photo showing building on fire at moment of plane impact
The towers’ robust structural systems enabled them to stand long enough for many survivors to escape. (Photograph © REUTERS/Sara K. Schwittek) 

By Robert L. Reid

On Sept. 11, 2001, the world changed and with it so did the civil engineering profession. Over the two decades that followed, engineers and designers have worked to redesign, rebuild, and remember what was lost.

The terrorist attacks on Sept. 11, 2001, are one of those historical events that burns itself so deeply into the public consciousness — like the bombing of Pearl Harbor or the assassination of President John F. Kennedy — that years later people recall with great clarity where they were and what they were doing when they first heard, or saw, the news.

Steven Plate, the chief of major capital projects at the Port Authority of New York and New Jersey — which owns the World Trade Center complex — survived the day because his teenage son forgot his homework. Returning home to retrieve the assignment, Plate missed his usual train from New Jersey to Manhattan and thus was not in his office on the upper floors of the WTC’s north tower when the first hijacked plane smashed into the building at 8:46 a.m. Some 84 of Plate’s PANYNJ colleagues died that day as well as thousands of other people who worked at or were visiting the site.

Dan Eschenasy, P.E., F.SEI, M.ASCE, is today the chief structural engineer for the New York City Department of Buildings. Twenty years ago, however, he held the same position for the city’s Department of Design and Construction, the city agency given control over construction and engineering operations at the disaster site. In that capacity he was called to ground zero early on 9/11, where he and his colleagues began to work 12-hour days, seven days a week, managing the engineering and construction activities.

Don Dusenberry, P.E., F.SEI, F.ASCE, is now a private engineering consultant in Wakefield, Massachusetts, retired from Simpson Gumpertz & Heger. On Sept. 11, 2001, he had been on a conference call with several ASCE-member colleagues as they discussed a proposed blast- and impact-resistant design standard. One colleague was in a Manhattan office with a view of the twin towers as the planes struck; another had pulled off the road in northern Virginia, where he soon reported a large cloud of smoke rising over the Pentagon.

William Fife, P.E., M.ASCE, has spent much of his career in the aviation sector, including 30 years at the PANYNJ as well as 10 years with DMJM Harris, now AECOM, where in 2001 he was the director of aviation services. On perhaps the most fateful day ever in commercial aviation, Fife had been sitting in a plane in Montreal ready to fly home from a meeting when heavily armed members of the Royal Canadian Mounted Police boarded the aircraft and told everyone to get off. With the border closed and the transportation system in chaos, Fife would not return home to New York for four days — and then only by train.

Ed DePaola, P.E., F.SEI, M.ASCE, the president and CEO of Severud Associates, had attended a meeting at the WTC on Sept. 10, 2001, but was excused from another meeting scheduled for the next day. Just before boarding his train from New Jersey to Manhattan, he heard on the radio that an airplane had struck one of the twin towers. Initially assuming the report involved a small private plane, he was surprised by the enormous cloud of smoke rising over the city that he saw from the train’s windows. Once in Manhattan — having learned what happened through a cellphone call with his wife — DePaola felt “like a salmon swimming upstream” as he tried to reach his office on Fifth Avenue while it seemed everyone around him was trying to get off the island. Working into the evening, he then headed back to Penn Station to take a train home — moving through an eerily deserted city like a scene from a disaster movie. “I walked right down the middle of Fifth Avenue ... no cars, no people.”

On 9/11, Antony Wood, now the CEO of the Chicago-based Council on Tall Buildings and Urban Habitat, was starting his first day as an associate professor of architecture at the University of Nottingham in the United Kingdom. Instead of the lessons planned for the day, “I sat there with my students and watched the towers collapse.”

Steven Scalici, P.E., M.ASCE, now retired but formerly a senior associate who led the eastern branch of STV’s transportation discipline, had a clear view as the twin towers fell on 9/11. At that time, he worked for a small engineering firm in midtown Manhattan, had friends and colleagues who worked at the WTC, and had often attended meetings there with PANYNJ clients. So he knew how difficult it would be to evacuate such enormous structures.

