The World Trade Center

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The World Trade Center

• The Design, the Collapse, and Influence on the
  Structural Engineering Profession
  
• January 6, 2004
• Lafayette College
• By Steve Kurtz, Ph.D.
• Dept. of Civil Environmental Engineering

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Main Topics

• The 1960s Groundbreaking Design of WTC 12
• The Causes of Collapse
• Influence on the Future of Structural Engineering

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WTC 1 2 A Groundbreaking Design
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WTC 1 2 A Groundbreaking Design

• In order to appreciate the landmark structural
  design that was seen in the Twin Towers, we must
  first study

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WTC 1 2 A Groundbreaking Design

• In order to appreciate the landmark structural
  design that was seen in the Twin Towers, we must
  first study
  
• High Rise 101

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High Rise 101 The Empire State Building
Floor Plans
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High Rise 101 The Empire State Building
Floor Plans
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High Rise 101 The Empire State Building
Columns Spaced in 30 grid Long
spans avoided for efficiency

Floor Plans
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High Rise 101

• Columns support Beams

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High Rise 101

• Columns support Beams

Beam
Column
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High Rise 101

• Columns support Beams
• Beam stresses increase with their span SQUARED

Beam
Column
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High Rise 101

• Columns support Beams
• Beam stresses increase with their span SQUARED
• This makes long spans EXPENSIVE

Beam
Column
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High Rise 101 The Empire State Building
THEREFORE Many Interior Columns
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High Rise 101 The Empire State Building
THEREFORE Many Interior Columns Maximu
m Structural Economy

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High Rise 101 The Empire State Building
THEREFORE Many Interior Columns Maximu
m Structural Economy NOT popular with Arch
itects

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High Rise 101 The Empire State Building
THEREFORE Many Interior Columns Maximu
m Structural Economy NOT popular with Arch
itects
NOT popular with Developers
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Inside the Empire State Building
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Inside the Empire State Building
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Inside the Empire State Building
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Inside the Empire State Building
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Inside the Empire State Building
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High Rise 101 The Lateral System

• Lateral System the group of members (beams,
  columns, and braces) that resist wind or
  earthquake forces.

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High Rise 101 The Lateral System

• Whereas

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High Rise 101 The Lateral System

• Whereas
• Gravity forces grow linearly, with building height

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High Rise 101 The Lateral System

• Whereas
• Gravity forces grow linearly, with building
  height
  
• Example A 20-story building supports about
  twice as much weight of occupants as a 10-story
  building

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High Rise 101 The Lateral System

• Whereas
• Gravity forces grow linearly, with building
  height
  
• Example A 20-story building supports about
  twice as much weight of occupants as a 10-story
  building
  
• Lateral forces grow exponentially, with building
  height

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High Rise 101 The Lateral System

• Whereas
• Gravity forces grow linearly, with building
  height
  
• Example A 20-story building supports about
  twice as much weight of occupants as a 10-story
  building
  
• Lateral forces grow exponentially, with building
  height
  
• Example A 20-story building may need to sustain
  4 times as much wind force as a 10-story building

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High Rise 101 The Lateral System

• Hence, the amount of structural steel needed

Wind Steel
Tons of Steel
Gravity Steel
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10
15
20
25
30
35
40
45
50
55
60
65
70
75
80
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High Rise 101 The Lateral System

• Hence, the amount of structural steel needed is
  huge for tall buildings due to wind (or
  earthquake)

Wind Steel
Tons of Steel
Total Steel
Gravity Steel
5
10
15
20
25
30
35
40
45
50
55
60
65
70
75
80
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High Rise 101 Lateral System Types

• Simple Braced Frame

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High Rise 101 Lateral System Types

• Simple Braced Frame
• Beam-column connections do not resist moment.
• Connections designed for shear, only.
• Act as pins

Rentable Space
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High Rise 101 Lateral System Types

• Simple Braced Frame
• Beam-column connections do not resist bending.
• Connections designed for shear, only.
• Act as pins
• Bracing provides primary means of resisting
  lateral forces

Bracing in the Core
Rentable Space
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High Rise 101 Lateral System Types

• Simple Braced Frame
• Resists wind by putting columns and braces in
  TENSION or COMPRESSION (no bending)

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High Rise 101 Lateral System Types

• Simple Braced Frame
• Resists wind by putting columns and braces in
  TENSION or COMPRESSION (no bending)

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High Rise 101 Lateral System Types

• Simple Braced Frame
• Resists wind by putting columns and braces in
  TENSION or COMPRESSION (no bending)
  
• EFFICIENT, because no bending

Bracing in the Core
Rentable Space
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High Rise 101 Lateral System Types

• Simple Braced Frame
• Resists wind by putting columns and braces in
  TENSION or COMPRESSION (no bending)
  
• EFFICIENT, because no bending
• But architecture may not always allow.

Bracing in the Core
Rentable Space
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High Rise 101 Lateral System Types

• Rigid Frame

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High Rise 101 Lateral System Types

• Rigid Frame
• No bracing
• Beam-column connections resist bending
• Moments in beams and columns are primary means of
  resisting lateral forces.
  

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High Rise 101 Lateral System Types

• Rigid Frame

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High Rise 101 Lateral System Types

• Rigid Frame
• Resists wind by putting columns and beams in
  BENDING
  
• INEFFICIENT, because of bending
• Often requires huge beams much bigger then
  needed to support the gravity loads.
  

