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The World Trade Centre was built between 1966 and 1970 in Lower Manhattan,
New York. The Twin Towers were an iconic symbol of New York and when built,
surpassed the Empire State Building as the tallest buildings in the world, at 110
stories tall. The project was to be built on a 16 acre site with a floor space of 12
million square feet. The architect Yamasaki decided on a design consisting of two
towers, maximising on floor space and taking advantage of the surrounding
views. Yamasaki wanted the World Trade Centre to act as “a living symbol of
man's dedication to world peace” and become a “representation of man's belief in
To achieve a lightweight structure with large floor space the towers had two main
structural elements. The first an outer steel lattice façade, acting like a hallow
tube, and an inner core that took the gravity loading only. The outer structure
consisted of many steel columns spaced at 100cm centres and acted as a bracing
system for lateral loading. This loading then did not have to be transferred to the
centre core resulting in a light structure. To prevent wind sway dampers were
added to absorb the energy.
Steel trusses were used as flooring connecting the outer support columns
to the inner core, giving more stability for lateral loading. Concrete was poured
over these web joists to form the floors.
The inner core was also made of steel columns and held the lift shafts and
stair wells. The Twin Towers were one of the first buildings to use sky lobbies
that enabled people to move from express lifts to local lifts. Two Sky lobbies
were used for this purpose and helped give more floor space by reducing the
amount of lift shafts.
Why did it collapse?
While it is obvious that the hijacked passenger planes were the initial cause of the
World Trade Centre collapse it is not quite so clear how the airplane impact
resulted in such an extraordinary failure.
The series of events leading up to the collapse of the WTC are probably
best examined in order, as they happened:
The Impact:
Shortly after 9am on September 11th 2001 a passenger jet collided with the north
tower of the world trade centre. The plane crashed through the façade, damaged
many of the external support columns, and caused massive dynamic lateral loads
in the structure. Less than twenty minutes later a second airplane collided with
the south tower, causing similar devastation. For the sake of simplicity only the
north tower will be discussed here, as both the impact and method of failure were
similar in both.
Witnesses who survived the collapse reported that when the plane hit the building
it “sprang back and forth like a car antenna, door frames twisted and jammed
shut, trapping a number of them [the witnesses] in a conference room.” (New
York Times)
The columns at the impact level were considerably smaller and less
massive than those at ground level. Many of the external columns (designed to
resist lateral loading) were damaged, and it is also possible that the impact
damaged the fire protection on the columns and severed the water pipes for the
sprinkler system.
Although the impact caused significant damage to the building, however, it was
not enough to cause collapse. The reasons the building withstood the initial
impact so well are as follows:[1]
When the plane broke through the façade it most likely fragmented. As there
are relatively few components in an aircraft which can do serious damage to a
structure, the core of the building was probably left intact. It was the core of
the tower that supported the weight of the building.
The design of the towers, and the façade in particular, had a lot of structural
redundancy. This meant that the load on the damaged columns was taken up
by other, undamaged, structural members. It can be seen from the video
footage that the corners of the structure were left unharmed by the aircraft
The towers were designed to withstand high lateral loading from wind, of
which there was very little on the day.
The Explosion:
The initial impact was closely followed by an explosion as the fuel, which had
been on board the plane, ignited. The explosion further weakened the beams and
columns of the building and may have blown away much of the lightweight fire
protection which surrounded the steel members.
The Fire:
The remaining fuel caused a fire within the building. This fire is generally
considered to be main factor in the failure of the structure, but there has been
some argument over how exactly it caused the collapse.
Initial speculations suggested that the temperature of the fire was enough
to melt the steel, and that that was the reason for collapse. Later calculations and
examination of the photographic evidence showed that the fires in the tower were
not intense enough for the steel to melt.[1] It is certain, however, that the
structural steel softened in the blaze and lost up to half of its strength.
Even with the reduced strength and numbers of the perimeter columns the
structure could still withstand the stresses involved. The eventual collapse was
caused by a combination of weakening of the steel members, and also buckling in
the members caused by the non-uniformity of the blaze. This caused the
members to expand at the side nearest the fire, while the outside area remained
at a relatively low temperature.
Progressive Collapse:
The impact of the airplane, causing stress on the floors, along with the weakening
and eventual buckling of the columns, caused the joints connecting the floors and
columns to fail as the columns buckled outward. Once one floor had collapsed
there was less restraint on the external columns, which allowed them to buckle
out even further, thus removing the support from lower and higher floors. This
effect, along with the impact of falling floors on those below, caused a domino
effect down the height of the building.
The lightweight design of the Twin Towers meant that they were able to implode,
and collapse directly down on themselves. It also explains the fact that the rubble
was only a few storeys high after the collapse.
What Was Learnt & Prevention.
1- Fireproofing:
When the Boeing 767-200ER stuck the 96th of the north tower on September the
11th 2001 it was traveling at about 440mph. On striking the building, pieces of
aircraft continued inwards severing and heavily damaging external and core
columns. The inward motion caused the fire proofing insulation applied to the
floors trusses and the columns to be scraped off by shapmel like air craft debris.
In total, insulation was stripped from trusses covering covering 60000 ft^2 of
floor area.
