Materials Analysis: Assessing Twin Towers' Collapse
Following the September 11, 2001 Terror Attack
In Fulfillment of the First Laboratory Activity of
MEC136 - Materials Science and Engineering
for Mechanical Engineering
Leanard B. Armada
J-Lou Avila
Mark Danielle N. Bahinting
Chapter 1
Introduction and Background
I. Historical context of the World Trade Center construction
In the years following World War 2, the World Trade Center was built. The Port Authority
of New York and New Jersey constructed it as a facility for companies and governmental
organizations that are heavily involved in the global trading system. Rapid economic expansion,
a significant rise in manufacturing, and a focus on urban development (such as in lower
Manhattan) were all hallmarks of this time period.
II. Brief overview of the 9/11 Twin Towers collapse
On the morning of September 11, 2001, Individuals associated with the terror group Al
Qaeda hijacked four airliners, one crashing in a field on route to the White House, another hit
the Pentagon in Arlington, Virginia, and the last two, American Airlines Flight 11 and United
Airlines Flight 175 targeted New York. The two aircraft intentionally collided with the North and
South Towers of the World Trade Center Complex in New York City, killing everyone on board.
The two enormous airliners collided, causing severe damage to the two towers' structural
integrity as well as engulfing them in a blaze of burning aluminum and jet fuel. These incidents
ultimately contributed to the tragic collapse of both buildings, which resulted in the deaths of
2,977 people and the injuries of thousands more.
III. Description of the aircraft impact on the Twin Towers
The American Airlines Flight 11 first crashed into the North Tower of the World Trade
Center between floors 93 and 99 at around 440 mph causing a massive fireball and significant
damage to the tower as well as a wide hole that spewed extensive fires on other floors. United
Airlines Flight 175 soon crashed next into the South Tower between floors 77 and 85 at around
590 mph causing another fireball and immense structural damage to the said tower.
IV. Importance of materials science analysis in understanding the collapse
Material Science plays a big role in investigating and understanding the catastrophic
event on the World Trade Center tower on 9/11, It provides insights into how the material works
in certain conditions. Especially steel, when it is exposed to heat, impact and load. Material
science also examined the material and the structural damage majorly caused by the impact
and fire.
V. Purpose and scope of the paper
The purpose of the paper was to conduct a comprehensive materials science analysis of
the collapse of the Twin Towers during the 9/11 terrorist attacks. With the objectives being to
examine available data and existing sources and research, analyzing the effects of the airliner
impact, examine the subsequent collapse, fire and heat effects, and to form a conclusion on
whether - given the circumstances - the towers should have collapsed in the scenario.
Chapter 2
Analysis
I. Materials Used in Construction ← Link
A. Overview of materials and construction methods in the Twin Towers
The world trade building center was built on the top of a bedrock 21m below the ground.
It was supported by giganti foundations. On the construction, more than million cubic yards of
earth and rock were removed to build a basement that was 299m x 155m x 21m. Underground
walls were built all the way down and into the bedrock to withstand external pressure such as
water, earth pressure and to prevent the undermining of adjacent buildings and streets. The
walls were 7 story high and heavily reinforced concrete walls.
B. Examination of the structural materials (e.g., steel, concrete)
The world trade center twin towers exterior walls were built of closely spaced steel
columns to perform as load bearing walls. The outer walls provided resistance to the lateral
external forces such as wind, earthquakes and impact, it also carried the vertical load for the
building. The interior columns are only located on the core of the building containing elevators.
C. Fireproofing and its role in the collapse
Mostly of the structural material used on the Twin tower was A36 structural steel. In the
lower elevation, higher strength steel was used but some selected floors, with normal strength
concrete was employed. Fire resistance of the perimeter was implemented by a layer of sprayed
concrete around the three sides of each column (about 5 cm thick including ceramic fiber mix).
The interior was protected with vermiculite plasters, the exterior side of each column was
covered by aluminum to which window frames were fixed.
