NIST NCSTAR 1-6

Federal Building and Fire Safety Investigation of

the World Trade Center Disaster

Structural Fire Response and

Probable Collapse Sequence of the

World Trade Center Towers

(Chapter 7- Appendix B)

John L. Gross

Therese P. McAllister

National Institute of Standards and Technology • Technology Administration • U.S. Department of Comm

Chapter 7

Structural Response of Major Tower Subsystems to

Aircraft Impact Damage and Fire

7.1 introduction

Prior to conducting the analysis of the global structural response of each tower, major structural subsystems were analyzed to provide insight into their behavior within the WTC global system'. The

three major structural subsystems, the core framing, a single exterior wall, and full tenant floors, were

analyzed separately for their response to impact damage and fire. The hat truss was not analyzed

separately as its structural behavior did not require significant reductions in model complexity for the

global analysis. The component analyses provided a foundation for these large, nonlinear analyses with

highly redundant load paths by determining component behavior and failure modes and enabling a significant reduction in finite element model complexity and size. While the component models used

preliminary estimates of elevated temperatures, the major subsystem models used final estimates of

impact damage and elevated temperatures determined from the aircraft impact analysis and the fire dynamics and thermal analyses.

The capacity of each subsystem to sustain loads for the imposed damage and elevated temperatures was

evaluated. The isolated subsystem models lacked the restraint and load paths to other subsystems found in the global analysis. Even so, the isolated subsystem response was useful for refining the global models

and interpreting subsystem behavior in the global system. For instance, when a column buckled in the

isolated core subsystem model, the only load path available to carry that column's load was the floor

system within the core structure. However, in the global structure, the hat truss at the top of the core would transfer loads to other core columns or the exterior walls, assuming the connections between the core columns and hat truss remained intact.

7.2 core subsystem

The core subsystem in the WTC towers was designed to carry gravity loads, which included the weight of

the structure, equipment, furnishings, and occupants. The core system was not designed to carry lateral wind loads. The core columns were 3 stories in length (36 ft) and were either box columns or wide flange

columns. At the aircraft impact floors, box columns transitioned to wide flange columns as loads and

member sizes decreased with height. Column connections used either welded or bolted splice plates and

were designed for compressive loads only; tensile, shear, or moment load transfer between columns was

not intended. The core floor slab was typically 4.5 in. normal weight concrete that was composite with

the floor beams through shear studs. The core floor was supported by wide flange beams with either a simple shear connection or a moment connection to the core columns.

All information and data related to the design and construction of the WTC towers were obtained from contract drawings

provided to NIST by The Port Authority or New York and New Jersey. Refer to NIST NCSTAR 1-2A for a complete

description of the WTC structural system and an index of all structural drawings.

NIST NCSTAR 1-6. WTC Investigation 185
Chapter 7

It is important to note that in the global structure, the core floors provided a secondary load path for load

redistribution between the core columns. The primary load path for load redistribution was through the

hat truss to adjacent coluinns. If columns were severed or a column or hat truss connection failed under

tensile loads, the floors above the damage area provided the load redistribution between columns.

The isolated core models provided insight into expected behavior for situations where the hat truss was

not able to redistribute loads. It also provided insight into the effects of impact damage and thennal

weakening over time of affected areas of the core structure.

7.2.1 Model and Method of Analysis

Isolated core models extended from Floor 89 to Floor 106 for WTC 1 and from Floor 73 to Floor 106 for WTC 2, shown in Figs. 7-1 (a) and (b), and did not include the hat truss, which extended from Floor 107

to the top of each building. The models included core columns and floor beams and slabs. Floor slabs were modeled as membrane elements with a relatively coarse mesh, which resulted in approximate slab

openings for elevators and mechanical shafts. The meshing did not affect the floor's ability to provide a load path between columns. For the purposes of the isolated core model, only the floor beams with partial

moment connections were included. It was assumed that simple shear connections were not capable of

transferring significant loads between columns. At the base of the models, vertical springs were provided

to represent the stiffness of columns below the model. The core subsystem model included large

deflection, temperature-dependent material properties with plasticity and creep for all structural framing

and plastic buckling behavior for columns.

The core subsystem was first analyzed for stability under gravity loads. The structural impact damage and

gravity loads were applied to the core subsystem for all four damage cases, WTC 1 Cases A and B and

WTC 2 Cases C and D (see Chapter 5). Impact damage was modeled by removing severed core columns

and damaged floor areas. The gravity loads included dead load, superimposed dead load, and service hve

load (25 percent of design live load). The use of 25 percent of design live load as the service live load is discussed in NIST NCSTAR 1-2.

The WTC 1 isolated core model was stable for Case A structural damage and applied gravity loads. The

isolated core model solution did not converge for WTC 1 Case B structural impact damage, which had more severed columns than Case A. The core structure was not able to tolerate the imposed damage

without shedding some load to the exterior walls. For the WTC 2 isolated core model to reach a stable solution under Case C structural damage and gravity loads, horizontal restraints were required in the east and south directions at each floor to represent the lateral restraint provided by the office area floors and

the exterior framed tube. Without the horizontal restraints, the WTC2 core model tilted significantly to the southeast due to the severed columns in that comer of the core. The isolated core model did not converge for WTC 2 Case D structural impact damage, which had more severed columns than Case C.

Each isolated core model was subjected to two temperature conditions as follows:

• WTC 1 (Case A impact damage) was subjected to Case A and Case B temperature histories

• WTC 2 (Case C impact damage) was subjected to Case C and Case D temperature histories

186 NIST NCSTAR 1-6, WTC Investigation
structural Response of Major Tower Subsystems

Case B and Case D impact damage could not be used for the isolated core models as no stable solution was obtained. Instead, for WTC 1, Case A impact damage was used for both Case A and Case B

temperature histories and, for WTC 2, Case C impact damage was used for both Case C and Case D

temperature histories. This approach allowed for comparison of the differences between Case thermal

loads for the same impact damage for each tower.

Temperamre histories were input as nodal temperatures at each node in the strucmral model that was

subject to heating. The temperatures predicted in the structural members depended on the extent of

insulation assumed to be in place and on the material properties and geometry of the structural members.

In each tower, the temperature in the structural members varied through the length and cross section and

changed with time. The temperature at every node in the global models was calculated by interpolation of

temperatures from the thermal analysis, which had a much finer mesh than the global structural models.

A linear temperature gradient was assumed across column cross sections and along the length of

members. To reduce data handling, the continuous temperature time histories were replaced with

piecewise linear time-histories without significant loss of accuracy. Elevated temperatures were applied

to the damaged core structure in 10 min inter\'als, where a temperature state was given for all structural components at a given time and linearly ramped to the next temperature state. Examination of temperature

histories indicated that no significant fluctuations between temperature states occurred for the 10 min

inter\'als selected for analysis. Temperature data were provided at 10 min intervals up to 100 min for WTC 1 and up to 60 min for WTC 2 (see NIST NCSTAR 1-5G for more discussion of the 10 min

intervals).

NIST NCSTAR 1-6, WTC Investigation 187
Chapter 7

WTCl Core .

WTC2 Isolated Core Model

(a) WTC 1

(b) WTC 2

Figure 7-1. Isolated core models.

7.2.2 WTC 1 Core Analysis Results

Figure 7-2 shows the vertical displacement of the WTC 1 isolated core model with impact damage (prior

to applying thermal loading) for Case A, where the colors represent the magnitude of vertical

displacement. The maximum vertical displacement was approximately 5 in. along the north side of the

188 NIST NCSTAR 1-6, WTC Investigation
structural Response of Major Tower Subsystems

core where core columns were severed. Figures 7-3 and 1-4 show the WTC 1 core structural response to Case A and Case B temperature histories, respectively. Case A resulted in column buckling at the northwest comer and the center of the south side between Floors 94 and 97. There was a 21 in. vertical

displacement at the northwest comer and a 12 in. vertical displacement at the center of the south side of

the core. Case B resulted in colimin buckling between Floors 94 and 98 and a 44 in. vertical displacement

at the center of the south side of the core.

For Case A temperatures, the core stmcture was first weakened on the northeast side as fires started in that area after the aircraft impact and then on the south side as the fires spread. For Case B temperatures,

the core stmcture weakened at the center of the north side and then the south side. The core stmctural

responses to these two temperature conditions illustrate the slight difference in core areas that were weakened by the elevated temperatures (northeast versus center of north side). When the results of each

isolated core model were compared to the observed behavior of WTC 1, the weakening on the south side

of the core was best matched by Case A impact damage and Case B temperatures (Case A impact damage

and Case A temperatures showed core weakening at the northwest comer of the core).

Figure 7-2. Vertical displacement of WTC 1 isolated core model with impact damage and

gravity loads.

NISTNCSTAR 1-6, WTC Investigation 189
Chapter 7

(a) North and east sides

(b) Northwest corner (5X magnification)

Figure 7-3. North side vertical displacement of the WTC 1 core model at 100 min for

Case A temperatures.

190 NISTNCSTAR 1-6, WTC Investigation
structural Response of Major Tower Subsystems

(a) South and east sides

(b) South side (10X magnification)

Figure 7-4. South side vertical displacement of the WTC 1 core model at 100 min for

Case B temperatures.

NISTNCSTAR 1-6, WTC Investigation 191
Chapter 7

7.2.3 WTC 2 Analysis Results

Figure 7-5 shows the vertical displacement of the WTC 2 isolated core model with impact damage for

Case C. The maximum vertical displacement was approximately 5.6 in. at the southeast comer of the core where core columns were severed. Figures 7-6 and 7-7 show the response (vertical displacement)

of the WTC 2 core model to Case C and Case D temperature histories, respectively. Case C resulted in a

6.1 in. vertical displacement at the southeast comer. Case D resulted in an 8.1 in. vertical displacement at the southeast comer. Without horizontal restraints, the core would have tilted more toward the southeast

comer, and the vertical displacement would have been larger. No columns buckled in either Case C or Case D.

For both Case C and Case D temperatures, the core stmcture was weakened at the southeast comer and

along the east side of the core. The core structural responses to these two temperature conditions

illustrate the slight difference in core weakening by the larger deflection in the southeast comer for Case

D, which had more column damage. The WTC 2 response for Case C and Case D temperatures was

similar, with a 2 in. increase for Case D. When the results of each isolated core model were compared to the observed behavior of WTC 2, both Cases provided a reasonable match.

Figure 7-5. Vertical displacement of the WTC 2 isolated core model with impact damage

and gravity loads (south and east faces).

192 NISTNCSTAR 1-6, WTC Investigation
structural Response of Major Tower Subsystems

NISTNCSTAR 1-6. WTC Investigation 193
Chapter 7

Figure 7-7. South and east side vertical displacement of the WTC 2 core model at 60 min

for Case D temperatures.

7.3 FULL FLOOR SUBSYSTEM

7.3.1 Model and Method of Analysis

Figure 7-8 shows the full floor model (described in Chapter 4), which included 1) exterior and core columns extending one story below and one story above the floor model, 2) spandrels, 3) floor slab,

4) floor trusses (bridging trusses were retained only in the two-way floor areas), 5) strap anchors, 6) core beams, 7) deck support angles, and 8) break elements to capture failure modes. Both core and exterior

columns were fixed against vertical movement at the lower ends and free to displace at the upper ends.

Exterior columns were restrained from out of plane displacement and all three components of rotation at the both column ends. The core columns were free to displace horizontally. Note that the two-way zones shown in Fig. 7-8 extended only to the comers of the core rather than beyond the comers as shown in Fig. 4-7. The extent of two-way action shown in Fig. 7-8 was believed to better represent actual

structural boundary conditions.

194 NISTNCSTAR 1-6, WTC Investigation
structural Response of Major Tower Subsystems

Figure 7-8. Full floor model.

The full floor subsystem models included large deflection and temperature-dependent material properties

with plasticity for all steel components. The model was used to evaluate structural response under dead

and live loads and elevated temperatures, identify failure modes and associated temperatures and times to

failure, and identify reductions in modeling complexity for global models and analyses. The structural response included thermal expansion of steel and concrete members, temperature-dependent properties of

steel and concrete that affected material stiffness and strength, and bowing or buckling of structural members. Creep was not included in the full floor models, as this analysis feature did not work with the

BEAM 188/189 elements in version 8.0 of ANSYS (the detailed truss model had 3D finite strain elements

that were changed to beam elements in the full floor model). Creep was included for beam elements in ANSYS 8.1, and subsequent analyses of the core and exterior wall subsystems included creep

deformation.

The floor slab was modeled as lightweight concrete across the entire floor (tenant and core floor areas)

with a bilinear stress-strain constitutive model that did not account for cracking or crushing. The concrete

material model used the compressive strength as the yield point, with the same yield strength in both

tension and compression (the reinforcing steel was assumed to provide the tensile capacity in the

composite floor). With this material model, tensile strength of the concrete slab was not represented

accurately, and the actual floor stiffness was overestimated. In the full floor models, bending stresses in the concrete slab that exceeded the actual tensile strength of concrete were found in few locations. This

phenomenon was typically observed when the temperature of the top of the slab was higher than the

temperature at the bottom of the slab, and the concrete slab still deflected down due to large thermal

expansion of the truss. However, when the temperature is higher at the bottom, the simplified truss model

NISTNCSTAR 1-6, WTC Investigation 195
Chapter 7

with this material model showed a very similar behavior to the detailed truss model, and the key failure

modes of the floors were not significantly affected.

Failure modes of the full floor model included truss diagonal buckling and weld failure, exterior and

interior truss seat failure, stud failure, strap anchor weld failure, connection failure between primary and

bridging trusses, and connection failure between long-span and transfer trusses.

Separate floor models were created from the Floor 96 structural model by imposing the different damage

and temperature conditions for WTC 1 Floors 93 to 99 and WTC 2 Floors 79 to 83. Structural

components that were severed due to the aircraft impact were removed from each floor model, based

upon four initial damage cases, WTC 1 Case Aj and Case B, and WTC 2 Case Ci and Case D,. Each full floor model was analyzed for stabiUty under gravity loads consisting of dead load, superimposed dead

load, and service live load (25 percent of design live load), which varied from 55 psf to 85 psf No

column loads were applied.

Each floor model was then subjected to the coiTesponding temperature conditions for each Case in 10 min

intervals, as described previously. Temperatures were assigned at structural component nodes. A

uniform distribution of temperatures through a cross section was assumed for truss members and

spandrels. For columns, a linear gradient in two directions was assumed, and the slabs had temperatures

defined at 5 nodes tlirough the slab depth.

Some members were removed from the model to improve computational performance. They were found

to fail in the early stages of thermal loading and caused convergence problems. Removal of the following

members did not affect the stability and uhimate failure mode of the full floor system under fire:

• Deck support angles and bridging trusses, which buckled due to thermal expansion.

• Shear studs and welds between strap anchors and truss top chords, which failed due to shear

force caused by differential thermal expansion between the floor and the exterior wall.

Final damage Cases A, B, C, and D were completed after the initial set of floor analyses were conducted,

as described above. The fire simulations did not change between initial Cases Ai to D, and final Cases A

to D, which resulted in the same concrete slab temperatures for the initial and final Cases. The truss temperatures differed as a result of the different estimates of dislodged fireproofing. The full floor models were not rerun for Cases A through D as comparisons showed that the structural temperature histories of

the floors were nearly identical for most floors and only slightly different for a few floors. The floor

analysis results for Cases A, to D, were used for Cases A to D in the exterior wall subsystem (Section 7.4)

and global analyses (Chapter 8).

For WTC 1, only Floor 97 showed a significant increase in damage to truss fireproofing on the south side

between Case A, and Case A, where the fireproofing damage over 1 1 trusses increased from just beyond

the core to two thirds of the floor span, as shown in Fig. 7-9. Figure 7-10 shows the temperature

distributions for WTC 1 Floor 97 trusses for Case A, and Case A. Analysis of Floor 97 for Case A

damage and temperature histories showed little difference in the floor behavior. Case Bj and Case B

structural and fireproofing damage were similar for all floors.

196 NISTNCSTAR 1-6, WTC Investigation
structural Response of Major Tower Subsystems

A review of WTC 2 Cases showed that the differences in truss fireproofing damage between Cases Cj and

C and Cases Dj and D (mostly on the east side) would cause little difference in the floor temperatures or

in the structural behavior. The exception was Floor 83 for Cases C, and C, where the fireproofing damage

increased from half to three quarters of the floor area. However, observations from photographs

suggested that the floor was disconnected immediately after the aircraft impact and fireball in this area. Since Floor 83 was assumed to be disconnected at the exterior wall over the area that would be heated in Case C, the analysis was not rerun for this case.

NISTNCSTAR 1-6. WTC Investigation 197
Chapter 7

101 103 106 ine 112 115 lie 121 1M 127 130 133 136 130 145 MS 161 IM tST 156

Case A

357 3&t 34e M& 3«2 33d 336 333 3a0 327 3» 321 3ie 31S 312 3QS aoe 303 301

198

Figure 7-9. Fireproofing damage to WTC 1 Floor 97 for Case Aj and Case A.

NIST NCSTAR 1-6, WTC Investigation
structural Response of Major Tower Subsystems

TMIN-lf .;K

Twa-so:-.:-!!'

950°C ' 950 °C

(a) Case A, top view at 10 min (b) Case A top view at 10 min

AN

950 °n

(c) Case A, top view at 50 min

AN

TEKPEKATURES

South West

950' °Q

(d) Case A top view at 50 min

AN

SEP 1 200<

o°c 950°C

(e) Case A, top view at 100 min

AN

TEMPERATURES

TT-yv(-ei9C.95

South , .

' '-hi Hi West

North

OX ' 950 °C

(f) Case A top view at 100 min

Figure 7-10. WTC 1 Floor 97 comparison of truss temperatures for Case Aj and Case A.

NISTNCSTAR 1-6, WTC Investigation 199
Chapter 7

7.3.2 WTC 1 Analysis Results

Areas of the floors that were subject to the combined effects of dislodged fireproofing and exposure to

fire were found to have two primary failure mechanisms: buckling of diagonal web members and

associated sagging of the floor or disconnection of exterior truss seat connections. When the vertical

support of truss seat connections failed, the floor would hang or sag between the remaining intact supports. The following descriptions of floor responses to structural impact damage and temperature

histories for Cases A„ B„ C„ and Dj present the time, locations, and maximum vertical displacements of

the sagging predicted in the floors.

Case Aj

As shown in Fig. 7-9, the intact fireproofing on the south floor trusses resulted in delayed heating of the

trusses. The WTC 1 floors in the impact zone had upgraded fireproofing thickness of 2.5 in. (modeled as

a thermally equivalent 2.2 in. to account for variability in thickness. Chapter 2). The maximum

temperatures shown for Floor 97 trusses on the south side ranged from 300 °C to 400 °C at 100 min. At

these temperatures, the steel expanded thermally but had only a modest reduction in stiffness and strength

(see Chapter 4).

The maximum vertical displacements of WTC 1 floors are listed in Table 7-1. Floor 95 to Floor 98

showed a significant vertical deflection (sag) in the north office area near the impact damage. The

vertical deflection in the south office area was found to be insignificant for all floors. Many diagonals of

Floor 95 to Floor 98 buckled in the hottest zones of the north office area. Although gusset plates

fractured at several locations, a complete disconnection of the floor from the exterior wall was not

predicted. Slab thermal expansion at 100 min (across the entire floor) ranged between 4 in. and 8 in. Since the floors did not sag except in areas adjacent to the impact zone on the north side, almost all the

exterior columns were pushed out by the floors. The high level of restraint imposed on the slab expansion

by the exterior columns, due to their fixed boundary conditions at column ends, resulted in compressive

forces developing in the slab. These compressive forces would likely have been smaller in the towers, as

the exterior columns would have expanded outward over the four to five floors subject to fires and

provided minimal restraint against thermal expansion of the slab.

Case Bj

Table 7-2 gives the maximum vertical displacement of WTC 1 floors for Case B,, and Figs. 7-1 1 through

7-15 show the vertical displacement contours at time of maximum displacement for Floors 95 to 99. Floors 93 and 94 had no fireproofing damage to the south floor trusses. The maximum deflection of

Floors 95 and 96 occurred just after the aircraft impact on the north side next to the damage area. Floors

97 and 98 maximum deflection occurred at 100 min on the south side. The time and location of

maximum floor deflections illustrate the movement of the fires from the north side just after impact to the

south and the effect of the truss fireproofing. The large deflections on the south side of Floors 97 and 98 were caused by the exterior seat failures that began at 90 min, due to reduction of vertical shear strength

under the elevated temperatures. Figure 7-16 shows the extent of truss seat failures for Floor 97 and

Floor 98, which was a loss of 18 percent to 25 percent of exterior connections for the two floors on the

south face. The average slab expansion across the entire floor ranged from 5 in. to 8.5 in. The interaction

of the floor slab and exterior columns was the same as described for Case A,.

200 NISTNCSTAR 1-6, WTC Investigation
Structural Response of Major Tower Subsystems

Table 7-1. Maximum vertical displacement of WTC 1 floors for Case Aj.

Floor Maximum

Displacement

(in.)

Location of

Displacement on WTC 1 noor

Time after Aircraft

Impact

(min)

93 5.4 North side 30

94 13.5 North side 100

95 30.9 North side 10

96 23.3 North side 10

97 31.5 North side 60

98 26.4 North side 30

99 7.0 North side 50

Table 7-2. Maximum vertical displacement of WTC 1 floors for Case Bj.

Floor Maximum

Displacement

(in.)

Location of

Displacement on WTC 1 noor

Time after Aircraft

Impact

(min)

93 -5.8 South side 100

94 12.7 South side 100

95 29.2 North side 10

96 28.6 North side 10

97 37.4 South side 100

98 49.0 South side 100

99 6.8 North side 100

Note; Negative value represents upward displacement.

NISTNCSTAR 1-6, WTC Investigation 201
Chapter 7

-29.244 -21.21 -13.176 -5.143 2.891 -25.227 -17.193 -9.159 -1.126 6.908 WTCl FL95 - Maximum Damage Case Temperature at 600 sec Displacements magnified 5Xs

Figure 7-1 1 . Vertical deflection of WTC 1 Floor 95 for Case Bj at 1 0 min.

-24.69 -16.771 -8.853 -.934992 6.983 WTCl FL96 Displacements magnified 5Xs

Figure 7-12. Vertical deflection of WTC 1 Floor 96 for Case Bi at 10 min.

202 NISTNCSTAR 1-6, WTC Investigation
Structural Response of Major Tower Subsystems

-37.368 -28.095 -18.823 -9.55 -.277601

-32.731 -23.459 -14.186 -4.914 4.359

V'TZl F197 Displacements masnified 5Xs

Figure 7-1 3. Vertical deflection of WTC 1 Floor 97 for Case Bj at 1 00 min.

-43.113 -31.249 -19.386 -7.522 4.341

v:tci Fi9e Disolacements maenified 5Xs

Figure 7-14. Vertical deflection of WTC 1 Floor 98 for Case Bj at 100 min.

NISTNCSTAR 1-6, WTC Investigation 203
Chapter 7

-6.762 -4.392 -2.022 .347425 2.717 -5.577 -3.207 -.83744 1.532 3.902 WTCl FL99 - Maximum Damage Case Temperature at 6000 sec Displacements magnified 5Xs

Figure 7-15. Vertical deflection of WTC 1 Floor 99 for Case Bi at 100 min.

(a) Floor 97 (b) Floor 98

Figure 7-16. Loss of vertical supports in WTC 1 Floor 97 and Floor 98 for Case Bj.

204 NISTNCSTAR 1-6, WTC Investigation
structural Response of Major Tower Subsystems

7.3.3 WTC 2 Analysis Results

Case Cj

Table 7-3 lists the maximum vertical displacement of WTC 2 floors for Case C„ and Figs. 7-17 through

7-21 show the vertical displacement contours at time of maximum displacement for Floors 79 to 83. Floors 79 to 83 had impact damage at the south side of the east floor area, but Floors 80 and 81 had many

interior truss seats severed from the south exterior wall to the east side of the core perimeter. The

maximum vertical deflection occurred in the southeast floor area near the impact damage for all floors,

with the exception of Floor 82. which had a maximum deflection in the northeast floor area. The

maximum deflection occurred at 60 min for all floors. The location of the maximum deflection was

primarily due to the combined effects of impact damage and elevated temperatures. Floor 82 had a large

span of unsupported floor along the exterior wall resulting from heat-induced truss seat failures, which led

to floor sagging in the northeast comer (see Fig. 7-20).

The west office area of Floors 79 to 83 had vertical deflections ranging from 12 in. to 18 in. at 60 min,

due to the combined effect of hot gases that spread throughout the floors and the 0.75 in. fireproofmg on

the trusses. The average thermal expansion of slabs across the entire floor ranged from 2.5 in. to 5.5 in. at 60 min.

A significant number of truss web diagonals buckled in the east floor area of Floor 81 to Floor 83. Truss

seat failures were not observed on Floor 79 to Floor 81. Figure 7-22 shows the truss seat failures for Floor 82 and Floor 83, which extended over 15 percent to 30 percent of the exterior wall width.

Table 7-3. Maximum vertical c isplacement of WTC 2 f oors for Case Cj.

Floor Max. Displacement

(in)

Location of Displacement

On East Floor

Time After Aircraft

Impact

(min)

79 19.0 South side 60

80 30.1 South side 60

81 31.0 South side 60

82 45.2 North side 60

83 38.9 South side 60

NISTNCSTAR 1-6, WTC Investigation 205
Chapter 7

-19.016 -14.005 -8.994 -3.983 1.028 -16.511 -11.5 -6.489 -1.478 3.533 WTC2 FL7 9 DisDlacements maenified 5Xs

Figure 7-17. Vertical deflection of WTC 2 Floor 79 for Case Ci at 60 min.

-30.062 -20.675 -11.289 -1.902 7.484 -25.368 -15.982 -6.596 2.791 12.177 WTC2 FL80 Displacements magnified 5Xs

Figure 7-18. Vertical deflection of WTC 2 Floor 80 for Case d at 60 min.

206 NISTNCSTAR 1-6, WTC Investigation
structural Response of Major Tower Subsystems

-30.976 -23.517 -16.058 -8.598 -1.139

-27.246 -19.787 -12.328 -4.869 2.59

KTC2 FL81 Displacements magnified 5Xs

Figure 7-19. Vertical deflection of WTC 2 Floor 81 for Case Cj at 60 min.

-39.861 -29.276 -18.692 -8.107 2.478 WTC2 FL82 - Displacements magnified 5Xs

Figure 7-20. Vertical deflection of WTC 2 Floor 82 for Case Ci at 60 min.

NISTNCSTAR 1-6, WTC Investigation 207
Chapter 7

-38.935 -29.625 -20.314 -11.003 -1.693

-34.28 -24.969 -15.659 -6.348 2.963

WTC2 FL83 - . Displacements magnified 5Xs

Figure 7-21 . Vertical deflection of WTC 2 Floor 83 for Case Cj at 60 min.

(a) Floor 82 (b) Floor 83

(3X displacement magnification)

Figure 7-22. Loss of vertical supports in WTC 2 Floor 82 and Floor 83 for Case Cj.

208 NISTNCSTAR 1-6, WTC Investigation
structural Response of Major Tower Subsystems

Case Dj

Table 7-4 lists the maximum vertical displacement of WTC 2 floors for Case Dj, and Figs. 7-23 through

7-27 show the vertical displacement contours at time of maximum displacement for Floors 79 to 83. Floors 79 to 83 had impact damage at the southeast floor area, but Floors 80 and 81 had many interior truss seats severed from the south exterior wall to the east side of the core perimeter. As Case D, had more impact damage near the southeast comer of the core than did Case Ci, Floors 80 and 8 1 had much

greater vertical deflections. At 50 min they were 66 in. and 97 in., respectively, in the southeast floor

area. The maximum temperatures were similar for Case C, and Case D„ but differences in times and

locations of maximum temperatures led to differences in locations of maximum vertical displacements.

