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On 11 September 2001, the collapse of the twin towers at New York’s World Trade Center cast a long shadow over the future of high rise construction and led to changes in engineering design.
On the day, now known as 9/11, the towers collapsed killing more than 2,600 people after terrorists flew two hijacked passenger jets into them.
Official investigations showed that the towers collapsed after the heat generated by aircraft fuel ignited fires and weakened the steel cores of the buildings, eventually triggering progressive collapses.
Designs have evolved
Twenty years on, tall building design has evolved to improve structural stability as well as emergency access and escape.
With the World Trade Center practically on its doorstep, WSP’s New York office responded to the 9/11 disaster almost immediately.
“My partners and myself were on the site in the early days,” says Shard designer WSP director of building structures Ahmad Rahimian. “Other staff from the engineering community were down there with us, helping to identify and minimise the risks to the search and rescue mission.
“Also, we had volunteered to inspect and assess the other buildings on the World Trade Center site, several of which had been damaged. It involved walking up many storeys.”
At the time WSP had a number of tall building projects on the drawing board. Clients naturally sought reassurance about the safety of the proposed designs.
Burnging aviation fuel weakend the Twin Towers triggering progressive collapses
“This wasn’t easy to give, as information on the collapses came in gradually as the in depth study by the National Institute of Standards & Technology (NIST) proceeded,” Rahimian says.
“We went back and looked at projects to see if there was anything we could do in the light of what we knew at the time. For those in the design stage, we took a hard look at the structural redundancy, hardened core, and hardened means of egress.”
Serendipitously, WSP had always favoured concrete cores for tall buildings, be they all concrete or hybrid steel/concrete structures. Rahimian points out that concrete cores add stiffness and mass and could offer a better structural solution. However, there was some resistance to these concepts among contractors who were more familiar with an all steel solution – such as that used in the twin towers. A high strength concrete core was seen as either too demanding on local expertise and/or too slow to erect.
We went back and looked at projects to see if there was anything we could do in the light of what we knew at the time
The original 110 storey steel framed twin towers used extensive prefabrication and were seen as a dazzling construction achievement. The first tenants moved into the North Tower less than 30 months after construction began.
At the centre of each tower full height steel box section columns embraced the service core, including the three emergency staircases.
These cores were protected by nothing more substantial than fire resistant gypsum wallboard.
“After 9/11 it became much easier to convince clients and contractors of the benefits of concrete cores,” Rahimian reports.
It would be seven years before NIST published its definitive report on the disaster. In the shorter term, however, it was soon clear that one of the major factors in the terrible loss of life had been the congestion on the escape stairs caused by panicked occupants attempting to descend as fast as possible as firefighters tried to force their way upwards.
Major revisions to local and national building codes would eventually emerge, with the New York City code being the first. In the meantime, concrete cores began to evolve rapidly. Emergency staircases grew wider – by 25% or more – and became more numerous. Dedicated firefighter access began to feature, via dedicated stairs or even fire resistant lifts. More attention was given to the challenge of evacuating handicapped or infirm occupants. Building management systems became increasingly sophisticated.
The real difference from 20 years ago is the vastly more powerful tools for structural analysis
As the years passed new challenges arose. From the Empire State Building to the twin towers, high rise had been synonymous with 100% office towers. Architects and engineers would go to great lengths to maximise net lettable floor space. But as technology evolved, as higher strength concrete and steel became readily available, very tall buildings that combined different forms of accommodation in a single tower became more common.
Typically, a “mixed use” development might have retail space at the lower levels, office floors above and expensive residential accommodation on the highest levels.
Hotel accommodation and hospitality floors could even be included in the mix. Coping with the different safety imperatives for the individual sections has been far from straightforward, to say the least. Fitting all the services and shafts into the core is akin to a three dimensional jigsaw.
Rahimian says that the initial post-collapse fears and anxiety among tall building occupants soon subsided but the challenges remained for those buildings still at the design stage.
Recent fires
He also cites the examples of recent fires in tall buildings – all completed post-9/11 – in the United Arab Emirates. Despite being up to 79 storeys high, the stricken towers’ occupants were all evacuated safely.
“What’s essential is trained building management and occupants’ familiarity with emergency procedures,” Rahimian adds.
“Obviously, skilled firefighters are also essential.”