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World Trade Center Lateral System

• Exterior columns spaced at 34 on center

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World Trade Center Lateral System

• Exterior columns spaced at 34 on center
• Rigid connections to very deep beams

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World Trade Center Lateral System

• Exterior columns spaced at 34 on center
• Rigid connections to very deep beams
• A kind of rigid frame,

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World Trade Center Lateral System

• Exterior columns spaced at 34 on center
• Rigid connections to very deep beams
• A kind of rigid frame, but with 1000 times less
  bending deformation

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World Trade Center Lateral System
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World Trade Center Lateral System

• Columns are principally in TENSION or COMPRESSION

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World Trade Center Lateral System

• Columns so close that the entire BUILDING behaves
  as one massive tube-shaped beam.
  
• Engineering Terms The BUILDING has a huge
  Moment of Inertia

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World Trade Center Gravity System
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World Trade Center Gravity System

• Because the entire lateral system was contained
  in the exterior walls, the floor beams did not
  need to resist bending.
  

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World Trade Center Gravity System

• Because the entire lateral system was contained
  in the exterior walls, the floor beams did not
  need to resist bending.
  
• Because the entire lateral system was contained
  in the exterior walls, the floor beams did not
  need to resist bending.
  

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World Trade Center Gravity System

• Because the entire lateral system was contained
  in the exterior walls, the floor beams did not
  need to resist bending.
  
• Small floor beams (on 114 Floors!)

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World Trade Center Gravity System

• Because the entire lateral system was contained
  in the exterior walls, the floor beams did not
  need to resist bending.
  
• Small floor beams (on 114 Floors!)
• Floor bar joists at 134 in each direction

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World Trade Center Gravity System

• Because the entire lateral system was contained
  in the exterior walls, the floor beams did not
  need to resist bending.
  
• Small floor beams (on 114 Floors!)
• Floor bar joists at 134 in each direction

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World Trade Center Gravity System

• Because the entire lateral system was contained
  in the exterior walls, the floor beams did not
  need to resist bending.
  
• Small floor beams (on 114 Floors!)
• Floor bar joists at 134 in each direction
• Very long spans! Nearly 60
• No intermediate columns in office area

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World Trade Center Gravity System

• Because the entire lateral system was contained
  in the exterior walls, the floor beams did not
  need to resist bending.
  
• Small floor beams (on 114 Floors!)
• Floor bar joists at 134 in each direction
• Very long spans! Nearly 60
• No intermediate columns in office area
• Joists spanned between exterior wall and core,
  simply-supported
  

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• WTC Floor Plan

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• SE Corner
• Columns at 34 on-center
• Bar-Joists at 134 on center

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Details Simple Connections
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Details Simple Connections

• A simple connection acts like a pin it resists
  forces but is free to rotate

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Details Simple Connections

• Two types of simple connections in the World
• Simple-Frame
• Seated Connection

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Details Simple Connections

• Two types of simple connections in the World
• Simple-Frame
• Seated Connection

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Details Simple Connections

• Two types of simple connections in the World
• Simple-Frame
• Seated Connection
• These two types of simple connections are
  considered equivalent. Both are
  
• Designed for vertical forces only. No bending
  resistance
  
• Widely used.

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Details Simple Connections

• WTC used Seated Simple Connections for the Bar
  Joists
  

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• Bar Joist Details

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• Exterior Wall during construction

Seat
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• Exterior Wall during construction

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The Causes of the September 11th Collapse
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• Satellite Photograph
• 9/17/02

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Sept. 11th Damage Impact
Structural Model
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Gravity Load Redistribution
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Gravity Load Redistribution
A Computer-based Structural Analysis was
performed Impacted Columns removed from the model
Result Analysis showed that remaining column
s were within limit states. Conclude Extremely
high redundancy was critical in soft
redistribution of forces
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Simple Connections, Revisited
Seated Connection
Simple Frame Connection
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Simple Connections, Revisited
Seated Connection Resists vertical forces
Simple Frame Connection Resists vertical forces
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Simple Connections, Revisited
Seated Connection Resists vertical forces Allo
ws rotation
Simple Frame Connection Resists vertical forces
Allows rotation
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Simple Connections, Revisited
Seated Connection Resists vertical forces Allo
ws rotation
Simple Frame Connection Resists vertical forces
Allows rotation Resists horizontal forces
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Simple Connections, Revisited
Seated Connection Resists vertical forces Allo
ws rotation DOES NOT Resist Horizontal Fo
rces
Simple Frame Connection Resists vertical forces
Allows rotation Resists horizontal forces
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Sept. 11th Damage Fire
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Sept. 11th Damage Fire
High Temperatures affects the stiffness of steel


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Sept. 11th Damage Fire
High Temperatures affects the stiffness of steel

Results Reduced buckling capacity of columns
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Sept. 11th Damage Fire
High Temperatures affects the stiffness of steel
Results Reduced buckling capacity of columns
Huge stretching of beams
Stretching beams (cable action)
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Cable Action in other buildings
WTC 5
Simple Shear Connection (not a seated connection!
)
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Catenary Action in other buildings
WTC 5
Huge Deformations Development of Catenary
action

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Cable Action in WTC Towers?
Seated Connections not strong in tension.
No cable action developed. Failure of building in
itiated by falling slabs and pancaking

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Conclusions and Future Influences on Structural
Engineering
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Conclusions and Future Influences on Structural
Engineering

• High redundancy present in the WTC towers was
  extremely critical in sustaining the initial
  impact by soft redistribution of axial column
  forces to adjacent columns.

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Conclusions and Future Influences on Structural
Engineering

• Although simple shear and seated connections are
  the same for normal design (to resist shear
  forces), seated connections may not be
  sufficiently strong in tension to develop
  cable-like action in extreme events such as 9/11