Bridging trusses that had been removed in Case C, were replaced in Floors 80 and 81 to provide support

to the primary trusses in the single-span (one-way) floor area during greater vertical deflections.

The slab expansion across the entire floor ranged from 1 in. to 5 in. Gusset plates and bolts at more than

75 percent of all the exterior seats along the east face of Floors 82 and 83 failed. These connection

failures were due to horizontal shear, parallel to the exterior wall, which was caused by differential thermal expansion between the floor framing, the floor slab, and the exterior wall. The truss at Column

357 of Floor 81 was the only one that lost its vertical support at the exterior seat among all the floors. This truss w-alked off the truss seat.

Several columns along the east and west sides of Floor 80 were pulled inward by the floor sagging in the

southeast area. (The inward pull on the west face was due to the lack of horizontal restraint for the core columns in the floor model; in the global model, the inward pull would be resisted by the core. The west

face inward pull was not applied in the global model). Floor 79 and Floor 81 showed similar behaviors to Floor 80 in terms of column horizontal reaction forces. Many columns of the west face of Floor 82 were

pulled inward, while reaction forces at many columns of the east face were close to zero. As described

above, gusset plates and seat bolts failed at a number trusses on the east face of Floor 82. Because

columns at these locations were not supported in the horizontal direction by the floor, the reaction force

became zero at these columns.

Table 7-4. Maximum vertical c isplacement of WTC 2 floors for Case Di. Floor Maximum

Displacement

(in)

Location of

Displacement on East

Floor

Time After Aircraft

Impact

(min)

79 35.8 South side 60

80 65.6 South side 40

81 96.7 South side 50

82 49.4 South side 60

83 44.6 South side 60

NISTNCSTAR 1-6, WTC Investigation 209
Chapter 7

-30.805 -20.758 -10.712 -.664742 9.382 WTC2 FL7 9 Displacements magnified 5Xs

Figure 7-23. Vertical deflection of WTC 2 Floor 79 for Case Di.

WTC2 FL80 Displacements magnified 5Xs

Figure 7-24. Vertical deflection of WTC 2 Floor 80 for Case Dj.

210 NISTNCSTAR 1-6, WTC Investigation
Structural Response of Major Tower Subsystems

-85.596 -63.431 -41.266 -19.101 3.064 KTC2 FL81 Displacements magnified 5Xs

Figure 7-25. Vertical deflection of WTC 2 Floor 81 for Case Dj.

-43.524 -31.693 -19.861 -8.03 3.802 WTC2 FL82 Displacements magnified 5Xs

Figure 7-26. Vertical deflection of WTC 2 Floor 82 for Case Dj.

NISTNCSTAR 1-6, WTC Investigation 211
Chapter 7

WTC2 FL83 Displacements maenified 5Xs

Figure 7-27. Vertical deflection of WTC 2 Floor 83 for Case D,.

7.4 EXTERIOR WALL SUBSYSTEM

The primary function of the exterior walls of the WTC towers was to resist wind loads, but they also

carried approximately 50 percent of the building gravity loads. For the WTC tower response to impact

damage and ensuing fires, the performance of the exterior wall under gravity loads was of interest and is addressed in this section. NIST NCSTAR 1-2 addresses the performance of the exterior wall under

gravity and wind loads.

The exterior wall was assembled with panels that were 3 stories high and 3 columns wide with spandrel

beams at each floor level. Adjacent panels were staggered by one floor to avoid alignment of the bolted

panel connections across a given floor level. Panel geometry was generally uniform at the upper stories

of the towers, but the steel grade and plate thickness varied within and between panels, depending upon a panel's location in the tower face. Examination of structural drawings showed that the plates at the

impact floors had the same yield strength. Therefore, in the translation process, all plates in the same

column cross section were considered to be of the same material. Panel connections were designed for

compressive and tensile loads.

Floor trusses were attached to every other column, with the same set of columns loaded at every floor. The exterior columns and deep spandrel beams resulted in a rigid frame that was efficient at redistributing

loads in the plane of the wall. The columns redistributed their loads within three to four floors. The

floors and spandrels provided lateral support to the exterior columns. Loss of lateral support through

failed floor connections increases the possibility of column instability (buckling), depending upon a column's load and temperatures. The hat truss was connected to each exterior wall at Floor 108 with four

212 NIST NCSTAR 1-6, WTC Investigation
Structural Response of Major Tower Subsystems

truss members extending from the core perimeter. The hat truss provided a load path between the core and exterior walls. As discussed previously, the hat truss was the primary load path between the core and

exterior walls, and the floors provided a secondary load path.

The analysis of a single exterior face provided insight into the conditions that would result in the inward

bowing of the south wall of WTC 1 and the east wall of WTC 2 observed in photographs (see Chapter 6). Conditions examined included pull-in forces resulting from sagging floors, disconnected floors resulting

from truss seat failure, additional vertical loads simulating load transfer to the exterior wall, and elevated

temperatures.

7.4.1 Finite Element Model and Methods of Analysis

The exterior wall models extended over 1 8 floors for the full width of a single face and were centered

around the areas of impact and fire damage. The south face of WTC 1 extended from Floor 89 to Floor

106. and the east face of WTC 2 extended from Floor 73 to Floor 90, as shown in Fig. 7-28. The exterior

panel that was severed during the aircraft impact and found south of the tower was removed from the

south face of WTC 1 . No structural damage to the panels was observed on the east wall of WTC 2. The

same boundary conditions were applied for both exterior wall models, shown in Fig. 7-29. Springs were

included at the base of the global models to represent the response of the exterior wall below the model.

The exterior wall models included temperature-dependent plasticity, creep, and plastic buckhng behavior.

The exterior wall models were first analyzed for gravity loads with aircraft impact damage. The loads in the columns right after aircraft impact included a set of axial forces at the top of the isolated wall model

that accounted for the columns above the top of the model. These loads were taken from an initial global

model that did not include creep or column buckling. This global model is further described in NIST

NCSTAR 1-6C. Floor gravity loads were also applied at each colunm with a floor connection.

The exterior wall models were then subjected to two temperature conditions for each tower: Case A and

Case B for WTC 1 and Case C and Case D for WTC 2. Elevated temperatures were applied to the wall

structure in 10 min interv als, as described previously. At the beginning of each 10 min interval, floor

connections were either (1 ) disconnected where observed in photographs and videos or computed in the

full floor analyses, or (2) loaded with an inward force where inward bowing was observed during that time inter\'al. At later stages of the WTC 2 analysis, where additional floor disconnection occurred, the

inward pull at that connection was removed.

Temperatures of strucmral components were based upon the combined effects of member size,

fireproofing damage, and fire size and duration. For the exterior columns and spandrels, the interior face was heated directly by fires through radiation and convection and the adjacent faces were heated through

conduction and cooled by convection. Elevated temperatures caused thermal expansion of heated

columns and modified the stresses in affected and adjacent structural members. Elevated temperatures

above 400 °C to 500 °C resulted in a reduction of load-carrying capacity and an increase in plastic and

creep deformations.

Inward pulling forces were estimated through a trial and error procedure. In each trial, a magnitude of

inward pull force was assumed and the model results were checked. The magnitude was kept constant

until the end of the analyses unless a floor connection became disconnected (see Section 7.3), at which

point the inward pulling force was set to zero. The inward bowing displacements were compared to the

NIST NCSTAR 1-6. WTC Investigation 213
Chapter 7

displacements measured from photographs at the same time points. The wall response was significantly

affected by accumulated plastic and creep strains, which were themselves functions of temperature and

inward pull over time.

COL359 COL301

COL359 COL301

' '— FL106

—FL99 —FL97 —FL95

Base -J

springs —FL93

\. FL91

^ ' " —FL89

I

Base

springs

— FIJ89 — FL87 — FL85 — E=L83 — FL81 — FL79 — FL77 — FL75

-r- FL73

Model of WTC 1 south wall Model of WTC 2 east wall

Figure 7-28. Isolated exterior wall segments from WTC 1 and WTC 2.

Apply axial

forces of the

above colurms

as nodal forces

to the top of the

isolated region

I J,

ANSYS

DEC 28 2004

Restrain only

UYatcolurm

locations at

floor levels

Full restraint at

\the base of the vertical springs

WTCl Isolated South Wall

Figure 7-29. Boundary conditions applied on the isolated exterior wall segment.

214 NISTNCSTAR 1-6, WTC Investigation
structural Response of Major Tower Subsystems

7.4.2 WTC 1 Analysis Results

The magnitude and distribution of inward pull forces on the south wall, which resulted in inward bowing

similar to that observed in photographs, were estimated from the WTC 1 exterior wall subsystem

analyses. The final magnitude thus obtained was used in the WTC 1 global analysis.

Initial trials with Case A and Case B damage and temperatures limited pull-in forces to areas with floor sagging sufficient in the full floor models to cause pull-in forces. However, such limited areas of pull-in

forces did not produce results that were consistent with the observed inward bowing. This was primarily

due to the lack of fireproofing damage to the south exterior wall and floor truss on the south side in Case

A impact damage estimates. With the thermally equivalent 2.2 in. of fireproofing intact on the south

trusses, these trusses did not heat appreciably, and the floors did not sag.

Case B had elevated temperatures for the south floor trusses and exterior columns where fireproofing was damaged betvv'een Floors 94 and 98. A second set of trials applied a pull-in force uniformly across Floors 95 to 99, except where the floor connections had failed. This extent of pull-in forces from floor sagging was greater than that shown by the full floor analyses, but produced a better estimate of the inward bowing as evidenced from the photographs. The smaller extent of floor sagging in Floors 95 to 99

that was predicted by the full floor analyses was likely due to the conservative estimates of fireproofing

damage. This assumption produced a lower bound on the bare steel surface area, thereby making it more

difficult to heat the steel to the point of failure. Greater fireproofing damage from structural accelerations

caused by the aircraft impact and subsequent vibrations as well as possible damage to the concrete slab from high thermal gradients near the slab surface may have contributed to the more extensive inward

bowing of the exterior wall that was observed.

In one trial, the magnitude of the pull-in force was increased over time until the the wall became unstable

at 90 min. When the magnitude of the pull-in force reached 9.37 kip per column, the analysis stopped due

to non-convergence. At the end of analysis, the maximum inward bowing was 24.7 in.

In another trial, the effect of thermal loading in combination with pull-in forces was examined. Pull-in

forces were applied at 80 min, since temperatures on the south side began to increase around that time,

and the thermal loading was continued to 100 min. In this analysis, the magnitude of the pull-in force was set to 6 kip per column so that the wall would not become unstable as a result of pull-in forces alone.

Figure 7-30 shows the locations of floor disconnections and pull-in forces. After applying a 6 kip pull-in

force per column from 80 min to 100 min, the maximum inward bowing increased from 12.2 in. to 31.3 in., as shown in Figs. 7-31 and 7-32. This analysis demonstrated that the thermal softening

increased existing inward bowing. Analysis results also showed that, at 100 min. Columns 320 to 346

had reached their load capacity for their plastically buckled shape and steel temperatures and were

shedding their loads to adjacent colurrms.

The maximum inward bowing of 31 in. was smaller than the observed maximum bowing of 55 in., and

the bowed wall was still stable in the analysis at 100 min. The magnitude of pull-in forces was expected

to be less than 6 kip in the global analysis with the addition of gravity loads from the core subsystem as it also weakened; therefore, pull-in forces of 4 kip to 5 kip were used in the global model analyses.

NIST NCSTAR 1-6, WTC Investigation 2 1

5
Chapter 7

100

99

98

a>

3

g 96

95

94

93

<XXXXXXXXXXXXXX)0(XOCXXXXXD00C)0000C)CK^^

000C)0C)0CXX)0CO00®C)00®®®®®®®00<X)(X)C^^

OCOOOOOOOOC)CO(XXXXX)OOOOOCKX)»»«CK^^

<X)OOOOOOOC>COOOOO<X)OOOOOOCOO»»»a^^ • Disconnection

O Inward Pull • Impact Damage

360 350 340 330

Colum ID

320 310 300

(a) Between 80 min and 90 min

100

99

98

0) f

97 3

§ 96

95

94

93

OC)OOC)OOCXXXXXXXXXX)OCOC)OCXDOOCK^^

COOCKDOOCOOO®®®®®®®®®®®C)OCXXXXXXXXXXXXXX)aD^^

OOOOCOOCOCKXXXXXHDOOCOOCXDOOO«»^^

OOOCOOOOOOCXDOOOOCOOOCOOOOOO^^ • Disconnection

O Inward Pull • Impact Damage

360 350 340 330 Colum ID320 310 300

(b) Between 90 min and 100 min

Figure 7-30. Locations of WTC 1 disconnections and pull-in forces over five floors for

Case B.

216 NISTNCSTAR 1-6, WTC Investigation
structural Response of Major Tower Subsystems

Figure 7-31. Inward displacement of the WTC 1 south wall at 80 min of the Case B

temperatures with floor disconnections and 6 kip pull-in forces over five floors.

NISTNCSTAR 1-6. WTC Investigation 217
Chapter 7

Figure 7-32. Inward displacement of the WTC 1 south wall at 100 min of the Case B

temperatures with floor disconnections and 6 kip pull-in forces over five floors.

7.4.3 WTC 2 Analysis Results

The magnitude and distribution of inward pull forces on the east wall, which resulted in inward bowing

similar to that observed in photographs at approximately 20 min and 50 min after the aircraft impact, were

estimated with these analyses. The final estimated magnitude and distribution was used in the WTC 2

global analysis.

Initial trials with Case C and Case D damage and elevated temperatures limited pull-in forces to areas with floor sagging sufficient in the full floor models to cause pull-in forces. However, such limited areas of pull-in forces did not produce results that were compatible with the observed inward bowing. While

damage to truss and exterior column fireproofing was similar for Cases C and D, the fire spread and

growth was not the same and produced different temperature histories for structural elements. Case C full floor models sagged and had pull-in forces at the north side of the east wall, whereas Case D had floor

sagging and pull-in forces at the south side of the east wall. Review of full floor model resuhs showed

that Case D temperature histories more closely matched the observed inward bowing of the east face. Case D was used for the global analyses.

218 NISTNCSTAR 1-6, WTC Investigation
structural Response of Major Tower Subsystems

A second set of trials with Case D elevated temperatures applied a pull-in force uniformly across Floors 79 to 83 of the east wall, except where the floor connections had failed. In two separate analyses,

pull-in forces of 0.5 kip and 5 kip were held constant as the temperature histories were applied until the

analysis failed to converge. The analysis with the 0.5 kip pull-in force failed to converge at 32 min. As

shown in Fig. 7-33, the wall bowed primarily outward (positive displacement direction is inward) as the

pull-in force was insufficient to cause inward bowing. The analysis with the 5.0 kip pull-in force failed to converge at 18 min. The inward "bowing of the exterior wall had reached 31 in., as shown in Fig. 7-34.

This \'alue is about three times larger than the 10 in. displacement measured in photographs at this time,

indicating that the assumed value of pull-in force was too large. Based on these two analyses, it was

concluded that the magnitude of the pull-in force for a uniform distribution was bounded by 0.5 kip and

5.0 kip. This range for the pull-in force is of the same order of magnitude as the tension calculated from

the detailed truss model (see Chapter 4).

The out-of-plane displacements shown in Fig. 7-33 at 20 min were inward on the south side and outward

in the middle and north section of the wall. This difference in behavior was due to the combined effects of column temperatures and column loads across the east wall. Temperatures were higher at the middle

and north half of the wall compared to the area near the southeast comer. The primary reasons for the

outward bowing on the north side of the east wall are as follows: (1) the higher temperatures in the north

side of the wall resulted in restrained thermal expansion and larger column loads; (2) the higher

temperatures of the inside face of the columns, relative to the outside, caused higher plastic and creep

strains and resulted in differential shortening of the inside relative to the outside; and (3) the plastic

softening and creep of the inside caused an outward shift in the neutral axis, and a resulting outward bow

of the columns.

This observation formed the basis for the next set of trials, where a step function was used to represent the

distribution of pull-in forces along the east wall. In each trial, the magnitude of pull-in force for each half

of the wall was assigned independently, with a higher magnitude on the north half of the east wall.

Two additional trials were analyzed. In the first trial, the magnitude of the pull-in forces was set to

1 .0 kip and 4.0 kip for the south and north halves of the east wall, respectively. Figure 7-35 shows the

out-of-plane displacements at 20 min and 50 min. As can be seen, the maximum inward bow calculated

at 20 min was 7.5 in. and located near the middle of Floor 81. This agreed well with the measured

displacements, which showed a maximum inward displacement of 10 in. near the middle of Floor 81.

The inward bowing started to decrease with time after 20 min and at around 40 min changed to outward

bowmg. The bowing at 50 min was mostly outward and did not agree with the measured displacements at

this time. This indicated that the assumed magnitudes of the apphed pull-in force were smaller than the

actual pull-in force on the east wall.

In the second trial, the magnitude of the pull-in force was increased to 1.5 kip and 5.0 kip on the south

and north portions of the east wall, respectively. Temperature histories were applied up to 50 min, at which point the analysis failed to converge. Figure 7-36 shows the magnitude of inward bowing at 20 min and at 50 min. The maximum inward bowing calculated at 20 min was 9.5 in. near the middle of

Floor 81. This observation agreed well with the 10 in. measured displacements at that time. The inward

bowmg continued to increase with increasing time and reached a maximum of 37 in. at 50 min! As seen

in Fig. 7-30, the location of the maximum displacements agreed well with the observations, but the

calculated magnitude of 37 in. was twice as large as the measured inward displacement of 20 in.

NISTNCSTAR 1-6, WTC Investigation 219
Chapter 7

This indicated that the magnitude of the applied pull-in force was close to the two sets of values assumed

for the step function distribution, 1.0 kip to 1.5 kip and 4.0 kip to 5.0 kip on the south and north portions

of the east wall, respectively. Considering the possible increase in column loads after impact for Case D

conditions, a pull-in force of 1.0 kip on the south half and 4.0 kip on the north half of the east wall was

selected as the initial estimate for the WTC 2 global model analysis.

NODAL SOLUTION

STEP=5

SUB =230 T1ME=47.39

UY (AVG) RSYS=0

DMX =25.824 SMN =-25.709

SMX =3.703

AIMSYS

MAR 3 2005 1 15:37:52 I

-25.709 -19.173 -12.637 -6.101 .434655 -22.441 -15.905 -9.369 -2.833 3.703 WTC2 Isolated Wall Model Lateral Pull at 1896s

Figure 7-33. Out-of-plane displacements of the WTC 2 east wall calculated with 0.5 kip

pull-in force with uniform magnitude distribution at 32 min.

220 NISTNCSTAR 1-6, WTC Investigation
structural Response of Major Tower Subsystems

NODAL SOLUTION

STEP=3

SUB =410 riME=25.725

UY (AVG) RSYS=0

DMX =31.249

SMN =-1.365

SMX -31.075

AIMSYSI

MAR 3 2005 15:14:31

wtm.

v====WKKmammm'K^mmm^mi^. ,.. J ii. i -1.365 5.844 13.053 20.262 27.47

2.24 9.448 16.657 23.866 31.075 WTC2 Isolated Wall Model Lateral Pull at 1069s

Figure 7-34. Out-of-plane displacements of the WTC 2 east wall calculated with 5.0 kip

pull-in force with uniform magnitude distribution at 18 min.

NISTNCSTAR 1-6, WTC Investigation 221
Chapter 7

NODAL SOLUTION

STEP=3

SUB =249 TIME=30

UY (AVG) RSYS=0

DMX =7.898

SMN =-2.066

SMX =7.4 65

-2.066 .051635 2.17 4.2£ -1.007 1.111 3.229

WTC2 Isolated Wall Model Lateral Pull at 1200s

ANSYS

FEB 22 2005 14:36:01

6.406

5.347 7.465

At 20 min

NODAL SOLUTION

STEP=6

SUB =214 TIME=75

UY (AVe) RSYS=0

DMX =15.425

SMN =-15.195

SMX =5.602 •

-15.195 -10.573

-12.884

ANSYS

FEB 22 2005 14:56:12

3.291

.980269 5.602 WTC2 Isolated Wall Model Lateral Pull at 3000s

At 50 min

Figure 7-35. Out-of-plane displacements of east wall calculated with pull-in force of 1.0

kip on the south half and 4.0 kip on the north half of the WTC 2 east wall.

222 NISTNCSTAR 1-6, WTC Investigation
NODAL SOLUTION

STEP=3

SUB =292 TIME=30

UY (AVG) RSYS=0

D^Q( =9.838

SKN =-1.402

SMX =9.483

Structural Response of Major Tower Subsystems

-1.402 1.017 3.436 5.855 -.192375 2.226 4.645 7.064 HTC2 - East Wall under Severe Case Damage and Temperatures

9.483

CAEAI

FEB 24 2005 17:23:44

-1.299 7.403 15.105 24.807 33.509

3.052 11.754 20.456 29.158 37.86

WTC2 - East Wall under Severe Case Damage and Temperatures

Figure 7-36. Out-of-plane displacements of east wall of WTC 2 calculated with pull-in

forces of 1.5 kip on the south half and 5.0 kip on the north half.

NISTNCSTAR 1-6. WTC Investigation 223
Chapter 7

7.5 SUMMARY OF SUBSYSTEM ANALYSES

The structural response of the isolated major subsystems (core, full floor, and exterior wall) to aircraft impact damage and fire are summarized here. These responses provided insight for the global model and

resuhs analysis.

Core Subsystem

• If core column connections to the hat truss failed, the core subsystem may have experienced

large vertical deflections in the local area of the connection failure due to loss of the primary

load path for the redistribution of loads and subsequent column plastic buckling and/or plastic

and creep deformations.

• The WTC 1 isolated core subsystem was stable with Case A aircraft impact damage and

gravity loads.

• To reach a stable solution for Case C structural damage and gravity loads, the WTC 2 isolated core model required horizontal restraints to be added in the east and south directions at each

floor representing the lateral restraint provided by the office area floors. Without the

horizontal restraints, the WTC 2 core model tilted significantly due to the severed columns in the southeast comer of the core.

• The isolated core models did not converge for WTC 1 Case B and WTC 2 Case D structural impact damage, which had more severed columns than Case A and Case C, respectively. The

core needed to redistribute loads to other areas in the global system for a stable solution with

Case B and Case D damage.

• The WTC 1 isolated core subsystem with Case A impact damage and Case B temperature

histories resuhed in column buckling between Floors 94 and 98 and a 44 in. vertical

displacement at the center of the south side of the core. The core structure was most

weakened from impact and thermal effects at the center of the south side of the core. Smaller

displacements occurred in the global model due to the constraints of the hat truss and floors.

• The WTC 2 isolated core subsystem with Case C impact damage and Case D temperature

histories resulted in an 8.1 in. vertical displacement at the southeast comer. Without the

horizontal restraints, the core would have tilted more toward the southeast comer, and the

vertical displacement would have been larger. No columns buckled. The core stmcture was

weakened from impact and thermal effects at the southeast comer and along the east side of

the core.

Full Floor Subsystem

• Final damage Cases A, B, C, and D were completed after the initial set of floor analyses were

conducted with Cases A„ B„ C„ and Dj. The fires did not-Change between initial Cases A, to D, and final Cases A to D. The concrete slab temperatures were the same for the initial and

final Cases. The tmss temperatures changed where the fireproofing damage changed. The

full floor models were not rerun for Cases A through D as comparisons showed that the

224 NISTNCSTAR 1-6, WTC Investigation
Structural Response of Major Tower Subsystems

structural temperature histories of the floors were nearly identical for most floors and only

slightly different for a few floors. The floor analysis results for Cases A, to D, were used for Cases A to D in the exterior wall subsystem and global analyses.

• The full floor model boundary conditions for the exterior columns constrained thermal

expansion of the concrete slab, which led to high compressive forces in the slabs, even with

sagging of floors.

• At lower elevated temperatures (approximately 100 °C to 400 °C), the floors thermally

expanded and displaced the exterior columns outward by a few inches; horizontal

displacement of the core columns was insignificant. None of the floors buckled as they

thermally expanded, even with the exterior columns restrained so that no horizontal

movement was allowed at the floors above and below the heated floor, which maximized

column resistance to floor expansion. Even with level of column restraint, the exterior

coluinns did not develop a sufficient reaction force (push inward to resist the expansion

outward) to buckle any of the floors.

• At higher elevated temperatures (above 400 °C), the floors began to sag as the floors'

stiffness and strength were reduced with increasing temperature and the difference in thermal

expansion between the trusses and the concrete slab became larger. As the floor sagging

increased, the outward displacement of the exterior columns was overcome, and the floors exerted an inward pull force on the exterior columns.

• The floors began to exert inward pull forces when floor sagging exceeded approximately

25 in. for the 60 ft floor span. This is based upon analysis results of both the detailed truss model and the full floor models (which showed a reduction in compression instead of tension

at the truss connections).

• Floor sagging was primarily caused by either buckling of truss web diagonals or disconnection of truss seats at the exterior wall or the core perimeter. Except for the truss seat failures near the southeast comer of the core in WTC 2 following the aircraft impact,

web buckling or truss seat failure was caused primarily by elevated temperatures of the

structural components.

• Sagging at the floor edge was due to loss of vertical support at the truss seats. The loss of

vertical support was caused by the reduction in vertical shear capacity of the truss seats due to elevated steel temperatures in most cases.

• When the gusset plates and bolts of the truss seat failed in the floor models due to horizontal

loads, it was rare that the truss also walked-off of the seats because the thermal expansion of

the floor was restrained by the exterior columns. The straps and studs at the exterior wall had

been removed from the floor models, which provided additional resistance to horizontal

loads; if the floor slab expansion had not been restrained by the exterior columns, the

horizontal loads between the slab and gusset plate and bolt would have been reduced.

j • The high surface temperatures in the concrete slabs of fire affected floors could have resulted

in delamination by spalling of the slab. This would possibly compromise knuckle strength,

NISTNCSTAR 1-6, WTC Investigation 225
Chapter 7

crack the slab, or cause loss of integrity of the floor system, contributing to greater floor

sagging.

• Case B impact damage and thermal loads for WTC 1 floors resulted in floor sagging on the

south side of the tower over floors that reasonably matched the location of inward bowing

observed on the south face. Case A impact damage and thermal loads did not result in sagging on the south side of the floors.

• Case C and Case D impact damage and thermal loads for WTC 2 both resulted in floor

sagging on the east side of the tower over floors that reasonably matched the location of

inward bowing observed on the east face. However, Case D provided a better match.

Exterior Wall Subsystem

• Inward forces were required to produce inward bowing that was consistent with

displacements measured from photographs. The inward pull was caused by sagging of the

floors. Heating of the inside face of the perimeter columns also contributed to inward

bowing. Thermal expansion occurred as soon as steel temperatures began to rise; column

shortening occurred when creep and plastic strains overcame thermal expansion strains,

typically at temperatures greater than 500 °C to 600 °C with accompanying high stresses and

duration of temperatures and stress levels.

• Models of exterior wall sections bowed outward in a pushdown analysis when several

consecutive floors were disconnected, the interior face of the columns was heated, and

column gravity loads increased (e.g., due to load redistribution from the core and hat truss).

