Massachusetts Institute of Technology Cambridge, MA, USA The Collapse of Twin Towers: Causes and Effects Keynote Lecture EFCA 2004 CONFERENCE AND GAM May

Massachusetts Institute of TechnologyCambridge, MA, USA

The Collapse of Twin Towers:

Causes and Effects

The Collapse of Twin Towers:

Causes and Effects

Keynote LectureEFCA 2004 CONFERENCE AND GAM

May 22-May 25, 2004Istanbul, Turkey

Keynote LectureEFCA 2004 CONFERENCE AND GAM

May 22-May 25, 2004Istanbul, Turkey

Dr. Oral BuyukozturkProfessor of Civil and Environmental Engineering

Dr. Oguz Gunes

Dr. Oral BuyukozturkProfessor of Civil and Environmental Engineering

Dr. Oguz Gunes

WTCWTC CausesCauses EffectsEffects LessonsLessons ConclusionConclusion

OUTLINE

• WTC DESCRIPTION

• CAUSES OF COLLAPSE– IMPACT

– FIRE

– PROGRESSIVE FAILURE

• EFFECTS OF COLLAPSE– CASUALTIES

– STRUCTURAL

– TRANSPORTATION

– ECONOMIC/BUSINESS

– COMMUNICATION

– LOGISTICS

• LESSONS LEARNED

• CONCLUSION

WTCWTC


WTC Facts

• Consist of Seven Buildings

• Own zipcode: 10047 & 10048

• Owner

Port Authority of NY and NJ

• Architect:

Minoru Yamasaki & Associates

• Engineer

John Skilling & Leslie Robertson

• Contractor

Tishman Construction Company

• Ground Breaking

August 5, 1966

• Opened

April 4, 1973

WTCWTC


WTC Facts

• Area of WTC complex65,000 m2 (16 acres)

• Total rentable office area1.1 million m2 (12 million foot2)

• Tower floor dimensions63 m (207 ft) sides

• Tower heights110 stories, 417 (N) and 415 (S) m (1368 and 1362 ft)

• Antenna110 m (360 ft)

• Earth work920,000 m3 (1.2 million cubic yards)

• Steel weight200,000 tons

• Concrete325,000 m3 (425,000 cubic yards)

• Total weight500,000 tons

WTCWTC


WTC Facts

• 3 exit stairways in the core of each tower

• 99 elevators and 16 escalators in each tower

• 43,600 windows/tower

• >350 businesses

• 50,000 employees in twin towers

• 150,000 daily visitors

WTCWTC


Innovations in Design of WTC

• A basement like a bathtub

• A building like a tube

• An elevator system like a subway system

• Viscoelastic dampers (10,000 in each tower)

• Outrigger space frame to support antenna

• Wind tunnel study for wind loads

• First commercial building designed to resist

plane impact

WTCWTC


Earthquake Load

Wind Load

Blast Load

ImpactLoad

GravityLoads

Structural Loads

• Gravity loads – Dead loads – Live loads– Snow loads

• Lateral loads– Wind loads– Seismic loads

• Special load cases– Impact loads– Blast loads

Design wind speed: 240 km/h (150 m/h)

Design impact object:Boeing 707

WTCWTC


Structural System

Framed tube construction principle: load bearing external walls stiffened by the floors to form a flexurally and torsionally rigid tube

WTCWTC

63.1 m (72’ 2”)

42 m (137 ft)

26.5 m (87 ft)

Core area(Steel Frame)

Outer steel latticeCross-braced floors


Exterior Column System

WTCWTC

• Assembly of the external wall units and floor units

• Wall units alternately staggered in one-storey heights

(FEMA 403)


Erection of Prefabricated Components

Prefabricated column units Erection of floor framing

WTCWTC

(FEMA 403) (FEMA 403)


Floor Joists (Trusses)

WTCWTC

(FEMA 403)(10,000 viscoelastic dampers used in each tower)


