9 - 1 1 R e s e a r c h

an attempt to uncover the truth about September 11th 2001
mirror of “NERDCITIES/GUARDIAN” site : disclaimer

Table of Contents

Executive Summary
 
1 Introduction
 
 1.1 Purpose and Scope of Study1-1
 1.2 WTC Site1-2
 1.3 Timeline and Event Summary1-4
 1.4 Response of the Engineering Community1-8
      1.4.1 Local Authorities1-8
      1.4.2 SEAoNY Participation1-10
 1.5 Overview of Building Codes and Fire Standards1-15
      1.5.1 Building Codes1-15
      1.5.2 Unusual Building Loads1-16
      1.5.3 Overview of Fire-Structure Interaction1-17
           1.5.3.1 ASTM E119 Standard Fire Test1-18
           1.5.3.2 Performance in Actual Building Fires1-18
 1.6 Report Organization1-20
 1.7 References1-21
 
2 WTC 1 and WTC 2
 
 2.1 Building Descriptions2-1
      2.1.1 General2-1
      2.1.2 Structural Description2-1
      2.1.3 Fire Protection2-11
           2.1.3.1 Passive Protection2-12
           2.1.3.2 Suppression2-12
           2.1.3.3 Smoke Management2-13
           2.1.3.4 Fire Department Features2-13
      2.1.4 Emergency Egress2-13
      2.1.5 Emergency Power2-14
      2.1.6 Management Procedures2-14
 2.2 Building Response2-15
      2.2.1 WTC 12-15
           2.2.1.1 Initial Damage From Aircraft Impact2-15
           2.2.1.2 Fire Development2-21
           2.2.1.3 Evacuation2-24
           2.2.1.4 Structural Response to Fire Loading2-24
           2.2.1.5 Progression of Collapse2-27
      2.2.2 WTC 22-27
           2.2.2.1 Initial Damage From Aircraft Impact2-27
           2.2.2.2 Preliminary Structural Analysis2-32
           2.2.2.3 Fire Development2-34
           2.2.2.4 Evacuation2-35
           2.2.2.5 Initiation of Collapse2-35
           2.2.2.6 Progression of Collapse2-35
      2.2.3 Substructure2-35
 2.3 Observations and Findings2-36
 2.4 Recommendations2-39
 2.5 References2-40
 
3 WTC 3
 
 3.1 Design and Construction Features3-1
      3.1.1 Project Overview3-1
      3.1.2 Building Description3-1
      3.1.3 Structural Description3-2
 3.2 1993 Attack3-5
 3.3 2001 Attacks3-5
      3.3.1 Fire and Evacuation3-5
      3.3.2 Building Response3-6
 3.4 Observations3-8
 3.5 Recommendations3-8
 3.6 References3-8
 
4 WTC 4, 5, and 6
 
 4.1 Design and Construction Features4-1
      4.1.1 Structural Design Features4-1
      4.1.2 Fire Protection Features4-2
 4.2 Building Loads and Performance4-4
      4.2.1 Impact Damage to WTC 54-7
      4.2.2 Fire Damage4-9
 4.3 Analysis of Building Performance4-10
      4.3.1 Steel and Frame Behavior4-10
      4.3.2 WTC 5 - Local Collapse Mechanisms4-15
 4.4 Observations and Findings4-16
 4.5 Recommendations4-21
 4.6 References4-21
 
5 WTC 7
 
 5.1 Introduction5-1
 5.2 Structural Description5-3
      5.2.1 Foundations5-3
      5.2.2 Structural Framing5-4
      5.2.3 Transfer Trusses and Girders5-4
      5.2.4 Connections5-8
 5.3 Fire Protection Systems5-10
      5.3.1 Egress Systems5-10
      5.3.2 Detection and Alarm5-10
      5.3.3 Compartmentalization5-11
      5.3.4 Suppression Systems5-12
      5.3.5 Power5-13
 5.4 Building Loads5-13
 5.5 Timeline of Events Affecting WTC 7 on September 11, 20015-16
      5.5.1 Collapse of WTC 25-16
      5.5.2 Collapse of WTC 15-16
      5.5.3 Fires at WTC 75-20
      5.5.4 Sequence of WTC 7 Collapse5-23
 5.6 Potential Collapse Mechanism5-24
      5.6.1 Probable Collapse Initiation Events5-24
      5.6.2 Probable Collapse Sequence5-30
 5.7 Observations and Findings5-31
 5.8 Recommendations5-32
 5.9 References5-32
 