II. Aircraft Impact Analysis, Effects of Fire and Heat
A. Detailed analysis of the aircraft impact on the Twin Towers
Several different factors affected the actual event of the crashes as well as the initial
aftermath of the impacts as detailed by Wierzbicki et.al (n.d.) and other related studies of the
event. From the initial crash, to structural damage, and effects of the resulting inferno, several
different occurrences within a short span of time catalyzed by the aircraft lead to what would
seem to be the unfortunate, yet - given the circumstance - inevitable collapse of both towers.
Aircraft Impact;
The paper states and details that the type of
aircraft that hit both towers were the Boeing
767-200ER.
Analyzing the dimensions and actual gross
mass of the aircraft at the time of impact, the authors
of the paper estimated that both aircraft had
approximate masses of 127 Tons each. With velocities reaching 240 and 200 meters/second
at the time of impact for both North and South Towers Respectively, the kinetic energies at
impact for both planes were as follows;
Given these energies at impact, as well as the fact that both planes rolled at angles of
approximately 26 and 35 degrees, these factors caused immense damage to several floors at
the moment of impact, with the fuselage alone (5.03 m in height) being taller than a single
floor of the tower (3.7 m), this would cause immense damage without even accounting for the
wings.
Fire and Heat;
Wierzbicki et.al (n.d.) added that the initial structural damage to the towers themselves
with respect to the structural shape of the buildings and locations of load bearing foundations,
caused enough structural damage themselves that it would be enough to take down the towers
already, the effects of the metastasized inferno that followed was also significant in affecting the
yield point of the different load bearing components of the structure. Though other researchers
like Vallero and Letcher (2013), argued that as can be seen from footage of the fires inside,
black billowing smoke indicated that only partial combustion of the fuels as well as other
flammable materials in the buildings took place. Nonetheless, the temperatures appear to
have reached 800 and 1000 °C with over 10,000 tonnes of aluminum melted in the process.
From a tertiary perspective, it is possible that the initial impact of the crash as well as
temperatures in the thousands of degrees could have a profound impact on the building's
structural integrity.
Structural Integrity and Building Failure;
A National Institute of Standards and Technology (NIST) Report on June 8, 2005
exclaimed that buildings are not designed to withstand a high speed collision with a large airliner
(the largest at the time) traveling at speeds of upwards of 600 mph. Also detailing that these
towers weren’t built to protect against fire
and facilitate an evacuation of magnitude
at the scale of the attacks.
This photo provided by the NIST
displays the amount of damage caused
by a single aircraft, destroying several
connection
points
and
deep inside the structure.
disintegrating
The Institute added that there
were several factors that aided in
enhancing the performance of the World
Trade Center buildings despite the
impact of aircraft and the subsequent
fires that followed. From the robust
closely
spaced
perimeter
columns,
exceeding wind load standards, among
others.
But
concluding
on
that
the
contrary,
due
to
also
aircraft
dislodging the inbuilt fireproofing due to the aircraft, the Towers’ central load bearing design was
allowed to tilt and shift its center of balance, hampering the exterior columns to hold load, and
contributing to the subsequent collapse.
III. Progressive Collapse
A. Events leading to the collapse
Weak link theory;
It is a fundamental principle of engineering design theory that a structure is only stable
as the weak link in a chain of events. This link may happen at a material or structure level
affecting the entire structural system stability if no provisions for redundancies have been
implemented in the system. In the collapse of the tower, there are many possible interacting
factors involved that lead to the catastrophic event.
Fire;
There has been speculation that with respect to the magnitude of temperature that may
have resulted from burning of the jet fuel possibly leading to the melting of steel in the World
trade center towers but thi is not true . Heat and temperatures are related but it should not be
confused as they vary. Heat is an extensive property, meaning it is dependent on the mass,
while temperature is an intensive property, meaning it is independent of the mass. The
temperature felt by the steel caused by the fire is not sufficient enough to melt the steel;
however, this fire and this level of temperature has a significant effect on the structural behavior.
Behavior of steel under high temperature;
When a steel is exposed to a high temperature environment, it does not perform well as
the structural material (steel) has a high thermal conductivity and the members made of steel
usually have thin cross sections. Steel that is subjected to high temperature undergoes a
substantial loss of strength and stiffness at a temperature level below the melting point, which is
referred to as thermal softening and thermal damage.