At lower temperatures, thermal expansion of the inside face was insufficient to result in

inward bowing of the entire exterior column. At higher temperatures, outward bowing

resulted from the combined effects of reduced steel strength on the heated inside face, which

shortened first under column gravity loads, and the lack of lateral restraint from the floors.

• The observed inward bowing of the exterior wall indicated that most of the floor connections

must be intact to cause the observed bowing.

• The floor levels predicted to have damaged fireproofing in the aircraft impact analysis

matched well with the floors that were identified from photographic and video analysis to have participated in the inward bowing of the exterior walls.

• The extent of floor sagging required at each floor was greater than that predicted by the full floor models. The estimates of the extent of sagging at each floor were governed by the

combined effects of fireproofing damage and fire; fireproofing damage was limited to areas

subject to direct debris impact. Other sources of floor damage from the aircraft impact and

fires (e.g., fireproofing damage from structural accelerations at impact and subsequent strong

vibrations or floor damage from concrete cracking and spalling from thermal effects) were

not included in the floor models.

• The exterior wall models were used to estimate the pull-in force magnitude and locations for each tower that would produce the observed bowing of the exterior wall.

226 NISTNCSTAR 1-6, WTC Investigation
structural Response of Major Tower Subsystems

• The WTC 1 isolated exterior wall analysis found that an inward pull force of 6 kip at each

column at Floors 95 to 99, starting 80 min after the aircraft impact, caused a maximum

inward bowing of 3 1 in. This inward deflection was smaller than the observed maximum

bowing of 55 in., and the bowed wall was stable at 100 min. The magnitude of pull-in forces was expected to be less than 6 kip in the global analysis with the addition of gravity loads

from the core subsystem as it also weakened; therefore, pull-in forces of 4 kip to 5 kip were

used in the global model analyses.

• The WTC 2 isolated exterior wall analysis found that an inward pull force of 1 .0 kip to 1 .5 kip and 4.0 kip to 5.0 kip on the south and north portions of the east wall, respectively, over

Floors 79 to 83, caused a maximum inward bowing of 9.5 in. at 20 min and 37 in. at 50 min.

The observ^ed deflections were 10 in. and 20 in., respectively. Considering the possible

increase in column loads after impact for Case D conditions, a pull-in force of 1.0 kip on the

south half and 4.0 kip on the north half of the east wall was selected for the initial estimate

for the WTC 2 global model analysis.

NISTNCSTAR 1-6. WTC Investigation 227
Chapter 7

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228 NISTNCSTAR 1-6, WTC Investigation
Chapter 8

Structural Response of the WTC Towers to Aircraft

Impact Damage and Fire

8.1 introduction

Prior to conducting global analysis of the structural response of each World Trade Center (WTC) tower, a tremendous amount of input data was obtained" and developed. Input data required for the structural response models included:

• Reference model of each WTC tower before the aircraft impact, based upon design and

construction documents (NIST NCSTAR 1-2)

• Steel and concrete material properties for room and elevated temperatures (NIST NCSTAR 1-3 and Chapter 4)

• Structural damage to columns and floors from the aircraft impact (NIST NCSTAR 1-2 and

Chapter 5)

• Passive fire protection conditions before and after impact (NIST NCSTAR 1-6A and Chapter 5)

• Temperature histories for all structural elements in the impact zone (NIST NCSTAR 1-5)

• Observed structural conditions and events from photographic and videographic records

(NIST NCSTAR 1-5 A and Chapter 6)

Input data was based on available records. For data that were not directly available, analytical and

experimental results were used to develop the required information. The tower design and construction

and the supplied structural materials were well documented. The passive fire protection conditions before

impact were less well documented, but review of Port Authority of New York and New Jersey (PANYNJ)

records and interpretation of photographs (from several sources throughout the life of the structure)

provided a documented basis for determining the likely conditions that existed in both towers before

September 1 1

, 200 1 . Steel and concrete temperature-dependent properties were developed from available

technical literature and from tests of samples recovered from the collapse site (steel tests were conducted

at National Institute of Standards and Technology (NIST) and concrete tests were conducted at Simpson

Gumpertz & Heger, Inc. (SGH) under contract to NIST) to assess conformance with specified properties.

Temperature histories were developed from fire dynamics simulations and thermal analyses conducted at NIST. The observed structural conditions and events defined the known events that occurred that day.

The structural response analyses helped determine the probable collapse sequence for each tower. These

sequences were validated using the observed structural events.

^ All information and data related to the design and construction of the WTC towers were obtained from contract drawings

provided to NIST by The Port Authority or New York and New Jersey. Refer to NIST NCSTAR 1-2A for a complete

description of the WTC structural system and an index of all structural drawings.

NIST NCSTAR 1-6. WTC Investigation 229
Chapter 8

The global analyses of WTC 1 and WTC 2 used final estimates of impact damage and elevated

temperatures to determine the structural response and sequence of component and subsystem failures that led to collapse initiation. Case B was used for WTC 1 and Case D was used for WTC 2 used, as described in previous chapters.

The global models and required input data are discussed in Section 8.2. The analysis methodology is presented in Section 8.3, and the results of the global analyses for WTC 1 and WTC 2 are presented in Sections 8.4 and 8.5, respectively. To better understand the relative contributions of impact damage and

fire to each tower collapse initiation, the hypothetical condition of the towers subjected to the same fires without aircraft impact damage is discussed in Section 8.6.

8.2 GLOBAL MODEL OF TOWERS

8.2.1 Model Description

The global model of each tower, which was used to determine the structural collapse sequence, was based on the reference structural models developed for baseline structural analyses of the towers (NIST

NCSTAR 1-2). The reference models were developed with SAP2000 and were used as a common basis

for the aircraft impact analysis, using LS-DYNA, and the structural response analysis, using ANSYS.

The SAP2000 global models were more detailed than models typically used for structural design

purposes. The models included exterior and core columns, the hat truss, and mechanical floors, but did not explicitly model the tenant floors due to model size limitations. The tenant floors were accounted for with constraint equations and concentrated floor loads at floor-to-column connection nodes.

The reference global models for WTC 1 and WTC 2 were translated into ANSYS models using an automated translator developed specifically for this effort by Computer Aided Engineering Associates,

Inc. (CAEA), as a subcontractor under the NIST contract to SGH. The coordinates of the nodes, crosssectional properties of members including orientation and offset of the cross-section, nodal loads, material

properties, and member end releases were automatically converted from the SAP2000 format to the

ANSYS format as described in NIST NCSTAR 1-6C.

The ANSYS models were truncated several floors below the impact floors, as previous analyses showed

that the structural response below the impact area remained elastic. WTC 1 was truncated at Floor 91, and

WTC 2 was truncated at Floor 77. The axial stiffness of the remaining structure below the line of

truncation was replaced with equivalent elastic springs. The global models of the two towers are shown in Fig. 8-1.

The truncated ANSYS global models were validated against the SAP2000 baseline global analyses for

gravity loads. The results from the translated ANSYS global models showed good agreement with the

baseline analyses: displacements were within 1 .4 percent for WTC 1 and 0.7 percent for WTC 2, and base

reactions were within 1.2 percent for WTC 1 and 0.3 percent for WTC 2. Based on these comparisons, it was concluded that the translation of the global models from SAP2000 to ANSYS was correct, and the

ANSYS models and their derivatives were used for the global analyses. Details of the translation and

validafion are found in NIST NCSTAR 1 -6C.

230 NIST NCSTAR 1-6, WTC Investigation
structural Response of WTC Towers

The core columns and exterior columns and spandrels were modeled with elements and features similar to those used in the isolated core and exterior wall analyses. Column analysis features included the effects of thermal expansion, plastic, and creep strains on column behavior within the global structural system.

When thermally-induced strains were sufficiently large, column loads increased if they were restrained.

Columns shortened and shed loads if either plastic or creep strains were large enough or if they buckled

plastically. Plastic (or inelastic) buckling describes the condition where a column becomes bowed

(displaced laterally between its ends) by plastic or creep strains, but continues to support a reduced

axial load. As the bowing becomes larger, the column's capacity to carry load diminishes further (see

Fig. 435) until the column no longer participates in carrying load in the global structure.

Floors in the global model were modeled by shell elements, which have their membrane stiffness equal to that of the full floor system. Floors in the global model functioned as diaphragms and transferred loads

between the exterior wall system and the core. Office area and core floors were modeled with an

equivalent floor slab thickness and modulus calculated to match the in-plane stiffness of the composite

floor system, including the concrete slab, floor trusses, and the floor seats. Bending stiffness of the floor

system was not matched because the floor loads were applied at the columns. Both core and office area

floor slabs were modeled with linear-elastic material properties for lightweight concrete.

Figure 8-2 shows the model of the core and tenant floors and the core beams that had moment

connections. Beams without moment connections cannot effectively transfer shear between columns

without significant relative displacement and, thus, were not modeled individually. The stiffness of

simply connected beams was smeared into that of the slab to capture the in-plane stiffness of the floor in the core. Inclusion of the core and office area floors was necessary for modeling force redistribution

within the core and between the core and the exterior columns. The core was effective in redistributing

loads from damaged core columns to adjacent core columns when the load path through the hat truss could not be developed due to either severed columns or column splices.

Floors in the global models were not intended to capture floor response and failure modes during fires. It was not practical, or in some cases not possible, to create computationally efficient global models that included all details of the floor system. The BEAM188/1 89 elements used in the full floor model caused

severe convergence problems when creep was included and those elements experience thermally-induced

buckling. Also, the extent of pull-in forces from sagging floors in the full floor models was less than

estimated from the observ ed bowing of the exterior walls in photographs and videos because the aircraft impact damage to thermal insulation of the floors was conservatively estimated by limiting the dislodged

thermal insulation to regions of direct debris impact.

Important failure modes were identified in the truss and fiiU floor analyses and incorporated into the

global models as floor/wall disconnections and pull-in forces at appropriate time intervals. Since the full floor models did not accurately estimate the pull-in forces at floor/wall connections, the fire-induced

damage obtained from the full floor model analyses were modified by observations obtained from the

examination of photographs and videos performed by NIST (NIST NCSTAR 1-5 A). See Chapter 7 in NIST NCSTAR 1-6C and Chapter 2 in NIST NCSTAR 1-6D for more details.

The global model included the hat truss at the top of each tower. The hat truss was designed to support an

antenna on top of the towers and transmitted loads to both the core and exterior columns. The loads were

distributed primarily to the core columns. There were four outriggers to each exterior wall that provided

rotational restraint for the antenna under wind loads. In addition, the outriggers provided a secondary

NIST NCSTAR 1-6, WTC Investigation 231
Chapter 8

load path between the core and exterior walls as determined from the structural response of the towers to impact damage and fires. Figure 8-3 shows the hat truss, with the outriggers labeled A through P. During the global structural response to impact and fire, the hat truss provided a primary path for transferring loads between the core columns and between the core and exterior walls.

0 1.132 2.264 3.396 4.528

.565976 1.698 2.83 3.962 5.094

(a) WTC 1 ANSYS Model Vertical Displacements

(b) WTC 2 ANSYS Model Vertical Displacements

Figure 8-1. Displaced shape of WTC 1 and WTC 2 at the end of gravity load analysis.

232 NISTNCSTAR 1-6, WTC Investigation
structural Response of WTC Towers

Figure 8-2. Office area and core floors and core beams.

Figure 8-3. Hat truss with labeled outriggers.

8.2.2 Model Modifications

The validated ANSYS models were modified to incorporate nonlinear material and geometric behaviors

required in areas subject to impact and fire. Modifications incorporated modeling enhancements and

refined input data based on numerous component and subsystem analyses. Temperature-dependent

modifications included material properties, coefficients of thermal expansion, plastic and creep strains. Nonlinear geometric behavior modifications included large displacements, plastic buckling and postbuckling of columns. Break elements were not included, based on the behaviors shown in the major

subsystem analyses of the isolated core, exterior wall, and full floors. Break elements for knuckles,

exterior wall bolted connections, and spandrel connections did not fail in the full floor and exterior wall

NISTNCSTAR 1-6, WTC Investigation 233
Chapter 8

analyses. Truss seat connection failures were imposed during analyses according to the time they were

calculated or observed to have occurred, rather than through use of break elements.

Preliminary global analyses had unacceptably slow rates of computation, due to the size of the models

and the computational effects of temperature-dependent material properties, especially creep. To reduce

the size of the global models and to increase the speed of the solution without adversely affecting the

analysis results, modifications were made to the models to improve computational efficiency.

The spandrels were modeled with BEAM 188 elements which experienced convergence problems when

thermal expansion caused them to buckle since there were not enough elements between two columns to capture the buckling detail. The buckling of spandrels did not compromise their ability to transfer shear

and bending moment. Based on visual evidence, buckling of spandrels did not play an important role in the collapse sequence, and increasing the number of spandrel elements would have unnecessarily

increased the model size. Therefore, the coefficient of thermal expansion for spandrels was set to zero, and the axial degree of freedom was released.

Since trusses were not modeled individually, the equivalent floor slab buckled easily when thermal

expansion was restrained by the exterior wall. Buckling of the equivalent floor slab often caused

convergence problems in the global analysis. The coefficient of thermal expansion of the floor in the

office area was set to zero to eliminate the unrealistic buckling problem. Neglecting the thermal

expansion of the office area floors introduced errors in bending of exterior coluinns between a heated

floor and a cool floor. The effect of this modeling assumption was small for columns extending between

two heated floors.

Neglecting the thermal expansion of the office area slabs did introduce small errors in the out-of-plane

bending of columns extending between a hot floor and a cool floor, but such errors were small for columns extending between two hot floors. The error introduced by this modification was not expected to change failure modes or collapse sequence in the global analysis, because thermal expansion of floors was

limited to less than a few inches (see Appendix A). The full floor models thermally expanded and pushed

outward on the columns until the thermal expansion was overcome by the floor sagging and the floors pulled inward on the exterior columns.

Construction sequence was not included in the global models with creep. The effect of neglecting

construction sequence was examined for both buildings. When construction sequence was not included in the analysis, the total axial loads in columns along the exterior walls increased by 7 percent to 15 percent.

Similarly, the total column loads supported by the core columns decreased by about 10 percent.

The calculations showed that the outriggers in the WTC 1 simulations were more highly stressed when

the construction sequence was not considered. Since it was believed that the hat truss played an important

role in transferring loads in WTC 1 , the yield strengths of the materials for these outriggers in WTC 1 were artificially increased to account for the incorrect increase in compressive stresses when construction

sequence was not considered.

The term "super-element" in ANSYS is used for sub-structuring in an analysis, where a portion of the

model with elastic behavior is condensed into a single element with a representative stiffness, damping

and mass matrix. The WTC 2 model was suitable for such a simplification as the section of the building

234 NISTNCSTAR 1-6, WTC Investigation
structural Response of WTC Towers

above Floor 86 was expected to remain elastic, based on the results of the isolated core analyses and

preliminary global analyses without creep strains or plastic buckling (see NIST NCTAR 1-6D).

The use of super-elements reduced the time to complete a single iteration by a factor of three. However,

if at later stages of the analysis the hat truss members became inelastic, the nonlinearities associated with

such inelastic behavior were not captured. A super-element cannot determine individual component

behavior as the group of components is represented by a single 'super' element. Moreover, the effects of

construction sequence on the load distribution between the core and wall elements could not be

represented, since the birth and death option could not be used in a super-element. (Birth and death refer

to the addition or removal, respectively, of an element during an analysis.) The effect of not including

construction sequence was evaluated and found to introduce an error of less than 12 percent for vertical

displacement. To evaluate whether the hat truss exceeded elastic limits, a separate model that included the

components at and above Floor 86 in the super-element was created. The stresses in all the components were calculated at the end of each 10 min time interval and compared with their capacities.

As the use of a super-element in the WTC 2 global model precluded the application of construction

sequence, construction sequence was not included in either the WTC 1 or WTC 2 global analysis.

Construction sequence refers to an analysis method where the self-weight loads are applied to the

structural model in steps to simulate the sequential loading that takes place as a building is constructed.

When construction sequence was not included, the total column loads in each exterior wall increased by 7 percent to 1 5 percent, and the total core columns loads decreased by about 10 percent for both models. It was also found that the outriggers of the hat truss were more highly stressed in the WTC 1 model without

construction sequence than in the translated ANSYS model which included construction sequence (see

Section 8.2.1). Since the hat truss played an important role in transferring loads, the yield strengths of

these outriggers in WTC 1 were increased to account for the artificially higher compressive stresses that resulted without consideration of construction sequence. The difference in the maximum displacement

between the models with and without construction sequence was within 12 percent for both WTC 1 and

WTC 2.

8.3 ANALYSIS METHODOLOGY

WTC 1 and WTC 2 global models were subjected to Case B and Case D aircraft damage and fires,

respectively. The results of the isolated wall, core, and full floor analyses indicated that structural responses to Case B and Case D more closely matched observed structural behavior in photographs and

videos than did Case A or Case C, respectively. Thus, Case B and Case D were chosen for the global

analysis of WTC 1 and WTC 2, respectively.

The global analysis was conducted in steps. Severed and heavily damaged core columns, floors, and

exterior columns and spandrels were removed from the model, and gravity loads were applied as concentrated loads at each column-floor node. Then, dead load and 25 percent of the design live loads were applied to the model without considering construction sequence. The solution for the first step,

which determined the structural condition of the tower after aircraft impact, provided the initial condition

for the application of temperature histories and thermally-induced structural damage.

Wind forces were not included in the global analysis of the WTC towers. Wind speeds were recorded at three nearby airports, and are shown in Table 8-1. The average wind speed on September 1 1, 2001,

NISTNCSTAR 1-6, WTC Investigation 235
Chapter 8

ranged from 7 knots to 1 1 knots (10 mph to 13 mph). In comparison, the design wind speed was 98 mph

averaged over 20 minutes at a height of 1,500 ft above ground (see Chapter 3 of NIST NCSTAR 1-2).

This speed is equivalent to a 3 s peak gust wind speed of about 8 1 mph to 90 mph at 33 ft above ground

in open terrain. Wind force is proportional to the wind velocity squared, therefore, the wind force on the

towers from the 13 mph winds on September 11, 2001, were approximately two percent of the average

design wind speed of 86 mph, which were negligible.

Table 8-1: Wind speeds recorded at airports near the WTC towers on

September 11, 2001.

Airport Time (a.m.) Direction Speed' (knots) Speed" (mph)

LaGuardia (NOAA 2001a) 8:51 320 9 13

9:51 340 9 13

John F. Kennedy (NOAA

2001b)

8:51 310 11 16

9:51 350 7 10

Newark, NJ (NOAA 2001c) 8:51 330 8 12

9:51 No data No data No data

Average Wind Speed 9 13

1 . Wind speed recorded as a 20 min average at a 33 ft elevation.

2. Wind speed converted to a 3 s peak gust at 33 ft (knots {20iTiin) * 1 .4375 = mph (3 s))

Temperature data were provided for heated structural components at 10 min intervals up to 100 min for WTC 1 and up to 60 min for WTC 2. The temperature histories were based on the combined effects of

impact damage to fireproofmg and fire spread and growth. The structural analysis used time steps

significantly less than 10 min, as a result the temperatures were linearly interpolated between the

temperatures at 10 min intervals.

Column-floor disconnections and pull-in forces that occurred during a time step were imposed at the start of the time step. In the global models, nodal couplings tied the exterior columns to the floors. The nodal

couplings were removed at locations of floor/wall disconnections. If disconnections were projected to occur or were observed in visual evidence at a time intermediate to the 1 0 min intervals used in the

analyses, for example, between 10 min and 20 min, they were imposed starting at the earlier time point, in

this example, at 10 min. Once a portion of a floor was disconnected from the exterior wall, it remained

disconnected for the remainder of the analysis. Similarly, pull-in forces were also applied to the global

models at the beginning of the 10 min time intervals in which they were predicted to occur or were

observed, and they were maintained at a constant level for the 10 min time interval.

Thermal expansion of the floors was not included in the global models. Floor analyses showed that the

floors initially pushed exterior column outward by a few inches. However, significant outward bowing was not observed and several inches of outward deflection of exterior columns would not affect the global

stability of the towers.

236 NIST NCSTAR 1-6, WTC Investigation
Structural Response of WTC Towers

8.4 RESULTS OF WTC 1 ANALYSIS

The global model of WTC 1 with creep, plastic buckling of columns, plasticity, and nonlinear geometry was analyzed with Case B structural damage and temperature histories.

8.4.1 WTC 1 Structural Response to Aircraft Impact Damage

Case B structural and passive fire protection damage after the aircraft impact was as follows:

• WTC 1 had 41 north exterior columns (Colunms 112 to 151) severed, 9 core columns on the

north central side of the core (Columns 503, 504, 505, 506, 604, 704, 706, 805, and 904), and one

exterior panel of the south face (Columns 329 to 331 between Floors 93 and 96 ) severed or heavily damaged between Floors 93 and 98

• Floor slabs and framing were severed or heavily damaged in the north office floor area through to the central north region of the core on Floors 94 to 97

• WTC 1 fireproofmg damage was centered primarily through the north face and floor area, the

core, and into the south floor area between Floors 94 to 99.

Figure 8-4 shows the vertical displacements at Floor 99, just above the impact area, with total displacements before aircraft impact and incremental displacements after impact. Figures 8-5 through

8-8 show the vertical displacement contours of the exterior walls and the core area before and after the

aircraft impact. Before the aircraft impact, the maximum vertical displacements of the exterior wall and

the core at Floor 99 were 2.5 in. and 3.6 in., respectively. Due to severe impact damage on the north face

and the north side of the core, WTC 1 tihed slightly to the north after the aircraft impact as can be seen in Fig. 8-4. The maximum displacement of the north wall increased from 2.5 in. to 5.7 in., and the

maximum displacement of the south wall decreased from 2.5 in. to 2.4 in. The vertical displacement of

the east and west wall slightly increased due to load redistribution.

NISTNCSTAR 1-6. WTC Investigation 237
Chapter 8

-2.5 in.

-3.6 in.

-2.8 H

in. o oo

~+0.5 in.

(3.3 in.)

-+ 3.2 in. (5 .7 in.) smammmaaumma

|~+0.5 in.

1(3.3 in.)

t

N

-+1.8 in. (5.4 in.)

-2.8

in.

-2.5 in. Before Impact

—0.1 in

(2.7 in.)

•

© © ©

-+0.1 in. (3.7 in.)

—0.1 in. (2.4 in.)

After Impact

Note: displacements are shown as positive downward

1-0.2 in.

'°(2.6 in.)

Figure 8-4. Vertical displacement at Floor 99 of WTC 1. Total displacements are shown

before aircraft impact and incremental displacements, with total displacements in

parentheses, are shown after impact.

Figure 8-5. Vertical displacement of WTC 1 east and north exterior walls before aircraft

impact.

238 NISTNCSTAR 1-6, WTC Investigation
Structural Response of WTC Towers

Figure 8-6. Vertical displacement of east and north exterior walls of WTC 1 after aircraft

impact for Case B.

NODAL SOLUTION

TIKE=. lOOE-02 i

RSYS=0

DKX =4.247

SMN =-4.246

>>c:>^ ^ FL99

FL95

-4.246 -3.303 -2.359 -1.415

-3.775 -2.831 -1.887

WTCl - Gravity before Impact w/o Const

ANSYS

FEB 14 2005 09:51:34

Figure 8-7. Vertical displacement of the east and north side of the WTC 1 core before

aircraft impact.

NISTNCSTAR 1-6, WTC Investigation 239
Chapter 8

Figure 8-8. Vertical displacement of the east and north side of the WTC 1 core after

aircraft impact for Case B.

The global analysis results showed that WTC 1 did not collapse following aircraft impact, as was

observed, and had considerable reserve capacity. The core columns were loaded to approximately

50 percent of their capacity prior to impact, and the exterior columns were loaded to approximately

20 percent of their capacity. The exterior columns were capable of large load transfers from the core columns.

Gravity loads were redistributed to adjacent core columns and the exterior walls, primarily through the hat

truss. Figures 8-9 and 8-10 show the demand-to-capacity ratios for the core columns before and after impact. The capacity of core and exterior columns was computed as the plastic (or inelastic) buckling

load according the American Institute of Steel Construction (AISC) Load and Resistance Factor Design

(LRFD) Specification procedures. Ratios less than 1.0 indicate that the column can carry additional

gravity loads, where as ratios greater than 1.0 indicate that the column is carrying more than its computed

capacity and, therefore, has plastic strains. Figure 8-10 shows that only two columns had a demand-tocapacity ratio greater than 1.0, and they were adjacent to severed or heavily damaged core columns.

After the aircraft impact, gravity loads that were previously carried by severed columns were redistributed

to other columns. Table 8-2 shows that the north wall at Floor 98 carried a total load of 10,974 kip

before aircraft impact, and 10,137 kip after the impact. The total load lost due to aircraft impact was

837 kip, or about 7 percent of the total load. Table 8-3 shows that the north wall at Floor 105 lost 732 kip

of axial force after impact. Therefore, most of the core loads (732 kip out of 837 kip) were transferred by

the hat truss, and the rest were redistributed to the adjacent exterior walls by the spandrels. Due to the

impact damage and the tilting of the building to the north after impact, the south wall also lost gravity

loads, and about 7 percent (604 kip) was transferred by the hat truss. As a result, the east and west walls

240 NISTNCSTAR 1-6, WTC Investigation
structural Response of WTC Towers

each gained about 7 percent (466 kip and 472 kip, respectively) and the core gained about 1 percent (400

kip) through the hat truss.

# Severed or highly damaged columns

Figure 8-10. Maximum demand-to-capacity ratio for axial force in core columns between

Floor 93 and Floor 99 of WTC 1 after aircraft impact for Case B.

NISTNCSTAR 1-6, WTC Investigation 241
Chapter 8

Table 8-2. Total column loads at Floor 98 of WTC 1 for Case B conditions.

North East South West Core 1 otal

/ 1 \ (1) Beiore Impact lu.y /4 O C /I c 11 AO ^

1 1 ,U23 T A AOA 34,029 73,144

(2) After Impact JU,1 j / Q Cil 1 1 A /lOA 34,429 /3,139

(3) 1 0 min y, /yo O A C\C\ o,4yu A O yl O 9,848 1 C All 36,4 /3 73,143

(4) 20 min 10,43 / n 1 AO 9,108 A AAA 9,900 34,495 73,143

(5) 30 min 1 A n 1 0 10,913 1 A A 10,034 1 A /I OA 10,420 9,715 32.060 TO 1 /I O 73,142

(6) 40 min 1 l,Uoo 1 A CAA iu,jyy 11 AA /I 1 1 ,U04 1 A 1 'TO 1 A O A /I 30.294 11 1 /I o 73,142

(7) 50 min 11 ^ Ac\

1 1,149 1 A AA O 10,908 111 AO1 1,192 1 A A ZO 10,458 "T A /I T C 29,435 1/11 73,141

(8) 60 min 1 1,205

11 1 Z' o

1 1,168 11 O C1 1.285 1 A T 1 10,716 73,141

(9) 70 mm 1 1,286 1 1,366 11 T /I 0

1 1 ,343 1 A ATA 10,939 28,205 1 O O 73,138

(10) 80 min 11 in a

1 1 ,j /6 11 c c c1 1 ,J J J

11/1 AA 1 1,41)9 11 1 1 A 11,119 2 /,681

-7 T ^ Af\ /3,140

(11) 90 min 10,916 1 1 AA 1 1 1.991 A A /I A 9,949 11 C ^1

1 1,657 o o con 28,587 nl AAA 73,099

(12) 100 min 10,828 12,249 9,638 1 1,905 28.478 73.098

(13) (2)-(l) -837 526 -668 574 400 -5

(14) (10) -(2) 1,239 2,484 1,052 1,973 -6,748 1

(15) (12) -(2) 692 3,178 -719 2,759 -5,951 -41

(16) (12)-(10) -548 694 -1,771 786 797 -42

Note: Compression is positive. Units are in kip.

