The events of 9/11 shook the world. Before that day, we could not imagine that someone would be bold and cruel enough to enact such violence. We could not imagine that two iconic 110-story skyscrapers would collapse in the middle of a U.S. city, gouging and crushing other buildings for hundreds of feet in all directions. We asked ourselves, “How could this possibly happen? How could they collapse?” These are natural questions that express the scope of the loss we felt on that day.
Structural engineers asked these questions, too, but they also asked the contrasting question: How did the World Trade Center towers manage to stand up to the attack at all, even for a short while? The damage was extensive. Commercial aircraft flying at nearly top speed crashed into the buildings, cutting wide swaths through the exterior walls and inflicting extensive interior damage. Shouldn’t that have been enough to cause immediate collapse?
The twin towers were not designed to resist the kind of damage they experienced. At most, when they were designed, there was concern that an errant aircraft might accidently hit one of the towers. Engineers might have assumed that fires in either building likely would be confined to one floor and that sprinkler systems would work properly.
What we saw was very different. Many key structural elements were destroyed instantly, and large fires ignited simultaneously on several floors with ruined sprinklers. The buildings stood, albeit briefly, primarily because their structures had redundant mechanisms to support the weight.
You have seen photographs of the towers pre-9/11. The exterior walls had narrow windows flanked by unusually closely spaced steel columns. There were also deep beams at the floor levels, forming a tight latticework of steel that covered the surfaces of the buildings.
Much of the welding was performed off-site. Three-story-tall by three-column-wide panels of the lattice were fabricated in a factory and transported to downtown Manhattan. To augment strength, these panels were staggered vertically, like pieces of a puzzle, so that all their tops and bottoms did not align at the same floor level.
The four sides of each building formed a tube with window slots This tube was designed to carry the weight of the building and its contents directly downward to the foundations. The walls acted as vertical beams, wide and tall as the buildings, that bent to the side to absorb wind loads.
When the aircraft cut out rows of columns and beams in the exterior walls, a field of the latticework above the damage provided horizontal beams several stories deep to span over the gash. At the same time, the damaged exterior walls were partially hung from structure above.
The strength of the tightly spaced structural elements allowed the walls to sag a bit without failing totally. In that state, the standing towers allowed many occupants below the collision floors and in surrounding buildings to escape.
Failure finally occurred when the intense fires inside the towers weakened floor systems that braced the exterior walls. When the walls buckled, the upper portions of the buildings fell as blocks and were driven through the lower floors as they fell, much like an axe head driven through wood to split it.
We also know that the Pentagon was attacked with an airplane on 9/11. Probably less well known is that some of the damaged area stood for about 20 minutes before it also collapsed. The Pentagon, at five stories tall, was undercut by a plane that crashed into its first floor, demolishing numerous columns there and several on the floor above. Yet it held up long enough for everyone in the upper three levels to evacuate before the collapse.
The Pentagon has a steel-reinforced concrete frame with a “two-way” floor system: beams run both ways between columns, providing a secondary mechanism to support the weight. When columns were destroyed, the building above sagged. But the crisscrossed floor beams acted as a net to support the area above the damage.
Essentially all concrete columns, including those of the Pentagon, encase embedded vertical steel bars that work together with the concrete to carry the weight. Concrete columns also have horizontal steel bars that wrap and brace the vertical bars and confine the concrete in the middle. As was common in the 1940s when the Pentagon was built, the horizontal steel bars were bent into spirals, wrapping around and around the vertical bars for the full height of each column.
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.
The structural engineering profession studies events such as those of 9/11 to improve practice. Many of these studies, conducted by organizations such as the Structural Engineering Institute at the American Society of Civil Engineers (SEI/ASCE), the National Institute of Standards and Technology and the Federal Emergency Management Agency, have documented characteristics, such as redundancy and ductility, that enhance resistance to extreme assaults. 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.
Of particular concern is the potential for “disproportionate collapse,” which is widespread collapse following very localized damage. We anticipate that gross damage could lead to collapse, as it did on 9/11, but we strive to prevent cascading collapse from events as small as a common fire, an explosion or a strike by an errant roadway vehicle.
Following the 1995 bombing of the Alfred P. Murrah Federal Building in Oklahoma City, SEI/ASCE published new guidance for designs to resist explosions. The fires that ultimately brought down the World Trade Center and the Pentagon led SEI/ASCE to advocate for better analysis of the effects of fires on buildings. Soon SEI/ASCE will release advice on mitigating the potential for disproportionate collapse. To gather and disseminate additional information, SEI/ASCE has set up Collaborative Reporting for Safer Structures U.S., a clearinghouse for engineers to share lessons learned from failures.
Like everyone, structural engineers mourn the human, physical and societal losses of 9/11, and we dread the possibility of catastrophic structural failures in the future. 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.