Watch 9/11 Timelin
TIMELINE OF SIGNIFICANCE
EVENTS OF SEPTEMBER
11
ON SEPTEMBER 11, 2001, FOUR COMMERCIAL The attacks highlighted vulnerabilities
AIRLINERS WERE HIJACKED BY TERRORISTS. TWO in U.S. national security and led to
PLANES STRUCK THE TWIN TOWERS OF THE significant shifts in military and foreign
WORLD TRADE CENTER AT 8:46 AM AND 9:03 AM, policy. Approximately 3,000 lives were
WHILE A THIRD HIT THE PENTAGON AT 9:37 AM, lost, making it the deadliest terrorist
WITH A FOURTH CRASHING IN PENNSYLVANIA act in history and prompting
AFTER PASSENGER INTERVENTION. widespread global repercussions
COMPARISON TERRORIST
TO THE 1993 ORGANISATION
WTC BOMBING S INVOLVED
THE 1993 BOMBING INVOLVED A The September 11 attacks were
SINGLE TRUCK BOMB, RESULTING IN orchestrated by al-Qaeda, a militant
Islamist organization led by Osama bin
6 DEATHS AND OVER A THOUSAND
Laden. This marked a significant
INJURIES. IN CONTRAST, THE 2001 operational shift in terrorism, moving
ATTACKS DEMONSTRATED A from isolated incidents to coordinated
COORDINATED STRATEGY USING large-scale attacks on civilian targets
COMMERCIAL AIRLINERS AS
WEAPONS, LEADING TO MASS
CASUALTIES AND A COMPLETE
STRUCTURAL COLLAPSE.
FLIGHT DETAILS IMPACT ON
AND HIJACKERS NORTH TOWER
• AMERICAN AIRLINES FLIGHT 11
AND UNITED AIRLINES FLIGHT At 8:46 AM, Flight 11 struck the
175, BOTH DEPARTING FROM North Tower between the 93rd and
BOSTON TO LOS ANGELES, WERE 99th floors at nearly full speed. This
HIJACKED BY FIVE TERRORISTS impact caused massive structural
EACH. THE PLANES WERE damage, instant fires, and chaos,
resulting in significant casualties and
PACKED WITH FUEL FOR THE
trapping many occupants in upper
CROSS-COUNTRY TRIP, floors.
MAXIMIZING POTENTIAL
DESTRUCTION UPON IMPACT
WITH THE WORLD TRADE
CENTER TOWERS
COMPARISON TO RESPONSE OF FIRE
KAMIKAZE TACTICS AND RESCUE
TEAMS
• THE HIJACKINGS DREW
PARALLELS TO KAMIKAZE TACTICS New York's fire and rescue organizations
USED IN WORLD WAR II, WHERE quickly mobilized after the attacks, facing
PILOTS CRASHED PLANES an unprecedented challenge with limited
access to the upper floors. The
DELIBERATELY INTO TARGETS.
unprecedented conditions included loss
THE ATTACKERS UTILIZED SIMILAR of power and functioning elevators,
TACTICS, TURNING COMMERCIAL making it difficult for teams to reach
AIRCRAFT INTO WEAPONS OF victims trapped above the impact zones
MASS DESTRUCTION, AIMED
DIRECTLY AT THE HEART OF THE
U.S. ECONOMY.
EVACUATION RECOVERY OPERATIONS
PROCEDURES AND POST ATTACK
CHALLENGES
EVACUATIONS FROM THE TWIN
In the aftermath, immediate recovery
TOWERS WERE CONDUCTED UNDER operations involved clearing over a
EXTREME STRESS, WITH MANY FACING million tons of debris from the site at a
DENSE SMOKE AND NON-FUNCTIONAL rate of 10,000 tons per day. Recovery
STAIRWAYS. DESPITE IMPROVEMENTS included assessing neighboring
MADE AFTER THE 1993 BOMBING, structures for structural integrity and
MANY PEOPLE STILL STRUGGLED TO logistical challenges for disposal of
ESCAPE EFFECTIVELY DUE TO THE hazardous materials and debris.