Table 8-3. Total column loads at Floor 105 of WTC 1 for Case B conditions.

North East South West Core Total

(1) Before Impact 8,026 6,562 8,092 6,604 20,361 49,645

(2) After Impact 7,294 7,028 7,488 7,076 20,761 49,646

(3) 1 0 min 6,944 6,461 6,981 6,469 22,790 49,646

(4) 20 min 7,551 7,075 7,057 7,158 20,806 49.647

(5) 30 min 8,020 7,998 7,569 7,685 18.377 49,648

(6) 40 min 8,193 8,571 8.129 8,147 16,608 49,649

(7) 50 min 8,285 8,878 8,315 8,428 15.743 49.650

(8) 60 min 8,351 9,130 8,414 8,687 15,069 49,650

(9) 70 min 8,435 9,319 8,481 8,914 14,502 49,651

(10) 80 min 8,528 9,497 8,551 9,097 13,978 49,651

(11) 90 min 8,096 9,847 7,327 9,506 14,876 49,652

(12) 100 min 8,023 10,076 7,066 9,720 14,767 49,653

(13) (2)-(l) -732 466 -604 472 400 1

(14) (10) -(2) 1,234 2,470 1,063 2,021 -6,783 5

(15) (12) -(2) 730 3,048 -422 2,644 -5,993 7

(16) (12)-(10) -504 579 -1,485 623 790 2 Note: Compression is positive. Units are in kip.

242 NISTNCSTAR 1-6, WTC Investigation
Structural Response of WTC Towers

8.4.2 WTC 1 Structural Response to Elevated Temperatures

In the early stages of the fire, temperatures of structural components in the core rose between 500 °C and

700 °C o\ er a 10 min to 20 min time interval (where fireprooflng was damaged), and the thermal

expansion of the core was greater than the thermal expansion of the exterior walls. The difference in the

thermal expansion between the core and the exterior walls increased the loads in the core columns at 20 min. After 20 min, the core continued to lose gravity loads due to thermal weakening and shortening

until the south wall started to bow inward. By 50 min, the core had displaced downward by 1.6 in on

average at Floor 99 due to creep and buckling of core columns. About 20 percent (6,748 kip) of the

gravity load was transferred by the hat truss to the exterior walls due to thermal weakening of the core at 80 min. as shown in Table 8-2; the north and south walls each carried about 10 percent more loads

(1,239 kip and 1,052 kip, respectively)and the east and west walls each carried about 25 percent more

loads (2.484 kip and 1,973 kip, respectively). Since the hat truss outriggers to the east and west walls were stiffer than the outriggers to the north and south walls, they transferred greater loads to the east and

west exterior walls. At 100 min, the core displaced downward at Floor 99 by 2.0 in. on the south side of

the core. As the core was weakened by creep and plastic buckling, gravity loads in the core were

transferred to the exterior walls.

The full floor analyses for WTC 1 Case B showed that the floors on the south side in the impact zone did not begin to sag and apply a pull-in force at the column connections until approximately 80 min after impact. Based upon the full floor and isolated south wall subsystem analyses, 5 kip of pull-in force was

applied to all columns across Floors 95 to 99 beginning at 80 min, as shown in Fig. 7-30. Figures 8-1

and 8-12 show the out-of-plane displacement contours of the south wall at 80 min and 100 min,

respectively. Figure 8-13 shows the time history of the inward bowing of the south wall. Until the 5 kip

pull-in forces were applied, no inward bowing had occurred. With the application of the 5 kip pull-in

force, the maximum inward bow increased to 15.5 in.

NOOAL SOLUTION

ST!iP=25 SUB =170 TlME-120

UY (AVG) MX =3.4 98 SMN =-2.207

SMX =2.42 6

¥iil;illiiiiiiiiliill ANSYS

MAR 2 2005 19:45:34

- FL99 - FL97 - FL95 - FL93

-2.207 -1.178 -.14807 .881588 1.911 -1.693 -.6629 .366759 1.396 2.426

rlTCl Severe Temperature at 4800 sec - South Face j,

Figure 8-11. Out-of-plane displacement of south wall of WTC 1 at 80 min for Case B.

NISTNCSTAR 1-6, WTC Investigation
Chapter 8

359

NODAL SOLUTION

STEP=33

SUB =1437 TIME=150 . . UY (AVG) \, . RSYS=0

DMX =42.979

SMN =-.701059 4.:..i SMX =42.826 •;•

301 ANSYS

MAR 30 2005 10:24:28

Figure 8-1 2. Out-of-plane displacement of south wall of WTC 1 at 1 00 min for Case B

conditions with 5 kip pull-in forces.

80 85 90

Time (min)

95 100

Figure 8-13. Time history of maximum out-of-plane displacement of WTC 1 south wall for

Case B with 5 kip pull-in forces.

Figure 8-14 shows that Columns 318 to 346 on the south wall unloaded due to inward bowing after 80 min. The loads increased on the east and west walls. Figure 8-15 shows the load increase for the east

244 NISTNCSTAR 1-6, WTC Investigation
Structural Response of WTC Towers

wall. Due to bowing of the south wall, the vertical displacement of the south wall increased as shown in Figs. 8-16 and 8-17, and the south wall lost about an additional 5 percent (1,771 kip) of load between

80 min and 100 min (see Table 8-2). As a result, the east and west walls and the core gained gravity

loads.

By approximately 87 min, the inward bowing increased significantly. As the bowing of the south wall

increased, a section of the south wall above the bowed-in area moved downward as can be seen in Fig. 8-1 8. By 90 min, the rate of increase in the inward bowing slowed down as the south wall

redistributed the gravity loads to the east and west walls and to the core. The inward bowing increased to 42.8 in. at 100 min. However, the south wall remained stable (had not buckled) at 100 min.

Isolated exterior wall and global analyses showed that varying the inward pull force by a small amount

caused a large difference in the amount of inward bowing. For a comparison, the inward bowing of the

south wall at 100 min from the analysis with a 4 kip pull-in force was only 14.5 in. at 100 min. Given that the inward bowing increased from 14.5 in. to 42.8 in. when the inward pull force was increased from

4 kip to 5 kip, a slight increase in the pull-in force over 5 kip would have resulted in instability of the

south wall.

360 350 340 330 320 310 300

Column ID

Figure 8-14. Distribution of axial force in exterior columns at Floor 98 of WTC 1 south

wall for Case B with 5 kip pull-in forces.

NISTNCSTAR 1-6, WTC Investigation 245
Chapter 8

Increased load at 100 min

—Before Impact —After Impact —20 min —50 min —80 min —100 min

260 250 240 230 220

Column ID

210 200

Figure 8-15. Distribution of axial force in exterior columns at Floor 98 of WTC 1 east wall

for Case B with 5 kip pull-in forces.

NODAL SOLUTION

STEP=25

SUB =170 TIME=120 C UZ (AVG) RSYS=0

DMX =7.323

SMN =-6.94

SMX =.154595

ANSYS

MAR 16 2005

17: 46:19

Figure 8-16. Vertical displacement of west and south exterior walls of WTC 1 at 80 min

for Case B.

246 NISTNCSTAR 1-6, WTC Investigation
structural Response of WTC Towers

• ••eoooeo 1-0.2 ~+3.7 -0.1

~+3.5

o o oo o o ©

o o oo o o o

o

o

o ©

@ ®

OOGO

® ®@

~+1.9

Q> -+1.8

OOOOOOOO

I

N

Note: downward displacement is sliown as positive displacement

Figure 8-18. Change in vertical displacement at Floor 99 of WTC 1 from the state before

impact to 100 min for Case B.

Plastic and creep strains played a significant role in the structural response of WTC 1 to the fires. Figures 8-19 to 8-21 show the maximum elastic-plus-plastic-plus-creep strain in each column between

Floor 93 and Floor 99 at 10 min, 40 min, and 100 min, respectively. Before the aircraft impact, the

columns had no plastic strain. After the aircraft impact, several columns that were close to severed and

NISTNCSTAR 1-6, WTC Investigation 247
Chapter 8

highly damaged columns experienced plastic strains. Plastic strain of the core increased for the first 40 min, and then remained almost constant to 100 min. Plastic strain of the south exterior columns

increased in almost all the bowed columns from 40 min to 100 min. However, creep strain was found to be far greater than plastic strain, especially in the core. At 40 min, 22 of 47 core columns had creep strain larger than 1 .0 percent. After 40 min, creep strain in core columns on the south side had increased. The

maximum elastic-plus-plastic-plus-creep strain at 100 min was 7.3 percent in Column 1006. As

temperature increased on the south exterior wall in the later stages of the fire, creep strain also increased

in about 20 columns on the south face. The maximum elastic-plus-plastic-plus-creep strain in the south

exterior columns reached 2.9 percent.

"cooccxxxxxxDoaxaxocxxcixcQjBXDCcaxxxxxxxxxx

Figure 8-19. Maximum elastic-plus-plastic-plus-creep strain for columns between

Floor 93 and Floor 99 of WTC 1 at 10 min for Case B (strain values are in percent).

248 NISTNCSTAR 1-6, WTC Investigation
structural Response of WTC Towers

Figure 8-20. Maximum elastic-plus-plastic-plus-creep strain for columns between

Floor 93 and Floor 99 of WTC 1 at 40 min for Case B (strain values are in percent).

Figure 8-21. Maximum elastic-plus-plastic-plus-creep strain for columns between

Floor 93 and Floor 99 of WTC 1 at 100 min for Case B with 5 kip pull-In forces (strain

value are in percent).

NISTNCSTAR 1-6, WTC Investigation 249
Chapter 8

Figures 8-22 and 8-23 show the axial force demand-to-capacity ratio for each core column at 80 min and

100 min, respectively. Compressive capacities of the core columns were calculated by AISC LRFD Eq.

E2-1 for plastic buckling with effective length factor of K equal to 1.0 and a resistance factor of 1.0. Comparison of Figs. 8-10 and 8-23 shows that the demand-to-capacity ratio increased for core columns

with relatively small elastic-plus-plastic-plus-creep strains and decreased for columns with high strains. Figure 8-2 1 shows that core columns with high creep strains had lower demand-to-capacity ratios. Columns with high compressive loads and large creep strains shortened and unloaded to stiffer columns

with less creep.

At 100 min, the core had weakened on the south side and shortened by 1.6 in. The south wall had bowed

inward approximately 43 in. and was unloading to the core and the adjacent east and west walls. As

discussed previously, a small change in the magnitude of the inward pull force changed the rate at which

the exterior wall bowed inward and reached a point of instabihty. Based upon observations and similar

results for WTC 2 at collapse initiation (described in the next Section), the following sequence of events

likely occurred as soon as the south wall reached instability and buckled.

The inward bowing of the south wall caused failure of exterior column splices and spandrels, and induced

column instability. The instability progressed horizontally across the entire south face. The south wall

unloaded and redistributed its gravity loads to the thermally weakened core through the hat truss and to the east and west walls tlirough the spandrels. The beginning of this load redistribution is illustrated in Tables 8-1 and 8-2. The building section above the impact zone began tilting to the south as column

instability progressed rapidly from the south wall along the adjacent east and west walls and increased the

gravity load on the core columns. The change in potential energy due to downward movement of the

building mass above the buckled columns exceeded the strain energy that could have been absorbed by

the structure. Global collapse then ensued.

IQQ1 # = Severed or highly damaged column 1QQ8

Figure 8-22. Maximum demand-to-capacity ratio for axial force in core columns between

Floor 93 and Floor 99 of WTC 1 at 80 min for Case B conditions.

250 NISTNCSTAR 1-6, WTC Investigation
structural Response of WTC Towers

# = Severed or highly damaged column

Figure 8-23. Maximum demand-to-capacity ratio for axial force in core columns between

Floor 93 and Floor 99 of WTC 1 at 100 min for Case B with 5 kip pull-in forces.

8.4.3 WTC 1 Hat Truss Members and Connections

The state of the hat truss members and the connections were checked since the global model did not include break elements to capture column and hat truss sphce failures or sufficient beam elements to capture buckling of the hat truss outriggers. The condition of the connections and the members in the

primary load path of the hat truss was evaluated at various time intervals. The evaluation included the core column splices for tension, outriggers and supporting columns for compression, and the hat truss connections that were in the primary load path for tension.

Core column splices had compressive forces before the aircraft impact. With the aircraft impact damage

and increasing plastic and creep strains, the core weakened and shortened, and some core columns were

supported from the hat truss. At 100 mm, nine core columns (503, 504, 505, 602, 603, 604, 605, 702, and

802) were in tension at Floor 105 as shown in Fig. 8-24. To evaluate the condition of the core column

splices at Floor 106, the tension capacity of these splices was compared to the tensile forces developed

during each 10 min time interval. To calculate the connection tensile capacity, AISC-LRFD procedures were used. It was found that tensile forces in the core columns were less than the capacities of the splices.

There were sixteen outriggers (four on each face), as shown in Fig. 8-25, that transferred gravity loads

between the core and the exterior walls. In the global model, each of these outriggers was modeled by

only one BEAM 24 element; therefore, buckling of the member was not captured although material

nonlinearity was included. Table 8^ lists demand-to-capacity ratios for the outrigger members over

time. Capacities of the outriggers were calculated using AISC LRFD procedures for plastic (inelastic)

buckling with effective length factor K equal to 0.75 (the outrigger end connections were different from

those used for the core columns) and a resistance factor of 1.0. As the outrigger members were not

NISTNCSTAR 1-6, WTC Investigation 251
Chapter 8

modeled with sufficient elements to capture plastic buckling, they yielded when the outrigger reached its compressive capacity; therefore, force redistribution to other outriggers was underestimated.

The hat truss connections within the hat truss itself were also checked. The hat truss connections in the

primary load path were identified, and their capacities were compared to their forces. The primary load

path was identified by selecting hat truss members with an absolute axial stress of 25 ksi or more at 80 min, as this was when maximum forces occurred. Only the comiections that were transferring tensile

forces were evaluated. In calculating the capacity of the connections, the AISC-LRFD procedures were

used. None of the hat truss connection capacities were exceeded. It was concluded that the hat truss redistributed loads between the core and the exterior wall columns as modeled in the global analysis.

Col 501 Coi508

o o ©Q o O o 0.15 O (J fo.4ij ^(J o o o

014 N

y 0.11 o oooo o o

o o o o o o I"

oo oooooo

oooooooo

Co! 1001 Col 1008

O = column in compression

Figure 8-24. Tension demand-to-capacity ratio for core column splices at WTC 1 Floor 106 at 100 min for Case B with 5 kip pull-in forces.

252 NISTNCSTAR 1-6, WTC Investigation
structural Response of WTC Towers

Figure 8-25. Location and label of outriggers and supporting columns for WTC 1.

Table 8-4. Demand-to-capacity ratio for axial force in outriggers of WTC 1 for Case B.

C)ulri;;<;ir II) Bfr Imp Mtr Imp 1 0 min 211 min 3(1 min 40 min 5(1 min 60 min 7(1 min 8(1 min 90 min 100 min

North A 0.26 0.35 0.26 0.40 0.49 0.53 0.56 0.59 0.62 0.65 0.54 0.52 B 0.22 -0.07 -0.13 -0.04 0.02 0.04 0.06 0.07 0.08 0,09 0,03 0,03 C 0.21 -0.05 -0.11 -0.02 0.04 0.06 0.08 0.08 0.09 0,11 0,05 0,04 D n.25 0.35 0.29 0 45 n.61 0.67 n 70 0.72 0.73 0,75 0,65 0,63

East E 0.32 0.48 0.38 0.56 0.75 0.83 0.88 0.92 0.94 0.97 0,95 0.96 F 0.23 0.28 0.21 0.30 0.41 0.48 0.52 0.55 0.57 0.59 0.62 0.64 G 0.23 0.26 0.20 0.26 0.35 0.42 0.46 0.49 0.51 0.54 0.58 0.61 H 0.33 0.?1 0.2? 0,25 0.36 n 46 0.51 0.55 0.58 0.60 0,72 0.77 South

I 0.25 0.12 0.04 0,03 0.12 0.20 0.23 0.26 0.27 0.28 0.28 0.28

J 0.21 0.10 0.01 0.03 0.13 0.24 0.28 0.29 0.30 0.31 0.04 -0.02 K 0.21 0.10 0.01 0.03 0.13 0.24 0.27 0.28 0.30 0.31 0.02 -0.04 L 0.26 n.12 0.02 0.05 0.11 0.20 0.24 0.27 0.29 0.31 0.19 0,18

VNcst M 0.33 0.33 0.22 0.29 0.36 0.44 0.49 0.53 0.56 0.58 0.69 0.73 N 0.24 0.27 0.20 0.28 0.34 0.40 0.44 0.47 0.50 0.52 0.56 0.58 0 0.24 0.29 0.22 0.31 0.38 0.43 0.47 0.50 0.53 0.55 0.59 0.61 P 0.33 0.48 0.39 0.54 0.64 0.71 0.75 0.79 0.83 0.87 0.88 0,90

8.5 RESULTS OF WTC 2 ANALYSIS

The global model of WTC 2 with creep, plastic buckling of columns, plasticity, and nonlinear geometry was analyzed with Case D structural damage and temperature histories.

NISTNCSTAR 1-6, WTC Investigation 253
Chapter 8

8.5.1 WTC 2 Structural Response to Aircraft Impact Damage

Case D structural and passive fire protection damage after the aircraft impact was as follows:

• WTC 2 had 34 severed columns on the south wall (Columns 407 to 440) and 1 1 core columns on

the south side of the core (Columns 701, 702, 801, 802, 803, 901, 903, 1001, 1002, 1003, and

1004) and 4 exterior columns on the north wall (Columns 253, 254, 257, and 258) that were

severed or heavily damaged between Floors 78 to 84.

• Floor slabs and framing were severed or heavily damaged in the south office floor area through

the east side of the core between Floors 78 and 84.

• WTC 2 fireproofing damage extended from the south exterior wall, through the east side of the

core, to the east and north exterior walls between Floors 78 and 84.

The vertical displacements of the exterior wall before the aircraft impact were about 2.0 in. to 3.0 in. (Fig. 8-26). After aircraft impact, the vertical displacements increased to 7.4 in. on the south wall

(Fig. 8-27). There was no horizontal (out-of-plane) displacement on the east wall before the aircraft impact. After aircraft impact, the south side of the east wall at Floor 86 displaced outward about 2.0 in.; whereas, the north side at the same floor did not displace.

In the core, the vertical displacements were about 3.5 in. to 4.2 in. before the aircraft impact, as shown in Fig. 8-28. After aircraft impact, the vertical displacements increased to 10 in. at the southeast comer of

the core where the aircraft impact had severed columns, as shown in Fig. 8-29.

Figures 8-30 and 8-31 show the north-south and east-west lateral displacements of the exterior wall

above Floor 86 after aircraft impact. Floor 1 10 moved toward the south about 5.1 in. and toward the east about 5.0 in. There was also a slight twist around the z-axis of the tower of about 0.07 percent at Floor 110. The twist around the z-axis was calculated by taking the difference between the average in- plane displacement of the two opposing exterior walls (such as the east and the west walls) at Floor 1 10

and dividing the result by the distance between these walls (-200 ft).

The global analysis showed that WTC 2 was stable following aircraft impact, as was observed, and had

considerable reserve capacity. Similar to WTC 1, the core columns were loaded to approximately 50

percent of their capacity prior to impact, and the exterior columns were loaded to approximately 20

percent of their capacity. The exterior columns were capable of large load transfers from the core columns after impact.

The loads in the severed exterior columns were transferred to adjacent exterior columns through the

spandrels and to the core through the hat truss. Several of the severed core columns at the southeast comer of the core were computed to have failed splice connections to the hat tmss (discussed in Section 8.5.3). The loads from these columns were transferred through the core floors to adjacent core columns and then to the east and south exterior walls through the hat tmss. Additionally, the severed core columns at the southeast comer resulted in the core leaning to the southeast. While the isolated WTC 2 core model was not stable with the stmctural impact damage, within the global system the core was

supported by the floors and exterior walls.

254 NISTNCSTAR 1-6, WTC Investigation
Structural Response of WTC Towers

The leaning of the core to the southeast contributed to the load redistribution in WTC 2, with a general

pattern of increased loads on the south and east columns (core and exterior) and decreased loads on the

north and west columns. Tables 8-5 and 8-6 show the total columns loads at Floors 83 and 105,

respectively, for the analysis stages from before impact to collapse initiation at 43 min. After impact, as

sho\\Ti in Table 8-5, the core carried 6 percent less loads (4,007 kip), and the east wall carried 24 percent more gravity loads (4,368 kip). The north wall loads decreased by 10 percent (1,374 kip), and the south

and west walls loads increased by 2 percent (227 kip) and 3 percent (604 kip), respectively.

The loads on the core columns before aircraft impact were distributed essentially symmetrically with

respect to the center of the core. There was a slight difference between comer columns on the south side

(501 and 1001) and north side (508 and 1008) due to slightly higher dead and live loads in the north side

columns. Columns 506, 507, 508, and 1008 at the northeast and northwest comers unloaded; the other

intact core columns increased in load (Figs. 8-32 and 8-33). The loads in Columns 904 and 1005, which

were adjacent to the se\'ered and heavily damaged columns, increased substantially at Floor 83 after impact. Column 904 increased from 660 kip to 1.506 kip and Column 1005 increased from 1,287 kip to 2,794 kip.

NODAL soi-jt:::; STEP=1 SUB =1 TIME=.100E-02

UZ (AVG) RSYS=0

DMX =3.032 SMN =-2.97

ANSYS

MAR 2 2005 10:51:30

-3.75 -1.25

-2.5 0 WTC2 Reduced Model Ac Bfrlmn

Figure 8-26. Vertical displacement before impact of WTC 2 exterior wall for Case D.

NISTNCSTAR 1-6. WTC Investigation 255
Chapter 8

256

Figure 8-28. Vertical displacement before impact of WTC 2 core for Case D.

NIST NCSTAR 1-6, WTC Investigation
structural Response of WTC Towers

Figure 8-29. Vertical displacement after impact of WTC 2 core for Case D.

NODAL SOLUTION

STEP=2

SUB =1 TIME=.002

UX (AVG) Rsys=o

DMX =11.112

SMN =-5.99

SMX =-.52582

ANSYS

MAR 14 2005 10:50:25

-5.99 -4.776 -3.561 -2.347 -1.133 -5.383 -4.168 -2.954 -1.74 -.52582 WTC2 Reduced Model At After Impact

Figure 8-30. Lateral displacements after impact above WTC 2 Floor 86 in the x-direction

(north-south) for Case D.

NISTNCSTAR 1-6. WTC Investigation 257
Chapter 8

Figure 8-31. Lateral displacements after impact above WTC 2 Floor 86 in the y-direction

(east-west) for Case D.

Table 8-5. Total column loads at WTC 2 Floor 83 for Case D (Compression is positive)

Row Analysis Stage West East North South Core Sum

(1) Before Impact 18065 18114 13567 13284 61828 124857

(2) After Impact 18670 22481 12193 13511 57821 124676

(3) 10 min 18728 22226 11896 13358 58413 124621

(4) 20 min 18914 22208 12052 13318 58124 124616

(5) 30 min 18876 23681 11770 13365 56967 124659

(6) 40 min 18531 23682 11906 13473 56825 124418

(7) 43 min 15667 15143 14215 16292 62422 123738

(8) (2)-(l) 604 4368 -1374 227 -4007 -181

(9) (3)-(l) 662 4112 -1670 74 -3415 -236

(10) (4)-(l) 849 4094 -1515 35 -3704 -241

(11) (5)-(l) 811 5567 -1797 81 -4861 -199 .

(12) (6)-(l) 466 5568 -1661 190 -5003 -439

(13) (7)-(l) -2398 -2971 648 3009 594 -1119

258 NISTNCSTAR 1-6, WTC Investigation
structural Response of WTC Towers

Table 8-6. otal column loads at WTC 2 Floor 105 for Case D (Compression is positive)

Row Analysis Stage West East North South Core Sum

(1) Before Impact 8497 8572 7382 7169 17123 48742

(2) After Impact 9170 11272 6487 8432 13382 48742

(3) 10 min 9182 11061 6250 8275 13975 48742

(4) 20 min 9279 11120 6311 8351 13682 48742

(5) 30 min 9370 11859 6416 8553 12544 48742

(6) 40 min 9198 11927 6524 8691 12402 48742

(7) 43 min 7086 8026 6546 9169 17915 48742

(8) (2)-(l) 674 2699 -895 1263 -3741 0

(9) (3)-(l) 685 2489 -1132 1106 -3148 0

(10) (4)-(l) 783 2547 -1071 1182 -3441 0

(11) (5)-(l) 873 3287 -965 1384 -4579 0

(12) (6)-(l) 702 3355 -858 1522 -4721 0

(13) (7)-(l) -141

-547 -835 2000 792 0

501

1924

745) (861

2096 1881

(m2

(ijc

(1195)

(l205) 3 © @ ^

782) ^fl^

(1168) \3

m54)

1001

2320 ^354) (12^ ( 2205

N

1844 3265

1008

Figure 8-32. Core column loads (kip) before impact at WTC 2 Floor 83 for Case D

(compression is positive).

NISTNCSTAR 1-6. WTC Investigation 259
Chapter 8

501 508

1001 N _ 1008

After Impact

Figure 8-33. Core column loads (kip) after impact at WTC 2 Floor 83 for Case D

(compression is positive).

8.5.2 WTC 2 Structural Response to Elevated Temperatures

In contrast to the fires in WTC 1 , which generally progressed from the north side to the south side over approximately one hour, the fires in WTC 2 started and remained on the east side of the building until it collapsed, with the fires spreading from south to north. With fireproofing dislodged over much of the same area, the structural temperatures increased in the core, floors, and exterior walls at similar times.

During the early stages of the fires, columns with dislodged fireproofing elongated due to thermal

expansion. As the structural temperatures continued to rise beyond 500 °C, the thermal expansion was

overcome by plastic and creep deformations under compressive loads.

Vertical displacement of the exterior walls before impact were 2.0 in. to 3.0 in. Vertical displacements of

the south and east walls after impact were around 7.3 in. on the south face (over the severed columns) and

about 3.5 in. on the east face, as shown in Figs. 8-34 and 8-35. These vertical displacements remained

essentially constant after impact until the east wall became unstable at 43 min (Figs. 8-36 and 8-37).

After impact, the core and the north wall unloaded, and their load was redistributed to the south, west, and

east walls. Table 8-7 shows that about 94 percent (3,740 kip/4,000 kip) of the load from the core was

redistributed through the hat truss to the east, south, and west walls and 6 percent was redistributed

through the floors to the east wall. A similar calculation for the east wall indicates that about 62 percent

(2,699 kip/4,368 kip) of the load increase came through the hat truss and 38 percent was transferred

through the spandrels to the north and south walls. Comparison of loads shown in Rows 8, 9, 10, and 1

260 NISTNCSTAR 1-6, WTC Investigation
structural Response of WTC Towers

in Tables 8-5and 8-6 show that the column loads did not significantly change until the core unloaded at 30 min. Prior to this point, the thermal expansion of the core columns caused loads to increase. When the

plastic and creep strains exceeded the thermal strains, the core columns shortened and unloaded. Loads in

weakened core columns were redistributed to adjacent columns primarily through the hat truss.