Jon D. Magnusson, P.E., S.E., F.SEI, NAE, Dist.M.ASCE, is a senior principal at Seattle-based Magnusson Klemencic Associates (the successor firm to Worthington, Skilling, Helle & Jackson, the structural engineering firm for the original World Trade Center complex). On 9/11, Magnusson was getting ready for work when he saw a television news report about a plane striking one of the WTC towers — a report that suggested it might be an accident involving a small commuter plane. But as he watched, he saw the live broadcast of the second plane hitting the other tower, and he knew “this was not an accident — it was an attack.”

Marking history

This year, the nation and the world mark the 20th anniversary of the 9/11 attacks, which destroyed the World Trade Center and other structures in New York City, damaged a large section of the Pentagon, outside Washington, D.C., and led to the crash in rural Pennsylvania of a fourth hijacked jet after the passengers fought back.

building on fire with heavy black smoke coming out of side of structure
Subsequent investigations revealed that the towers withstood the impacts of the planes and would have remained standing were it not for the dislodged fireproofing combined with the conflagration. (Photograph © Getty Images/Jose Jimenez/Primera Hora)

As part of ASCE’s commemorations, Civil Engineering spoke to more than a dozen engineers and designers about how the events that day did — and did not — change the civil engineering profession. These experts explained how the lessons learned from that day were incorporated into the design of new buildings, especially tall buildings, as well as the renovations of existing structures. They discussed how the experiences led to new building codes, standards, and other engineering guidance. And they also talked about how the attacks resulted in the use of new technologies, new approaches and techniques, and a new focus on certain issues and concerns that suddenly became vital aspects of civil engineering work.

Some even explored what they believe to be the misunderstandings and missteps made in the aftermath of the attacks.

Surprising strength

Although the whole world knows that the twin towers at the World Trade Center collapsed on 9/11, numerous engineers point out that the surprising thing is not that the towers fell but that they stayed standing for as long as they did after being struck by the aircraft.

Given the footprint of the towers — which measured 209 ft by 209 ft in plan — and the roughly 156 ft wide wingspan of the Boeing 767 airliners that smashed into the buildings, each tower lost approximately two-thirds of its structural columns on one side, says Magnusson. After such damage, most people would have expected an instantaneous collapse, he adds. Instead, the south tower remained standing for 56 minutes, and the north tower stood for 1 hour, 42 minutes.

The unique structural systems of the towers worked to their advantage, Magnusson says. “The load was distributed — the heavy grid of beams and columns and the exterior tubes bridged over the hole in the side of each tower,” he explains. (For a more detailed discussion of the structural systems of the twin towers, see “A remembrance: The World Trade Center towers and the engineers who designed them.”)

Ultimately, Magnusson says, the towers collapsed because of the fires caused by the jet fuel that engulfed multiple floors in each tower simultaneously. The impacts destroyed the buildings’ sprinkler systems and blew off the spray-on fireproofing material, exposing the bare steel of the structure to the flames, Magnusson adds.

At the Pentagon, a Boeing 757 jet was flown into the western side of the five-story structure, near the dividing line between a portion of the building that had recently been renovated — in part, to improve protection against an external blast — and an unrenovated portion, says Paul F. Mlakar, Ph.D., P.E., F.SEI, Dist.M.ASCE, now retired but previously a technical director of the U.S. Army Corps of Engineers’ Engineer Research and Development Center in Vicksburg, Mississippi. Mlakar led ASCE’s investigation of the structural damage at the Pentagon and helped write ASCE’s Pentagon Building Performance Report, published in 2003.

Although the area struck by the aircraft was heavily damaged, the basic structure in that part of the Pentagon remained standing for approximately 20 minutes. As with the twin towers, the eventual collapse of that portion of the Pentagon building was caused not by the impact of the aircraft but by the fires ignited by the crash. The delayed collapse at the Pentagon enabled many people to escape the building, Mlakar says, adding that “everyone above the second story in the area of the crash survived.”