6 Bankers Trust Building
 
 6.1 Introduction6-1
 6.2 Building Description6-1
 6.3 Structural Damage Description6-4
 6.4 Architectural Damage Description6-6
 6.5 Fireproofing6-10
 6.6 Overall Assessment6-10
 6.7 Analysis6-11
      6.7.1 Key Assumptions6-12
      6.7.2 Model Refinement6-12
      6.7.3 Simulation of Nonlinear Behavior6-13
      6.7.4 Connection Details6-13
      6.7.5 Connection Behavior6-14
 6.8 Observations and Findings6-15
 6.9 Recommendations6-16
 6.10 References6-16
 
7 Peripheral Buildings
 
 7.1 Introduction7-1
 7.2 World Financial Center7-4
      7.2.1 The Winter Garden7-4
      7.2.2 WFC 3, American Express Building7-5
 7.3 Verizon Building7-7
 7.4 30 West Broadway7-13
 7.5 130 Cedar Street7-14
 7.6 90 West Street7-15
 7.7 45 Park Place7-17
 7.8 One Liberty Plaza7-17
 7.9 Observations and Findings7-19
 7.10 Recommendations7-19
 7.11 References7-19
 
8 Observations, Findings, and Recommendations
 
 8.1 Summary of Report Observations, Findings, and Recommendations8-1
 8.2 Chapter Observations, Findings, and Recommendations8-2
      8.2.1 Chapter 1: Building Codes and Fire Standards8-2
      8.2.2 Chapter 2: WTC 1 and WTC 28-2
           8.2.2.1 Observations and Findings8-2
           8.2.2.2 Recommendations8-5
      8.2.3 Chapter 3: WTC 38-6
           8.2.3.1 Observations8-6
           8.2.3.2 Recommendations8-6
      8.2.4 Chapter 4: WTC 4, 5, and 68-6
           8.2.4.1 Observations and Findings8-6
           8.2.4.2 Recommendations8-7
      8.2.5 Chapter 5: WTC 78-7
           8.2.5.1 Observations and Findings8-7
           8.2.5.2 Recommendations8-8
      8.2.6 Chapter 6: Bankers Trust8-8
           8.2.6.1 Observations and Findings8-8
           8.2.6.2 Recommendations8-9
      8.2.7 Chapter 7: Peripheral Buildings8-10
           8.2.7.1 Observations and Findings8-10
           8.2.7.2 Recommendations8-10
      8.2.8 Appendix C: Limited Metallurgical Examination8-10
           8.2.8.1 Observations and Findings8-10
           8.2.8.2 Recommendations8-11
 8.3 Building Performance Study Recommendations for Future Study8-11
      8.3.1 National Response8-11
      8.3.2 Interaction of Structural Elements and Fire8-11
      8.3.3 Interaction of Professions in Design8-12
      8.3.4 Fire Protection Engineering Discipline8-12
      8.3.5 Building Evacuation8-12
      8.3.6 Emergency Personnel8-12
      8.3.7 Education of Stake-holders8-13
      8.3.8 Study Process8-13
      8.3.9 Archival Information8-13
      8.3.10 SEAoNY Structural Engineering Emergency Response Plan8-13
 
A Overview of Fire Protection in Buildings
 
 A.1 IntroductionA-1
 A.2 Fire BehaviorA-1
      A.2.1 Burning Behavior of MaterialsA-1
      A.2.2 Stages of Fire DevelopmentA-4
      A.2.3 Behavior of Fully Developed FiresA-4
 A.3 Structural Response to FireA-5
      A.3.1 Effect of Fire on SteelA-5
           A.3.1.1 IntroductionA-5
           A.3.1.2 Evaluating Fire ResistanceA-6
           A.3.1.3 Response of High-rise, Steel-frame Buildings in Previous FiresA-9
           A.3.1.4 Properties of SteelA-10
           A.3.1.5 Fire Protection Techniques for SteelA-14
           A.3.1.6 Temperature Rise in SteelA-14
           A.3.1.7 Factors Affecting Performance of Steel Structures in FireA-17
      A.3.2 Effect of Fire on ConcreteA-19
           A.3.2.1 GeneralA-19
           A.3.2.2 Properties of Lightweight ConcreteA-19
      A.3.3 Fire and Structural ModelingA-22
 A.4 Life SafetyA-23
      A.4.1 Evacuation ProcessA-24
      A.4.2 AnalysisA-24
 A.5 ReferencesA-26
 