CAUSES OF COLLAPSE

• IMPACT

• FIRE

• PROGRESSIVE FAILURE

CausesCauses


Timeline of Events in 9-11

8:46 a.m. - Plane hits North Tower 9:03 a.m. - Plane hits South Tower9:17 a.m. - FAA shuts down NYC airports9:17 a.m. - Amtrak suspends all service9:17 a.m. - NY DOT shuts down highways9:21 a.m. - Port Authority closes bridges and tunnels9:40 a.m. - FAA grounds all flights9:43 a.m. - Plane hits Pentagon9:59 a.m. - South Tower collapses10:00 a.m. - Armed forces put on high alert10:20 a.m. - NYC Transit shut down10:29 a.m. - North Tower collapses10:30 a.m. - NJ Transit stops rail service to Penn Station10:37 a.m. - Fourth plane crashes in Pennsylvania10:45 a.m. - All PATH operations stop10:50 a.m. - All remaining bridges and tunnels close

CausesCauses


Impact Configuration

CausesCauses

(FEMA 403)


Target Flight AircraftImpact

TimeApproximate

Velocity

km/hr m/s

North Tower

AA-11 Boeing 767-200 8:46 AM 691 192

South Tower

UA-175 Boeing 767-200 9:03 AM 864 240

Impact Velocity

CausesCauses

Estimated Impact Velocities


Conservation of linear momentum

2

12 1

t

tF ma F dt m v v

Impulse = change in momentum

vm

Boeing 767-200ER

14 m (46 ft)

5.5 m (18 ft)

48.5 m (159.2 ft)

max 180 ton (395,000 lb)m 237 m/s (530 mph)cv

F = collision force F = mv / td = ½ mv2 / d ≈ 12,500 ton

d = distance traveled by plane to a stop ≈ 50 m

v = velocity of plane ≈ 250 m/s

m = mass ≈ 200 ton

(weight of each floor ≈ 2,500 ton)

212 6250 MJkE MV= =

Ek = kinetic energy

td = duration of collision = 2 d / v ≈ 0.4 s

Characteristics of Plane Impact

CausesCauses


Impact Damage to North Tower

Floors 94 - 98

CausesCauses

(FEMA 403)


Impact Damage to South Tower

Floors 78 - 84

CausesCauses

(FEMA 403)


Impact Induced Fires

Estimated 38,000 liters (10,000 gallons) of jet fuel in each plane at impact.

CausesCauses

(FEMA 403)


Column Safety Under Service Loads

Yield stress

Working stressY

YF

Safety against yielding

Buckling stress

Working stressb

bF

2

b

R IE R

L A

Buckling stress

Safety against buckling

CausesCauses


Behavior of Materials under Heat

0

0.2

0.4

0.6

0.8

1

0 200 400 600 800 1000

Temperature (C)

Mat

eria

l p

rop

erty

Modulus of Elasticity

Compressive Strength

Concrete

0

0.2

0.4

0.6

0.8

1

0 200 400 600 800 1000

Temperature (C)

Mat

eria

l P

rop

erty Yield

Strength

Modulus of Elasticity

Steel

CausesCauses

2

2

x

TD

t

T

intTTex

Tk ext

Diffusion

Conduction

KmWk

smD

//45

/107.133.11 26

KmWk

smD

//86

/108.34.2 26

Steel

Concrete

Substrate

Fire proofing

6-8 times more fire proofing required for steel


Fire Type and Protection

0

200

400

600

800

1000

0 20 40 60 80 100 120

Time (Minutes)

Tem

per

atu

re (

C) Fire Temperature

Unprotected steel

Protected Steel

CausesCauses

0

200

400

600

800

1000

1200

0 20 40 60 80 100 120

Time (Minutes)

Tem

per

atu

re (

C)

Cellulosic fire

Petrochemical fire

• Jet fuel fire provides more heat than typical office fire.

• Degree of fire proofing significantly affects the time for

evacuation


A Possible Failure Mechanism

CausesCauses


Collapse of the Towers

South tower: 9:59 a.m.

North tower: 10:29 a.m.