The steel was now exposed to extremely high temperatures because the plane
fuel immediately caught fire on impact with the building. The steel began to
soften due to the removal of the fire proofing and the intense heat. The steel
began to lose its strength and stiffness. The floors began to sag. The sagging
pulled the external columns inwards. An inward bowing of the external columns
At some stage a critical amount of inward bowing took place and the columns
snapped. This caused the above 10-15 floors to fall downwards. The building was
unable to support the downward momentum of the building.
The removal of the fire proofing from the trusses and the associated sagging of
the beams could have been prevented by;
The use of better fire proofing, which is able to withstand huge dynamic forces,
such as planes travelling into building.
Better inspection of fire proofing in building after it is applied, so that any fire
proofing that is supposed to be there is actually there.
By the development of new types of fire proofing which are more paint-like than
spray-like. This would provide certain advantages.
2-Active Fire Protection:
The spread of the fire was not halted by the sprinkler system which was
employed within the building. There was very little redundancy contained in the
system. For example, the sprinkler system on a particular floor was connected to
the water supply at one point only. So if there was failure at a single point, then
the entire floor would be devoid of water for the sprinkler system.
Also, the fire notification in the building was only capable of determining and
(a) Areas that at some point had reached alarm point conditions
(b) Areas that had not reached alarm point conditions.
Therefore the quality and reliability of the information available to the fire
command station was limited. The system also experienced degradation in
performance especially in the higher floors. Where communication was lost
completely. This resulted in fireman being sent to locations, where they had little
chance of reducing the effect of the flames.
Nonetheless, much of this was not a design problem, just that the building was
totally unprepared for a plane containing huge amount of air line fuel colliding at
speeds of over 600 mph.
The poor quality of the active fire protection in the building could be prevented
The redundancy of active fire protection in the building be enhanced to
accommodate the greater risks associated with increasing building height.
An improvement of fire alarms and communication systems in buildings so as to
provide continuous and accurate information. While also allowing the
communication of a larger quantity of information from active fire protection
3-Structural Issues:
When the critical amount of inward bowing of the external columns took place,
the columns snapped. The columns sprung back and out. Then, the top 10-15
floors then fell downwards.
The momentum of these floors falling on the supporting structure was unable
to stop or even slow the falling mass. And the result was the collapse of the
entire structure. The method of failure is known as progressive failure.
This collapse of the building could be prevented by;
The employment of greater redundancy in the building, so as the building can
remain standing even though it may not be happy about it.
The building used an innovative structural system in which the towers were
designed as framed tube structures, with columns grouped around the perimeter
and within the core. The perimeter columns supported virtually all lateral loads,
such as wind loads, and shared the gravity loads with the core columns. There
should be a more thorough evaluation of all new construction ideas and methods.
In the impact area of the first building, none of the stairwells were passable after
the airline impact. In the building only one of the ninety nine elevators was
functioning. However, this elevator only went to the 16th floor.
While, in the second building only eighteen people from above the floors of
impact were able to utilize the stairwell. In the second building, also only one of
the ninety nine elevators was functioning. However, this only went as far as the
40th floor.
Nevertheless, adequate stairwell capacity was provided, as the number of
building occupants leaving the building prior to the collapse of both buildings had
The number of people who dyed in the collapse of the world trade tower could of
been lower if;
The small number who escaped from above the impact region could have been
much greater if the stairwells were shielded with the use of reinforced concrete
instead of the gypsum cladding which was employed.
Adequate stairwell capacity to allow safe and rapid egress is provided. While also
specially designed elevators that are fire protected and structurally hardened.
Stairwell and elevator activity should be more robust and remote from each
other. So that if one is damaged the other is not.
There was stair cases and elevators specifically for fireman. This was an issue
highlighted in the evacuation because the progress of fireman was affected by
those evacuating the building as they had to share the one stairway.
There was a lack of reliable and accurate information among emergency
responder and building occupants.
For example, there was nearly 1,000 emergency workers on site, but there
was a limited number of communication frequencies. Therefore information could
not be relied to from rescue workers in the higher towers in the building, resulting
in confusion. And perhaps, sending rescue workers to locations which hazardous.
In reports, building occupants also spoke of the lack of communication and
confusion in the evacuation.
This could be improved by
Providing a more suitable communication system for rescue workers which could
cater for over 1,000 people on site.
Developing a system for building occupants to allow them to receive messages
containing information on their mobile phones.
[1] “Why did the World Trade Centre collapse” Thomas W. Eager and Christopher
Musso, JOM, 53 (12),(2001), pp. 8-11
[2] “The World Trade Centre and 9/11: A discussion on some engineering design
issues.” Tim Wilkinson, written for “safe buildings for this century” Australian
Institute of Building Surveyor National Conference, 12-13 August 2002.
[3] “World Trade Centre – Some Engineering aspects” Timothy Wilkinson, 2001
[4]”What Future For Tall Buildings?” The Structural Engineer, 73 (20), 16th
October 2001, pp. 13-15.
NIST NCSTAR 1: Federal Building and Fire Safety Investigation of the World Trade
Center Disaster: Final Report of the National Construction Safety Team on the
Collapses of the World Trade Center Tower, NIST, (, 2006.
Collapse of the World Trade Center, wikipedia,

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