B Structural Steel and Steel Connections
 
 B.1 Structural SteelB-1
 B.2 Mechanical PropertiesB-2
 B.3 WTC 1 and WTC 2 Connection CapacityB-4
      B.3.1 BackgroundB-4
      B.3.2 ObservationsB-5
      B.3.3 ConnectorsB-5
 B.4 Examples of WTC 1 and WTC 2 Connection CapacityB-7
      B.4.1 Bolted Column End PlatesB-7
      B.4.2 Bolted Spandrel ConnectionsB-8
      B.4.3 Floor Truss Seated End Connections at Spandrel Beam and CoreB-9
      B.4.4 WTC 5 Column-tree Shear ConnectionsB-12
 B.5 ReferencesB-14
 
C Limited Metallurgical Examination
 
 C.1 Introduction C-1
 C.2 Sample 1 (From WTC 7)C-1
 C.3 Summary for Sample 1C-5
 C.4 Sample 2 (From WTC 1 or WTC 2)C-5
 C.5 Summary for Sample 2C-13
 C.6 Suggestions for Future ResearchC-13
 
D WTC Steel Data Collection
 
 D.1 IntroductionD-1
 D.2 Project BackgroundD-1
 D.3 MethodsD-2
      D.3.1 Identifying and Saving PiecesD-2
      D.3.2 Documenting PiecesD-5
      D.3.3 Getting CouponsD-8
 D.4 Data CollectedD-10
 D.5 Conclusions and Future WorkD-13
 D.6 ReferencesD-13
 
E Aircraft Information
 
 General SpecificationsE-1
 
F Structural Engineers Emergency Response Plan
 
G Acknowledgments
 
H Acronyms and Abbreviations
 
I Metric Conversions
 
Tables
 
 Table 1.1 Timeline of Major Events.1-10
 Table 2.1 Estimated Openings in Exterior Walls of WTC 1.2-23
 Table 5.1 WTC 7 Tenants5-2
 Table 5.2 WTC 7 Fuel Distribution Systems.5-14
 Table 7.1 DoB/SEAoNY Cooperative Building Damage Assessment.7-3
 Table A.1 Peak Heat Release Rates of Various Materials.A-3
 Table A.2 Fire Duration in Previous Fire Incidents in Steel-frame Buildings.A-10
 Table A.3 Critical Temperatures for Various Types of Steel.A-15
 Table A.4 Test Methods for Spray-applied Fireproofing Materials.A-18
 
Figures
 
Chapter 1
 
 Figure 1-1 WTC site map.1-3
 Figure 1-2 Approximate flight paths of aircraft.1-5
 Figure 1-3 WTC impact locations and resulting fireballs.1-5
 Figure 1-4 Areas of aircraft debris impact.1-6
 Figure 1-5 Fireball erupts on the north face of WTC 2.1-7
 Figure 1-6 View of the north and east faces.1-7
 Figure 1-7 Schematic depiction of areas of collapse debris impact.1-9
 Figure 1-8 Seismic recordings at Palisades.1-11
 Figure 1-9A Satellite photograph of the WTC site taken before the attacks.1-12
 Figure 1-9B Satellite photograph of the WTC site taken after the attacks.1-13
 Figure 1-10 Comparison of high-rise building and aircraft sizes.1-19
 