CausesCauses

(FEMA 403)Collapse of South Tower


EFFECTS OF COLLAPSE

• CASUALTIES

• STRUCTURAL

• TRANSPORTATION

• ECONOMIC/BUSINESS

• COMMUNICATION

• LOGISTICS

EffectsEffects


Casualties

Estimated 58,000 Present2,830 Died

• All but four people above the crash zones died

• Nearly everyone below the crash zones lived

• Most people died in the North Tower, which was hit first

• Nearly 500 rescue workers died when two towers collapsed

• Victims were 78% male and 22% female


Structural Damage

EffectsEffects

Collapsed or DestroyedOne World Trade Center (North Tower)Two World Trade Center (South Tower)Marriott Hotel (3 WTC)Five World Trade CenterSeven World Trade Center

Partially CollapsedFour World Trade CenterSix World Trade Center

Major DamageOne Liberty PlazaEast River Savings BankN.J. Kalikow and Co. Building and Millennium HotelFederal BuildingN.Y. Telephone BuildingOne World Financial CenterTwo World Financial CenterThree World Financial CenterSt. Nicholas Greek Orthodox Church90 West StreetBankers Trust


Critical Infrastructures Affected by 9-11

EffectsEffects

• Government Operations

• Emergency Services

• Transportation

• Water Supply Systems

• Telecommunication

• Energy Supply

• Banking and Finance

• Gas & Oil Storage and Delivery


U.S. Air Traffic on 9/11 Morning

9:17 a.m. FAA shut down NYC

airports

9:40 a.m. FAA grounded all flights

Transportation

8:48 a.m. – AA-11 hit WTC 1 9:04 a.m. – UA-175 hit WTC 2 9.38 a.m. – AA-77 hit Pentagon10:20 a.m. – UA-93 crashed

First shut down of all air transportation

EffectsEffects

10:00 a.m. 3,181 Aircrafts aloft



10:40 a.m. 820 Aircrafts aloft

10:55 a.m. 680 Aircrafts aloft

11:40 a.m. No commercial aircrafts

aloft


Transportation

• 3 million commute to NYC everyday

• Damage to 1/9 Line (serves 600,000 people daily)

– WTC subway & PATH station destroyed

– 427 m (1400 feet) of tunnel destroyed

– two fan plants destroyed

– Transportation disruptions for up to 14 days

EffectsEffects


Economic/Business

• Big economy of the Big Apple

– Over 8 Million people reside in NYC

– $42.3 Billion budget

– 350,000 City employees in 38 Agencies

• 50,000 office workers at the WTC each day

• Six banks, five large investment firms, and three insurance companies had their headquarters in the towers.

• The Port Authority of New York and New Jersey had its headquarters in the building.

• American Express had three floors in the WTC.

• Major damage to World Financial Center next to the WTC site.

EffectsEffects


• Hardware & Software failure

• Data corruption

• Telecommunication failure

• Site physical facility problems

• Security failure

EffectsEffects

Business Continuity

What companies had planned for before 9-11

What was not anticipated?

• Collapse of the building

• Long-term outage

• Transportation failure

• Logistics failure

• People support issues

• Loss of key personnel

Many companies which did not have alternate sites and trained personnel are gone!


Telecommunications Challenges

• Significant destruction of Verizon Central Office at 140 West Street

– Between 9000 and 14000 businesses left without service (IDC Flash)

– 300,000 voice lines (200k for homes/small businesses)

– 3.5 million data circuits, 2 million that “pass through”

– 1,737 employees (all evacuated)

– Water from broken mains and fire hoses flooded basement vaults, shorting cables that had not been cut by the falling debris

EffectsEffects

– Ducts outside were covered by 30 foot high debris, denying Verizon access for several days

• Power outages


LESSONS LEARNED

• Vulnerability assessment

• Design against vulnerabilities

• Redundancy

• Structural

• Egress paths

• Transportation

• Communication

• Energy

• Business continuity

• Preparedness

LessonsLessons


Vulnerability and Risk Assessment

• Probabilistic risk assessment (PRA) and decision making have been effectively used in

• nuclear engineering, • manufacturing, • seismic loss estimation etc.

• Probabilistic, nonlinear, and coupled evaluation of building vulnerability is needed for identified hazards.