Shortly after impact. Floors 79 to 83 began to sag and pull inward on the east wall (except where truss seat connections had failed). At 20 min, the east wall had bowed inward 9.5 in. near the center of the east

wall, as shown in Fig. 8-38. The computed inward displacement agrees well with the observed inward

displacement (-10 in.) that was measured from photographs at 9:21 a.m. (approximately 20 minutes after the aircraft impact). Inward displacements of the east wall steadily increased until collapse initiation.

At 30 min, the core unloaded about 850 kip (from 4,861 kip to 4,007 kip), the east wall increased about

1,200 kip (from 5,567 kip to 4,368 kip), and the north wall unloaded about 420 kip (from 1,797 kip to 1,374 kip) at Floor 83. Floor 105 column loads remained almost constant after aircraft impact until the

east wall became unstable at 43 min. From 40 min to 43 min, the east wall suddenly unloaded about

8,540 kip, the west wall unloaded about 2,860 kip, the core load increased by about 5,600 kip, the north

wall load increased by about 2,3 10 kip, and the south wall load increased about 2,820 kip at Floor 83

(Table 8-8). Comparison of the load redistribution that took place at Floor 105 with that at Floor 83

indicates that essentially all the additional core load from the east and west walls was transferred through

the hat truss. For the east wall, about 46 percent (3,901 kip/8,539 kip) of the load shed was redistributed

through the hat truss to the core and 54 percent was redistributed primarily through the spandrels to the

south and north walls. After the load redistribution, the total load in the core columns increased to the same level as before the aircraft impact.

NISTNCSTAR 1-6, WTC Investigation 261
Chapter 8

NODAL SOLUTION

STEP=8

SUB =337 TIME=30

UZ (AVG) RSYS=0

DMX =9.831

SMN =-7

ANSYS

MAR 2 2005'

WTC2 Reduced Model At 1200s

Figure 8-34. Vertical displacement of exterior wall of WTC 2 at 20 min for Case D.

^r^mm )m)))t )m)m )»vx n»y in>Km^

-1.8 in

X

-7.3 in ^

# @ O OO O (i

O O o oo o ©

oooo o

O C

O oo o

O oo o

N

o

o

3.7 in -3.0 in

Figure 8-35. Vertical displacement at Floor 83 of WTC 2 at 20 min for Case D (note the tilt toward east and south).

262 NISTNCSTAR 1-6, WTC Investigation
structural Response of WTC Towers

Figure 8-36. Vertical displacement of exterior wall of WTC 2 at 43 min for Case D.

-4.5

^2.5 in Likely zone the -1.2 in lower tilts around

X

-10 in ^

Q O G

"^^^ O

O O O 03 : ^ O O O CD O o"^.

^

13 in 12 in 11 in — N

-11 in -8.5 in

) M ) M M mac M(((

X

-5.0 in

Figure 8-37. Vertical displacement at Floor 83 of WTC 2 at 43 min for Case D (note the tilt toward east and south).

NISTNCSTAR 1-6, WTC Investigation 263
Chapter 8

Table 8-7. Change in total column loads before and after aircraft impact.

(Loads After Impact) - (Loads Before Impact)

(Compression is positive).

Row Floor West East North South Core

(1) 83 604 4368 -1374 227 -4007

(2) 105 674 2699 -895 1263 -3741

(3) (2)-(l) 69 -1668 479 1035 266

Table 8-8. Change in total column loads between 40 min and 43 min.

(Loads at 43 min) - (Loads at 40 min)

(Compression is positive).

Row Floor West East North South Core

(1) 83 -2864 -8539 2309 2819 5596

(2) 105 -2112 -3901 23 479 5513

(3) (2)-(l) 752 4637 -2286 -2340 -84

Figure 8-38. Out-of-plane displacement of the east wall of WTC 2 at 20 min for Case D.

264 NISTNCSTAR 1-6, WTC Investigation
structural Response of WTC Towers

At 43 min, the east wall became unstable and the inward displacement increased to 62 in., as shown in Figs. 8-39 and 8^0. The south and east wall vertical displacements increased to 1 1.3 in. The northwest comer of the exterior wall displaced upward about 1.0 in. to 2.0 in., as the tower tilted to the southeast

around an axis passing through the southwest and northeast comers, as indicated in Fig. 8-37. The north

exterior wall displaced laterally by an additional 15.2 in. to the east, and the south exterior wall

displacement increased 6.7 in. to the south. The building section above the impact damage rotated about

the tower axis an additional 0.10 percent at Floor 1 10.

The core displacements suddenly increased to 13 in. at the southeast comer of the core, as shown in Figs. 8—41 and 8—42. Loads on the core columns increased significantly, especially at the northeast comer. For instance, at Floor 83 the load in core column 1008 increased from 2,826 kip after aircraft impact (Figs. 8-33 and 8^3) to 5,317 kip at 43 min (Fig. 8^4), the load in core column 907 increased

from 1,290 kip to 2,328 kip, the load in core column 805 increased from 950 kip to 1,483 kip.

Figure 8—45 shows the total displacements (deformed shape scaled by a factor of 20) above Floor 86

when the east wall buckled. The building section above the impact damage tilted to the southeast, and

collapse initiated. For reference, the original undeformed tower is also shown.

When the east wall buckled, the load distribution changed significantly, due to the increased tilting of the

building section above the impact damage towards the east. Figures 8-46 to 8^9 show how exterior

columns loads changed in the exterior walls from before impact to when the east wall became unstable at 43 min. The exterior columns of the east wall unloaded about 200 kip on average at Floor 83. Similarly,

the columns on the west face unloaded about 65 kip on average. Part of the load from the east and the west walls was redistributed to the east side of the south and the north walls. The column loads on the

east side of the south wall increased from about 500 kip to 800 kip. The column loads on the east side of

the north wall increased from about 250 kip to 400 kip.

Figures 8-50 and 8-51 illustrate the load redistribution among the exterior wall and core columns at Floor 83 before aircraft impact and at 43 min, respectively. The tilting of the building about an axis

through the shaded area in Fig. 8-51 followed the buckling of the east wall and weakening of the core. Comparison of column loads before aircraft impact and when the east wall became unstable shows the columns unloading over the width of the east face and increasing at the east side of the south and north

walls.

Figures 8-52 and 8-53 show the maximum elastic-plus-plastic-plus-creep strains in the columns between

Floor 78 and Floor 83 at 20 min and 43 min, respectively. The elastic-plus-plastic strains, which were

less than 0.05 percent before the aircraft impact, reached 0.60 percent in some exterior columns and

0.35 percent in some core columns after the aircraft impact, typically those adjacent to severed or heavily

damaged columns. With increasing temperatures the plastic and creep strains increased, especially on the

east wall and the east side of the core. When the east wall buckled, the elastic-plus-plastic strains reached

their maximum of 2.2 percent in the east wall and 0.9 percent in the east side core columns. Creep strains were 1 .0 percent to 2.0 percent in the east wall, about 2.0 percent to 6.0 percent in the core columns, and

about 4.0 percent to 5.0 percent in the east side of the north wall.

NISTNCSTAR 1-6, WTC Investigation 265
Chapter 8

20 25 30 35

Time, min

40 45

Figure 8-40. Variation of maximum out-of-plane displacement on the east wall of WTC 2 over time for Case D.

266 NISTNCSTAR 1-6, WTC Investigation
structural Response of WTC Towers

NISTNCSTAR 1-6, WTC Investigation 267
Chapter 8

501 508

(^^^ (^) QlQ)

^^30| ^0^(l^

-(D6 1014 (882|

(1207) (1243) M304|

3?

1001 N

2200 ) (^^^^ C^^^

(^2705^

1008

Figure 8-43. Core column loads (kip) at Floor 83 of WTC 2 at 20 min for Case D

(compression is positive).

501

(^^^ (^818^

f2346

508

2340

-a4

^210) (^85) (843) l^^^S'

747) 1483

1€

1001

1893 )(1 100) (2215

1850

N

2328 0

1008

Figure 8-44. Core column loads (kip) at Floor 83 of WTC 2 at 43 min for Case D

(compression is positive).

268 NISTNCSTAR 1-6, WTC Investigation
structural Response of WTC Towers

ANSYS

Undeformed

building edge

Figure 8-45. Total displacements of WTC 2 above Floor 86 at 43 min of Case D

(deformed shape magnified 20 times). Note the tilt toward east and south.

Figure 8-46. Axial force in the east wall columns at Floor 83 of

WTC 2 for Case D (compression is positive).

NISTNCSTAR 1-6, WTC Investigation 269
Chapter 8

1 \

1 u —Before Impact

- —After Impact - —20 min

, —43 min

1

400

300

200 o

160 150 140 130 120 110 100

Column ID.

Figure 8-47. Axial force in the west wall columns at Floor 83 of

WTC 2 for Case D (compression is positive).

1500

460 450 440 430 420 410 400

Column ID.

Figure 8-48. Axial force in the south wall columns at Floor 83 of WTC 2 for Case D

(compression is positive).

270 NISTNCSTAR 1-6, WTC Investigation
Structural Response of WTC Towers

600

260 250 240 230 220

Column ID.

210 200

Figure 8-49. Axial force in the north wall columns at Floor 83 of WTC 2 for Case D

(compression is positive).

Scale

5,000 kip

Q 2,500 kip

O 1,000 kip

O 300 kip

501 508 OOO03OOO

o o o oo o o o

Q Q OooO Q Q O O O oo O O

O O O OO o o o OOOCDOOO

1001 1008

N

Figure 8-50. Axial force in Floor 83 columns of WTC 2 before impact for Case D

(compression is positive).

NISTNCSTAR 1-6. WTC Investigation 271
Chapter 8

Scale

5,000 kip

Q I 2,500 kip

O 1,000 kip

® 300 kip

Figure 8-51. Axial force in Floor 83 columns of WTC 2 at 43 min for Case D

(compression is positive).

OJIOCXxaxaxxxxDOOOOCXxxxxDOOOoccxxxxxxxxxx^

Figure 8-52. Maximum elastic-plus-plastic-plus-creep strains at 20 min for columns

between Floor 78 and Floor 83 of WTC 2 for Case D (strain values are in percent).

272 NISTNCSTAR 1-6, WTC Investigation
structural Response of WTC Towers

XXXXXX30CXCOXOCOOOOCOCOCOCXXXXXXOOCIOOOOOOOOOOOOOO

Figure 8-53. Maximum elastic-plus-plastic-plus-creep strains at 43 min for columns

between Floor 78 and Floor 83 of WTC 2 for Case D (strain values are in percent).

At 43 min, the core had weakened on the east side and shortened by 3.0 in. at the southeast comer. The

east wall had bowed inward approximately 62 in. and unloaded to the core and the adjacent north and

south walls. The inward bowing of the east wall caused failure of exterior column splices and spandrels,

and induced column instability. The instability progressed horizontally across the entire east face. The

east wall unloaded and redistributed its loads to the thermally weakened core through the hat truss and to the east and west walls through the spandrels. This load redistribution is shown in Tables 8-5 and 8-6.

The building section above the impact zone began tilting to the east (and to the south, although to a lesser extent) as column instability progressed rapidly from the east wall along the adjacent north and south

walls, and increased the gravity load on the weakened east core columns. The change in potential energy

due to downward movement of building mass above the buckled columns exceeded the strain energy that could have been absorbed by the structure. Global collapse then ensued.

8.5.3 WTC 2 Hat Truss Members and Connections

The state of the hat truss members and the connections were checked as the global model did not include

break elements to capture column and hat truss splice failures or sufficient beam elements to capture

buckling of hat truss outriggers. The condition of the connections and the members in the primary load

path of the hat truss was evaluated at different time intervals. The evaluation included the core column

splices for tension, outriggers and supporting columns for compression, and the hat truss connections that were in the primary load path for tension.

In the WTC 2 global model, the hat truss was part of the super-element above Floor 86. The elastic model that generated the stiffness matrix for the super-element, referred to as the "top model" hereinafter, was used to determine component forces. The displacements obtained at the interface nodes between the

NISTNCSTAR 1-6, WTC Investigation 273
Chapter 8

super-element and the nonlinear portion of the building (Floor 86) were applied to the base of the top

model for each analysis step.

Figures 8-54, 8-55, and 8-56 show the loads on the core column splices at the hat truss level at different

steps of the analysis. Each splice was under compressive load before the aircraft impact. After the

aircraft impact, the splices at severed core column lines started to carry tensile loads. The tensile capacity

of the splices was compared to tensile forces at 40 min, which was when the maximum tensile forces

occurred. In calculating the tensile capacity of the connections, AISC-LRFD procedures were used.

501 508

1001 N 1008

Figure 8-54. Axial force in core columns (kip) at WTC 2 Floor 105 (at hat truss level)

before impact for Case D (compression is positive).

274 NISTNCSTAR 1-6, WTC Investigation
structural Response of WTC Towers

501 508

1001 N 1008

Figure 8-55. Axial force in core columns (kip) at WTC 2 Floor 105 (at hat truss level) after

impact for Case D (compression is positive).

NISTNCSTAR 1-6, WTC Investigation 275
Chapter 8

501 508

1001 N 1008

43 min

Figure 8-56. Axial force in core columns (kip) at Floor 105 (at hat truss level) of WTC 2

for Case D conditions (compression is positive).

The evaluation of core column splices required an iterative procedure as splice failures were not modeled

in the top model. In the first iteration, the top model reached equilibrium using the interface node

displacements at 40 min. Once equilibrium was reached, the columns exceeding their splice capacity were identified (in the first iteration columns 1001 and 1002 were identified) and removed from the top

model. Before removing the columns, the displacement boundary conditions applied at the bottom of

these column lines (at Floor 86) were replaced with the reaction forces that were obtained at the end of

the first iteration. This conversion from displacement to force boundary condition allowed the remaining

portion of the column lines to displace in the vertical direction when the columns were removed at Floor 105 to simulate splice failure. This iterative procedure was repeated until none of the remaining

splices exceeded their tension capacity. A stable state was reached at the end of the fourth iteration. Fig. 8-57 shows the state of the core column splices at the end of the fourth iteration. Splices for columns 1001 and 1002 failed after impact, and splices for columns 701, 801, 901, 902, and 1003 failed

either after impact or as the core responded to the fires.

In the global analyses, splice failures were not included in the super-element, which remained elastic throughout the analysis. However, based upon the analyses discussed below, it was concluded that the

inclusion of splice failures would not have significantly affected the load redistribution in the global

analysis. The core floors would have redistributed the loads in the failed columns to adjacent core columns, as occurred for columns with failed splice connections in the impact area. The adjacent

columns would have then transferred the loads to the hat truss.

276 NISTNCSTAR 1-6, WTC Investigation
structural Response of WTC Towers

501 508

o o o oo o o o

o o o oo o o o

• o ooo o o o

• o ooo o o

• • o oo o o o

• • • oo o o o

1001 ^ |y| 1008

Failing splices right after impact Splices failing after load redistribution

Figure 8-57. State of core column splices at Floor 105 of WTC 2.

To quantify the amount of load that was redistributed through the hat truss to the adjacent core columns,

the total column loads on each face and at the core area were extracted at Floor 105. After several

iterations, it was determined that about 73 percent of the released tension load was transferred through the

hat truss, and 27 percent was transferred through the core floors. As a result of this redistribution, the

loads on the southeast comer outriggers were reduced, but the loads on the adjacent outriggers were

increased.

The axial stresses in the outriggers increased as a result of the aircraft impact damage. The maximum

axial stress of 28.4 ksi before aircraft impact increased to 55.0 ksi after the buckling of the east wall. The

specified grade of steel for the outriggers was 50 ksi. The NIST tests conducted on this grade of steel resulted in an average yield strength of about 54 ksi. Considering this yield strength and the 10 percent

increase in the hat truss forces due to neglecting the construction sequence, it was concluded that the

outriggers of the hat truss did not exceed their elastic limits.

To check the buckling capacities of the outriggers, the AISC LRFD procedures were used with an

effective length factor K equal to 0.75 and a resistance factor of 1 .0. The calculated capacities were compared with the axial compressive forces and corrected to account for the axial load increase due to construction sequence. The location of the outriggers and the supporting columns are shown in Fig. 8-58.

Table 8-9 summarizes the demand-to-capacity ratios for the outriggers. Except for Outrigger L, none of

the outriggers exceeded their buckling capacities. Outrigger L was located at the southeast comer of the core (the comer where the core columns were severed due to aircraft impact).

1^

NISTNCSTAR 1-6, WTC Investigation 277
Chapter 8

Figure 8-58. Location and IDs of outriggers and supporting columns

Table 8-9. Demand-to-capacity ratios for outriggers of WTC 2 for Case D conditions

(outrigger IDs are shown in Fig. 8-3).

Outrigger ID Bfr. Imp. Aftr. Imp. 10 min 20 min 30 min 40 min 40* min 40** min 43 min

West

A 0.23 0.61 0.61 0.64 0.66 0.63 0.63 0.38 0.25 B 0.19 0.30 0.30 0.32 0.33 0.31 0.31 0.25 -0.03*** C 0.19 0.27 0.27 0.28 0.30 0.27 0.28 0.22 -0.07 D 0.21 0.18 0.18 0.19 0.21 0.18 0.20 0.19 -0.29 North

E 0.29 0.1

0.10 0.10 0.13 0.14 0.17 0.09 -0.01 F 0.21 0.10 0.08 0.08 0.10 0.12 0.13 0.11 0.08 G 0.20 0.11 0.08 0.08 0.11 0.13 0.14 0.13 0.10 H 0.30 0.21 0.16 0.18 0.19 0.18 0.19 0.24 0.24 East

I 0.22 0.39 0.35 0.34 0.45 0.43 0.42 0.41 -.0.18 J 0.18 0.56 0.52 0.53 0.66 0.65 0.61 0.71 0.02 K 0.18 0.62 0.58 0.59 0.72 0.72 0.66 0.79 0.09 L 0.22 1.12 1.09 1.12 1.24 1.30 1.11 0.00 0.72 South

M 0.30 0.68 0.64 0.64 0.70 0.75 0.52 0.72 0.87 N 0.22 0.34 0.32 0.33 0.36 0.38 0.28 0.34 0.40 O 0.22 0.30 0.28 0.30 0.32 0.33 0.27 0.31 0.33 P 0.31 0.51 0.50 0.52 0.53 0.53 0.50 0.57 0.49

* After load redistribution due to core column splice failures. After Outrigger L was removed.

* Negative value indicates tension

278 NISTNCSTAR 1-6, WTC Investigation
Structural Response of WTC Towers

With the identified splice failures in Columns 1001 and 1002 and adjacent core columns, the load in this outrigger would have been redistributed to other outriggers. Based on the computed load redistribution

after splice failures, the demand-to-capacity ratio on Outrigger L was estimated to be reduced from 1.3 to

1.1 (Column "40* min" in Table 8-9).

Outrigger L was removed from the top model after all the failed splices were removed to determine the

effect on adjacent outriggers. Removal of the Outrigger L represented an upper bound solution as the

load in the Outrigger would not have dropped down to zero. The adjacent outriggers increase in load; however, after the removal of the Outrigger L as presented in Column "40** min" of Table 8-9, none of

the remaining outriggers exceeded their buckling or yield capacities.

The cormections within the hat truss were also checked. The hat truss connections in the primary load

path were identified, and their capacities were compared to their forces. The primary load path was

identified by selecting hat truss members with an absolute axial stress of 25 ksi or more at 40 min, as this was when maximum forces occurred. Only the connections that transferred tensile forces were evaluated.

In calculating the capacity of the connections, the AISC-LRFD procedures were used. None of the hat

truss connection capacities were exceeded. Before redistribution of load due to the column-to-hat truss splice failure, none of the hat truss connections had exceeded their capacities except for the hat truss connections associated with the 1001 core column. After the load redistribution following the splice

failure, the demand on the hat truss connections for the 1001 column was less than the yield capacities of

all connections. It was concluded that the hat truss was capable of transferring loads from core columns

to the outriggers.

Based on this discussion, it was concluded that the hat truss transferred the majority of the loads between

core and exterior wall columns, even though some column splices may have failed and one outrigger may

have buckled.

8.6 STRUCTURAL RESPONSE OF THE WTC TOWERS TO FIRE WITHOUT

IMPACT DAMAGE

Whether the towers would have collapsed if subjected to an intense but conventional fire without aircraft impact was considered to better understand the relative roles of the impact damage and fires. This is not

to imply that the fire growth and spread observed in the towers could be obtained without aircraft damage

to the buildings and rapid ignition of multi-floor fires due to the dispersion ofjet fuel. NIST used the

observations, information, and analyses developed during the Investigation to enable the formulation of

probable limits to the damage from such a fire. Since a complete analysis beyond the actual collapse times

of the towers was not conducted, the findings in this section represent NIST's best technical judgment

based upon the available observations, information, and analyses.

In making the comparison, the following points were considered.

• Both WTC 1 and WTC 2 were stable after the aircraft impact. The global analyses showed

that both towers had considerable reserve capacity after structural impact damage. For

example. Figs. 8-9 and 8-10 show the core column demand-to-capacity ratios remained

nearly the same before and after impact, except for a few columns adjacent to the severed

columns. Global analysis produced similar trend for the exterior columns. This was

confirmed by analysis of video footage of the post-impact vibration of WTC 2, the more

NISTNCSTAR 1-6, WTC Investigation 279
Chapter 8

severely damaged of the two towers, which showed that the period of vibration of the building before and after impact were nearly the same, thus showing that the building had

significant reserve capacity. WTC 2 oscillated with a peak amplitude that was between 30

percent and 40 percent of the tower sway under design winds and at periods nearly equal to the first two translation and torsion mode periods calculated for the undamaged strucmre (see

NIST NCSTAR 1-2).

• Results of both the multi-workstation experiments and the simulations of the WTC fires showed that the combustibles in a given location, if undisturbed by the aircraft impact, would

have been almost fully burned out in about 20 min. Note that, for the occupancies in the

World Trade Center, the fuel load was estimated—and supported by fire dynamics

calculations and visual observations—to be approximately 4 psf (see NIST NCSTAR 1-5).

• The fires used in the Investigation (Cases A through D), estimated from fire dynamics

simulations, represented fires that were far more severe than an intense conventional fire (see

NIST NCSTAR 1-5).

• In WTC 1, if fires had been allowed to continue past the time of building collapse, complete

burnout would likely have occurred within a short time since the fires had already traversed

around the entire floor and most of the combustibles would already have been consumed (see

NIST NCSTAR 1-5). During the extended period from collapse to bumout, the steel temperatures would hkely not have increased very much. The installed insulation in the fire- affected floors of this building had been upgraded to an average thickness of 2.5 in.

• In a fire simulation of WTC 2, that extended Case D for 2 hours with all windows broken

during this period, the temperatures in the truss steel on the west side of the building (where

the insulation was undamaged) increased for about 40 minutes before falling off rapidly as the combustibles were consumed. Results for a typical floor (floor 81) showed that temperatures of 700 °C to 760 °C were reached over approximately 1 5 percent of the west

floor area for less than 10 minutes. Approximately 60 percent of the floor steel had

temperatures between 600 °C and 700 °C for about 15 minutes. Approximately 70 percent of

the floor steel had temperatures that exceeded 500 °C for about 45 min. At these

temperatures, the floors would be expected to sag and then recover a portion of the sag as the

steel began to cool. Based on results for Cases C and D, the temperatures of the insulated

exterior and core columns would not have increased to the point where significant loss of

strength or stiffness would occur during these additional 2 hours. With intact, cool core columns, any inward bowing of the west exterior wall that might occur would be readily

supported by the adjacent exterior walls and core columns.

• In the simulations of Cases A through D, none of the columns and trusses for which the

insulation was intact reached temperatures at which significant loss of strength occurred for the duration analyzed. The relative effects of the presence or absence of insulation on

structural components, subjected to the same fire conditions, are shown in Fig. 8-59 (see

NIST NCSTAR 1-5) for both adjacent trusses and exterior columns. As the plots indicate,

the rate of heating was found to differ significantly depending on whether the insulation was

intact or not.

280 NIST NCSTAR 1-6. WTC Investigation
structural Response of WTC Towers

Figure 8-59. Temperatures of two adjacent trusses (left) and two adjacent perimeter

columns (right) exposed to simulated fires in WTC 1. Data plotted in blue are for

structural steel components with fireproofing; data in red are for steel components

without fireproofing (from NIST NCSTAR 1-5).

• Structural computer simulations of the floor system (Chapter 7), supported by results of full- scale fire tests (NIST NCSTAR 1-5) and performance observed in standard fire tests (NIST

NCSTAR 1-6B), showed that structural steel, insulated with Va in. thick fireproofing, would

not have reached temperatures greater than 650 C prior to burnout of the combustibles (20

min as noted above). Simulations also showed that variations in thickness resulting from

normal application, even with occasional gaps in coverage, would not have changed this

result.

• The structural temperatures of core columns in WTC 1 and WTC 2 did not exceed 300 °C

where the fireproofing was intact. Thermal analysis of the WTC 1 and WTC 2 floors and

exterior columns indicated that the steel temperatures were generally lower than 300 °C, with

a few isolated members that rarely exceeded temperatures of 400 °C for WTC 1 and 500 °C

for WTC 2 (NIST NCSTAR 1-5). Under these temperatures, reductions of stiffness and

strength were small and creep effects and buckling were found not to be significant (Chapters

4 and 7). Insulated floors thermally expanded and pushed outward on the exterior columns as well as sag m the full floor analyses, but the floor sag was insufficient to exert an inward pull on the exterior columns.

• Inward bowing of the exterior walls in both WTC 1 and WTC 2 was observed only on the face with the long-span floor system. In WTC 1 , this was found to be the case even though

equally extensive fires were observed on all faces. The impact damage to the north face

reduced the area over which pull-in could occur. In WTC 2, fires were not observed on the

long-span west face and were less intense on the short-span faces than on the east face.

NIST NCSTAR 1-6, WTC Investigation 281
Chapter 8

• Inward bowing was a necessary but not sufficient condition to initiate collapse. In both WTC

1 and WTC 2, significant weakening of the core due to aircraft impact damage and thermal

effects was also necessary to initiate building collapse.

• The tower structures had significant capacity to redistribute loads (a) from bowed walls to adjacent exterior walls with short-span floors via the arch action of spandrels, and (b)

between the core and exterior walls via the hat truss and, to a lesser extent, the floors.

In evaluating how the undamaged towers would have performed in an intense, conventional fire, NIST

considered the following factors individually and in combination:

• The temperatures that would be reached in structural steel components with intact insulation.

• The extent of the area over which high temperatures (e.g., greater than 600 °C where

significant thermal weakening of the steel occurs) would be reached at any given time.

• The duration over which the high temperatures would be sustained concurrently in any given

area.

• The length of the floor span (long or short) where high temperatures would be reached.

• The number of floors with areas where high temperatures would be sustained concurrently in the long-span direction.

• The potential for inward bowing of exterior walls (i.e., magnitude and extent of bowing over

the width of the face and the number of floors involved) due to thermally-induced floor

sagging of long-span floors and associated inward pull forces.

• The capacity of the structure to redistribute loads (e.g., via the spandrels, hat truss, and floors)

if the thermal conditions were sufficiently intense to cause inward bowing of the exterior

walls.