An uncertain future

In the days and weeks that followed 9/11, many people speculated that the attacks meant the end of tall buildings. Even some CTBUH members — the tall building experts — thought that people would no longer want to go into a tall building again, “especially a trophy building — a building that culturally as well as physically pops its head above the urban skyline,” says Wood. And there was, at least initially, a lot of uncertainty and a definite slowdown immediately after 9/11. Some tall building projects were stopped outright, and others were reimagined as shorter structures, Wood recalls.

That anxiety over tall buildings went on for perhaps a year or two after 9/11, notes Ron Klemencic, P.E., S.E., NAC, F.SEI, NAE, Dist.M.ASCE, the chair and CEO of Magnusson Klemencic. “But now, 20 years later ... not only did we continue to build tall buildings, we built more tall buildings in the last 20 years” than in any period prior to 9/11, says Klemencic, who also served as the chair of CTBUH from 2001 to 2006.

Wood points to a series of CTBUH-produced charts on tall buildings that confirm Klemencic’s statement. In the 1990s, for example, there were only a dozen or so buildings greater than 200 m in height each year in the whole world, Wood says. But by roughly a decade after 9/11, some 112 such structures were being constructed globally every year. In the year 2000, Wood adds, there were only 262 buildings in existence 200 m tall or taller. Today, there are more than 1,700.

graph showing number of supertall buildings through the yearsit photo credit]
COMPLETION OF SUPERTALL TOWERS BY DECADE (Graph © Council on Tall Buildings and Urban Habitat)

One of the key factors behind this upsurge in tall buildings involves where the buildings are being constructed, Wood adds. Until the 1980s, almost all of the 100 tallest buildings in the world could be found in North America, and especially in the United States, he says. Today, only a fraction of such buildings are in America. But that does not mean the United States stopped or even slowed down its construction of tall buildings, Wood stresses. Instead, it reflects the fact that other parts of the world — especially Asia and the Middle East — suddenly started building tall buildings at an incredible rate.

Moreover, while the tallest buildings were once almost entirely office buildings made from steel, today the majority of new tall structures are residential or mixed-use buildings framed in either concrete or composite systems, Wood adds. Thus, not only did people around the world end up being unafraid to work in tall buildings after 9/11, they also seem quite comfortable living in them.

The recent tremendous growth in tall building construction can, at least partially, be attributed to 9/11 in the strange sense that the events of that day “were projected into the consciousness of almost every inhabitant on this planet,” Wood says. Thus, shocking as the tragedy was, it may also have made people around the world — especially developers or political leaders in other countries — start thinking about tall buildings for the first time in their lives. And once they started thinking in tall terms, they began building that way, too.

Increasing urbanization, population growth, and a desire for greater density and sustainability will likely continue to move the tall building trend upward, Wood adds.

Learning the lessons

Following the 9/11 attacks, various government agencies and private organizations investigated the events of 9/11, including how the buildings had performed that day. ASCE, the Federal Emergency Management Agency, and the engineering firm Greenhorne & O’Mara Inc., now part of Stantec, produced an early look report in May 2002, World Trade Center Building Performance Study: Data Collection, Preliminary Observations, and Recommendations. ASCE and its Structural Engineering Institute examined the damage to the Pentagon in a January 2003 report (see “The investigations: The Pentagon”). And the National Institute of Standards and Technology conducted a building and fire safety investigation of the World Trade Center attacks, producing a final report in September 2005 (see “The investigations: The World Trade Center towers”).  