CHAOS AND CONFUSION AS THE FIRES
ENGULFED THE BUILDINGS.
SKYSCRAPER Lightweight Modular
Construction
DESIGN
Constructed in the 1960s,
• THE WORLD TRADE CENTER (WTC) the WTC employed
REPRESENTS A SIGNIFICANT lightweight materials and
EVOLUTION IN SKYSCRAPER DESIGN modular methods, enabling a
faster build time. This
BY EMPHASIZING LIGHTWEIGHT
approach also notably
STRUCTURES AND MODULAR reduced construction costs
CONSTRUCTION, ALLOWING FOR without compromising safety.
RAPID ASSEMBLY WHILE STILL
MAINTAINING STRUCTURAL
INTEGRITY.
WIND LOAD Perimeter Tube Design
RESISTANCE
The innovative perimeter tube design
of the WTC employed 244 exterior
• ENGINEERED TO WITHSTAND columns, offering enhanced stability
and the ability to accommodate large
EXTREME WIND FORCES, THE WTC
window spaces. This framework was
WAS DESIGNED TO RESIST WINDS essential for distributing loads
OF UP TO 225 KM/H. THE
STRUCTURE'S LATERAL LOAD
RESISTANCE WAS A CRUCIAL
ASPECT OF ITS DESIGN DUE TO ITS
HEIGHT.
REDUNDANCY AND Core Structure and Functionality
RESILIENCE
The core of each tower housed elevators,
stairwells, and utilities, ensuring smooth
operation and occupant access. This
• THE DESIGN FEATURED SIGNIFICANT
structure was critical for both functionality
REDUNDANCY; IF ONE OR TWO and the load-bearing capacity of the
COLUMNS WERE COMPROMISED, towers.
NEIGHBORING COLUMNS WOULD
TAKE ON ADDITIONAL LOADS,
MAINTAINING THE OVERALL
STRUCTURAL INTEGRITY OF THE
TOWERS
COMPARISON TO
TRADITIONAL Initial Impact Resistance
DESIGNS
The towers initially withstood the
aircraft impact due to their lightweight
design, engineered to resist strong
• UNLIKE TRADITIONAL wind forces. With a mass over 1,000
SKYSCRAPERS THAT RELIED ON times greater than the aircraft, the
HEAVY MASONRY AND CLOSE-SET WTC towers were surprisingly resilient
at the initial impact phase.
COLUMNS, THE WTC EMPLOYED A
SOPHISTICATED LIGHTWEIGHT
STEEL FRAMEWORK, MARKING A
SHIFT IN HIGH-RISE BUILDING
METHODOLOGIES.
AIRCRAFT MASS
Role of Jet Fuel Explosion
VERSUS STRUCTURE
WEIGHT
• THE TOTAL WEIGHT OF EACH
TOWER WAS APPROXIMATELY Upon impact, the aircraft
500,000 TONS, COMPARED TO THE carried 90,000 liters of jet
MASS OF THE AIRCRAFT, WHICH fuel, resulting in a
WAS RELATIVELY MINOR. THE catastrophic explosion with
DESIGN HAD BEEN OPTIMIZED TO severe consequences. This
BEAR STRESSES GREATLY explosion significantly
EXCEEDING THOSE GENERATED BY contributed to the
THE PLANES, SHOWCASING THE destruction of structural
columns and ignited fires that
STRENGTH OF THE OVERALL
prolonged the crisis,
STRUCTURE. ultimately weakening the
towers.
MISCONCEPTION Combustion Science: Flame Types
S ABOUT STEEL
MELTING
There are three main flame types: jet
• THE STEEL IN THE WORLD TRADE
burner, pre-mixed flame, and di-use
CENTER DID NOT MELT DURING THE flame. The WTC’s fire was
FIRES. IT IS COMMONLY
characterized as a di-use flame, which
MISREPRESENTED THAT THE JET-FUEL
typically produces lower heat intensity
FIRE REACHED TEMPERATURES HIGH
compared to jet burners. This flame
ENOUGH TO MELT STEEL, WHICH
type burned fuel-rich, leading to
BEGINS AT AROUND 1,500°C.