Chapter 2
 
 Figure 2-1 Representative floor plan (based on 94th and 95th floors of WTC 1).2-2
 Figure 2-2 Representative structural framing plan, upper floors.2-4
 Figure 2-3 Partial elevation of exterior bearing-wall frame.2-6
 Figure 2-4 Base of exterior wall frame.2-7
 Figure 2-5 Structural tube frame behavior.2-7
 Figure 2-6 Floor truss member with detail of end connection.2-8
 Figure 2-7 Erection of exterior wall and floor deck components.2-9
 Figure 2-8 Erection of floor framing during original construction.2-9
 Figure 2-9 Cross-section through typical floor trusses, showing transverse truss.2-9
 Figure 2-10 Outrigger truss system at tower roof.2-10
 Figure 2-11 Location of subterranean structure.2-11
 Figure 2-12 Floor plan of 94th and 95th floors of WTC 1.2-14
 Figure 2-13 Zone of aircraft impact on the north face of WTC 1.2-16
 Figure 2-14 Zone of impact of aircraft on the north face of WTC 1.2-17
 Figure 2-15 Impact damage to the north face of WTC 1.2-18
 Figure 2-16 Impact damage to exterior columns on the north face of WTC 1.2-18
 Figure 2-17 Debris location on the 91st floor of WTC 1.2-19
 Figure 2-18 Landing gear found at the corner of West and Rector Streets.2-19
 Figure 2-19 Redistribution of load after aircraft impact.2-20
 Figure 2-20 Expansion of floor slabs and framing forces out columns.2-25
 Figure 2-21 Buckling of columns initiated by failure of floor joist connections.2-26
 Figure 2-22 Catenary action of floor joists initiates column buckling failures.2-26
 Figure 2-23 Aerial photograph of the WTC site after September 11 attack.2-28
 Figure 2-24 Southeast corner of WTC 2 shortly after aircraft impact.2-28
 Figure 2-25 Zone of impact of aircraft on the south face of WTC 2.2-29
 Figure 2-26 Impact damage to the south face of WTC 2.2-30
 Figure 2-27 Impact damage to exterior columns on the south face of WTC 2.2-30
 Figure 2-28 Conflagration and debris exiting the north wall of WTC 2.2-31
 Figure 2-29 A portion of the fuselage of United Airlines Flight 175.2-32
 Figure 2-30 North face of WTC 2.2-33
 Figure 2-31 Plot of column utilization ratio at the 80th floor of WTC 2.2-34
 Figure 2-32 The top portion of WTC 2 falls to the east, then south.2-36
 Figure 2-33 Damage to substructure slabs caused by collapses.2-36
 
Chapter 3
 
 Figure 3-1 Developed north and west elevations.3-2
 Figure 3-2 Developed south and east elevations.3-2
 Figure 3-3 Typical hotel floor plan.3-3
 Figure 3-4 Typical hotel floor framing plan.3-4
 Figure 3-5 Typical transverse bracing elevation.3-5
 Figure 3-6 Exterior columns from WTC 2 fall on WTC 3.3-6
 Figure 3-7 Partial collapse of WTC 3 after collapse of WTC 2.3-7
 Figure 3-8 Remains of WTC 3 after collapse of WTC 1 and WTC 2.3-8
 
Chapter 4
 
 Figure 4-1 Typical floor plan for WTC 5.4-2
 Figure 4-2 Typical column-tree system (not to scale).4-3
 Figure 4-3 Typical interior bay framing in WTC 5.4-3
 Figure 4-4 Stairway enclosure core locations in WTC 5.4-4
 Figure 4-5 Damage to WTC 4.4-5
 Figure 4-6 Damage to WTC 5.4-5
 Figure 4-7 Approximate locations of damaged floor areas of WTC 5.4-6
 Figure 4-8 Damage to WTC 6.4-8
 Figure 4-9 Impact damage to WTC 6.4-8
 Figure 4-10 Impact damage to the exterior facade of WTC 6.4-9
 Figure 4-11 WTC 5 facade damage.4-10
 Figure 4-12 Impact damage to WTC 5.4-11
 Figure 4-13 WTC 5 on fire.4-13
 Figure 4-14 Deformed beams in WTC 5.4-13
 Figure 4-15 Unburned bookstore in WTC 5.4-14
 Figure 4-16 Looking at undamaged stair tower in WTC 5.4-14
 Figure 4-17 Buckled beam flange and column on the 8th floor of WTC 5.4-15
 Figure 4-18 Internal collapsed area in WTC 5.4-16
 Figure 4-19 Internal collapsed area in WTC 5.4-17
 Figure 4-20 Internal collapsed area in WTC 5.4-18
 Figure 4-21 Closeup of connection failure at column tree.4-18
 Figure 4-22 Connection samples.4-19
 