Even taller buildings became feasible, Rahimian says, partly because of improvements in construction logistics.
“The real difference from 20 years ago is the vastly more powerful tools for structural analysis. I started my design career in the 1980s with a slide rule: these days advanced software means we can compare different structural alternatives in no time at all.”
One unusual feature of the twin towers was the elastomeric dampers installed between the lightweight composite floor trusses and the perimeter columns.
Designing for wind is critical for most super-tall towers: the vortices spiralling along the sides of the building rather than air pressure on the windward side can cause swaying motion on the upper floors which can be extremely uncomfortable for their occupants. Effective measures have to be taken to reduce the sway to acceptable levels.
“On the very slender Trump World Tower we used a 600t tuned mass damper at roof level,” Rahimian reports. By contrast, WSP went for a tuned liquid damper – basically three u-shaped concrete water tanks, also at roof level – for the slender 50 storey One Madison Park Building, also in New York.
He adds: “Modern damping devices do a phenomenal job for occupant comfort. On our ultra-slim residential towers we now have the possibility of designing structures that are adequately stiff with or without central cores, using auxiliary damping technologies. In such cases the emergency stairs can be well separated.”
One tool not available to designers decades ago was computational fluid dynamics (CFD). With CFD it is now possible to model air flow around a proposed building with amazing accuracy – not just the building in isolation, but the building on its proposed site. In city centre locations, with disturbed air swirling around neighbouring high rises, a proposed building can behave in a way that only CFD can predict with any degree of accuracy.
What’s essential is trained building management and occupants’ familiarity with emergency procedures
As designers have pushed beyond 600m heights, creating a new official category of mega-tall skyscrapers, totally new forms have begun to emerge. The geometrically rectangular boxes typified by the twin towers had reached their limit. Dubai’s Burj Khalifa, the first to break the 600m barrier, opted for a tri-lobed planform, while the yet to be completed Nakheel Tower, also in Dubai, could break all records at more than 1,000m.
This WSP-designed project was planned to consist of four separate towers in very close proximity linked by substantial sky bridges every 25 floors. From any distance it would appear as a single tower. Like the Burj Khalifa, the Nakheel Tower’s architecture was “tuned” with CFD to significantly minimise wind effects. Both are all concrete designs.
Rahimian says that one of the biggest long term impacts of 9/11 has been the development of increasingly comprehensive building codes and standards.
“Back in the 1980s and 1990s a typical code might run to 100 pages. These days they can be 1,000 pages or more. Coupled with all the other improvements, in analytical software, materials, contractor expertise, we are moving towards true ‘cities in the sky’.”
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20 years on and still the ICE or IStructE remains silent on the cause of failure of the three WTC towers. Even at the time the WTC towers were designed, they went through a rigorous design process with robustness in mind. As an engineer & structural designer I still cannot accept the official narrative that a relatively lightweight aluminum aircraft could have possibly cut through the very substantial structural steel perimeter load bearing columns. There is no explanation how the even flimsier aircraft wings could possibly remain in one piece & cut through the building structure like a knife through butter. Any material scientist knows that aluminum isn’t able to penetrate through steel. Then the next thing that defies structural logic is how progressive collapse of around 80 lightweight concrete floor slabs could possibly pancake down one over next all the way to the ground floor. How is it possible that these floors and structural steel beam/columns literally turned to dust when they were pulled down to grade. The only structures I’ve ever seen do that were all brought down by the use of explosives in controlled demolition. Indeed most such demolition jobs do so falling vertically on their own footprint. I’ve read & studied the NIST reports in detail and in my humble opinion they fall far short of a proper structural examination & forensic investigation. Indeed for the 47 stores WTC7 tower which was not struck by any aircraft, this building fell in free fall velocity, yet the NIST does not even look deeply into what was the cause of failure.
Free fall is not a velocity, neither can it be unless terminal velocity. This is a misnomer in respect of structural collapse. Acceleration due to gravity is the correct way of analysis for building collapse, slowed down by crush down until zero acceleration is achieved.
A quick examination of the connection between floor and external envelope is all you need to see to explain the collapse mechanism. Its in the report.
Yes, definitely a non-robust detail.
WTC7 is the real mystery, especially as it’s collapse was reported before it even happened!
