Posted by Drifter
The
collapse of the World Trade Center has been subject to intense public scrutiny
over the 20 years since the centre’s twin towers were struck by aircraft
hijacked by terrorists. Both collapsed within two hours of impact, prompting
several investigations and spawning a variety of conspiracy theories.
Construction
on the World Trade Center 1 (the North Tower) and World Trade Center 2 (the
South Tower) began in the 1960s. They were constructed from steel and concrete,
using a design that was groundbreaking at the time. Most high-rise buildings
since have used a similar structure.
The
investigatory reports into the events of September 11, 2001 were undertaken by
the US Federal Emergency Management Agency (FEMA) and
the National Institute of Standards and Technology.
FEMA’s
report was published in 2002. This was followed by the National Institute of
Standards and Technology’s three-year investigation, funded by the US Federal
Government and published in 2005.
Some
conspiracy theorists seized on the fact the NIST investigation was funded by
the federal government — believing the government itself had caused the twin
towers’ collapse, or was aware it would happen and deliberately didn’t act.
While
there have been critics of both reports (and the investigations behind them
weren’t flawless) — their explanation for the buildings’ collapse is widely
accepted. They conclude it was not caused by direct impact by the aircraft, or
the use of explosives, but by fires that burned inside the buildings after
impact.
Fire and rescue workers search
through the rubble of the World Trade Center in New York on 13 September 2001.
On 11 September 2001, two aircrafts were flown into the centre’s twin towers,
causing both to collapse. BETH A. KEISER/EPA
Why
did the towers collapse as they did?
Some
have questioned why the buildings did not “topple over” after being struck
side-on by aircraft. But the answer becomes clear once you consider the details.
Aircraft
are made from lightweight materials, such as aluminium. If you compare the mass
of an aircraft with that of a skyscraper more than 400 metres tall and built
from steel and concrete, it makes sense the building would not topple over.
The
towers would have been more than 1,000 times the mass of the aircraft,
and designed to resist steady wind loads more
than 30 times the aircrafts’ weight.
That
said, the aircraft did dislodge fireproofing material within the towers, which
was coated on the steel columns and on the steel floor trusses (underneath the
concrete slab). The lack of fireproofing left the steel unprotected.
As
such, the impact also structurally damaged the supporting steel columns. When a
few columns become damaged, the load they carry is transferred to other
columns. This is why both towers withstood the initial impacts and didn’t
collapse immediately.
Progressive
collapse
This
fact also spawned one of the most common conspiracy theories surrounding 9/11:
that a bomb or explosives must have been detonated somewhere within the
buildings.
These
theories have developed from video footage showing the towers rapidly
collapsing downwards some time after impact, similar to a controlled
demolition. But it is possible for them to have collapsed this way without
explosives.
It
was fire that caused this. And this 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 then deflected to an extent but resisted further movement.
This figure shows the expansion of
floor slabs and framing which likely happened as a result of the fires.
Source: FEMA
With
the columns resisting movement there was nowhere else for the concrete floors
to expand. This led to an increased buildup of stress in the sagging floors,
until the floor framing and connections gave in.
The
floors’ failure pulled the columns back inwards, eventually leading to them
buckling, and the floors collapsing. The collapsing floors then fell on more
floors below, leading to a progressive collapse.
The buckling of columns initiated by
floor failure. FEMA
This
explanation, documented in the official reports, is widely accepted by experts
as the cause of the twin towers’ 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.
The
debris from the collapse of the North Tower set at least ten floors alight in
the nearby World Trade Center 7, or “Building 7”, which also collapsed about seven hours later.
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
2007, the National Institute of Standards and Technology also published a best practice guide recommending
risk-reducing solutions for progressive collapse.
What
does this mean for high-rise buildings?
Before
9/11, progressive collapse was not well understood by engineers. The disaster
highlighted the importance of having a “global view” of fire safety for a
building, as opposed to focusing on individual elements.
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. In the decades since the World Trade
Center was designed, buildings have become taller and more complex, as
societies demand sustainable and cost-effective housing in large cities.
Some
86 of the current 100 tallest buildings in the world were
built since 9/11. This has coincided with a significant increase in building
façade fires globally, which have gone up sevenfold over the past three
decades.
This
increase can be partly attributed to the wide use of flammable cladding. It is
marketed as an innovative, cost-effective and sustainable material, yet it has
shown significant shortcomings in terms of fire safety, as witnessed in
the 2017 Grenfell Disaster.
The
Grenfell fire (and similar cladding fires) are proof fire safety in tall
buildings is still a problem. And as structures get taller and more complex,
with new and innovative designs and materials, questions around fire safety
will only become more difficult to answer.
The
events of 9/11 may have been challenging to foresee, but the fires that led to
the towers’ collapse could have been better prepared for.
Written by David Oswald, Senior Lecturer in
Construction, RMIT University; Erica Kuligowski, Vice-Chancellor's Senior
Research Fellow, RMIT University, and Kate Nguyen, Senior Lecturer, ARC DECRA Fellow
and Victoria Fellow, RMIT University