In addition, NIST considered the following known facts about the performance of the WTC tower

structures in fires:

• Historical fires also provided evidence that the towers would not collapse if subjected to a major fire without accompanying impact damage. WTC 1 did not collapse during the major

fire in 1975, which engulfed about 9,000 ft" on the southeast quadrant of Floor 11. The fire spread mostly via utility closets to ten floors. At the time, office spaces in the WTC towers were not sprinklered. The fire caused minimal damage to the floor system with the Vi in. specified insulation thickness applied on the trusses (four trusses were slightly distorted) and

at no time was the load carrying capacity of the floor system compromised. The fire "did not damage a single primary, fireproofed element. Some top chord members (not needed for

structural integrity), some bridging members (used to reduce floor tremor and the like) and some deck support angles (used only as construction elements) were buckled in the fire—all were unfireproofed steel." (SCHR Letter Report 1975).

282 NISTNCSTAR 1-6, WTC Investigation
structural Response of WTC Towers

• Additionally, the four Standard Fire Tests (ASTM E 1 19) of floor assemblies like those in the

WTC towers showed that the load carrying capacity of the short span 35 ft floor system with

a 0.75 in. insulation thickness was not compromised by heating for two hours at furnace

temperatures with applied loads that exceeded those on September 1 1

, 200 1 by a factor of

two. It took about 90 minutes of sustained heating in the furnace for temperatures to exceed

600 °C on steel truss members with either Yi in. or in. insulation thickness. The high

temperature conditions in the furnace tests were at least as severe and lasting as long as the

WTC fires, although the top of the slab was not heated. Although some web members

buckled and the floor test assembly sagged up to 14 in. during the tests, the insulation

remained intact during the tests.

From these points and observed performance, NIST concluded:

• In the absence of structural and insulation damage, a conventional fire substantially similar

to or less intense than the fires encountered on September 11, 2001 likely would not have led

to the collapse of a WTC tower.

• The condition of the insulation prior to aircraft impact, which was found to be mostly intact,

and the insulation thickness on the WTC floor system contributed to, but did not play a governing role, in initiating collapse of the towers.

• The towers likely would not have collapsed under the combined effects of aircraft impact and

the subsequent multi-floor fires encountered on September 11, 2001 if the insulation had not been widely dislodged or had been only minimally dislodged by aircraft impact.

These conclusions apply to fires that are substantially similar to or less intense than those encountered on

September 1 1 , 2001 . They do not apply to a standard fire exposure or an assumed fire exposure which

has (a) uniform high temperatures over an entire floor or most of a floor (note that the WTC floors were

extremely large) and concurrently over multiple floors and (b) high temperatures that are sustained

indefinitely or for long periods of time (greater than about 20 min at any location), and (c) combusted fire loads that are significantly greater than those considered in the analyses. They also do not apply if the

capacity of the undamaged structure to redistribute loads via the spandrels, hat truss, and floors were not accounted for adequately in a full 3-dimensional simulation model of the structure.

8.7 SUMMARY OF STRUCTURAL RESPONSE OF THE WTC TOWERS

The structural analyses conducted of floors, isolated exterior walls and cores, and global models of

WTC 1 and WTC 2 found that the collapse of the towers was due to the combined effects of structural and insulation damage from aircraft impact and the subsequent fires.

Impact damage alone did not cause collapse of the towers, as they were stable after the aircraft impact and

analyses showed that they had substantial reserve capacity. The fires alone also would not have caused

collapse of the towers. Without impact damage, there would not have been extensive dislodging of

insulation, and the structural steel temperatures would have been generally less than 300 °C, with a few

steel temperatures reaching 400 "C in WTC 1 floors and 500 °C in WTC 2 floors. The core would not have weakened, the floor sag would have been insufficient to pull inward on the exterior columns, and as

a consequence the exterior walls would not have bowed inward.

NISTNCSTAR 1-6, WTC Investigation 283
Chapter 8

Collapse occurred after the fires weakened areas of the core, floors, and exterior walls that had dislodged

insulation, and the core and exterior columns were unable to support the gravity loads with their reduced

capacity.

The towers would likely not have collapsed under the combined effects of aircraft impact and the

subsequent multi-floor fires if the insulation had not been dislodged or had been only minimally

dislodged by aircraft impact. The existing condition of the insulation prior to aircraft impact and the

insulation thickness on the WTC floor system did not play a significant role in initiating collapse of the towers.

8.8 REFERENCES

National Oceanic and Atmospheric Administration (NOAA)/National Climatic Data Center (NCDC),

(NOAA 200 la)" Automated Surface Observing System Data; Unedited Surface Weather Observations

- Hourly Data, LaGuardia Airport, September 11, 2001"

National Oceanic and Atmospheric Administration (NOAA)/National Climatic Data Center (NCDC),

(NOAA 200 lb)" Automated Surface Observing System Data; Unedited Surface Weather Observations

- Hourly Data, John F. Kennedy Airport, September 1 1, 2001"

National Oceanic and Atmospheric Administration (NOAA)/National Climatic Data Center (NCDC),

(NOAA 2001c)"Automated Surface Observing System Data; Unedited Surface Weather Observations

- Hourly Data, Newark, NJ Airport, September 1 1

, 200 1

284 NISTNCSTAR 1-6, WTC Investigation
Chapter 9

Probable Collapse Sequences

9.1 INTRODUCTION

World Trade Center (WTC) 1 and WTC 2 were subjected to aircraft impact and uncontrolled fires and

experienced a series of events that required complex analyses to determine their probable collapse

sequences. The analysis of these events required a formal approach to integrate multiple disciplines

effectively, to discern which parameters significantly influenced the analysis methods and results, and to determine the probable sequence of events leading to the initiation of structural collapse. These methods

were applied as appropriate to different scales of modeling—component, subsystem, and global scales

for the aircraft impact damage, fire dynamics, thermal, and structural response analyses.

To identify the probable collapse sequences, National Institute of Standards and Technology (NIST)

adopted an approach that combined mathematical modeling, statistical-based analysis methods, laboratory

experiments, and analysis of photographs and videos. The approach accounted for variations in models,

input parameters, analyses, and observ ed events. It included the evaluation and comparison of possible

collapse hypotheses based on various damage states, fire paths, and structural responses to determine the

following:

• The probable sequence of events from the moment of aircraft impact until the initiation of

global building collapse;

• How and why WTC 1 stood nearly twice as long as WTC 2 before collapsing (102 min for WTC 1 versus 56 min for WTC 2), although they were hit by virtually identical aircraft (Boeing 767-200ER);

• What factors, if any, could have delayed or prevented the collapse of the WTC towers.

Section 9.2 describes the methodology used to conduct the aircraft impact, fire dynamics, thermal, and

structural response analyses for determining the probable collapse sequence of each tower, which is presented in Section 9.3. Section 9.4 presents a discussion and summary of the collapse sequences.

To determine the probable collapse sequence for each tower, the following steps were required:

• identification of key observables, primarily from photographs and videos

• development of collapse hypotheses, which were updated periodically through the course of

9.2 METHODOLOGY

the investigation with the acquisition of new data and analysis results

NISTNCSTAR 1-6. WTC Investigation 285
Chapter 9

• sensitivity studies to identify influential parameters through the application of a formal

statistical approach, orthogonal factorial design (OFD)

• development and refinement of mathematical modeling—finite element analyses and

computational fluid dynamics

• evaluation of analysis results against observed and expected structural behavior, with

adoption of the event tree technique, and pruning and updating of the tree branches based

upon comparisons with observed data

These steps were applied to the degree needed in each phase of the analyses, from aircraft impact to fire spread, thermal loads, and structural response.

9.2.1 Key Observed Events and Conditions

Observations and data about the events following the aircraft impact were primarily obtained from three sources:

• Photographic and video records that had been catalogued and time stamped for the NIST

Investigation (NIST NCSTAR 1-5A)

• Interviews of individuals in the towers during the event and those contacted by individuals in the towers during the event (NIST NCSTAR 1-7)

• Interviews of emergency response persoimel and emergency communication records (NIST

NCSTAR 1-8)

Observations were used to develop timelines and refine collapse hypotheses for each tower. Key

observations were used to guide the towers' structural analysis and are summarized in the structural timelines (Chapter 6). Structural analyses were used to develop and refine understanding of the

sequences of events, particularly events near or in the core that could not be observed.

Observations were classified into two groups: key observations and noted observations. Key observations were significant structural events that were explicitly addressed in or used to validate the structural analyses. Noted observations were events that may have been linked to a structural response, but their significance could not be conclusively assessed.

Observables were used in all the analyses in three ways: (1) to determine input parameters, such as the

aircraft speed and direction at impact, (2) to impose time-related constraints on the analysis, such as imposing observed broken windows over time to constrain the spread of fire, or (3) to validate analysis

results, such as global stability after impact and during thermal loading.

9.2.2 Collapse Hypotheses

Collapse hypotheses were developed over the course of the NIST Investigation. The first hypotheses were published in the May 2003 NIST Progress Report, and were updated in the June 2004 Progress

Report and October 2004 Public Meeting at NIST. The Probable Collapse Sequence for each tower was

286 NIST NCSTAR 1-6, WTC Investigation
Probable Collapse Sequences

presented at the April 2005 Public Meeting in New York City. The stages of hypothesis development are summarized as follows:

• Possible Collapse Hypotheses (May 2003) - not building specific; key events not identified

• Working Collapse Hypothesis (June 2004) - single hypothesis for both WTC towers;

identified chronological sequence of major events

• Leading Collapse Hypotheses (October 2004) - separate hypothesis for each WTC tower;

identified building-specific load redistribution paths and damage scenarios in addition to chronological sequence of major events

• Probable Collapse Sequences (April 2005) - refined building-specific collapse sequences

with chronological sequence of major events, load redistribution paths, and damage

scenarios.

Over the course of the investigation. NIST continued to investigate technical issues and modify or refine

the collapse hypotheses for each tower as needed. Technical issues that were analyzed and refined during

the investigation included:

• Aircraft impact damage to structural components, insulation, and partition walls.

• Dispersion of aircraft debris and damage to building contents.

• Thermal effects on core columns and floors, especially extent and movement of fires.

• Thermal effects on exterior columns, especially temperature gradients in columns.

• Extent of load redistribution within and between core columns and exterior wall columns and

their reserve capacity to accommodate added gravity loads with thermal effects.

• Capacity of hat truss to accommodate load redistribution from severed columns.

• Capacity of bolted splices in the severed core columns to carry tensile loads to the hat truss.

• Relative magnitude of the load redistribution provided by the hat truss, local core floor, and

the truss floor system for each tower.

• Axial/shear/bending capacity of floor connections to core and exterior columns.

• Mechanisms to propagate instability laterally in the exterior columns

• Capacity of spandrels, including splices, to transfer shear in the exterior walls.

• Role of bolted splices in the instability of exterior columns.

• Comparison and reconciliation of hypotheses with observed facts (photographs and videos,

eyewitness accounts, emergency communication records).

NISTNCSTAR 1-6. WTC Investigation 287
Chapter 9

The possible collapse hypotheses published in May 2003 were developed by NIST and considered several

leading hypotheses that had been postulated publicly by experts. These are summarized in Appendix C,

Table C-1. One hypothesis suggested that the load carrying core columns were weakened by the fires and

failed, initiating overall building collapse without the need for any weakening or failure of the steel truss floor system. Another hypothesis suggested that significant portions of one or more floor truss systems

sagged, as they were weakened by fires, pulling the exterior coluinns inward via the connections to

initiate overall building collapse through combined compression and bending failure of the exterior

columns. A variation of this hypothesis suggested that the sagging floor system failed in shear at its connections to the columns, leading to overall building collapse initiation through buckling failure of the

exterior columns. Load eccentricities introduced by partially damaged floor systems could also have

contributed to buckling failure of the columns.

The working collapse hypothesis published in June 2004 was developed to explain the collapse initiation of the WTC towers. The working hypothesis (summarized in Appendix C, Table C-2) identified the

chronological sequence of major events as the WTC tower structures redistributed loads from one

structural element to another to accommodate the aircraft impact and subsequent fire damage until no

further load redistribution was possible, thus, leading to collapse. The working hypothesis was based on

analysis of the available evidence and data, consideration of a range of hypotheses (including those

postulated publicly by experts), and the understanding of structural and fire behavior at that time. It allowed for multiple load redistribution paths and damage scenarios for each building.

The leading collapse hypotheses for WTC 1 and WTC 2 that were presented in October 2004 are shown

in Appendix C, Figs. C-3 and C-4. A separate collapse hypothesis was developed for each tower that identified load redistribution paths and damage scenarios for each major event. The leading hypotheses

accounted for the WTC structural system, aircraft impact and subsequent fires, post-impact condition of

insulation, the quality and properties of the structural steel and concrete, and the relative roles of the

exterior and core columns and the composite floor system, including connections. The hypotheses were

consistent with evidence held by NIST (at that time). They were based on the subsystem analysis

described in Chapter 7.

The Probable Collapse Sequences for WTC I and WTC 2 were presented in April 2005 following

completion of global structural response analyses and are shown in Section 9.3. The structural sequences

of events were consistent with evidence held by NIST.

9.2.3 Mathematical Modeling - Analysis Interdependencies

Events that played a significant role in the structural performance of the towers were the aircraft impact,

rapid ignition of fire on multiple floors, and the growth and spread of fire in each tower. To determine the

structural response, detailed information was required on the condition of the structural system and its passive fire protection system both before and after the aircraft impact and during the ensuing fires that elevated temperatures in the structural members.

The interdependence of the various analyses is illustrated in Fig. 9-1. Reference structural models were

developed before other structural models to determine the baseline performance of each tower prior to September 1 1, 2001. The reference models were used as a basis for the aircraft impact damage models

and the structural response and failure models to ensure consistency between structural models. The

288 NISTNCSTAR 1-6, WTC Investigation
Probable Collapse Sequences

aircraft impact analysis determined damage to the exterior and the interior of the building and included

the strucmral system, insulation, partition walls, and fiimishings for each tower. The analysis also

provided an estimate of the fuel dispersion in the towers. These results provided initial conditions for the

fire dynamics analysis, thermal analysis, and structural analysis. The fire dynamics analysis simulated the

growth and spread of fires and produced gas temperature histories for each floor subjected to fire. The fire dynamics model accounted for damage to interior partition walls and floors (which affected ventilation

conditions) and the distribution of debris and fuel.

Reference Structural

Models Baseline

Performance

SAP Analysis

1 SAP

Reference

Model

Conversion

J

Resolution

1-4 in. lO'^s

Aircraft Impact

Damage Analysis

LS-DYNA

Time scale range be^A/een analyses:

10 orders of magnitude

Length scale range between analyses:

5 orders of magnitude

/Reference / Model

\ Conversion

Fire Dynamics

Analysis

FDS

Gas Temperature

Histories

Thermal Analysis

ANSYS

JL

Structural Temperature

Histories

Resolution

50 cm

Resolution

1-2 cm

1 s

Structural Model

Conversion

Structural Response and

Failure Analysis

ANSYS

Resolution

1 to 60 in. 600 s

Collapse Sequence

Figure 9-1. Critical analysis inter-dependencies.

The thermal analysis used a solid element heat transfer model to determine temperature histories for the

various structural components accounting for the presence or dislodgement of insulation. The thermal

analysis required input from the structural analysis model, fire dynamics analysis results, the analysis of

damage to insulation, and temperamre-dependent thermal material properties. The structural temperature

histories, also referred to as thermal loads, were input to the structural analysis, along with the structural impact damage and temperature-dependent material properties, to determine the structural response of

each tower.

NISTNCSTAR 1-6, WTC Investigation 289
Chapter 9

9.2.4 Sensitivity Studies to Identify Influential Variables

Sensitivity studies were conducted for the aircraft impact, fire dynamics, and thermal analyses to identify

the most influential parameters for component, connection, and subsystem behavior. To identify the most

influential parameters, an orthogonal factorial design process was used to design analysis 'experiments'

(Box, 1978). Numerical experiments with an orthogonal factorial design (OFD) method were conducted

for detailed models of components and subsystems to identify parameters that strongly influenced the

analysis results:

• Only parameters whose values were not accurately known were selected (parameters that were known with near certainty were set to the known values).

• Selected parameters were varied within a range of likely values, determined from available

data and assigned three alternative values: lower value (-), central value (0), upper value (+).

The OFD approach allowed for identification of influential parameters that reduced the number of

analysis runs at the global level. The influential parameters for the structural response analyses included

the aircraft impact analyses through the impact damage and temperature histories that were part of the

required input data. To determine structural response to damage and thermal loads, numerous component

and subsystem studies were conducted that identified critical structural behavior and failure mechanisms

and how they varied with temperature. Structural behaviors that were studied included restrained thermal

expansion, thermal weakening of columns and floors, floor sagging and associated inward pull on exterior

wall, and load redistribution through major structural subsystems. Failure mechanisms that were studied

included, for example, tensile failure of core column splices and hat truss connections, column buckling,

or loss of composite action in the floor system.

The influential parameters that were identified for each analysis, based on available information, were

used to create three input data sets. Figure 9-2 illustrates the analysis tree with all influential parameter

combinations resulting from this procedure for the three likely values, a lower value, a central value, and

an upper value. It is apparent that analysis of all possible combinations required the number of analyses

at each level to increase by a factor of three. The number of global structural response analyses was

prohibitive with this approach.

However, computational analyses provided valuable insight into the relationship between input and

output data for the aircraft impact, fire dynamics, thermal, and stmctural response analyses. These

insights, along with the sensitivity studies, enabled significant reduction of the number of scenarios that were analyzed. Figure 9-3 shows the final pruned analysis tree, which was obtained as follows. After the

aircraft impact analysis results were evaluated for the three sets of input parameters, the less severe damage case was discarded (pruned) as it did not reasonably match key observables. The base and more

severe damage cases were each analyzed for fire growth and spread (FDS) and for the corresponding

temperature histories of structural components (FSI). The linkage between the aircraft impact, fire dynamics, and thermal analyses for each damage case created highly correlated sets of input data and

analysis results. For instance, the damage from the severe aircraft impact case provided input data for analysis of the fires corresponding to the severe impact damage, and both analyses provided input data for

the thermal analysis of structural components subject to severe impact damage and the corresponding

fires. The high level of correlation between the linked sets of aircraft impact, fire, and thermal analyses,

as well as similar results for alternative fire conditions for the same impact damage, led to a single branch

290 NISTNCSTAR 1-6, WTC Investigation
Probable Collapse Sequences

at each successive analysis, as shown in Fig. 9-3. The temperature histories for the base and more severe

cases (referred to as Case A and Case B for WTC 1 or Case C and Case D for WTC 2 elsewhere in this report) were used in the structural analysis of major subsystems—the isolated core, a full floor, and the

exterior wall analyses. The results of the subsystem analyses showed that the more severe case impact

damage results better matched key observables. The subsystem analysis results led to the prunmg of the

global structural analysis for the base case impact damage sub-tree, as shown in Fig. 9-3. Consequently,

only the more severe cases (Cases B and D) were used in the global analysis of each tower.

Tables 9-1 to 9-4 list the observables used for the validation of analysis results, the significant input data,

influential parameters, and significant output for each analysis.

Tower before Impact

Less Severe Damage

Combustibles, partitions

Fireproofing

Base Impact Damage

Combustibles, partitions

Fireproofing

More Severe Damage

Combustibles, partitions

Fireproofing

FDS FDS FDS FDS FDS FDS FDS FDS FDS

FSI

FSI

FSI

(0)

FSI (+)

FSI

FSI

(0)

FSI

)

FSI (0)

FSI

FSI

FSI

FSI

(0)

FSI

(-)

FSI

FSI

(0)

FSI

FSI

FSI

:o)

FSI iH

{•) FSI (-)

FSI

FSI

FSI 0)

FSI

FSI (+)

FSI (0)

FSI

SIR

STR

-il STR

ST

ST

STR

ST ~- STR

Hi STR

{);

m

STR

STR !0) J

\1 STR (+) 1J ^ 'Hi STR

STR

STR

» -

o)-:J

STR

STR iO)

[1 STR

STR

STR 0) J

STR (+)L:

I

STR

: str|o)

C STR

STR

STR

J: STR [+?

Figure 9-2. Full analysis tree for influential parameter effects

NISTNCSTAR 1-6. WTC Investigation 291
Chapter 9

Tower before Impact

r

Influential

Parameters

Highly

Correlated by

Aircraft

Impact

Influential

Parameters

Largely

Independent

of Aircraft

Impact Result!

Less Severe Damaqe Base Impact Damaqe More Severe Damaqe

Combustibles, Combustibles, Combustibles,

partitions partitions partitions

Fi reproofing Fireproofing Fireproofing

Structural Structural Structural

Pruned FDS

FSI

Pruned

FDS

FSI

STR

Figure 9-3. Pruned

Table 9-1.

analysis tree for influential parameter effects.

Aircraft impact analysis parameters.

Validation Data Model and Analysis

Observables from

Photo/Video

Observables

from

Interviews

Significant Input Influential

Parameters

Significant Output

Impact damage to exterior wall

Engine/landing gear

exit location and

speed

Aircraft impact

conditions for model

input (velocity,

location, orientation

to building)

Stairwell damageStructural model with

service loads

Aircraft model with proper contents and mass

Floor content layout

Material properties for high strain rates

Aircraft velocity

Aircraft pitch

Aircraft mass

Aircraft material failure

strain

Tower steel failure

strain

Partition strength

Live load weight

Structural damage

Debris path

(insulation and

partition damage)

Fuel path

292 NISTNCSTAR 1-6, WTC Investigation
Probable Collapse Sequences

Table 9-2. Fire dynamics analysis parameters

I alidation Data Model and Analysis

Observ ables from

Photo/Video

Observables

from

Interviews

Significant Input Influential Parameters

Significant Output

Fire near window s None Ventilation sources from Average fuel density VJ ex. k> IvIllL/WlCllLll t vs location and time debris damage

Distribution of intact Smoke out w indows Added fuel from aircraft contents vs rubble

vs location and time

Fuel distribution after Shaft ventilation in Window breakage vs aircraft impact core

location and time

Window openings vs time

Floor content layout

Partition damage

Table 9-3. Thermal analysis parameters

V alidation Data Model and Analysis

Observ ables from

PhotoA ideo

Observables

from

Interviews

Significant Input Influential Parameters

Significant Output

None None Thermal models of Insulation initial Structural structure condition temperature histories

Insulation initial condition Estimated insulation damage

Estimated insulation damage

Gas temperatures

NIST NCSTAR 1-6. WTC Investigation 293
Chapter 9

Table 9-4. Structural response analysis parameters.

Evidence/ Data Model and Analysis

Observables from

Photo/Video

Observables

from

Interviews and

Recordings

Significant Input Influential Parameters

Significant Output

Initial Stability

Floors sagging at windows

Exterior wall inward

bowing and

instability

NYPD Aviation

Unit first responder

communications

Initial structural condition after impact

Structural temperature

histories

Pull-in force location and magnitude

Pull-m force location and magnitude

Floor disconnections

Creep strain

Probable collapse

sequence

Sequence of

component and

subsystem failures,

including instability

of exterior wall

Tilt of building

section above

impact during

collapse

Global stability vs time

9.2.5 Evaluation of Collapse Hypotheses

Development and validation of the probable collapse sequence for each tower was shaped by evidence

gathered in the investigation, including photographs and videos, design and maintenance documents, and

eyewitness accounts. Photographs and videos provided knowledge about aircraft impact damage to the

tower exterior walls, fire growth and spread at the building exterior, inward bowing of an exterior wall in each tower, and the direction of tih for the building section above the impact zone as the towers collapsed.

Eyewitness accounts provided some information about the interior conditions surrounding the impact

areas, but the descriptions tended to be general in nature and often did not provide locations or specifics

within a floor level. Figure 9-4 lists data (primarily based on photos and videos except for the

metallurgical measurements) used to determine input data, impose time-related constraints, and validate

analysis results for determining the probable collapse sequences.

The use of observables as a constraint had the important effect of reducing the uncertainty in the analysis

results. The time and frequency of the applied constraints affected the degree to which the analysis

uncertainty was reduced.

Figure 9-5 illustrates conceptually how the variance (or uncertainty) of the global stability of the towers

(indicated here by the global reserve capacity RC) changed from the time of impact to the time of

collapse. The shaded band qualitatively indicates the degree of uncertainty in RC at each time / after considering the analysis results and the observations made prior to /, except for collapse. The aircraft impact caused a reduction in the towers strength, but substantial reserve capacity remained afterward.

The combined effect of the impact damage and fires caused a gradual reduction of the global capacity.

The initial period of heating caused minimal changes in the structural capacity, but as time progressed,

294 NISTNCSTAR 1-6, WTC Investigation
Probable Collapse Sequences

various events occurred that caused a sudden or more rapid loss of global capacity. For instance, failure of critical columns from thennal weakening or inward bowing of an exterior wall may are events

associated with a rapid loss of global capacity.

Based only on model predictions, the variance (or uncertainty) of the global reserve capacity grew with

time. However, whenever an observ able matched analysis results, it reduced the uncertainty in the analysis results. Ahematively, when the observables were used to constrain model parameters and adjust

results to be consistent with observations, the variance of the global reserve capacity and the sequence of

events that took place were reduced. As the structural analyses approached the time to collapse, the

ability of the analyses to match the time to failure depended upon the variance in the analysis results. WTien considering the sequence of structural events and time to failure, it was more important to match

the sequence of events as the time to collapse initiation was influenced by adjustments in influential parameters and imposition or matching of observables. As a result of using observables to constrain

model parameters and analysis results, NIST believes that the probable collapse sequences that were

determined are highly robust. The times to failure for the collapse sequences, however, are subject to considerable variability, particularly since they are sensitive to small changes in the magnitude of the

pull-in forces.

Aircraft Impact

• Impact damage to exterior wall

• Engine exit location and speed

• Exit areas for debris

• Aircraft impact velocity, location, and orientation to building

• Stairwell damage

Fire/Thermal

• Fire m windows vs. location and time

• Smoke vs. location and time

• Window breakage vs. location and time

Material Properties

• Mechanical and metallurgical properties of recovered steel

Structural Response

• Global stability after impact and during thermal loading

• Floors draped in windows

• Inward bowing of exterior columns

• Tilting of building section above impact and fire zone

• Time to coUaose

Figure 9-4. Data used for input, constraints, and validation of probable collapse

sequences.

NISTNCSTAR 1-6, WTC Investigation 295
Chapter 9

Global

Reserve

Capacity

RC(t)

RC(t=0)

Capacity

Before

Impact

RC(te)-0

Capacity'

After

Impact

Events with significant

contribution to change in global capacity

Variability in time

to collapse

0 10 20 30.... tcollapse Time (min)

Figure 9-5. Variability in global reserve capacity using model predictions and

observables for sequential analyses with imperfect information.

9.3 PROBABLE COLLAPSE SEQUENCES

The following four structural events that were common to both towers are part of the sequence of events

described:

• Floor sagging was caused by elevated steel temperatures resulting from loss of insulation.

Substantial sagging of the floor resulted in pull-in forces at column connections, and led to inward bowing of the exterior wall. Calculations, supported by the four Standard Fire Tests,

showed that the most likely cause of floor sagging was buckling of the truss web diagonals,

as shown in Fig. 9-6. In the figure, the left portion of the truss maintained flexural stiffness,

but the right end lost some flexural stiffness as a result of extensive web buckling. The

resultant sagging produced tensile forces in the floor system which was approximated by a combination of flexural and catenary behaviors as shown in Fig. 9-7. A floor system with

tensile forces at its connections does not restrain the exterior wall from bowing inward.

• Bowing and plastic buckling of an exterior wall under the combined effects of elevated

temperatures, redistributed gravity loads, pull-in forces from sagging floors, and loss of

lateral support due to failure of truss seat connections.

