Above 1, degrees Fahrenheit, it loses a significant amount of its strength. He said the extreme heat from the fires might have caused the steel floors to expand and bow, which may have caused the support columns to bend inward and buckle.
Heat also may have caused the steel flooring to separate from the columns, or the columns themselves may have heated up and buckled outward.
Hamburger and his colleagues have not yet determined which of these scenarios occurred on Sept. As the tower collapsed, the trusses just fell apart," he observed, noting that trusses are difficult to fireproof. Hamburger noted that each tower was constructed using a novel tube frame system designed to resist winds of up to 80 miles per hour.
But the connections of the tube frame were weak, causing them to break apart and become three-pronged missiles that crashed into the street and into nearby buildings. Future designs. He pointed out that fires frequently occur in high-rise buildings and noted that between and , 30 fires occurred in the United States in buildings that were 50 stories or taller.
Right now, structural engineers know very little about fire. He and his colleagues will attempt to answer those questions when the ASCE report is made public next spring. Does it make sense to design buildings to withstand such events as Sept. It was fire that caused this. And the fire is believed to have come from the burning of remaining aircraft fuel. According to the FEMA report, fire within the buildings caused thermal expansion of the floors in a horizontal and outwards direction, pushing against the rigid steel columns — which deflected to an extent but resisted further movement.
With the columns resisting movement, there was nowhere else for the concrete floors to expand to. This led to an increased buildup of stress in the sagging floors, until the floor framing and connections gave in. The collapsing floors then fell on more floors below, leading to a progressive collapse. It is understood the South Tower collapsed sooner because it suffered more damage from the initial aircraft impact, which also dislodged more fireproofing material.
While there are different theories regarding how the progressive collapse of Building 7 was initiated, there is consensus among investigators fire was the primary cause of failure. Both official reports made a range of fire safety recommendations for other high-rise buildings, including to improve evacuation and emergency response. In , the National Institute of Standards and Technology also published a best practice guide recommending solutions to reduce risk of progressive collapse.
There have since been changes to building codes and standards on improving the structural performance of buildings on fire, as well as opportunities to escape such as added stairwell requirements.
At the same time, the collapse of the twin towers demonstrated the very real dangers of fire in high-rise buildings. The reinforcing pattern in the columns created ductility—meaning they could distort sharply without fatal rupture. Tightly confined inside the cage of vertical and horizontal steel bars, the concrete in the middle of many of the damaged columns stayed in place even though the columns were bent into the shape of a banana, curving out at mid-height to as much as three times the diameter of the cage.
Even with that much deformation, many heavily damaged columns still supported weight, thereby limiting the area of the floor that needed to behave as a net. The portion of the Pentagon that ultimately collapsed was far less than the total undercut by the plane. Like the towers, it also fell because intense fire further weakened critical beams and columns. Building on that information are researchers and practicing structural engineers striving to make our structures safe and economical for everyday use and also survivable when damaged.
Following the bombing of the Alfred P. But engineers and other partners to the construction industry are not complacent when there are failures. We study, we learn, and we improve. This is an opinion and analysis article, and the views expressed by the author or authors are not necessarily those of Scientific American.
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