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Science, Engineering, and Speculation: The Collapse of the World Trade Center

Science, Engineering, and Speculation: The Collapse of the World Trade Center. Common Questions and Reports. Why didn’t the Towers topple at impact? “The towers…could remain standing if hit by a 707” Les Robertson, WTC Structural Engineer (Chicago Tribune)

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Science, Engineering, and Speculation: The Collapse of the World Trade Center

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  1. Science, Engineering, and Speculation: The Collapse of the World Trade Center

  2. Common Questions and Reports • Why didn’t the Towers topple at impact? • “The towers…could remain standing if hit by a 707”Les Robertson, WTC Structural Engineer (Chicago Tribune) • “24,000 gallons of aviation fluid melted the steel.”Hyman Brown, WTC Construction Manager (BBC Americas) • Why did the buildings fall straight down? • Were the Towers defectively designed? • How should engineering design change? February 20, 2002 - TMS Presentation

  3. Tower Facts • Cost: ~$1 Billion each • 110 Stories • 1,362-1,368 ft (411 m) tall, ~70 ft (21 m) below grade • 208 ft (63.5 m ) on each side • 90 ft x 130 ft (27 m x 40 m) core for ducting & elevators • ~500,000 t each February 20, 2002 - TMS Presentation

  4. WTC Facts • Ground Breaking: August 5, 1966 • Opened: April 4, 1973 • Architect: Minoru Yamasaki • Engineer: John Skilling • Owners: Port Authority of New York & New Jersey • 7 Buildings, 10 Million Square Feet February 20, 2002 - TMS Presentation

  5. WTC Facts • Area of WTC complex • 65,000 m2 (16 acres) • Total rentable office area • 1.1 million m2 (12 million foot2) • Tower floor dimensions • 63 m (207 ft) sides • Tower heights • 110 stories, 417 (N) and 415 (S) m • (1368 and 1362 ft) • Antenna • 110 m (360 ft) • Earth work • 920,000 m3 (1.2 million cubic yards) • Steel weight • 200,000 tons • Concrete • 325,000 m3 (425,000 cubic yards) • Total weight • 500,000 tons

  6. 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

  7. 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 WTC

  8. Design wind speed: 240 km/h (150 m/h) Design impact object: Boeing 707 Structural Loads • Gravity loads • Dead loads • Live loads • Snow loads • Lateral loads • Wind loads • Seismic loads • Special load cases • Impact loads • Blast loads Impact Load Wind Load Gravity Loads Blast Load Earthquake Load WTC

  9. WTC Plan February 20, 2002 - TMS Presentation

  10. Structural System Framed tube construction principle: load bearing external walls stiffened by the floors to form a flexurally and torsionally rigid tube 63.1 m (72’ 2”) 42 m (137 ft) Core area (Steel Frame) 26.5 m (87 ft) Cross-braced floors Outer steel lattice WTC

  11. Lightweight Perimeter Tube Design “One of the most redundant and one of the most resilient skyscrapers” February 20, 2002 - TMS Presentation

  12. Exterior Column System (FEMA 403) • Assembly of the external wall units and floor units • Wall units alternately staggered in one-storey heights

  13. Prefabricated Components Prefabricated column units Floor framing

  14. Floor Joist Schematic Outer Perimeter Box Column Inner Core Floor Joists Angle Clips Viscoelastic Dampers February 20, 2002 - TMS Presentation

  15. Floor Joists (Trusses) (10,000 viscoelastic dampers used in each tower)

  16. Eggcrate Construction • Inner core carried gravity load- 500,000 t (109 lbs) • 44 heavy box section steel columns • Outer core carried wind load- 11,000,000 lbf (4.9x107N) • 59 14 in (36 cm) box section columns on each face, on 39 in (99 cm) centers • 5/3 safety factor…less than 1/3 design load on outside columns on low wind day On a low wind day, outer columns must lose 80% of strength to exceed design February 20, 2002 - TMS Presentation

  17. September 11 Impacts • 8:45 am: North Tower hit by American Flight 11, north face 96th floor. • 9:03 am: South Tower hit by United Flight 175, south face, 80th floor. February 20, 2002 - TMS Presentation

  18. CAUSES OF COLLAPSE • IMPACT • FIRE • PROGRESSIVE FAILURE

  19. Impact Configuration Causes

  20. Enormous Impact Energy • Kinetic energy of 767 (worst case): 350 mph (156 m/s ), 350,000 lbs (150,000 kg ) Ki = 1.35 x 109 ft-lb (1.825 x 109 J) • Very concentrated impact area-forces absorbed by bending, tearing, distortion of steel and concrete • Perimeter tube design redistributed loads to nearby columns Tower design dissipated impact energy February 20, 2002 - TMS Presentation

  21. Towers Don’t Topple • 1.35 billion ft-lb like bullet hitting tree • Concentrated energy penetrated instead of pushing • Inertia: Each Tower had over 2500 times more mass than aircraft. Fire was clearly principle cause of collapse February 20, 2002 - TMS Presentation

  22. Boeing Aircraft Boeing 707-320B Boeing 767-200ER Similar sized aircraft-Design for 707 impact also handled 767 impact February 20, 2002 - TMS Presentation