Structural performance of columns and slabs under high temperature;
Slab systems primarily carry load in bending. It is well known that the bending members
subjected to fire undergo a large deformation. With heating, the structural members reach the
yield limit of strength of the steel faster and resulting in a large plastic deformation. But ductile
deformation would have further delayed the structural collapse, or would have prevented it and
ductile deformation could hardly happen in the towers. This concluded that the slab didn't
undergo thermal softening and thermal damage which essentially led to a ductile failure of the
slab system.
B. Existing conspiracies on the collapse
The common theory and what most others thought was that the reason for the collapse
of the towers was because of controlled demolition.The use of bombs and explosives are the
reason why the towers collapse. This claim was based on the iron rich sphere in the dust of the
collapses and the speed and symmetry of the collapse of buildings. However, scientific analysis
shows that these spheres are consistent with the steel and other materials present in the
buildings and do not provide evidence of explosives. The collapse was the effect of the
structural damage in the building.
C. How the initial failures propagated throughout the structures
The initial impact of the two aircrafts initially caused significant structural damage to the
North and South Towers of the World Trade Center. These impacts created large holes in the
building and ignited fires especially due to the amount of jet fuel they have carried. These fires
burned at temperatures hot enough to be able to weaken the steel beams and columns that
supported the structure of the buildings.
As the structural components of the towers weakened, the upper floors of each building
above the impacted zones started to sink. This combination of structural damage, intense heat,
and increased load towards the lower floors led to a “progressive collapse” meaning that the
collapse of the upper floors propagated downward causing the floors below to collapse as well.
A global failure of the overall structures resulted from this progression wherein the buildings,
from the impacted zone towards the ground level seemingly began to collapse simultaneously.
The release of stored potential energy from the upper floors of the buildings as they fell
contributed to the acceleration of the total collapse of both the North and South Towers.
D. Examination of key collapse mechanisms
A few mechanisms such as based on the mechanics point of view of Buyukozturk & Ulm
(2003) stated that the initial impact and penetration of the aircrafts disrupted the continuity of the
force flow in the outer structure of the buildings from which a significant stress distribution can
be observed from damaged members of the structure unto the undamaged parts creating a new
force balance. Since the absence of equilibrium results in failure, it can be inferred that this
force balance was maintained during the time until complete collapse, that is 56 minutes for the
South Tower and 102 minutes for the North Tower, both being after the initial impact.
Another likely scenario provided by Buyukozturk & Ulm (2003) was due to the failure of
the support structures of the slabs, initiated through insufficient fireproofing or higher thermal
damage and softening of the bolts. A zipper effect may have been produced by the failure of
one or many supports resulting in the loss of the slab’s bearing load capacity. When a floor
system fails, the lower floors have to carry the additional weight.
Chapter 3
Conclusions
Conclusion
A. Summary of key findings and insights
These findings shed light on significant elements of the building and collapse of the Twin
Towers on 9/11. The building process included strong foundations, well spaced steel columns in
the external walls, and fireproofing techniques. The Boeing 767-200ER planes inflicted
significant damage, according to the aircraft impact analysis, with kinetic energy and angles of
impact being important factors. The jet fuel-fueled fires that followed severely damaged the
buildings. As structural elements deteriorated, a gradual progressive collapse eventually caused
a breakdown on a grand scale. The intricate series of events that led to the towers' eventual
collapse are highlighted, underscoring the significance of structural resilience in high-stress
situations. In summary, it can be concluded that the impact of the two airliners catalyzed and
contributed to the collapse and ultimate failure of the World Trade Center Towers.
B. Implications of the materials science analysis
Material Science informs efforts to prevent similar incidents in the future. Understanding
the limitations, vulnerabilities, and failure mechanisms, engineers and constructors can develop
strategies and approaches to mitigate the impact of such events especially on tall structures and
infrastructure like the World Trade Center. Highlighting the importance of fire-resistant building
material on such tall and high structures; quality control to ensure the structural integrity of the
building to ensure the design is more resistant to progressive collapse.
References
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&
Facts.
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https://www.scientificamerican.com/article/what-structural-engineers-learned-from-9-11/
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https://www.nist.gov/pao/questions-and-answers-about-nist-wtc-7-investigation
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