“Given the way video footage clearly shows WTC7 falling in accordance with a controlled demolition, the idea that there were pre-planted explosives inside the building is a thought pondered by many families of victims, scientists, physicists, engineers, and more.
A study from the University of Alaska Fairbanks, where professor and Chair of the Department of Civil and Environmental Engineering, Dr. Leroy Hulsey, provides “definitive,” proof that the building must have been blown up.
“Our study found that the fires in WTC 7 could not have caused the collapse recorded on video,” […] “We simulated every plausible scenario, and we found that the series of failures that NIST claimed triggered a progressive collapse of the entire structure could not have occurred. The only thing that could have brought this structure down in the manner observed on 9/11 is the near-simultaneous failure of every column in the building below Floor 17.”
Professor Hulsey
The significance of ‘near-simultaneous failure of every column in the building’ is great because it leads one to posit that explosives must have been used to bring it down. Explosives would mean someone had access to the towers main structures to plant all of the explosives.
Unlike NISTS’s non transparent model of explaining how WTC7 came down, the research team at UAF made all of the data used and generated during the study available to the public. NIST’s official explanation for why they did not release their own model? It “might jeopardize public safety.”
I suggest you take another, not so “quick” examination outside of the NIST report. Besides, why would you rely on a report that has never been peer reviewed and can not be peer reviewed because NIST refuses to release all the data they used in their “models” and “conclusions”? If you don’t want to take the effort, then just answer 1 question. How is it possible that the floor trusses were able to overcome the 5x redundancy that was designed into the outer steel columns and the 3x redundancy that was designed into the inner-core steel columns? In other words, these buildings were designed to withstand the collective weight EIGHT TIMES the weight and stress that what was necessary and remain – structurally intact ? Is that what we see in the collapse of these 2 towers that took about 10 seconds each to collapse a 110 stories?
Nor should you believe the official narrative. Did you know that the official NIST report on 9/11 DOES NOT explain the progressive collapse of the twin towers ? As they state in their very own report, their “analysis” is confined to events up to the *initiation of the collapse*. When asked about the cause of such destructive and progressive collapse that followed, they state that the do not have an explanation for it. You want to learn more? Let over 3,800+ Architects and Engineers walk you through the many many issues left unanswered – Search AE911Truth. In particular, watch their 3 part presentation on YT discussing many of these issues. Very eye opening. And please, pass AEf911Truith along to others as well. And no, I have NO affiliation with this organization – they are the only ones who back up what they say.
Yes , I’ve need to hear why the aircraft wings etc did not collapse?
Yes, I find it hard to believe, but the only structures which I have ever seen collapse vertically with no sign of toppling were due to controlled demolition.
According to news reports the lessons described above regarding escape stairs, building management and firefighting which were apparent then would have been invaluable in reducing loss of life at Grenfell Tower 6 years later, especially in view of similar deaths in a previous London fire where victims could have escaped but had been told to stay in their apartments to allow access for fire brigade. That was compounded in the case of Grenfell by the LFB statements that they had found it unsafe to approach the building.
Asymmetrical damage DOES NOT result in symmetrical collapse. View the collapse of Building 7. The official reason is that thermal expansion pushed ONE beam off its girder in the NW (must confirm which corner) corner on floor 12 of WTC 7. This initiated the collapse of the building. With that in mind, watch the video of the collapse and ask yourself 1) if the initiation of the collapse was on floor 12 of the NW corner, why is it the penthouse on the roof of the building starts it decent before any other part of the building? 2) With the collapse initiated at floor 12 of the NW corner, how is it possible that top left of the building starts it’s decent at the same time the top right of the building ? If asymmetrical damage results in symmetrical collapse – demolition experts would be out of business, because it wouldn’t matter where/when you triggered explosives in a building – it would always fall into it’s own footprint. Pure nonsense. As far as the twin towers – the issues with the official conspiracy theory regarding their collapse is beyond astounding. Research AE911Truth – 3800+ Architects and Engineers will walk you through them all. I agree with the other commenter regarding WTC 7 – to learn – by SCIENCE – what DID NOT bring down that building see the study from the University of Alaska Fairbanks professor and Chair of the Department of Civil and Environmental Engineering Dept, Dr. Leroy Hulsey multi year, multi – computer modeled review of WTC 7’s collapse.
Seems ChatGPT knows fairly well what happened:
Here is an image of his response: https://ibb.co/VwSfQW5