HOWEVER, THE ACTUAL MAXIMUM
incomplete combustion and lower
TEMPERATURES REACHED DURING temperatures.
THE FIRE WERE LIKELY AROUND 750–
800°C, INADEQUATE FOR MELTING,
BUT SUFFICIENT FOR REDUCING
STRUCTURAL INTEGRITY.
MAXIMUM FLAME Impact of Fuel-Rich Fires
TEMPERATURES
The WTC fires were highly fuel-rich,
leading to the production of thick black
smoke indicative of incomplete
• THE MAXIMUM FLAME TEMPERATURE combustion. This fuel-rich condition
WHEN BURNING JET FUEL IN AIR IS limited maximum temperatures,
generally keeping them below levels
APPROXIMATELY 1,000°C, WHICH IS
capable of melting steel, but high
INSUFFICIENT TO MELT STEEL. IN
enough to reduce its strength
WELL-MIXED CONDITIONS LIKE A JET significantly
ENGINE, TEMPERATURES CAN EXCEED
3,000°C, BUT SUCH CONDITIONS
WERE NOT PRESENT IN THE WTC
FIRES. THE FUEL-RICH ENVIRONMENT
CONTRIBUTED TO LOWER OVERALL
STEEL STRENGTH Thermal Expansion and
AND TEMPERATURE Structural Integrity
EFFECTS
Non-uniform temperature
• STEEL LOSES STRENGTH AT ELEVATED distribution caused thermal
TEMPERATURES, BEGINNING TO expansion in the WTC structure,
SOFTEN AROUND 425°C AND LOSING leading to distortions and
APPROXIMATELY HALF ITS STRENGTH potential buckling failures. The
AT 650°C. ALTHOUGH THE greatest weakening occurred in
TEMPERATURES IN THE WTC FIRES areas not uniformly exposed to
heat, causing differential stresses
WERE INSUFFICIENT TO MELT THE
that ultimately contributed to the
STEEL, THEY CAUSED SUBSTANTIAL structural collapse.
WEAKENING, AFFECTING THE
STRUCTURAL INTEGRITY AND
RESILIENCE OF THE BUILDING.
ROLE OF Domino Effect of Structural Failures
REDUNDANT
DESIGN
• WHILE THE PERIMETER TUBE
STRUCTURE PROVIDED As several exterior columns were
REDUNDANCY AGAINST lost, loads shifted to adjacent
ISOLATED COLUMN FAILURE, members, prompting a chain
THE IMPACT AND FIRE reaction. This progressive collapse,
likened to falling dominoes, began
CREATED MULTIPLE
with floors giving way under excess
SIMULTANEOUS FAILURES. weight from above, triggering further
CRITICAL CONNECTIONS, failures.
PARTICULARLY ANGLE CLIPS,
DID NOT ADEQUATELY
SUPPORT THE EXCESSIVE
LOADS DURING THE COLLAPSE
SEQUENCE.
CONCLUSION: DESIGN IMPLICATIONS AND
FUTURE CHANGES
• THE WTC COLLAPSE HIGHLIGHTED CRITICAL FLAWS IN BUILDING DESIGN
WHICH LACKED CONSIDERATION FOR EXTREME CONDITIONS. NEW
BUILDING CODES AND DESIGN PRINCIPLES FOCUSING ON ENHANCED
SAFETY, STRUCTURAL REDUNDANCY, AND FIRE RESISTANCE ARE NOW
CRITICAL FOR FUTURE CONSTRUCTION.
LESSONS LEARNED FOR FUTURE ENGINEERING
• THE WTC DISASTER REINFORCED THE NECESSITY FOR ENGINEERS TO
PRIORITIZE SERIOUS QUANTITATIVE ASSESSMENTS OVER SPECULATIVE
DESIGNS. FUTURE SKYSCRAPERS WILL INCORPORATE RIGOROUS SAFETY
STANDARDS AND ENHANCED RESILIENCE TO WITHSTAND EXTREME
SITUATIONS, ENSURING BETTER PROTECTION FOR OCCUPANTS.