  23. Impact Velocity Estimated Impact Velocities Causes

  24. Impact Damage to North Tower Floors 94 - 98 Caues

  25. Impact Damage to South Tower Floors 78 - 84 (FEMA 403) Causes

  26. Impact Induced Fires Estimated 38,000 liters (10,000 gallons) of jet fuel in each plane at impact. (FEMA 403) Causes

  27. Characteristics of Plane Impact Conservation of linear momentum v m Impulse = change in momentum d = distance traveled by plane to a stop ≈ 50 m v= velocity of plane ≈ 250 m/s 14 m (46 ft) m= mass ≈ 200 ton F = collision force F = mv / td = ½ mv2 / d≈ 12,500 ton 5.5 m (18 ft) 48.5 m (159.2 ft) (weight of each floor ≈ 2,500 ton) td= duration of collision = 2 d / v ≈ 0.4 s Boeing 767-200ER Ek = kinetic energy Causes

  28. Heat vs. Temp • Heat: Energy, measured in calories or Joules. Extensive property related to temperature. • Temperature: Measure of vibrational and rotational energy stored in atoms, measured in degrees. Intensive property. • Temperature and heat related by density and heat capacity. Small stove burner can’t heat a large pot February 20, 2002 - TMS Presentation

  29. Understanding Flames Jet Burner: 2000W/cm2 Stoichiometric burn temperature Premixed Flame: 1000 W/cm2 Stoichiometric burn temperature Diffuse Flame: 0.1-10 W/cm2 Lower burn temperature;non- stoichiometric February 20, 2002 - TMS Presentation

  30. Tower Fire was Diffuse Flame • Stoichiometric hydrocarbon burn~ 5480o F (~3027o C) • Maximum temperature in air: 1520o F (~825o C) • WTC fire was fuel rich—could not have been 1520o F Temperature in Tower fires about the same as typical office fires February 20, 2002 - TMS Presentation

  31. Steel Weakened and Distorted • Tm of Steel: ~2750o F (1500o C) • Steel could not have melted, but was weakened • 30%-40% of RT strength at 1500o F • Thermal gradients on outside columns create yield level stresses • Cool outside, hot inside induced thermal expansion mismatch between inner and outer faces • Non-uniform heating on long floor joists Combination of softening, loss of structural integrity, distortion induced buckling February 20, 2002 - TMS Presentation

  32. Structural Failure (1) Fire spreads very quickly, with high heat load February 20, 2002 - TMS Presentation

  33. Structural Failure (2) Structural Members begin to deform and sag February 20, 2002 - TMS Presentation

  34. Structural Failure (3) Weakening and deformation cause joist to separate from column February 20, 2002 - TMS Presentation

  35. Structural Failure (4) Weakened, unsupported outer column buckles Falling joist impacts joist below, separates it from column February 20, 2002 - TMS Presentation

  36. Fire softens and distorts beams Structural elements begin to fail, leaving columns unsupported Buckling occurs on a few floors Floors above impact fall onto lower floors Pancake effect begins Structural Failure Sequence Impact changes loading on many beams February 20, 2002 - TMS Presentation

  37. Structure Couldn’t Handle Falling Load • Floors rated for ~1300 t • At least 15 floors above impact site >45,000t • Bazant and Zhou (Northwestern University) estimate overload ratio of 64.5 at time of impact! Buildings fell in approximately 10 seconds - - very close to free fall. February 20, 2002 - TMS Presentation

  38. Towers Fell Straight Down • Bending strain severed backside columns • Tube design: 90% air • Lower floors could not support falling load • Cg would have to sway over 100 ft (30 m ) to topple! February 20, 2002 - TMS Presentation

  39. Towers Were Not Defectively Designed Extraordinary Circumstances: • Heavy structural damage • Fire insulation ripped away in impact • Very high heat load • Very fast and thorough dispersion of fire Towers Survived: • South Tower: 47 minutes • North Tower: 1 hour 44 minutes WTC endured impact and inferno long enough for most people to escape February 20, 2002 - TMS Presentation

  40. “The buildings displayed a tremendous capacity to stand there despite the damage.” -Robert McNamara Scientific American, October 9, 2001

  41. WTC Questions • Why didn’t the Towers topple at impact? • Could the Towers have handled a 707? • Did the steel melt? • What caused the final failure? • Why did the towers fall straight down? • Were the towers defectively designed? • How should engineering design change? February 20, 2002 - TMS Presentation

  42. February 20, 2002 - TMS Presentation

  43. Responsibility of Engineers: Safety • Focus on Survivability: • Better evacuation plans and systems • Better communications systems • Reasonable structural fortification • Improved fire insulation materials Safety does not mean invincibility February 20, 2002 - TMS Presentation

  44. 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 Lessons

  45. Responsibility of Engineers: Redundancy • In Buildings: • Redundant fire and communications systems • Contingency designs for any public artery: • Transportation: subways, bridges • Necessities: food supplies, power grid, gas pipelines Redundancy undermines terrorism February 20, 2002 - TMS Presentation

  46. Responsibility of Engineers: Training & Leadership • Engineers understand failure modes, must train: • Rescue workers • Code writers • Law makers • Maintenance groups • Engineering guidance and leadership is critical to crisis management Better training prevents triage in the courtyard February 20, 2002 - TMS Presentation

  47. Responsibility of Engineers: Thoughtful Communication • Public needs and deserves answers • Complete analysis not always needed • Breadth of understanding required • Speaking a “new language” Analysis eliminates myths, strengthens public confidence February 20, 2002 - TMS Presentation

  48. Moving Forward “Greatness is found when American character and American courage can overcome America’s challenges” February 20, 2002 - TMS Presentation

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