• Weakening of the core columns (which was resisted by the hat truss) was caused by the

combined effects of structural impact damage, redistributed gravity loads, elevated

temperatures, plastic and creep strains, and plastic buckling of core columns.

296 NISTNCSTAR 1-6. WTC Investigation
Probable Collapse Sequences

Redistribution of gra^ity loads resulted from impact damage, restrained thermal expansion,

core weakening, leaning of the tower section above the impact damage, and bowing and

buckling of exterior walls. Redistribution of gravity loads between the core to the exterior

walls occurred primarily through the hat truss, while load redistribution between adjacent

exterior walls occurred primarily through the spandrels. Restrained thermal expansion

occurred in the exterior wall when heated columns were restrained by adjacent cooler

columns. Restrained thermal expansion also occurred when the core columns were restrained

by the hat truss connection to the exterior wall; elongation of the core columns transferred

loads from the exterior wall to the core.

Exterior seat

Buckling of diagonals

Interior seat

-42.11 -32.603 -23.095 -13.588 -4.081

-37.357 -27.849 -18.342 -8.834 .673211

Figure 9-6. Vertical displacement contour of the detailed truss model under thermal

loading.

V=0.5qL

M

V=0.5qL

T=r0.5qx(L-x)-Ml/A

Figure 9-7. Combined flexural and catenary action in the floor system.

9.3.1 Probable Collapse Sequence of WTC 1

The aircraft impacted the north wall of WTC 1 at 8:46 a.m. The aircraft impact severed exterior columns

and floors on the north side of the tower and into the core between Floors 93 and 98. The subsequent

fires weakened structural subsystems, including the core, floors, and exterior walls. The core weakened,

the floors sagged, and the south exterior wall bowed inward. At 10:28 a.m., about 102 min after the

aircraft impact, WTC 1 began to collapse.

NISTNCSTAR 1-6, WTC Investigation 297
Chapter 9

A sequence of main structural events that led to the collapse of WTC 1 starting from aircraft impact is discussed below. The WTC 1 collapse sequence consisted of five main events: aircraft impact, core weakening, floor sagging and disconnection, inward bowing of the south wall, and collapse initiation. Each event is discussed in terms of (1 ) the factors and sub-events that led to the event and (2) the

consequential structural changes that were caused by the event. Observations for WTC 1 are presented

again in Table 9-5. The probable collapse sequence is presented in Fig. 9-8.

Table 9 -5. Observations for WTC 1.

Time

Timefrom

Impact Observation

8:46:26 0 min WTC 1 was impacted by a Boeing 767 between Floors 93 and 99

and Columns 109 and 152. Fig 6-1 shows Columns 120 to 159.

9:25:28 39 min Fire on west side of south wall.

9:40 69 min No inward bowing of perimeter columns was visible

10:22:59 97 min Inward bowing of the south perimeter wall was visible from Floor

95 to about Floor 99, with a maximum inward bowing of ~ 55 in. at Column 315 and Floor 97.

10:28:18 102 min Smoke and debris clouds out of the north, east, and west walls on

Floor 98. Fire out of windows on the north, east, west, and south

walls between Floor 92 and Floor 98, and Floor 104.

10:28:20 102 min Tower began to collapse - first exterior sign of collapse was at Floor 98. Rotation of at least 8 degrees to the south occurred before

the building section began to fall vertically under gravity.

10:28:48 102 min Remaining portion of core collapsed.

298 NISTNCSTAR 1-6. WTC Investigation
Probable Collapse Sequences

1. Aircraft Impact Damage:

• Aircraft impact severed a number of exterior columns on the north wall from Floor 93 and

Floor 98, and the wall section above the impact zone moved downward.

• After breaching the building's exterior, the aircraft continued to penetrate into the building,

severing floor framing and core columns at the north side of the core. Core columns were also

damaged toward the center of the core and, to a limited extent on the south side of the core. Fireproofmg was damaged from the impact area to the south exterior wall, primarily through

the center of WTC 1 and at least over a third to a half of the core width.

• Aircraft impact severed a single exterior panel at the center of the south wall between floors 94

and 96.

• The impact damage to the exterior walls and to the core resulted in redistribution of severed

column loads, mostly to the columns adjacent to the impact zones. The hat truss resisted the

downward movement of the north wall, and rotated about the east-west axis.

• As a result of the aircraft impact damage, the north and south walls each carried about 7 percent

less gra\ ity loads after impact, and the east and west walls each carried about 7 percent more

loads. The core carried about 1 percent more gravity loads after impact.

2. Effects of Subsequent Fires and Impact Damaged Fireproofing:

A. Thermal Weakening of the Core:

• The undamaged core columns developed high plastic and creep strains over the duration the

building stood, since both temperatures and stresses were high in the core area. The plastic

and creep strains exceeded thermal expansion in the core columns.

• The shortening of the core columns (due to plasticity and creep) was resisted by the hat

truss, which unloaded the core over time and redistributed loads to exterior walls.

• As a result of the thermal weakening (and subsequent to impact and prior to inward bowing

of the south wall), the north and south walls each carried about 10 percent more gravity

loads, and the east and west walls each carried about 25 percent more loads. The core

carried about 20 percent less gravity loads after thermal weakening.

B. Thermal Weakening of the Floors:

• Floors 95 to 99 weakened with increasing temperatures over time on the long-span floors and sagged. The floors sagged first and then contracted due to cooling on the north side;

fires reached the south side later, the floors sagged, and the seat connections weakened.

• Floor sagging induced inward pull forces on the south wall columns.

• About 20 percent of the connections to the south exterior wall on floors 97 and 98 failed

due to thermal weakening of the vertical supports. C. Thermal Weakening of the South Wall:

• South wall columns bowed inward as they were subjected to high temperatures and inward

pull forces in addition to axial loads.

• Inward bowing of the south wall columns increased with time.

Figure 9-8. WTC 1 probable collapse sequence.

NISTNCSTAR 1-6, WTC Investigation 299
Chapter 9

3. Collapse Initiation

• The inward bowing of the south wall induced column instability, which progressed rapidly

horizontally across the entire south face.

• The south wall unloaded and tried to redistribute the loads via the hat truss to the thermally

weakened core and via the spandrels to the adjacent east and west walls.

• The entire section of the building above the impact zone began tilting as a rigid block (all four faces, not only the bowed and buckled south face) to the south (at least about 8°) as column instability progressed rapidly from the south wall along the adjacent east and west

walls.

• The change in potential energy due to downward movement of building mass above the

buckled columns exceeded the strain energy that could be absorbed by the structure. Global collapse then ensued.

Figure 9-8. WTC 1 probable collapse sequence (cont).

Aircraft Impact

WTC 1 was impacted by an aircraft on the north wall. Columns 1 12 to 151 between Floors 94 and 98 were severed or heavily damaged on the north wall. After breaching the building's perimeter, the aircraft continued to penetrate into the building. The north office area floor system sustained severe structural damage between Columns 112 and 145 at Floors 94 to 98. Core columns were severed or heavily

damaged (nine were predicted) between Floor 92 and Floor 97. The aircraft impact also severed a single

exterior panel at the center of the south wall from Columns 329 to 331 between Floor 93 and Floor 96. In

addition, insulation on floor framings and columns were damaged from the impact area to the south

perimeter wall, primarily through the center of WTC 1 and over one-third to one-half of the core width.

Gravity loads on severed columns were redistributed mostly to columns adjacent to the impact zone. Due

to the severe impact damage to the north wall, the wall section above the impact zone moved downward.

The hat truss resisted the downward movement of the north wall and rotated about its east-west axis,

which reduced the load on the south wall. As a result, the north and south walls each carried about 7 percent less gravity loads at Floor 98 after impact, the east and west walls each carried about 7 percent more loads, and the core carried about 1 percent more gravity loads at Floor 98 after impact, as shown in Table 9-6.

Core Weakening

Temperatures in the core area rose quickly; therefore, the thermal expansion of the core was larger than

the thermal expansion of the exterior walls in early stages of the fire, resulting in an increase in the

gravity loads in the core columns until 10 min (8:56 a.m.), as shown in Table 9-6. The additional loads

due to impact damage and high temperatures resulted in high plastic and creep strains in the core columns

during early stages of the fire. Creep strain continued to increase until the collapse initiated. By 30 min

(9: 16 a.m.), the plastic-plus-creep strains exceeded thermal expansion strains. Due to high plastic and

creep strains and plastic buckling of some core columns, at 100 min (10:26 a.m.), the core structure at Floor 99 had displaced downward 2.0 in. on average.

300 NISTNCSTAR 1-6, WTC Investigation
Probable Collapse Sequences

Table 9-6. Total column loads at Floor 98 and Floor 105 of WTC 1 for Case B

North East South West Core

Floor 98 Floor 105 Floor 98 Floor 105 Floor 98 Floor 105 Floor 98 Floor 105 Floor 98 Floor 105

(1) Before Impact 10.974 8,026 8.545 6.562 1 1,025 8,092 8,572 6,604 34,029 20,361

(2) After Impact 10.137 7.294 9,071 7.028 10.356 7,488 9,146 7,076 34.429 20,761

(3) 10 min 9.796 6.944 8.490 6.461 9.848 6,981 8,536 6,469 36,473 22,790

(4) 20 min 10.437 7,551 9,108 7,075 9,900 7,057 9,202 7,158 34,495 20,806

(5) 30 min 10.913 8,020 10,034 7,998 10.420 7,569 9,715 7,685 32,060 18,377

(6) 40 min 1 1.068 8,193 10,599 8,571 1 1 ,004 8,129 10,178 8,147 30,294 16,608

(7) 50 min 1 1.149 8.285 10.908 8,878 1 1.192 8,315 10,458 8,428 29.435 15,743

(8) 60 min 1 1 ,205 8.351 1 1.168 9,130 1 1.285 8,414 10,716 8,687 28,766 15,069

(9) 70 min 1 1,286 8,435 1 1.366 9,319 1 1.343 8.481 10,939 8,914 28,205 14,502

(10) 80 mm 1 1.376 8,528 1 1.555 9,497 1 1 .409 8,551 1 1.1 19 9,097 27,681 13,978

(11) 90 min 10,916 8,096 1 1 .991 9.847 9,949 7,327 1 1.657 9,506 28,587 14,876

(12) 100 min 10.828 8.023 12.249 10.076 9.638 7,066 11.905 9,720 28.478 14,767

(13) (2)-(l) -837 -732 526 466 -668 -604 574 472 400 400

(14) (10) -(2) 1.239 1,234 2.484 2.470 1,052 1,063 1,973 2,021 -6,748 -6,783

(15) (12)-(2) 692 730 3.178 3.048 -719 -422 2.759 2,644 -5,951 -5,993

(16) (12)-(10) -548 -504 694 579 -1.771 -1.485 786 623 797 790 Note : Compression is positive. Units are m kip The shortening of core columns was resisted by the hat truss, which unloaded the core with time and

redistributed the gravity loads from the core to the exterior walls, as can be seen in Table 9-6 at 80 min.

As a result, the north, east, south, and west walls carried about 12 percent, 27 percent, 10 prcent, and 22

percent more gravity loads, respectively, for Floor 98 at 80 min than the state after impact, and the core

carried about 20 percent less loads. At 80 min, the unloading of the core columns was at its maximum.

Sagging of Floors and FloorA/Vall Disconnections

The floors thermally expanded in the early stages of the fires. However, the thermal expansion was

overcome by the significant sagging of the floors, which then pulled inward on the exterior columns.

Floor 95 to Floor 99 sagged due to elevated temperatures in the south floor areas with long-span trusses. While the north floors first sagged and then contracted due to cooling on the north side, the fires reached

the south side later, and the south floors sagged. Figure 9-9 shows vertical displacement contours of

Fl oor 95 to Floor 98 predicted by the full floor models at 100 min (10:26 a.m.). Floor sagging induced

pull-in forces on the south wall columns over Floors 95 to 99. In addition, about 20 percent of the exterior seats of Floors 97 and 98 on the south wall failed due to their reduced vertical shear capacity, as shown in Fig. 9-10.

Bowing of South Wall

Exterior columns on the south wall bowed inward as they were subjected to high temperatures, pull-in

forces from the floors (beginning at about 80 min), and additional loads redistributed from the core. The

observed inward bowing of the south wall at 10:23 a.m. was 55 in. while the calculated inward bowing was 31 in., as shown in Figs. 9-1 1 and 9-12. Since no bowing was observed on the south wall at 9:55 a.m., the south wall was considered to begin bowing inward around 10:10 a.m. when the floors on

the south side began to experience large sagging. The inward bowing of the south wall increased with

time due to additional gravity loads caused by core weakening and increased temperatures on the south

wall. As the floor applied inward pull to the south exterior wall at approximately 80 min, the south wall

began to unload to adjacent walls and the core.

NISTNCSTAR 1-6. WTC Investigation 301
Chapter 9

(c) Floor 96 (d) Floor 95

Figure 9-9. Vertical displacement of Floors of WTC 1 for Case B' at 100 min.

(a) Floor 97 (b) Floor 98

Figure 9-10. Loss of vertical supports observed In Floor 97 and Floor 98 of WTC 1 for

Case B' (1x displacement magnification).

302 NISTNCSTAR 1-6, WTC Investigation
Probable Collapse Sequences

liiiklfiliiiiinii

Figure 9-11. Inward bowing of the WTC 1 south wall of WTC 1 at 10:23 a.m.

I NISTNCSTAR 1-6, WTC Investigation 303
Chapter 9

NODAL SOLUTION

STEP=33

SUB =1437

TIME=150

UY (AVG)

RSYS=0

DMX =42 . 979 SMN =-.701059

SMX =42.826

ANSYS

MAR 30 2005 10:23:38

-FL99

Col359 MN

-.701059 8.972 18.644 28.317 37. 99^ 4.135 13.808 23.481 33.153 42.826

WTCl Severe Temp at 6000s w/5kip pull - South Face (5X)

Figure 9-12. Inward bowing of south wall of WTC 1 global model with creep at 100 min

for Case B with 5 kip pull-in forces (5x displacement magnification).

Buckling of South Wall and Collapse Initiation

The inward bowing of the south wall increased as the post-buckling strength of bowed columns continued

to reduce. The bowed columns increased the loads on the unbuckled columns on the south wall by shear

transfer through the spandrels. Consequently, instability progressed horizontally, and when it engulfed

the entire south wall, it progressed along the east and west walls. Moreover, the unloading of the south

wall resulted in further redistribution of gravity loads on the south wall to the east and west walls and to the thermally weakened core via the hat truss. At 100 min, the north, east, and west walls at Floor 98

carried about 7 percent, 35 percent, and 30 percent more gravity loads than the state after impact, and the

south wall and the core carried about 7 percent and 20 percent less loads, respectively. The increased

loads on the east and west walls were due to their relative higher stiffness compared to the impact

damaged north wall and bowed south wall. The section of the building above the impact zone began

tilting to the south at least about 8° as column instability progressed rapidly from the south wall along the

adjacent east and west walls, as shown in Fig. 9-13. The gravity loads could no longer be redistributed, nor could the remaining core and perimeter columns support the gravity loads from the floors above. The

change in potential energy due to downward movement of building mass above the buckled columns

exceeded the strain energy that could have been absorbed by the structure. Global collapse ensued.

304 NISTNCSTAR 1-6, WTC Investigation
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'.7..W.~ iTlrT^airwMike Ba llo

Smoke and debris

expelled on north face at Floor 98

$Tim Mam / Mike Ballou ® 2001

WTC 1 building

section above impact

damage zone tilts to the

south

Figure 9-13. Expulsion of smoke and debris at WTC 1 Floor 98 on the east, north, and

west faces.

NISTNCSTAR 1-6, WTC Investigation 305
Chapter 9

9.3.2 Probable Collapse Sequence of WTC 2

The aircraft impacted the south wall of WTC 2 at 9:03 a.m. The impact mostly severed columns and

floors that were toward the east side of the building between Floor 78 and Floor 84. The subsequent fires were also observed on the east side of the building. At 9:59 a.m., about 56 min after the aircraft impact,

the building started to collapse, with the east wall buckling inward followed by tilting of the building

portion above Floor 82 toward the east and south.

The section below discusses the sequence of main structural events that led to the collapse of WTC 2

starting from aircraft impact. Each event is discussed in terms of ( 1) the factors and sub-events that caused the event and (2) the structural changes that were caused by the event. The probable collapse

sequence consists of five main structural events: aircraft impact, sagging and disconnections of floors,

inward bowing of the east exterior wall, unloading and tilting of the core, and initiation of collapse.

Observations for WTC 2 are presented in Table 9-7. The probable collapse sequence is presented in Fig. 9-14.

Table 9-7. Key observations on WTC 2.

Time

Time from

Impact

(min)

Observation

9:03 0 min WTC 2 was impacted by a Boeing 767 between Floors 77 and 85 and Columns 404 and

443.

9:23 20 min Inward bowing of east face, maximum deflections of 10 in. at Floor 80. 9:53 50 min Bowmg in of columns, maximum deflections of 20 in. at Floor 80. East side of Floor 83

draped between Columns 310 and 342. 9:58:02 Perimeter coluinns bowing inward on east face. 9:58:59 WTC 2 began to collapse.

9:58:59 55 min -

56 min

Building section above the impact area tilted to the east and south. Tilting appears to take place around Floor 82. Rotation of approximately 4 to 5 deg to the south and 20 to 25 deg to the east occurred before the building section began to fall vertically.

Aircraft Impact

The aircraft impacted the south wall of WTC 2 and severed a significant number of exterior columns on

the south wall from Floor 78 to Floor 84. The floors on the south side sustained severe structural damage

between Columns 410 and 436 from Floors 79 and 83. Core columns were severed or heavily damaged

( 1 1 were predicted) between Floor 77 and Floor 84. The aircraft impact also severed two columns in the

north wall. The aircraft impact caused damage to the floor framing and core columns at the southeast comer of the core. Insulation was damaged from the impact area through the east half of the core to the

north and east perimeter walls. The floor truss seat connections over about one-quarter to one-half of the

east side of the core were predicted to be severed by the impact analysis on Floor 80 and Floor 81. Based

on the hanging object in the photographs, about one-third of the east perimeter wall floor connections on

Floor 83 were assumed to be severed.

306 NISTNCSTAR 1-6, WTC Investigation
Probable Collapse Sequences

1. Aircraft Impact Damage:

• Aircraft impact severed a number of exterior columns on the south wall from floors 78 to 84,

and the wall section above the impact zone moved downward.

• After breaching the building's exterior, the aircraft continued to penetrate into the building,

severing floor framing and core columns at the southeast comer of the core. Fireproofmg was damaged from the impact area through the east half of the core up to the north and east exterior walls. The floor truss seat connections over about 1/4 to 1/2 of the east side of the core were severed on floors 80 and 81 and over about 1/3 of the east exterior wall on floor 83.

• Aircraft impact severed a few columns near the east comer of the north wall between floors 80

and 82.

• The impact damage to the exterior walls resulted in redistribution of severed column loads,

mostly to the columns adjacent to the impact zones. The impact damage to the core columns

resulted in redistribution of severed column loads mostly to other intact core columns and the

east exterior wall. The hat truss resisted the downward movement of the south wall, and

rotated about the east-west axis.

• As a result of the aircraft impact damage, the core carried 6 percent less gravity loads after impact and the north face carried 10 percent less loads. The east face carried 24 percent more

gravity load, while the west face and the south face carried 3 percent and 2 percent more

gravity load, respectively.

• After impact, the core was leaning toward the east and south exterior walls. The exterior

walls acted to restrain the core structure.

2. Effects of Subsequent Fires and Impact Damaged Fireproofing:

A. Thermal Weakening of the Core:

• Several of the undamaged core columns near the damaged and severed core columns

developed high plastic and creep strains over the duration the building stood, since both

temperatures and stresses were high in the core area. The plastic and creep strains exceeded

thermal expansion in the core columns.

• The core continued to tilt toward the east and south due to the combination of column

shortening (due to plasticity, creep, and buckhng) and the failure of column splices at the hat

truss in the southeast comer.

• As a result of thermal weakening (and subsequent to impact), the east wall carried about 5 percent more gravity loads and the core carried about 2 percent less loads. The other three

walls carried between 0 and 3 percent less loads. B. Thermal Weakening of the Floors:

• Floors 79 to 83 weakened with increasing temperatures over time on the long-span floors on

the east side and sagged.

• Floor sagging induced inward pull forces on the east wall columns.

• About an additional 1/3 of the connections to the east exterior wall on floor 83 failed due to thermal weakening of the vertical supports. C. Thermal weakening of the east wall:

• East wall columns bowed inward as they were subjected to high temperatures and inward pull

forces in addition to axial loads.

• Inward bowing of the east wall columns increased with time.

Figure 9-14. WTC 2 probable collapse sequence.

NISTNCSTAR 1-6, WTC Investigation 307
Chapter 9

3. Collapse Initiation

• The inward bowing of the east wall induced column instability, which progressed rapidly

horizontally across the entire east face.

• The east wall unloaded and tried to redistribute the loads via the hat truss to the weakened

core and via the spandrels to the adjacent north and south walls.

• The entire section of the building above the impact zone began tilting as a rigid block (all four faces; not only the bowed and buckled east face) to the east (about T to 8") and south

(about 3° to 4°) as column instability progressed rapidly from the east wall along the

adjacent north and south walls. The building section above impact continued to rotate to the

east as it began to fall downward, and rotated to at least 20 to 25 degrees.

• The change in potential energy due to downward movement of building mass above the

buckled columns exceeded the strain energy that could be absorbed by the structure. Global collapse then ensued.

Figure 9-14. WTC 2 probable collapse sequence (cont).

As a result of the impact damage, dead and live loads carried by severed columns on the south wall and in the southeast comer of the core were redistributed to adjacent intact columns and also to the columns on

the east wall (see Table 9-8). After redistribution, the total axial load on the core columns reduced by

6 percent, and the total axial load on the north wall columns reduced by 10 percent. The total axial load on the east wall columns, however, increased by 24 percent, and the total axial load on the west and south

wall columns slightly increased by 2 percent to 3 percent.

Just below the hat truss level (Floor 105), analyses predicted that about seven column splices failed for

columns at the southeast comer of the core. This increased the core tilting toward the southeast and also

increased the vertical downward displacement of the core at the impact zone. After the failure of the core column splices, the remaining core columns transferred 73 percent of the loads released in the failing core columns to the exterior walls through the hat tmss and 27 percent of the loads were transferred through

the core floors.

Even though some column loads on the south wall were reduced after impact, the total load did not change, as some of the loads from the core area were redistributed to that wall through the hat tmss (see

Table 9-9). At the end of the load redistribution after impact, the core was leaning toward the east and

south. The perimeter walls acted to restrain the core stmcture in the lateral direction.

Sagging of Floors and FloorAA/all Disconnections

Thermal expansion of the floors also occurred early in the fires, but as floor temperatures increased, the

floor sagged and began to pull inward on the exterior columns. As a result of the aircraft impact damage

and increasing temperatures due to subsequent fires. Floor 79 through Floor 83 sagged over time. The

amount of sagging was more significant at Floor 80 and Floor 8 1 where the tmss seats on the east side of

the core were failed due to aircraft impact (see Fig. 9-15). Increased temperatures also weakened the

tmss seats on the east exterior wall and caused additional disconnections at Floor 82 and Floor 83, which

further increased the floor sag (see Fig. 9-16). Floor sagging induced pull-in forces on the east wall

columns, and started shortly after impact and grew with time.

308 NISTNCSTAR 1-6, WTC Investigation
Probable Collapse Sequences

Bowing of East Wall

The east wall columns bowed inward as a result of increasing temperatures (reduced strength and

stiffness) and pull-in forces induced by sagging floors (see Figs. 9-17 and 9-18). The amount of inward

bowing in the east wall steadily increased with time due to the combined effects of pull-in forces from

sagging floors, increased axial loads, and a continuous increase in thermally induced plastic and creep

strains (see Fig. 9-19). The load in bowed columns decreased, with some load transferring to adjacent

unbowed columns, but the total column load on the east wall remained more or less constant for the

duration after aircraft impact (see Fig. 9-20).

Unloading and Tilting of Core

With increasing time and temperatures, the core columns developed high compressive plastic and creep

strains, especially on the east side of the core. Plastic and creep strains started to exceed the thermal

expansion strains approximately 30 min after the aircraft impact (see Fig. 9-21). High plastic and creep

strains caused unloading on the east side core columns. This increased the core tilt toward the southeast

and transferred more loads to the east wall. As a result, at Floor 83, the total axial load carried by the core columns were reduced by 8 percent, the east wall loads increased by 29 percent, and the north wall loads

decreased by 12 percent, relative to the total loads before aircraft impact. The total loads on the south and

west walls did not change significantly (see Tables 9-8 and 9-9).

Buckling of East Wall and Collapse Initiation

With continuously increased bowing and axial loads, the east wall became unstable. The instability started at the center portion of the wall and rapidly progressed horizontally on both sides. As a result of

buckling, the east wall significantly unloaded, redistributing its loads to the weakened core through the

hat truss and to the east side of the south and north walls through the spandrels (see Figs. 9-22 through

9-24 and Table 9-10). Furthermore, the portion of the building above the buckled columns suddenly

moved downward, and the building tilt towards the east increased.

The section of the building above the impact zone began tilting to the east (about 7° to 8°) and south

(about 3° to 4°) as column instability progressed from the east wall to the adjacent south and north walls. The building section above impact continued to rotate to the east as it began to fall downward, and rotated

to at least 20 degrees to 25 degrees. The gravity loads could no longer be redistributed, nor could the

remaining core and perimeter columns support the gravity loads from the floors above. The change in potential energy due to downward movement of the building mass above the buckled columns exceeded

the strain energy that could be absorbed by the structure. The building portion above the impact zone became unstable, and building collapse ensued.

NISTNCSTAR 1-6. WTC Investigation 309
Chapter 9

Table 9-8. Total column loads at Floor 83 of WTC 2 for

Case D. Compression is positive.

Row Analysis Stage West East North South Core

(1) Before Impact 18065 18114 13567 13284 61828

(2) After Impact 18670 22481 12193 13511 57821

(3) 1 0 min 18728 22226 11896 13358 58413

(4) 20 min 18914 22208 12052 13318 58124

(5) 30 min 18876 23681 11770 13365 56967

(6) 40 min 18531 23682 11906 13473 56825

(7) 43 min 15667 15143 14215 16292 62422

(8) (2)-(l) 604 4368 -1374 227 -4007

(9) (6)-(2) -138 1201 -287 -38 -996

(10) (7)-(6) -2864 -8539 2309 2819 5596

Table 9-9. Total column loads at Floor 105 of WTC 2 for

Case D. Compression is positive.

Row Analysis Stage West East North South Core

(1) Before Impact 8497 8572 7382 7169 17123

(2) After Impact 9170 11272 6487 8432 13382

(3) 1 0 min 9182 11061 6250 8275 13975

(4) 20 min 9279 11120 6311 8351 13682

(5) 30 min 9370 11859 6416 8553 12544

(6) 40 min 9198 11927 6524 8691 12402

(7) 43 min 7086 8026 6546 9169 17915

(8) (2)-(l) 674 2699 -895 1263 -3741

(9) (6)-(2) 28 656 37 259 -980

(10) (7)-(6) -2112 -3901 23 479 5513

310 NISTNCSTAR 1-6, WTC Investigation
Probable Collapse Sequences

Floor 82 Floor 83

Floor 79

Figure 9-15. Vertical displacements of Floors 79 through Floor 88 of WTC 2 at 40 min

(Case D).

