Collapse of the World Trade Center
Group 2
History
The World Trade Center towers were completed in 1973 after 7 years of
construction. They stood at 110 storeys high. The two iconic towers were part
of a larger complex of 7 buildings situated in Lower Manhattan, New York
City.
Yamasaki, one of the most prominent architects of the 20th century,
completed the architectural design. The towers were to be the tallest buildings
in the world, larger than the Empire State building.
Structural design:
The structural engineers had to employ a tube-frame structural system to
implement Yamasaki’s ambitious design. This system meant that the towers
were capable of resisting lateral forces in any direction by cantilevering a
vertical tube-like structure from the foundation, this allowed open floor plans
with no interior columns.
The tube-frame structural system consisted of high-strength, load-bearing
perimeter steel columns. These were closely spaced to form a rigid structure
which supported virtually all lateral loads, such as wind loads, and shared the
gravity loads with the core columns.
The perimeter was made up of 236 narrow, hollow steel columns. These
resisted lateral loading from wind and some of the vertical loading. These are
connected together with spandrel plates on each floor that
transmitted shear forces between the columns. There were less of these in
the lower levels to allow for doors
Fig 1. Perimeter and core
The inner core consisted of 47 columns and was 27m by 41m. These reduced
gradually in size in the higher stories. The four corner columns of the core
took a fifth of the total gravity load on the core columns. The core columns
were interconnected by steel beams to support the core floors.
The floor system connected the core to the perimeter columns.
As well as the gravity and lateral loads the buildings were designed to take
the impact load of a plane. The New York Port Authority stated that the impact
load of a Boeing 707 hitting the builing at 600 MPH was also considered in the
design of the towers. This load was considered because in 1945 a B-25
aircraft crashed into the Empire State building. It struck the 78 th and 79th
floors. The building withstood the impact and the resulting fires and was
reoccupied one week after the crash.
Airline Impact
The impact of the passenger jet did not cause the towers to collapse
immediately. This is hardly surprising given that the mass of each tower was
1,000 times the mass of the aircraft and that each tower had been designed to
resist steady wind loads of 30 times the weight of the aircraft.
Furthermore, since there was no significant wind on the morning of
September 11th 2001 the outer perimeter columns were only stressed before
the impact to around 1/3 of their 200 MPa design allowable.
The impact of the aircraft undoubtedly destroyed several columns in the WTC
perimeter wall. However, the number of columns lost on the initial impact was
not large and the loads were shifted to remaining columns in this highly
redundant structure. Structural engineers design structures to survive the loss
of some load bearing members. The fact that the world trade center was so
redundant meant that it survived this loss.
The buildings remained standing after impact. However, upon impact 90,000
L gallons of jet fuel exploded. The resulting fire was the main cause of
collapse.
Collapse of the towers
The collapse of both towers was very similar. The impact of the aircraft
severed several exterior columns and severely damaged others. As the
aircraft continued to penetrate the building it also severed core columns as far
as the centre of the core. It also removed the fireproofing from other columns
and the floor trusses.
The removal of the fireproofing allowed the fire to weaken the steel in the
trusses and cause them to sag and also weaken the columns. The loss of
columns, both core and exterior caused a redistribution of loads.
The sagging of the floor trusses pulled the south perimeter walls inwards. The
perimeter columns became overloaded and collapsed. As the exterior wall
collapsed the loads were transferred to the core through the hat truss. The
tower would probably not have collapsed if the fireproofing had not been
dislodged.
The collapse of WTC2 occurred in a shorter space of time due to the speed
and angle at which the aircraft struck the buildings. Flight AA 11 that struck
WTC1 hit at approximately 443mph at a vertical approach angle of 10.6°
downwards and a roll angle of 25°. Flight UAL 175 that struck WTC2 hit at
approximately 542mph at a vertical approach angle of 6° downwards and a
roll angle of 38°. The collapse of WTC2 took only 56 minutes to collapse
compared to the 102 minutes that it took for WTC1 to collapse. This was due
to the stronger force of impact of flight AA 11.
Conclusions
There were myriad reports commissioned following the collapse of the twin
towers. Most of these reports absolved the structural designers of blame for
the collapse. The designers did not anticipate an explosion of 90,000 L
gallons of jet fuel within the buildings. The towers remained standing for
between 1 and 2 hours. The design life for a Skyscraper on fire if the sprinkler
system fails to operate is 3 hours. The towers stood for less than the design
life, but only because the fire fuel load was so large. It was an extraordinary
fire that could not have been anticipated at the design stage. It has been
suggested that greater structural redundancy could have helped the building
remain standing for longer. However, the twin towers were already greatly
redundant structures.
The key event that precipitated collapse was the stripping off of fireproofing
that was protecting the steel. To prevent this occurring at another building in
the future designers could:
-
Use stronger fireproofing capable of withstanding greater forces
-
Use a different type of fireproofing (spray was used in the twin towers)
Many occupants and emergency workers died on September 11th 2001.
Lessons can also be learned about evacuation from buildings on fire or in
danger of collapse. Many people were trapped on floors above impact. The
stairwells were too damaged to use and no lifts were in use either. Looking
forward there should be:
-
Better exit planning
-
A more fire- and shock-resistive enclosure of fire stairs and elevators
than the currently allowed gypsum board enclosure
-
Shielded stairwells with the use of reinforced concrete
-
Adequate stairwell capacity to allow safe and rapid egress
-
Stairwell and elevator activity should be more robust and remote from
each other
References
 Collapse of the World Trade Center, Wikipedia
 Fire Engineering, Collapse Lessons
(http://www.fireengineering.com/articles/article_display.html?id=163411)
 Final report on the collapse of the world trade center, NIST
 Why Did the World Trade Center Collapse?
(http://www.tms.org/pubs/journals/JOM/0112/Eagar/Eagar-0112.html)
Group 2
Breiffni Fitzgerald
Peter Kehoe
Eamon Sexton
Steven Wyer