The lessons learned from such investigations helped change various local and international building codes as well as existing standards and other engineering documents. It was a slow and methodical process, however. One such standard — Blast Protection of Buildings: Overview, Blast Effects, and Design Considerations (ASCE 59-11) — was not published until 2011. A new manual of practice on physical security — Structural Design for Physical Security: State of the Practice (MOP 142) — came out this year. And a new standard on disproportionate collapse — in which, for example, the failure of one structural element leads to the total collapse of a building — is not expected until sometime in 2022, notes Dusenberry, who chairs the ASCE committee that is developing the standard.

collapsed side of a building
At the Pentagon, an area covering roughly 50 ft by 60 ft of the upper floors above the point of impact collapsed. (Photograph © Getty Images/Brooks Kraft)

Dusenberry also worked on the Pentagon Building Performance Report and the blast protection standard, which he says “lays out a procedure for blast resistance, provides guidance on what forces ought to be considered, and provides guidance on detailing connections and structural elements” for new buildings and upgrading existing structures.

Previous information on designing for blasts had primarily been for use by the government, Dusenberry says, while the ASCE standard was designed for the private sector. It’s hard to say how much of the content of the standard — in the works prior to 9/11 — was directly influenced by the events of that day, Dusenberry says. “This was not a standard about impact resistance of airplanes against buildings,” he says. Instead, the document was informed by other explosion-induced terrorist events, such as the first attack on the WTC in 1993, the 1995 bombing of the Alfred P. Murrah Federal Building in Oklahoma City, and similar attacks throughout the world.

“The events of 9/11 were another example of the need for the private sector and public sector to be aware of the threats and cognizant of methods to respond,” he explains. “And certainly the participation of people in (the development of the standard) was very strongly influenced by 9/11 — there was no difficulty getting together a committee of people interested in working on the preparation of this standard.”

The physical security manual of practice was written by Peggy Van Eepoel, P.E., F.SEI, M.ASCE, a senior principal and the office director of Thornton Tomasetti’s Washington, D.C., office, and Sharon Gallant, P.E., M.ASCE, a principal in the San Francisco office of KPFF Consulting Engineers. Intended to update and expand on a 1999 technical report on physical security, the new manual of practice reflects the dramatic changes of the past two decades, Van Eepoel notes.

The 1999 document was an attempt to provide some sort of physical security information for commercial building owners at a time when the topic mostly involved military construction, which also meant that much of the information at that time was classified, Van Eepoel says. By contrast, the new manual of practice was prepared for a world in which security is a key concern for civilian developers and building owners — especially those whose buildings are possible terrorist targets or are located in close proximity to possible targets. There has also been a great increase in public research into the topic of security over the past 20 years.

Moreover, while certain information remains classified, the government — from the Department of Defense to the General Services Administration — has also created new resources on physical security specifically for public release. “They realize there is a use for their documents that is much broader,” Van Eepoel says, “and so they need to provide access to design professionals all over the United States and around the world.”

Adopting improvements

The NIST report resulted in more than 20 recommendations that were ultimately adopted into the International Building Code, notes Therese McAllister, Ph.D., P.E., F.SEI, M.ASCE, the community and resilience group leader at NIST. These include such measures as installing an additional exit stairway for buildings more than 420 ft tall, increasing the width of exit stairways, and requiring that exit stairway enclosures be separated by a certain distance to ensure that a single event — a fire, an explosion, etc. — does not put all the stairs out of commission.

The recommendations also now permit the use of elevators to help evacuate occupants in tall buildings and require methods to keep those elevators functioning, including hardening the shafts and preventing water from entering and damaging the elevator hoistways. Another provision called for the installation of luminous markings on exit paths in certain buildings to facilitate rapid egress, even if the lights go out.

two book covers about standards

In the aftermath of the 9/11 terrorist attacks, ASCE developed new standards and other documents to help civil engineers design buildings to better meet the challenges posed by such events. These included Blast Protection of Buildings: Overview, Blast Effects, and Design Considerations (ASCE 59-11), Structural Design for Physical Security: State of the Practice (MOP 142), and a standard on disproportionate collapse, expected next year.

