Group 2
World Trade Centre
Collapse
History
•Construction completed in 1973 after 7 years of
construction
•Two tallest buildings in the world at time of completion
•110 Stories high
•Two Identical Towers
•Architectural design by Yamasaki Architects
Structural Design
•Idea was to have as much uninterrupted
office space as possible
•Tube frame system used where central core
was cantilevered from the foundations
•236 perimeter steel columns connected
together by spandrels
Structural Design
• Hat trusses between the 107th floor and the
roof to support an antenna
• Perimeter columns were also load bearing and
resisted much of the lateral loads
Structural Design
•Core was made up of 47 columns that
reduced gradually in size in the higher
stories
•The four massive corner columns took one
fifth of the total gravity load on the core
•Floor systems consisted of concrete laid on
a metal deck that was supported by a truss
system
September 11th
Flight AA11 struck WTC 1 (North Tower) at
8:46 am
Flight UAL 175 struck WTC 2(South Tower) at
9:03am
Damage similar in both towers but worse in
South tower due to the speed and angle at
which the plane hit
Collapse of the towers
• The North tower collapsed at 10:28am 102
minutes after being hit
• The South tower collapsed at 9:59am 56
minutes after being hit
Collapse of the towers
• Several perimeter columns severed or badly
damaged
• The aircraft also severed core columns as far
as the centre of the core
• The load was redistributed among the other
columns
Collapse of the towers
• The impact removed much of the fireproofing
on the columns and the floor trusses
• 90,000 Gallons of jet fuel exploded
• The fire weakened the steel in the floor
trusses and they began to sag
Collapse of the towers
• The sagging of the floor trusses pulled the south
perimeter wall inwards
• The perimeter columns became overloaded and
collapsed
• The hat truss transferred the loads to the core but this
also became overloaded, bowed and collapsed
• Pancake collapse occurred
Future design
• RC core for better protection of stairwells and
structural safety
• Different types of fireproofing for structural
members
• Emergency stairwells in other parts of the
building