4A6 CASE STUDY PRESENTAION
World Trade Centre Collapse
NEW YORK SKYLINE, 2000
The Twin Towers
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Two 110-storey towers:
North Tower 1,368ft tall
South Tower 1,362ft tall
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North tower struck between floors
94 & 98 with an impact speed
470mph
South tower struck between floors
78 & 84 with an impact speed
590mph
•
North tower collapsed 56 minutes
after it was struck
South tower collapsed 1 hour 43
minutes after it was struck
What Happened . . .
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The planes struck each tower causing extensive structural damage, including
localised collapse at the several floors directly impacted by the aircraft.
•
As the planes impacted each building jet fuel on board ignited, causing intense
fires throughout the upper portions of the building.
•
The steel columns, softened in the heat, separated from each other and
buckled.
•
Weighted down by debris, furniture, concrete and steel, floors progressively
collapsed.
•
The structural damage sustained by each tower from the impact, combined with
the ensuing fires, resulted in the total collapse of each building.
The Technical Factors Contributing to the
Disaster
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WTC was in effect a vertical steel
tube
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Strength of the structure lay in its
external steel framework of columns
and Vierendeel trusses
•
These supported light steel girder
floors
Progressive Collapse
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Each tower remained standing immediately after it was hit
•
Immediate damage to the structure was not disproportionate in the
circumstances
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On impact the aviation fuel caught fire
•
Intense heat weakened structure in the crash location to such an extent that it
was unable to support the building above
Progressive Collapse
•
•
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A progressive collapse of the whole building followed, the increasing kinetic
energy being sufficient to cause catastrophic damage to propagate downwards
through the previously undamaged lower stories
Under intense heat the structure above failed, allowing the building to fall under
gravity onto the section below
This descending section of building which gained momentum as potential energy
was converted to kinetic energy
Structural Engineering Implications
‘Structural engineering is the science and art of
designing and making, with economy and elegance,
buildings, bridges, frameworks, and other similar
structures so that they can safely resist the forces to
which they may be subjected’
The Institute of Structural Engineers, U.K.
Structural Engineering Implications
•
Clearly WTC towers did not ‘safely
resist’ aircraft impacts
•
Structural engineers readily
recognise that extreme events will
cause structural damage and place
people in jeopardy
•
Damage cannot be avoided entirely
by design
Structural Engineering Implications
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•
•
•
•
•
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The term ‘safely resist’ must be qualified
Safety is not an absolute as perceived by the general public
Virtually always some risk of loss in structures
Perhaps the definition of risk should also be included in the definition of
structural engineering
The scenario for aircraft needs to include not only the impact event but also
subsequent explosion and fire
Aim is to design the structure, its fire resistance, means of escape and in-use
management systems so that lives are saved and damage minimised
Loss of life and damage should not be disproportionate to the cause of collapse
Human and Managerial Factors
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Aircraft impact
Fire deformation
Stored energy
DESIGN
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Air space monitoring
Tannoy broadcast
Security measures
HUMAN
Conclusions
The actual performance of a building depends on . . .
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•
•
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The passage of time
The behaviour of users
The natural elements
Unnatural events
Is it possible for a tall building to . . .
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•
•
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Withstand the impact of an aircraft
Survive a catastrophic fire
Have adequate fire protection
Have adequate modes of evacuation
Lessons to be learnt from the disaster
“We learn more from buildings that
fall down than from buildings that
stand up.”