Other measures increased the bond strength of fireproofing, especially for tall buildings, and required that all members of the primary structural frame have the same fire resistance rating required for the columns they support — including bracing members essential to stability. The latter point was an especially critical change, McAllister notes, because previously certain bracing elements were seen as secondary members that needed less fire protection than the primary structural columns they supported. As a result, such elements could fail, leaving the columns vulnerable to buckling. So, treating these bracing elements as primary members is an important distinction, McAllister adds.

How much change?

When compared with the scale of the 9/11 attacks, however, many of the resulting changes to codes and standards do not seem especially dramatic, engineers note. For example, Ronald Hamburger, P.E., F.SEI, the San Francisco-based senior principal and board chair of Simpson Gumpertz & Heger and an author on the ASCE/FEMA report about the World Trade Center, defined the code changes since 9/11 as “pretty minimal.”

Scalici disagrees with those who say the post-9/11 code changes were not significant. STV served on the design team for a new transportation hub at the WTC and as a consultant on other projects, including for the design of streets around the WTC campus.

In fact, Scalici felt that just keeping up with all the code changes could be exhausting. “During my work, adherence to design codes was a constant concern because changes were happening right away, and the (Port Authority) was always on us to ensure we were up to date. In fact, before we started any work, the (Port Authority) required us to draft a Basis of Design report, a bible for us to constantly update and use as our/their guiding light for the entire project.”

Perhaps the most significant structural change to the International Code Council’s IBC involved structural integrity requirements, says Hamburger. These set minimum levels of interconnectivity for different structural elements, such as the floor slabs, columns, beams, walls, and other features. Introduced in the 2006 IBC, the measures were designed to provide alternative load paths within a structure “so that if a primary, gravity-load-carrying element were compromised by an event — a terrorist attack, a fire, whatever it might be — there would be some ability of the structure to redistribute the load to the other elements and avoid a structural collapse,” explains Hamburger.

This measure generated significant opposition during the ICC hearings, Hamburger adds, and was ultimately restricted to just certain high-rise buildings that represent “just a small fraction of the building population.” The changes were not adopted for ASCE’s key structural standard, Minimum Design Loads for Buildings and Other Structures (ASCE 7), Hamburger adds, but a performance-based procedure for evaluating the fire protection of structural systems in buildings was introduced as Appendix E to ASCE 7-16.

Regulatory changes involving structural performance and fire protection after 9/11 were less noteworthy, says Jose Torero Cullen, Ph.D., CPEng, M.ASCE, a professor of civil engineering at University College London in the United Kingdom and the chair of ASCE’s fire protection committee. Basically, nothing changed involving fire, Torero Cullen says. “There is no code in the United States now that requires you, no matter how complex your building is, to conduct an explicit structural analysis for fire performance,” he states. Torero Cullen contrasts that lack of a fire-focused structural analysis requirement with the analyses that are required in building codes for earthquakes on the West Coast or wind analyses for tall buildings.

Torero Cullen does not blame anyone today for this oversight. Instead, he argues that structural behavior in fire was divorced from structural analysis back in 1928 when the engineering community adopted the concept of fire resistance, which focused primarily on the use of insulating materials to prevent a structure from heating. But “fire resistance is completely blind as a concept to thermal expansion, to failure of connections, to large deformations and localized buckling — all of which are fundamental components of structural behavior,” Torero Cullen explains.

As a result, important lessons from 9/11 may be lost because structural engineers know so little about fire and fire engineers know so little about structural behavior, he concludes.

Hamburger takes a similar position, noting that he is personally disappointed that the structural engineering profession “did not step up in a larger way and embrace taking responsibility for fire protection engineering of structures.” He understands the reluctance on the part of the profession to take on additional responsibilities, perhaps without any additional compensation, but he argues that fire protection engineering heavily involves structural engineering. Accepting that would have been “an opportunity not only to improve the design of our buildings but also to expand our scope,” Hamburger concludes.

While Klemencic agrees that the fire protection and structural engineering communities have been siloed in the past, he takes a more optimistic view. Since 9/11, he says, “we’re seeing those two communities coming together ... (with) a greater commingling of the two disciplines to come up with more specific and appropriate responses on a building-by-building basis.” But such a shifting in the way each side thinks will take time, he concedes.

