise buildings have other types of serious issues to consider. The most feared problems often stem from non-spectacular scenarios. For example, a firefighter entering a high-rise building may lose communications with his team because tall buildings are not conducive to radio wave transmission and therefore do not allow firefighters' radios to work properly—a problem that has not been adequately addressed during the recent trend of building ever higher.
Even something as simple as an external fire on a high-rise cannot be defended with conventional firefighting techniques. Unless helicopters or aircraft are deployed, a fire on the 75th floor could wreak havoc. These safety concerns were documented by fire service representatives long before September 11th. As early as 1995, retired FDNY Fire Chief Vincent Dunn wrote that the World Trade Centers lacked proper evacuation avenues, and that if the buildings ever encountered a serious fire, existing procedures would be useless and people above a certain threshold would not be able to evacuate.
As Taipei 101 prepared to lose the title of the world's tallest building, the majority of skyscrapers around the world fell under safety codes implemented for ten-story buildings. New safety standards must be created for buildings of 100 or more stories, or occupants will never be completely safe. As building trends continue, new safety considerations will be needed. Not only is an airplane attack a reason for concern—a basic fire can be just as deadly.
"Technical truss methods and their failure points"
Prior to September 11th, engineers would have sworn by the truss construction system as a reliable high-rise technique. No test of truss construction had even conceived of the situation created by the crashing of two hijacked jetliners turned missiles.
Depending on the type of construction used in newer high-rise buildings, designers take into consideration a number of different types of collapse scenarios. Even though designers were aware of the possibility of fire when they used truss techniques in the Twin Towers, they most likely only tested the possibility of construction failure due to normal fire exposure. The September 11th events triggered temperatures well above the norm. Therefore, the type of collapse that occurred that day was beyond the experience and established history of truss construction methods.
Truss technology has come under scrutiny. However, although the design and construction of the World Trade Center towers is open to criticism, the fact that the Twin Towers took over an hour to eventually fall after sustaining direct hits from two fast-moving jetliners provides some sense of how strong the buildings actually were. "The fact that the structures were able to sustain this level of damage and remain standing for an extended period of time is remarkable" (Graham-Rowe, 2003).
Buildings are getting taller. The fact that a 110-story building collapsed in only 8 seconds should put fear in the heart of any firefighter. Once a firefighter makes the decision to enter a building to help those in need, he must understand ahead of time that 8 seconds will most likely not provide enough time to escape.
From a firefighter's perspective, the Twin Towers departed from accepted high-rise building standards when it came to the construction process. New skyscrapers do not bear loads in the same manner as conventional skyscrapers did in the 19th century, which were built with a skeleton of interior supporting columns that supported the structure. In modern construction, exterior walls of glass, steel, or synthetic material do not carry any load.
"Summary of ongoing high-rise safety concerns"
"Ranked data tables of world's tallest buildings"
Always verify citation format against your institution’s current style guide requirements.