The Twin Towers at the World Trade Center as seen in 1987. Image: Yann Forget/Wikimedia Commons

It is now almost 20 years since the Twin Towers of the World Trade Center in New York were destroyed during a terrorist attack, killing 2,606 people within the two buildings. As is often the case following both natural and human-made disasters, the collapse of the Twin Towers prompted sweeping regulatory changes in how tall buildings are designed, constructed, and operated.

Although the collapse of the Twin Towers would prompt broad reforms to building safety in the years which followed, that is not to say that the towers themselves were absent of collapse mitigation measures. As noted by FEMA, the Twin Towers were “the first structures outside of the military and nuclear industries whose design considered the impact of a jet airliner, the Boeing 707.”

Undoubtedly, many hundreds of lives were saved because of the redundancy and robustness built into the structure.

Although each tower was struck by the much heavier, faster, and fuel-equipped Boeing 767, both towers withstood the initial impact of the jet without immediate collapse, with WTC 1 and WTC 2 ultimately collapsing 102 and 56 minutes after impact, respectively. In their 2011 research paper on the collapse, the Council on Tall Buildings and Urban Habitat explained that “despite the terrible loss of life and property, WTC 1 and WTC 2’s ability in particular to survive initial impact loads and the resulting structural damage is notable. Undoubtedly, many hundreds of lives were saved because of the redundancy and robustness built into the structure.”

In the case of WTC 2, studies after the collapse found that although more than 30% of south-facing columns were destroyed over five stories, the building would have remained standing had it not been for the ensuing fire damage caused to the tower’s structural steel. Both towers suffered from a “progressive collapse,” where steel columns lost strength due to exposure to high temperatures caused by burning jet fuel, thus surpassing their structural load capabilities, and leading to a “pancaked” collapse, floor-by-floor.

In the aftermath of the disaster, a series of investigations were launched with the goal of improving building performance and safety. An initial report by the Structural Engineering Institute, ASCE, and FEMA was followed by an investigation from the NIBS and NIST, a federal body which is also currently leading an investigation into the recent collapse of the Chamberlain Towers at Surfside, Miami. Ultimately, 23 major building and fire code changes recommended by NIST were approved by the International Code Council (ICC), the primary creator of building codes and standards in the United States.

Among the changes to building codes adopted after the Twin Towers collapses are considerations which many architects today would consider second nature.

Among the changes to building codes adopted after the Twin Towers collapses are considerations which many architects today would consider second nature. These include the requirement for at least three exit stairways for buildings more than 420 feet high, a 50% increase in the width of exit stairways, and the permitting of elevators to be used to evacuate occupants and transport firefighters during a fire. To aid this final measure, elevators supporting heavy firefighting equipment were also required in all high-rise buildings over 120 feet.

Changes to fire access lobbies included a minimum area requirement of 150 square feet, and a requirement to keep such lobbies free of storage. The required area of Fire Command Centers also increased from 96 square feet to 200 square feet, including a requirement that such centers provide information cards with critical response information. Building codes were also modified to ensure that tall buildings permitted full radio coverage to be used by emergency responders.

In addition to measures designed to aid evacuations and emergency responders, the collapse of the Twin Towers has also altered our approach to structural engineering. “Until those attacks, most buildings had been built with defences against total collapse, but progressive collapse was poorly understood, and rarely seen,” said Shih-Ho Chao, an Associate Professor of Structural Engineering and Applied Mechanics and the University of Texas Arlington in a 2016 thought-piece. “Since 2001, we now understand progressive collapse is a key threat. And we’ve identified two major ways to reduce its likelihood of happening and its severity if it does: improving structural design to better resist explosions and strengthening construction materials themselves.”

To improve the structural design of tall buildings, engineers have taken inspiration from buildings in earthquake-prone areas.

To improve the structural design of tall buildings, engineers have taken inspiration from buildings in earthquake-prone areas. Chao explains how building codes from engineering institutes such as the American Society of Civil Engineers require all structural supports to be designed with enough flexibility to withstand a once-in-a-thousand-year earthquake. The collapse of the Twin Towers has equally evolved the industry’s appreciation for structural redundancy: the ability for key beams and columns to remain stable despite the loss of neighboring structural elements.

For the integrity of materials themselves, the 9/11 attacks have led to a preference for concrete structural columns in tall buildings, offering higher fire resistance than the steel columns used in the Twin Towers. One World Trade Center, for example, uses three-foot-thick reinforced concrete walls across the full height of the building, equipped with large amounts of reinforcing bars.

Chao also notes the increasing prevalence of high-strength, needle-like steel microfibers mixed with concrete, which is highly resistant to blast damage. “As a result, we can expect future designers and builders to use this material to further harden their buildings against attack,” Chao explains. “It’s just one way we are contributing to the efforts to prevent these sorts of tragedies from happening in the future.”