Chapter 5
 
 Figure 5-1 Foundation plan - WTC 7.5-3
 Figure 5-2 Plan view of typical floor framing.5-4
 Figure 5-3 Elevations of building and core area.5-5
 Figure 5-4 Fifth floor diaphragm plan showing T-sections.5-6
 Figure 5-5 3-D diagram showing relations of trusses and transfer girders.5-6
 Figure 5-6 Seventh floor plan showing locations of transfer trusses and girders.5-7
 Figure 5-7 Truss 1 detail.5-8
 Figure 5-8 Truss 2 detail.5-9
 Figure 5-9 Truss 3 detail.5-10
 Figure 5-10 Cantilever transfer girder detail.5-11
 Figure 5-11 Compartmentalization provided by concrete floor slabs.5-12
 Figure 5-12 Sequence of debris generated by collapses of WTC 2, 1, and 7.5-17
 Figure 5-13 Pedestrian bridge.5-18
 Figure 5-14 Spread of debris around WTC 7.5-18
 Figure 5-15 Debris from the collapse of WTC 1.5-19
 Figure 5-16 Damage to the southeast corner of WTC 7.5-19
 Figure 5-17 Building damage to the southwest corner of WTC 7.5-20
 Figure 5-18 WTC 7, with a large volume of dark smoke rising from it.5-21
 Figure 5-19 Fires on the 11th and 12th floors of the east face of WTC 7.5-22
 Figure 5-20 View of WTC 7 with both mechanical penthouses intact.5-24
 Figure 5-21 East mechanical penthouse collapsed.5-25
 Figure 5-22 East and now west mechanical penthouses gone.5-25
 Figure 5-23 View from the north of the "kink" or fault developing in WTC 7.5-26
 Figure 5-24 Areas of potential transfer truss failure.5-27
 Figure 5-25 Debris cloud from collapse of WTC 7.5-27
 Figure 5-26 Debris generated after collapse of WTC 7.5-28
 
Chapter 6
 
 Figure 6-1 Bankers Trust building with impact damage.6-1
 Figure 6-2 Closeup of area of partial collapse.6-2
 Figure 6-3 Floor plan above the 2nd level (ground floor extension not shown).6-3
 Figure 6-4 Area of initial impact of debris at the 23rd floor. ..... 6-4
 Figure 6-5 Approximate zones of damage.6-5
 Figure 6-6 Moment-connected beams to columns.6-7
 Figure 6-7 Column with the remains of two moment connections.6-7
 Figure 6-8 Failed shear connection of beam web to column web.6-8
 Figure 6-9 Suspended column D-8 at the 15th floor.6-8
 Figure 6-10 Area of collapsed floor slab.6-9
 Figure 6-11 Bankers Trust lobby (note debris has been swept into piles).6-9
 Figure 6-12 Office at north side of the 8th floor.6-10
 Figure 6-13 3-D ANSYS model of flange and shear plate moment connection.6-14
 Figure 6-14 3-D ANSYS model of flange and seat moment connection.6-14
 
Chapter 7
 
 Figure 7-1 NYC DDC/DoB Cooperative Building Damage Assessment Map.7-2
 Figure 7-2 Southeast corner of WFC 3.7-5
 Figure 7-3 View of Winter Garden damage from West Street.7-6
 Figure 7-4 View of Winter Garden damage from West Street.7-6
 Figure 7-5 Interior damage at floor 20 of WFC 3.7-7
 Figure 7-6 Verizon building - damage to east elevation.7-8
 Figure 7-7 Verizon building - damage to east elevation.7-9
 Figure 7-8 Verizon building - damage to east elevation.7-9
 Figure 7-9 Verizon building - column damage on east elevation.7-10
 Figure 7-10 Verizon building - damage to south elevation.7-11
 Figure 7-11 Verizon building - localized damage to south elevation.7-11
 Figure 7-12 Verizon building - detail of damage to south elevation.7-12
 Figure 7-13 30 West Broadway - south facade, 6th floor to roof.7-13
 Figure 7-14 130 Cedar Street and 90 West Street.7-14
 Figure 7-15 Interior of 90 West Street showing typical construction features.7-16
 Figure 7-16 Buckling damage at top of column on floor 8 of 90 West Street.7-17
 Figure 7-17 Buckling damage at top of column on floor 23 of 90 West Street.7-18
 Figure 7-18 One Liberty Plaza - south elevation, lower floors.7-19
 Figure 7-19 One Liberty Plaza - south elevation, upper floors.7-20
 