Updating ‘antiquated’ codes

On 9/11, New York City was still using building codes from 1968 that had become “pretty antiquated,” notes Eschenasy. By 2008, the city had adopted updated codes that incorporated many of the NIST recommendations from 2005, subsequent NIST documents, and other expert sources, he adds.

The new city codes focused on several areas to enhance structural performance under extreme event scenarios. The provisions of structural integrity and continuity (focused on tying together structural elements horizontally and vertically) were intended to apply to all new construction, Eschenasy says. To prevent disproportionate collapse of tall buildings, the code required special structural integrity key element analyses involving either specific load resistances or alternative load paths and a peer review of the structural systems, he adds.

Separately, New York City also adopted a law mandating automatic sprinkler systems in all office buildings 100 ft or taller — a requirement that applied to both new construction and existing buildings (see “Engineers Meet Challenges of NYC Sprinkler Law,” Civil Engineering, June 2019, pages 32-34).

Although the events of 9/11 led to “an in-depth reanalysis of how tall buildings should be designed,” Eschenasy notes, some of the city’s new code provisions were meant to be applied to a wider range of structures. The special calculations and peer review requirements, for example, can be applied to any structure with a height-to-base ratio of 7-to-1, regardless of the actual height, Eschenasy says.

Upcoming code changes will also consider adding peer review requirements for geotechnical and wind tunnel reports — a “natural corollary” to the 9/11-inspired structural integrity provisions, Eschenasy notes.

Raising questions

Many of the recent changes to codes and standards, as well as other actions, were clearly aimed at correcting specific problems identified from 9/11. But some engineers, like Magnusson, do not think such efforts were necessary.

The existing codes in 2001 were sufficient, Magnusson says, and the tall buildings of that time were safe — against the types of hazards that engineers generally design for. But the events of 9/11 were something engineers cannot design against — a “military-style attack” rather than a structural failure, he says.

Engineers can design buildings to protect against probabilistic events — hurricanes, earthquakes, conventional fires — Magnusson explains. But it is “impossible to design” for a deliberate, human-caused, military-style attack, he says, because no matter what size of event the engineer might plan for, the actual attack could always be more powerful than expected, or it could involve multiple attacks or other unanticipated factors.

For Magnusson, the solution is not structural but policy-related: Societies must work to avoid military attacks on civilian buildings. Toward that end, he says, engineers can become “vocal with the policymakers” who deal with such issues.

“There’s no way to outbuild determined terrorists,” Magnusson concludes, unless we make every building a military-style, bomb-proof bunker.

Going beyond

Although the regulatory systems have, so far, made only relatively small changes in the aftermath of 9/11, individual project developers — public and private — “have gone beyond what the codes and standards require and provided additional protection against terrorist attacks” on their facilities, says Hamburger. These voluntary, ad hoc efforts range from structural enhancements to improved security systems to better fire protection and even “limiting the ability of traffic to get in and around a building,” Hamburger explains. Much of this work has been kept secret, he adds, because the facility owners and operators want to keep potential terrorists from knowing how to defeat the new protections.

Perhaps not surprisingly, New York City has been a leader in these enhanced protection systems, Hamburger says. And many of the major buildings, especially the tall buildings, developed over the past 20 years incorporated protective measures that most likely would not have been used prior to 9/11, he adds.

The rebuilding of the World Trade Center campus is one of the best examples of such actions. Starting from a site that had been reduced to a roughly 200 ft tall pile of rubble, the new WTC rose from its ashes like the proverbial phoenix. The project involved “some of the most complex engineering and design and construction efforts in the history of the United States or the world,” says the PANYNJ’s Plate.

The Department of Design and Construction had authority for all construction and engineering work at the site, says Eschenasy. For these efforts, the department engaged Thornton Tomasetti and relied on volunteers from the Structural Engineers Association of New York, he adds.