Hazard identification,

prioritization and evaluation

Vulnerability analysis

Risk assessment &

Loss estimation

Optimum mitigation strategy

Decision &

Implementation

LessonsLessons


Design for Fire

• Old: Prescriptive-Based Design– Design based on fire rating of

materials used– Fire rating of material from tables– Compliance with a code specified

value

• New: Performance-Based Design– Evaluate the strength and stiffness for a particular

design fire– Coupled stress-thermal analysis– Specialized design for fire effects– Use of fire retardant materials, advanced coatings

and ceramics

LessonsLessons


Performance Evaluation Under Fire

Coupled structural/fire analysis

Structural loads

Thermal analysis Stress analysis

Fire modeling

Deformations, damage, collapse

Elastic/strength properties

Thermalproperties

Structural Model Geometry

DemandTime: 20 min

Onset of fire

Time: 35 min

Time: 45 min

Weakest link

LessonsLessons


Design for Impact Loading

Engineering problems related to impact loads:

• Modeling of impact

• Assessment of impact damage

• Evaluation of structural safety after impact

• Modeling of potential fire after impact

• Coupled evaluation of structural integrity

and collapse potential

LessonsLessons


Impact Modeling

220 m/sV »

212 3460 MJkE MV= = 3.0 MNcuttingP »

Velocity

Total kinetic energy Fuselage cutting force

MIT Impact and Crashworthiness Laboratory

Exteriorcolumns

Corecolumns

Boeing 767-200

Floor

Floor

Core area

Boeing 767-200

Boeing 767-200Max. takeoff weight: 395,000 lb (180 ton)Max. fuel capacity: 24,000 gal (91,000 liter)Cruise speed: 530 mph (237 m/s)

VV

LessonsLessons


The initial kinetic energy of the plane is dissipated through

• Permanent plastic deformation (crushing)

• Generated Heat

• Fracture and fragmentation

(creating new surfaces)

• Friction

• Residual velocity

• Elastic vibrations

Energy Dissipation During Impact

Core columns

28%

Aircraft25%

Exterior columns3%

Floorstructure

53%

Estimated distribution of energy dissipation

May be used asa design tool

MIT Impact and Crashworthiness Laboratory

LessonsLessons


Redundancy and Progressive FailureRedundancy in column system

Redundancy in floor system

System Redundancy(Global frame)

Local Redundancy(Local joints)

Improved local

redundancy

FEMA403

LessonsLessons


Emergency Egress Strategies

• Elevated passages to neighboring buildings

• Refuge floors/rooms with fire escape elevators

• Perimeter wall rescue vehicles

• Fire resistant escape chutes

• Flying rescue platforms

• Individual fire resistant parachutes

LessonsLessons


Lessons for Business Continuity

• Establish multiple physical sites (redundancy)

• Assign same level workers over two or more physical sites.

• Favor distributed networks

• Cross train high level manages (IT) in each other’s duties.

• Have a clear order of succession.

• Regularly rehearse full disaster drills.

LessonsLessons


Lessons for Logistics

• What can go wrong?• What is the likelihood of a disruption?• What are the consequences if it happens?• How do we recover?

Assess vulnerability to special events

• Keywords: Redundancy and flexibility

• Strategic planning - locations

• Operational security

• Public-private partnerships

• Awareness culture

Reduce probability of disruption

LessonsLessons


Conclusions

ConclusionConclusion

Causes

Effects

Lessons

• Large scale casualties, structural damage, infrastructure disruption, and economic loss

• Worldwide impact on way of human life

• Rigorous engineering paradigms• Preparedness• Effective emergency management and

disaster recovery planning

• Vulnerability to unexpected events in physical and social dimensions

• Progressive and time-sensitive nature of destruction


Lesson Learned

• Expect the unexpected.

• Be prepared!

ConclusionConclusion

Documents

Massachusetts Institute of Technology Cambridge, MA, USA The Collapse of Twin Towers: Causes and Effects Keynote Lecture EFCA 2004 CONFERENCE AND GAM May