Group 2 - Disaster Project
The World Trade Centre – 9/11
Introduction:
Once dominating the New York skyline, the twin towers atrocity is one we are all familiar with. The
towers were part of a larger development consisting of another four buildings. Building of the towers
commenced in August 1966 and was completed in April 1973, taking just under seven years to
complete. This report aims to describe the structural design of the towers, the damage caused by the
impact of the planes and the design details which led to the spectacular collapse of both towers.
In addition to this some comments are made on other design details which prevented the escape of
people in the stories above the impact zone. Finally this report discusses the changes to the way in
which we build that will hopefully ensure that if we cannot design for such impacts, then at least we
can ensure that more people get out safely.
What happened:
On Tuesday September 11th 2001 hijackers took control of two planes and crashed them into the twin
towers of the world trade centre complex. It was the largest terrorist attack ever committed on
American soil with the loss of almost 3000 lives and over 6000 more injured.
At 8:46 am American Airlines Flight 11 was crashed into the north tower. Seventeen minutes later the
south tower was also struck. The south tower collapsed first, less than an hour after it was hit. The
North tower soon followed at 10:28 am.
As a result of the world trade centre disaster there were numerous changes in both security and
building design. The attacks initiated the United States “War on Terror” which began with the
invasion of Afghanistan in October that year and still continues to this day.
Although it would be uneconomical to design buildings to withstand the structural damage the world
trade centre endured, buildings are now designed with better escape systems to ensure minimum loss
of life.
Structural Design of the Twin Towers:
The architect who designed the world trade centre was Minoru Yamasaki and the structural
engineering team was lead by John Skilling and Leslie Robertson.
The skyscrapers were each 110 stories high (about 411m) and had six basement floors. The basement
contained train and subway stations as well as underground parking and technical services such as an
air-conditioning plant. The upper floors contained open plan offices which were free of internal
supports. Each floor had about 2900m2 of open space.
Each tower was square (63.5m x 63.5m) and had an internal core of 24m x 42m.
Picture 1: External Columns
Picture 2: Structural Layout of Towers
The basic structural design of the towers was as follows:
59 box shaped columns forming a dense outer wall were spaced at 1.02m. These columns formed a
rigid tube fixed to the foundations which resisted wind loads and some of the gravity loads. The
majority of the gravity loads were taken by the internal core.
Composite floors spanned between the outer wall and core without internal columns. The floors
comprised of trusses (spaced at 2m), covered by troughed steel decking with a 100mm concrete layer
on top. Secondary joists braced the trusses transversely. In order to minimise the wind induced
vibrations, viscoelastic shock absorbers were placed between the trusses and external columns.
It should be noted that this structure was highly redundant.
Fire Protection
Originally each of the towers was sprayed with an asbestos-free fire retardant and each of the
members of the steel trusses were covered in about 20mm of the material.
According to the Federal Emergency Management Agency Report on the tragedy, it was decided to
upgrade the fire protection (in the 1990’s) by applying extra material to the trusses in order to increase
the thickness to 25mm – 38mm. The fireproofing was added to floors as they became available and by
9/11 the entire impact zone of tower 1 was upgraded but only one floor of the second tower impact
zone was completed.
In addition to the fire proofing material, automatic sprinkler systems were installed in each tower and
every stairwell had a standpipe bringing water to each floor, pumps were located both outside and
inside the towers in order to provide a suitable water pressure.
Mode of failure:
Initial impact of planes did not knock the buildings immediately. The wing span of the Boeing 767 is
48m and the width of the towers was 64m, therefore the corners of the building were not significantly
damaged by impact. The impact did however destroy the columns around the impact zone.
The Towers were redundant structures so although some of the members were wiped out the rest
could compensate by redistributing the loads to adjacent columns.
The impact of the planes removed much of the fire retardant covering the steel members around the
damaged zones. The Boeing planes that crashed contained approx. 38,000 liters of aviation fuel. This
fed a massive fire burning at 800°C - 1500°C. Steel loses half its strength at 1200°C. Due to this high
heat, the steel in the columns and trusses around the impact zone would have deformed, this was the
beginning of the collapse.