“Staffing from the SEAoNY community evaluated the condition of approximately 400 buildings surrounding the WTC site,” says Victoria Arbitrio, P.E., SECB, F.SEI, M.ASCE, an associate partner at Gilsanz Murray Steficek LLP. “They then followed up with more detailed visual assessments and, when warranted, performed detailed engineering assessments on the buildings which were severely damaged. SEAoNY teams also assessed loads to be imposed on the underground infrastructure and designed supports for the cranes and other equipment brought in to clear the debris. They (also) assessed the stability of the ‘pile’ of debris.”

SEAoNY members “also worked with FEMA and subsequently NIST to salvage critical building components for further investigation, even traveling to the scrap yards to decide whether or not to salvage individual steel elements,” Arbitrio adds.

The first new skyscraper on the site — the 52-story 7 World Trade Center — opened in 2006, followed by other tall structures, most notably One World Trade Center, colloquially known as the Freedom Tower, which at 1,776 ft is the tallest building in the Western Hemisphere.

The National September 11 Memorial opened on the 10th anniversary of the attacks and the 9/11 Memorial Museum opened in 2014. The World Trade Center Transportation Hub — featuring the white-ribbed Oculus structure designed by Santiago Calatrava — opened in 2016, replacing the original Port Authority Trans-Hudson rail station that was destroyed on 9/11 and providing connections to a dozen New York City subway lines.

Other buildings at the new WTC site, including 2 WTC and the new St. Nicholas Greek Orthodox Church, are still in the process of being rebuilt.

One WTC especially demonstrates a host of changes in tall building design based on lessons learned from 9/11, notes Plate, who stresses that “we met or exceeded code in everything we did at the WTC site.”

These measures include extra-wide stairs that are encased within a fire-rated enclosure of reinforced concrete that is pressurized to help keep out smoke in the event of a fire. The fireproofing on the building’s structural steel far exceeds the code requirements for density, adhesion, and durability. The structure features multiple redundant systems, including redundant standpipes within the building’s protected core — made from several feet of high-strength, 14,000 psi reinforced concrete — that have special control valves that shut off automatically if the pipe is breached, preventing the fire reserve water tanks from draining.

The building also features a dedicated stairway just for the use of firefighters and other first responders, so that they can access the building “unimpeded by the public trying to come down the stairs,” Plate says. The PANYNJ has welcomed local firefighters to the new WTC site so that they can become familiar with the facility, locate the dedicated stairway, familiarize themselves with some of the safety systems, and take other actions that in the event of an actual emergency might “save vital minutes,” he adds.

One WTC also features coaxial cables throughout the stairs and corridors leading to stairs that transmit radio signals so that smartphones and first responder radios will function in those spaces for better communications during emergencies.

Simulations and solutions

The new WTC transportation hub also features redundant fire protection systems and was designed using a software modeling system that simulates how pedestrians would move through the site under normal and emergency conditions, explains Scalici.

Called STEPS, which stands for Simulation of Transient Evacuation and Pedestrian movementS, the software system was created by the engineering firm Mott MacDonald, and its use on the project was required by the PANYNJ, Scalici notes. With the software, STV’s team built a model of the physical background settings of the transportation hub and simulated pedestrian movements, including identifying potential locations that might impede normal operations and evacuations.

STV was also able to examine various “what if” scenarios: What if this stairway was closed? What if that exit wasn’t available? Or what if an “incident” were to happen at key dense pedestrian mixing areas?

The goal was to evacuate the transportation hub in 7.5 minutes per a directive of the Fire Department of New York, Scalici says. “We assessed a seven-level terminal populated by upwards of 50,000 people,” something never done before, he explains. “The software helped determine the number and locations of stairs, corridors, and portals and helped determine other factors as well.”