Appendix A
 
 Figure A-1 Heat release rate for office module.A-2
 Figure A-2 Fire growth rates.A-3
 Figure A-3 Comparison of exposure temperatures in standard tests.A-6
 Figure A-4 Thermal properties of steel at elevated temperatures.A-11
 Figure A-5 Stress-strain curves for structural steel (ASTM A36).A-11
 Figure A-6 Strength of steel at elevated temperatures.A-12
 Figure A-7 Modulus of elasticity at elevated temperatures for structural steels.A-13
 Figure A-8 Reduction of the yield strength of cold-formed light-gauge steel.A-13
 Figure A-9 Steel temperature rise for unprotected steel column.A-16
 Figure A-10 Effect of one inch of spray-applied fire-proofing.A-16
 Figure A-11 The modulus of elasticity strength of different types of concretes.A-20
 Figure A-12 Compressive strength of concrete at elevated temperature.A-21
 Figure A-13 Variation of the volume-specific heat of concretes.A-21
 Figure A-14 Specific flow rate as a function of density.A-25
 Figure A-15 Estimated evacuation times for high-rise buildings.A-26
 
Appendix B
 
 Figure B-1 Exterior column end plates.B-1
 Figure B-2 Tensile stress-strain curves for three ASTM-designation steels.B-3
 Figure B-3 Expanded yield portion of the tensile stress-strain curves.B-3
 Figure B-4 Effect of high strain rate on shape of stress-strain diagram.B-4
 Figure B-5 Column tree showing bolt bearing shear failures.B-6
 Figure B-6 Shear fracture failure of fillet welds.B-7
 Figure B-7 Bent and fractured bolts at a column four-bolt connection.B-8
 Figure B-8 Typical truss top chord connections.B-10
 Figure B-9 (A) Visco-elastic damper angles.B-11
 Figure B-9 (B) failed bearing seat connection.B-11
 Figure B-10 (A) Bracket plate.B-11
 Figure B-10 (B) horizontal plate brace with shear connectors.B-11
 Figure B-11 Shear failure of floor joist connections.B-12
 
Appendix C
 
 Figure C-1 Eroded A36 wide-flange beam.C-1
 Figure C-2 Closeup view of eroded wide-flange beam section.C-2
 Figure C-3 Section from the wide-flange beam shown in Figure C-1.C-2
 Figure C-4 Optical microstructure near the steel surface.C-3
 Figure C-5 Another hot corrosion region near the steel surface.C-3
 Figure C-6 Microstructure of A36 steel.C-4
 Figure C-7 Deep penetration of liquid into the steel.C-4
 Figure C-8 Qualitative chemical analysis.C-5
 Figure C-9 Qualitative chemical analysis.C-6
 Figure C-10 Grain boundary corrosion attack.C-6
 Figure C-11 Region showing the corrosion attack of Sample 2.C-7
 Figure C-12 Higher magnification of the region shown in Figure C-10.C-7
 Figure C-13 Regions where chemical analysis was performed.C-8
 Figure C-14 Gradient of sulfides into the steel from the oxide-metal interface.C-12
 
Appendix D
 
 Figure D-1 Mixed, unsorted steel upon delivery to salvage yard.D-2
 Figure D-2 Torch cutting of very large pieces.D-3
 Figure D-3 Pile of unsorted mixed steel. D-3
 Figure D-4 Engineer inspects and marks promising pieces.D-4
 Figure D-5 Stenciled markings on WTC 2 perimeter column from floors 68-71.D-5
 Figure D-6 Steel pieces marked "SAVE."D-6
 Figure D-7 Engineers measuring and recording steel piece dimensions.D-6
 Figure D-8 Engineer measuring spandrel plate thickness (ts).D-7
 Figure D-9 Measurement of 1/4 inch for web thickness (tw).D-7
 Figure D-10 Measured dimensions of the steel pieces.D-8
 Figure D-11 Burnt steel piece marked for cutting of coupon.D-9
 Figure D-12 Coupon cut from WTC 5 showing web tear-out at bolts.D-9
 Figure D-13 WTC 1 or WTC 2 core column (C-74).D-10
 Figure D-14 WTC 7 W14 column tree with beams attached to two floors.D-11
 Figure D-15 Built-up member with failure along stitch welding.D-11
 Figure D-16 Engineer inspecting fire damage of perimeter column tree.D-12
 Figure D-17 Seat-connected in fire-damaged W14 column from WTC 7.D-12
 Figure D-18 WTC 1 or WTC 2 floor-truss section with seat connection.D-13


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