The exact way in which the towers collapsed seems to differ in various sources but one possible
mechanism is as follows:
The steel lost strength due to the intense fire, this led to deformations and sagging of the floor trusses
and columns, stresses caused by thermal expansion caused a failure of a joint which connected the
floor to the external columns. Since the trusses served to support the columns from buckling
outwards, once this restraint was removed the columns buckled and the floor fell downwards.
When the weakened floors started to collapse they initiated the
collapse of the floors beneath them causing a pancake effect.
The floors started crashing down on the floors below increasing
the weight and momentum of the collapse. The mode of failure
was the same for both towers.
The towers were 95% air that’s why they looked so flat after the
collapse.
Buckling of columns leading to progressive failure (Source: FEMA Report)
Design “Flaws”:
After the collapse, the Federal Emergency Management Agency (FEMA) and several other agencies
formed a group to study the response of the buildings to the impacts and to form recommendations for
the future.
It is very interesting to note that the main finding of this report was that no building could have been
reasonably designed to withstand such an attack and it was the buildings extra redundancies which
allowed the towers to remain standing long enough to allow people to escape at all. However the
report also makes references to factors which caused the collapse and prevented escape of people
from the upper floors but insists that these should not be considered as “weaknesses”
The report highlights the following design details:
1. The steel floor truss system and it’s redundancy when compared to other structural elements
2. The use of dry-wall around escape paths
3. The use of fire protection which was not designed to provide resistance to impacts and blasts
The first point illustrates the fact that the only thing keeping the outer columns from buckling was the
floor trusses – once these trusses failed entire collapse was imminent. The second point is very
important with regards to evacuating people. The following witness account is taken from the Sydney
Morning Herald (June 1, 2002):
“The stairs were blocked by huge pieces of the light gypsum drywall, often called sheetrock that had
enclosed the stairwell”
With regards to the fire protection (point 3), it was never designed to withstand the impact from the
attack. The fact that it was blown away from the elements it was protecting was the main factor which
resulted in the collapse of the two towers.
In addition to these four points highlighted by the FEMA report, survivor accounts also give some
other clues as to how the protection mechanisms failed after the impact.
Due to the initial impact, the building “sprang back and forth like a car antenna”, causing twisting
and buckling of internal members – including door frames and fire doors which were jammed shut
trapping people inside.
The twin towers were both equipped with sprinkler systems, which were activated when a fire was
detected. However, there was very little redundancy within the system, which meant when the plane
crashed into the building it severed pipes of the system causing it to become ineffective. Also, a factor
that contributed to the hindrance of the firemen to enter the building was the fact there was only one
stairwell present for people evacuating the building and the firemen trying to enter the building.
What have we learned & how this can be avoided in the future:
A tragic legacy of September 11 is that buildings in New York City must now adhere to more
demanding building codes. Based on suggestions outlined in a lengthy government report published
by the National Institute of Standards and Technology (NIST), New York's building codes have been
adopted by cities across the United States.
 New codes have now been introduced for designing and evaluating fire resistant structures.
 Codes have been updated to improve the design, performance, reliability, and redundancy of fire
protection systems such as sprinklers, standpipes/hoses, fire alarms, and smoke management
systems.
 All evacuation stairwells are planned to be encased in reinforced concrete to allow maximum time
to escape safely.
 Improved evacuation procedures, emergency communications, and emergency preparedness
measures were also introduced into the new building regulations of New York by the NIST
 Response operations, emergency communications, access to buildings, and coordination of largescale emergency response measures were also added to the codes
References:
1. The World Trade Centre and 9/11: A Discussion on Some Engineering Design Issues – Tim
Wilkinson, 2006
2. Picture 1: Internet - http://z.about.com/d/architecture/1/0/i/p/ExteriorDetailiStock.jpg
3. Picture 2: Internet - http://i2.tinypic.com/otkear.gif
4. http://architecture.about.com/od/disastersandcollapses/a/twintowerfall.htm
5. Federal Building and Fire Safety Investigation of the World Trade Centre Disaster – Design and
construction of structural systems – NIST
6. Wikipedia September 11 - http://en.wikipedia.org/wiki/September_11_attacks
7. Wikipedia World Trade Centre - http://en.wikipedia.org/wiki/World_Trade_Center