Although the simulations showed that the site could be emptied within the allotted time, Scalici was able to verify the design in a more direct way as well. Roughly one year after the opening of the transportation hub, he was visiting the site with a colleague who had worked on the project with him. Overlooking the Oculus from the east balcony, they heard an alarm go off for what turned out to be just a minor smoke incident. As people began to evacuate, Scalici and his colleague “got on the case,” waiting and timing how long it would actually take to empty the site. Sure enough, everyone exited the building within seven minutes, and Scalici said to his colleague: “We did our job — it worked!”

The STEPS software was relatively new at the time STV used it on the transportation hub, and the firm worked closely with Mott MacDonald to provide feedback and update the system, Scalici says. STV has since used the software on multiple projects in New York City, including the Long Island Rail Road’s near-complete East Side Access terminal under Grand Central Terminal, and elsewhere. So, while the tragedy of 9/11 can never be undone, Scalici says, engineers can create better-designed buildings in the future, thanks to one of the tools used to redesign and rebuild the WTC.

The airport experience

While many building developers and operators can decide whether to adapt their structures based on the events of 9/11, one particular type of facility has almost universally made considerable changes: airports.

In the past, the teams that designed airports often did not include security personnel from the Federal Aviation Administration, and threat assessments were not generally performed, notes Fife. In the aftermath of 9/11, however, Fife made sure to always have security experts from the FAA (and later the Transportation Security Administration) attend the peer review meetings that he helped plan at various airports around the country. The peer reviews were focused on airport planning, design, and construction plans — and following 9/11, security became one of the leading topics, Fife says.

Suddenly, the job of an engineer designing a new airport terminal involved new requirements aimed at keeping vehicles a certain distance from the terminal building and designating space for screening vehicles when necessary. All the new security equipment had to be installed within the footprint of the terminal, a process that could require fitting enormous machines for screening bags within spaces that previously had just required a small conveyor system, Fife says. Retail and food services had to be repositioned beyond the security checkpoints to accommodate the passengers who now needed to arrive at the airport hours ahead of their scheduled departures, adds Alan Reiss, a PANYNJ director of major capital projects.

Changing the work

For some engineers, the events of 9/11 changed the way they do their work. Van Eepoel, for instance, used to design primarily government structures, such as new embassy buildings. Now, she works on a range of structures, public and private, and even existing buildings that need to be retrofitted.

DePaola finds more and more clients interested in protecting against explosions, perhaps caused by someone carrying a bomb into their building or parking an explosives-filled vehicle just outside. “Even after 20 years, (blast protection) comes up on just about every project,” he says. He also sees a much greater interconnection of disciplines involving the entire design team — from the architects to the mechanical systems engineers to the contractors — to fully address the clients’ concerns.

That sort of broader perspective also affects McAllister, whose position at NIST before 9/11 had been as a research structural engineer. Gradually, and then officially in 2015, her role took on more and more aspects of community resilience. The change was based on the fact that “when the towers collapsed, it affected all of lower Manhattan,” she notes. “It took out power and water and communications ... it was not just a building owner’s problem or an occupant’s problem.” That led to “this broader understanding about buildings having a very large impact on the surrounding community,” McAllister explains.

For Plate, the resilience of lower Manhattan is one of the proudest achievements of the work to rebuild the World Trade Center complex. Before 9/11, the area around the WTC generally emptied out as soon as the stock market closed for the day, he says. But now with the memorial, the museum, the Oculus, and other new amenities, the location has become a major tourist site around the clock; thousands of people live in the area and even more come to work.

The work of the PANYNJ as well as all the architects, engineers, contractors, and others didn’t merely rebuild the World Trade Center buildings, Plate notes. “We dedicated ourselves and rebuilt out of a national and global tragedy. Ordinary people did extraordinary things, and we left a legacy for the next generation.” Ultimately, he stresses, “it’s about resilience and showing the world how you respond to incredible challenges and dark days.”

Robert L. Reid is the senior editor and features manager of Civil Engineering.

This article first appeared in the September/October 2021 issue of Civil Engineering.

Read the next article in the collection: "From the ashes: One World Trade Center."