NISTNCSTAR 1-6, WTC Investigation 311
Chapter 9

Damage a few minutes before collapse

Figure 9-16. Floor sagging observed on the east wall of WTC 2 at different stages.

312 NISTNCSTAR 1-6, WTC Investigation
Probable Collapse Sequences

At 20 minutes

At 40 minutes

Figure 9-17. Out-of-plane displacements on the east wall of WTC 2 (Case

NISTNCSTAR 1-6, WTC Investigation 313
Chapter 9

359 355

Blocked by another bldg.

Displacement measurements at 9:21 a.m.

355 350 345 3)0

83

82

81

80

79

78

77

Blocked by another bldg. f

010

Displacement measurements at 9:53 a.m.

Figure 9-18. Out-of-plane displacement estimates of the east wall of WTC 2 from

photographs.

314 NISTNCSTAR 1-6. WTC Investigation
Probable Collapse Sequences

Figure 9-19. Variation of maximum out-of-plane displacement on the east wall of WTC 2

(Case D).

25000

20000 -

.9-

1 5000 - - - ; - ; o

o

2 10000 - X<

5000 - -

-

0 —'—I 1—'

'i—'—'

>—'—' '—'——'

Before After 10 min 20 min 30 min 40 min

Impact Impact

Time

Figure 9-20. Total column loads at Floor 83 of the east wall of WTC 2 at different stages

(Case D).

NISTNCSTAR 1-6, WTC Investigation 315
Chapter 9

3000COGOOCXXXXXX3000CX»OOOOOOQO

At 40 min

Figure 9-21. Maximum elastic + plastic + creep strains for columns between Floor 78

and Floor 83 of WTC 2 at different stages (Case D) (strain values are in percent).

316 NISTNCSTAR 1-6, WTC Investigation
Probable Collapse Sequences

Figure 9-22. Inward bowing of the east wall of WTC 2 when buckled at 43 min for Case D

(4x displacement magnification).

NISTNCSTAR 1-6, WTC Investigation 317
Chapter 9
Probable Collapse Sequences

Table 9-10. Change in total column loads when the east wall of WTC 2 buckles (Case D,

Row Floor West East North South Core

(1) 83 -2864 -8539 2309 2819 5596

(2) 105 -2112 -3901 23 479 5513

(3) (2)-(n 752 4637 -2286 -2340 -84

NODAL SOLUTION

STEP=7

SUB =1 TIME=.007 USUM (AVG) RSYS=0

DMX =21.601

SMN =3.298

SND( =21.601

WTC-2 Severe Case Temperature Analys

ANSYS

^4AR 14 2005 12:38:28

Undeformed

building edge

15.5 19.568

13.467 17.534 21.601

Figure 9-24. Total displacements of WTC 2 above Floor 86 at 43 min for Case D. toward east and south (20x displacement magnification).

Note tilt

9.3.3 Events Following Collapse Initiation

Failure of the south wall in WTC 1 and east wall in WTC 2 caused the portion of the building above to tilt

in the direction of the failed wall. The tilting was accompanied by a downward movement. The story

immediately below the stories in which the columns failed was not able to arrest this initial movement as evidenced by videos from several vantage points.

The structure below the level of collapse initiation offered minimal resistance to the falling building mass

at and above the impact zone. The potential energy released by the downward movement of the large

building mass far exceeded the capacity of the intact structure below to absorb that through energy of

deformation.

NISTNCSTAR 1-6, WTC Investigation 319
Chapter 9

Since the stories below the level of collapse initiation provided httle resistance to the tremendous energy

released by the falling building mass, the building section above came down essentially in free fall, as seen in videos. As the stories below sequentially failed, the falling mass increased, further increasing the

demand on the floors below, which were unable to arrest the moving mass.

The falling mass of the building compressed the air ahead of it, much like the action of a piston, forcing

material, such as smoke and debris, out the windows as seen in several videos.

NIST found no corroborating evidence for alternative hypotheses suggesting that the WTC towers were

brought down by controlled demolition using explosives planted prior to September 11, 2001. NIST also

did not find any evidence that missiles were fired at or hit the towers. Instead, photographs and videos

from several angles clearly showed that the collapse initiated at the fire and impact floors and that the

collapse progressed from the initiating floors downward, until the dust clouds obscured the view.

9.4 DISCUSSION AND SUMMARY

The results of structural analyses conducted in this study on components, subsystems, isolated exterior

walls and cores, and global models of WTC 1 and WTC 2 showed that the collapses of the towers were

initiated by the combined effects of the structural and insulation damage from aircraft impact and the

subsequent intense fires.

The impact damage alone did not cause collapse of the towers, as they stood for a period of time, and

collapse occurred after the fire-induced weakening of core, floor systems and exterior walls. Global

analyses showed that both towers had substantial reserve capacity after the aircraft impact.

Similarly, the fires alone did not cause the collapse of the towers. In the absence of insulation damage, the

weakening of the core columns and sagging of the floors sufficient to pull in on the exterior walls would

not have occurred.

9.4.1 Structural Response to Impact Damage and Fire

All three major subsystems played a role in the structural collapse sequence for WTC 1 and WTC 2 as described herein.

Role of the Building Core

The core columns were designed to carry the building gravity loads and were loaded to approximately

50 percent of their capacity before the aircraft impact.

The core columns were weakened significantly by the aircraft impact damage and thermal effects. Thermal effects dominated the weakening of WTC 1 . As the fires moved from the north to the south side

of the core, the core was weakened over time by significant creep strains on the south side of the core. Aircraft impact damage dominated the weakening of WTC 2. Immediately after impact, the vertical

displacement at the southeast comer of the core increased 6 in. (from 4 in. to 10 in.). With the impact

damage, the core subsystem leaned to the southeast and was supported by the south and east floors and

exterior walls.

320 NISTNCSTAR 1-6, WTC Investigation
Probable Collapse Sequences

Gravity loads redistributed from the core to the exterior faces primarily through the hat truss due to

aircraft impact and thermal effects. The WTC 1 core carried 1 percent less loads after impact but

20 percent less after thermal weakening. The WTC 2 core carried 6 percent less loads after impact and

2 percent less loads after thermal weakening.

Additional axial loads redistributed to the exterior columns from the core were not significant (only about

20 percent to 25 percent on average) as the exterior columns were loaded to approximately 20 percent of

their capacity before the aircraft impact.

Role of the Building Floors

The floors were designed to support occupancy loads and transfer them to the core and perimeter

columns. They were also designed to act as horizontal diaphragms when the buildings were subject to high winds. In the collapse of the towers, the primary role of the floors was to provide inward pull forces

after sagging that induced inward bowing of exterior columns (South face of WTC 1; East face of

WTC 2).

The floors provided inward pull forces as they sagged significantly under thermal loads. However, the

sagging floors continued to support their floor loads despite the dislodged insulation and extensive fires. Some truss seat connections with dislodged insulation at the exterior columns did fail and disconnect

from the exterior wall under thermal loads. Floor disconnections increased the unsupported length of the

exterior columns and distributed floor loads to adjacent truss seats. There were no inward pull forces

where the floors were disconnected.

Role of Exterior Walls

Column instability over an extended region of the exterior face ultimately triggered the global system

collapse as the loads could not be redistributed through the hat truss to the already weakened building

core. In the area of exterior column buckling, loads transferred through the spandrels to adjacent columns

and adjacent exterior walls. As the exterior wall buckled (south face for WTC 1 and east face for WTC 2), the column instability propagated to adjacent faces and caused the initiation of the building

collapse.

The exterior wall instability was induced by a combination of thermal weakening of the columns, inward

pull forces from sagging floors, and to a much lesser degree, additional axial loads redistributed from the

core.

9.4.2 Structural Response to Fire Without Impact Damage

Without insulation to delay the heating of steel components, steel temperatures began to rise upon

exposure to fires. Thermal expansion occurred as temperatures increased; members restrained against

thermal expansion increased in compressive load and may have caused a local or global load

redistribution, depending upon the compressive load increase and extent of heating. Once steel temperamres exceeded 500 °C to 600 °C, the steel experienced significant reductions in stiffness and

strength. The thermal analysis found that temperatures of floor trusses and columns with intact insulation

rarely exceeded 400 T during Case B fires (100 mm long) for WTC 1 and 500 °C during Case D fires (60

NISTNCSTAR 1-6. WTC Investigation 321
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min long) for WTC 2 . Insulated floors thermally expanded, pushed outward on the exterior columns, and

sagged in the full floor analyses, but the floor sag was insufficient to exert an inward pull on the exterior

columns.

Steel members with dislodged insulation were found to have temperatures greater than 600 "C and often

higher than 800 °C within 10 min to 15 min after exposure to a nearby fire. Fire exposures considerably

longer than the 60 to 100 min exposure in WTC 2 and WTC 1, respectively, were required for insulated

members to reach these temperatures. Reductions in modulus of elasticity, yield strength, and ultimate

tensile strength of steel in the WTC towers were predicted to be 13 percent, 20 percent, and 10 percent,

respectively, at 400 °C, and 35 percent, 92 percent, and 80 percent, respectively, at 700 °C. Steel loses its strength significantly at 700 °C. At these temperatures, the long-span floors were found to sag

sufficiently to exert and inward pull on the exterior walls, primarily due to buckling of truss web diagonal

members. In addition, creep in steel columns becomes significant when the steel temperatures are greater

than 500 °C and subject to high stresses for a period of time.

Inward bowing of an exterior wall was a necessary but not sufficient condition to initiate collapse. In both WTC 1 and WTC 2, significant weakening of the core due to aircraft impact damage and thermal

effects was also necessary to initiate building collapse. The tower structures had significant capacity to redistribute loads (a) from bowed walls to adjacent exterior walls with short-span floors via the arch

action of spandrels, and (b) between the core and exterior walls via the hat truss and, to a lesser extent, the

floors. Without the impact damage, the towers' capacity to redistribute loads would have been even

greater.

As shown in the analysis results, the temperatures in steel components without insulation damage were

lower for the same fire. Lower temperatures resulted in reduced creep, plasticity, and buckling. Without

insulation damage, floor sagging was insufficient to exert pull-in forces on the exterior wall; the core columns maintained their stiffness and strength; and the exterior wall did not bow inward. The lack of

thermally induced damage would result in negligible load redistributions, and the towers would have

remained stable.

9.4.3 Time to collapse

The difference in the time it took for each WTC tower to collapse was due primarily to the differences in structural damage, the time it took the fires to travel from the impact area across the floors and core to

critical locations, and the time it took to weaken the core and exterior columns. WTC 2 had asymmetric

structural damage to the core, including the severing of a comer core column, and WTC 1 had more

symmetrical damage. The fires in WTC 2 reached the east side of the building more quickly, within 10 to 20 minutes, than the 50 to 60 minutes it took the fires in WTC 1 to reach the south side.

9.4.4 Comparison with Other Collapse Hypotheses

Alternate analyses and collapse hypotheses were performed and reported by other studies. A comparison

of NIST and other hypotheses is presented to review assumptions, methodologies, and results. The

comparison includes analyses performed by

• Northwestern University,

322 NISTNCSTAR 1-6, WTC Investigation
Probable Collapse Sequences

• Weidlinger Associates, Inc. with Hughes Associates and ArupFire,

• University of Maryland at College Park and the Israel Institute of Technology,

• Edinburgh University, and

• Amp.

The NIST structural response analyses included the effects of aircraft impact damage to the structure and

thermal insulation, fire growth and spread, the resulting time-varying temperatures of the structural components, and the progression of local structural failure leading up to collapse initiation. The analyses

included the effects of construction sequence, thermal expansion, plastic and creep strains, temperaturedependent material properties, and relevant failure modes for structural members and connections.

With the exception of the Weidlinger-led study, the analyses for the other collapse hypotheses presented

here ignored impact damage, assumed time-temperature curves for structural subsystems (i.e. floor trusses and exterior columns), and conducted analyses of components or subsystems but did not conduct global

analyses of the entire structure (i.e., core, floors, exterior walls, and hat truss) that considered all of the

load redistribution paths as local members and subsystems were thermally weakened over time. The

Weidlinger study included impact damage and assumed time-temperature curves for structural subsystems for their global analyses of each tower.

Northwestern University

The study performed by Northwestern University (Bazant 2002) was a simplified approximate analysis of

the overall collapse of the WTC towers which addressed the question of why a total collapse occurred.

The analysis addressed the results of prolonged heating which would have caused the columns of a single

floor to lose their load carrying capacity and initiated the collapse of the building. The analysis assumed

loss of thermal insulation during impact, uniform temperatures of 800 °C for a uniform column size and

load across a single floor, and creep buckling and loss of load carrying capacity in over half of the

columns. The analysis included evaluation of the dynamic amplification of the loads and the ability of the

columns in the lower floors to dissipate the kinetic energy of the falhng upper building mass through

formation of plastic hinge mechanisms. The analysis found that the ratio of the kinetic energy of the upper

building section dropping one floor to the deformation energy of plastic hinge rotation in the lower

building columns was approximately a factor of eight.

The study by Northwestern did not address the details of impact damage, fire dynamics, or structural response of the towers. Rather, a generalized condition was assumed of heated columns, and the question

of why there was total collapse was addressed. NIST agrees with the assessment of the tower's required

structural capacity to absorb the released energy of the upper building section as it began to fall as an approximate lower bound. The likelihood of the falling building section aligning vertically with the

columns below was small, given the observed tilting, so that the required capacity would be greater if interaction with the floors was also considered, as pointed out in the study.

NISTNCSTAR 1-6, WTC Investigation 323
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Weidlinger Associates, Inc. with Hughes Associates and ArupFire

The study led by Weidlinger Associates, Inc. (Abboud 2003, Post 2002a, Post 2002b, Glanz 2002) used

the SAP2000 and FLEX finite element code to calculate the aircraft impact damage to both towers and

their structural response to damage and elevated temperatures. FLEX is an explicit, nonlinear, large

deformation transient analysis finite element code for the analysis of structures subject to blast, impact,

and shock loads. The fires were evaluated by Hughes Associates and ArupFire Inc. The fires were found

to be less than fully developed office fires, with gas temperatures ranging from 400 °C to 700 °C in the

impact regions and well ventilated regions near the exterior walls; exterior locations with persistent fires were assumed to have 1000 °C temperatures. Based upon study of smoke plumes and fire spread, it was

concluded that the floors did not fail or have a significant role in the collapse of the towers. The

structural response analysis found that the impact debris dislodged thermal insulation and that the hat

truss played a significant role in transferring loads between the core and exterior walls. The analysis

identified the specific cause of each towers' collapse to be the failure of core columns that either lost insulation or were destroyed during the aircraft impact. WTC 2 exterior columns on the east side began to

fail first and redistributed loads to the core columns until the loads could no longer be supported, due to successive failures of core columns. WTC 1 core columns began to fail first due to damage and thermal

weakening and attempted to redistribute loads to the exterior walls through the hat truss. WTC 2 was

found to collapse first primarily because the damage was off-center and compromised the southeast comer of the core.

NIST agrees with many of the findings by the Weidlinger Associates, Inc. led study. However, there were some differences in the modeling approach and assessment of contributing factors. The most

significant difference was that the floors were not analyzed for their response to fire, so that the collapse

hypothesis did not account for floor sag and its contribution to inward bowing of the exterior columns.

The reason for WTC 2 collapsing before WTC 1 was attributed to the off-center damage, particularly the

damage to the southeast comer core column. NIST found that in addition to the differences in impact

damage between the two towers, the time it took the fires to travel from the impact area across floors and

the core to critical areas, and the time it took for the core and exterior columns to become thermally

weakened, also contributed to the difference in times to collapse initiation.

University of Maryland and Israel Institute of Technology

The study performed by the University of Maryland and the Israel Institute of Technology (Quintiere

2002) was based upon a thermal conduction analysis of truss web members subjected to a uniform gas

temperature and a stmctural failure analysis based on buckling of the truss web member due to a temperature-induced reduction in stiffness. Gas temperatures were estimated to be approximately 900 °C

for the duration of the fires in each tower. A thermal conduction analysis of web members was conducted

to estimate the temperature of the web member as a function of time and insulation thicknesses (0.75 in. and 1.5 in.). A web member with an assumed load was calculated to buckle when temperatures of about

630 °C to 770 °C were reached, due to a reduced modulus of elasticity. The time at which the insulated

members reached temperatures that met the buckling criteria fell within the observed collapse time of

each tower. It was noted that a bare steel web member would fail by this criteria in 10 min to 15 min, and

that this time did not match the observed time to collapse initiation. Given the failure of truss web

members, it was postulated (not supported by calculations) that the floors would sag and fail at their connections to the columns and that progressive collapse would ensue as the floors below also failed.

324 NISTNCSTAR 1-6, WTC Investigation
Probable Collapse Sequences

NIST findings differed from those given in this study. NIST also found that web members in the floor

trusses buckled when heated sufficiently, which led to sagging of the floor, but did not find appreciable

floor sagging when the truss insulation remained intact. The Maryland study suggested that the sagging

floors resulted in failure of the floor to column connections; NIST analyses found that the sagging floors did not cause floor connections to fail, except at a few isolated locations, but rather produced an inward

pull on the exterior walls. An inward pull of exterior columns would not occur if floor to column

connections had failed. To produce the inward bowing of the exterior walls that was observed, the floors had to sag and exert an inward pull well before collapse initiated, not at the time of collapse as proposed

in the Maryland study. Additionally, analysis of a floor collapsing onto a floor below, which was

unlikely gi\'en the required event of all floor connections failing nearly at the same time, was not found to result in failure of the impacted floor.

University of Edinburgh

The University of Edinburgh study (Usmani 2003, Usmani 2005) performed a nonlinear, large

displacement finite analysis of a t>'pical 2D slice of the tower structure that encompassed twelve floors around the impact level of WTC 1 using ABAQUS. However, there were also some simplifying

assumptions to reduce the model complexity, such as restraining lateral movement of the floor at the core end and a pinned connection to the external columns at the other end. The truss diagonals were modeled

with a single axial element and connections were not explicitly modeled. It was assumed that the core columns were relatively cool and that the collapse would initiate at the exterior columns. A generalized

exponential curve represented the time-temperature relationship, and assumed temperature profiles were

applied to the floors for \'arious fire scenarios. The exterior columns were linearly ramped from ambient

temperatures. The analysis found that the heated floors expanded and pushed the exterior columns

outward and that the outward movement was resisted by tension in the cool floors above and below the

fire floors. The analysis also found that a floor buckled at 400 °C, causing the exterior column to 'rebound', resulting in large compressive loads on the floors above and below, which in turn buckled. It was stated that the same mechanism propagated to adjacent floors until it was arrested or caused collapse.

Consideration of the hat truss, its capacity for redistribution of loads between the exterior walls and core, and its role in delaying the collapse mechanism until the structure's redistribution capacity was exhausted was discussed, but no supporting analyses were presented. These results were cited as a possible fire- induced collapse mechanism for a tower without impact damage that was based on thermal expansion

rather than fire-induced loss of strength and stiffness.

NIST findings also differed from the findings of the University of Edinburgh study. NIST included

thermal expansion in its detailed analyses of full floor systems, and did not find that buckling of any floor

system occurred. Rather, as truss web members began to buckle, the floors began to sag, which increased over time. The sudden bucklmg of the first floor m the Edinburgh analysis, followed by the sudden

subsequent failure of floors above and below, does not match the observed inward bowing of the exterior

walls which increased over time. Further, NIST found that failure of an exterior wall was necessary but

not sufficient to initiate the collapse of either tower. In both WTC 1 and WTC 2, significant weakening

of the core due to aircraft impact damage and thermal effects was also necessary to initiate building

collapse.

NISTNCSTAR 1-6, WTC Investigation 325
Chapter 9

Amp

The study by Arup (Lane 2005) was conducted to determine if the WTC towers had any collapse

mechanisms specific to their structural system features. Based upon information available through

presentations, studies included 2D analysis of a twelve-floor slice of the exterior column and floors to the

core, a twelve-floor slice across the entire tower (from exterior column to exterior columns), and 3D

quarter floor, half floor, and quarter-floor seven-story section models. None of the models included the

hat tmss; all of the models included individual floor trusses and the floor slabs. Temperatures from the

fires for structural members were assumed, where the floor trusses reached 800 °C in a "very short time",

the exterior columns and spandrels heated linearly to 400 °C by 3600 s. Slab temperatures were not

reported. Three floors were heated to 800 °C, the floors sagged and the exterior wall section was pulled

inward. The inward bowing of the exterior wall was considered to be a collapse mechanism for the towers. Arup stated that the behavior was calculated for the duration of the fire with no user intervention

and without inclusion of any aircraft impact damage, including damage to thermal insulation.

The description of Arup analyses is based on presentations since no published reports by Arup were

available prior to the release of this report. The study by Arup found that the composite truss floors sagged as they were heated and pulled inward on the exterior wall, similar to the findings by NIST.

However, the NIST analyses did not find uniform temperatures across an entire floor nor simultaneously on multiple floors, as assumed in the Arup analyses. Further, NIST did not find any insulated truss members reaching temperatures of 800 °C prior to the collapse of either tower. NIST thermal analyses

showed that steel temperatures in areas where the insulation remained intact rarely exceeded 400 °C in WTC I and 500 °C in WTC 2. The Arup 3D seven-floor model did not include load transfer

mechanisms, including the hat truss, the core, and sufficient portions of the exterior wall to provide the

arching action observed in the impact faces. NIST found that failure of an exterior wall was necessary

but not sufficient to initiate the collapse of either tower. In both WTC 1 and WTC 2, significant

weakening of the core due to aircraft impact damage and thermal effects and redistribution of loads

between the core and exterior wall were found to be necessary to initiate building collapse.

9.4.5 Factors that Affected Performance

From the collective knowledge and insights gained through the Investigation of the collapse of the WTC

towers, the following factors were identified that affected performance of both towers on

September 11, 2001:

• The closely spaced columns, along with deep short spandrels, allowed a redistribution of

loads as a result of aircraft impact damage to the exterior wall.

• Because there was effectively no wind on the morning of September 1 1, 2001, the capacity of

the exterior wall provided to accommodate design wind loads was available to carry

redistributed gravity loads.

• The large dimensional size of the WTC towers helped the buildings withstand the aircraft impact.

• The composite floor system with primary and bridging trusses forming a 2-way grid, and the

two layers of welded wire fabric in the slab, acted to bridge over damaged areas without

326 NISTNCSTAR 1-6, WTC Investigation
Probable Collapse Sequences

propagation of collapse from areas of aircraft impact damage to other locations, thereby

avoiding larger scale floor collapse upon impact.

• The hat truss played a major role in the post-impact perfonnance of the building. This was

accomplished through redistribution of the loads from the significant weakening of the core, due to aircraft impact damage and subsequent thennal effects, by redistributing loads from

the damaged core columns to adjacent intact columns and, ultimately, by redistributing loads

to the exterior walls from the thermally weakened core columns that lost their ability to support the buildings' weight.

• The buildings would likely not have collapsed under the combined effects of aircraft impact

and the subsequent jet-fuel ignited muUi-floor fires, if the insulation had not been dislodged

or had been only minimally dislodged by aircraft impact. The existing condition of the

insulation prior to aircraft impact and the insulation thickness on the WTC floor system did

not play a significant role in initiating collapse on September 11, 2001.

9.5 REFERENCES

Zdenek Bazant and Yong Zhou (2002), Ji^y Did the World Trade Center CoUapse?-Simple Analysis,

Journal of Engineering Mechanics, Vol. 128, No. 1

Abboud, Najib et.al., (2003), Anatomy of a disaster: A structural investigation of the World Trade Center

collapses. Proceedings of the Third Congress Forensic Engineering, San Diego, CA, United States, Oct

19-21 2003, p 360-370

Nadine Post (2002a), Report Ties WTC Collapses to Column Failures, Engineering News Record,

McGraw Hill Construction. 10/25/2002

Nadine Post (2002b), Study Absolves Twin Tower Trusses, Fireproofing, Engineering News Record,

McGraw Hill Construction, 11/04/2002

James Glanz and Eric Lipton (2002), In Data Trove, A Graphic Look at Towers' Fall, The New York

Times, October 29, 2002

J.G. Quintiere, M. di Marzio, and R. Becker (2002), A suggested cause of the fire-induced collapse of the

World Trade Towers, Fire Safty Journal, Vol. 37, Issue 7, October 2002, p 707-716.

A.S. Usmani, Y.C. Chung, and J.L. Torero (2003), How did the WTC towers collapse: a new theory. Fire

Safety Journal, vol 38, Isue 6, October 2003, p 501-533.

A. S. Usmani (2005), Stability of the World Trade Center Twin Towers Structural Frame in Multiple

Floor Fires, Journal of Engineering Mechanics, Vol. 131, No. 6, Jun 2005, p 654-657.

B. Lane (2005), Comments on structural fire response and collapse analysis, Presentation given at the

Technical Conference on the Federal Building and Fire Safety Investigation of the World Trade Center

(WTC) Disaster, National Institute of Standards and Technology (NIST), Gaithersburg, Maryland,

September 13-15, 2005, http://wtc.nist.gov

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328 NISTNCSTAR 1-6, WTC Investigation
Chapter 10

Findings

There were many facets to the work reported herein. First, the thickness of the passive fire protection was

estabhshed from recorded measurements and interpretation of photographs of the originally applied

SFRM. This information was used, along with statistical analysis and thermal structural analyses, to establish the thickness of passive fire protection (insulation or fireproofing) to be used in finite element

thermal analyses. Next, standard fire resistance tests were conducted to establish the appropriate

classification (fire resistance rating) of the original design of the WTC floor system and structural performance of the floor system in standard fires for insight into perfomiance in actual fires. Characterization of the temperatures of the structural components, determined from simulated WTC fires,

allowed the calculation of the performance of major subsystems constituting the structural system of the towers. In turn, insights obtained from these analyses were used to formulate and execute global analyses

to analyze the collapse sequence of each tower. The structural analyses results were guided, and where

possible vahdated, by obser\'ations made from the review of thousands of photographs and video

recordings. This chapter reports the findings resulting from these efforts to characterize the conditions of

the WTC towers before the attacks, their weakening due to the aircraft impacts, their subsequent response

to the growth and spread of fires, and the progression of local failure that ultimately led to the total collapse of both towers.

10.1 PASSIVE FIRE PROTECTION

The passive fire protection applied to the steel structural components in the WTC towers was investigated

to provide information on the in-place condition of the thermal insulation before and after the aircraft impact. The specified and "as applied" thicknesses, the variability in thickness, the condition of the

insulation over a 30-year service life, and the effects that the variability and condition have on the

structural behavior of insulated steel members were studied. The rationale behind the selection of the

effective thickness of thermal insulation for use in thermal analyses was presented. Additionally, the

procedures and practices used to provide the passive fire protection for the floor system of the WTC

tower structures was documented.

10.2 BUILDING CODE REQUIREMENTS FOR STRUCTURAL FIRE

RESISTANCE

Finding 1 : The WTC towers were classified as Class IB, as defined by the 1968 New York City

Building Code. This classification required a 3 h fire rafing for columns and 2 h for floors. The towers

could have been classified as Class lA since both Class 1 A and IB permitted buildings of unlimited

height. Class 1 A required a 4 h fire resistance rating for columns and a 3 h rating for floors. In 1969, The

Port Authority of New York and New Jersey (PANYNJ) specified 0.5 in. fireproofing for all beams,

spandrels, and trusses, to maintain the Class 1-A Fire Rating of the New York City Building Code. A

condition assessment conducted in 2000 reported that the WTC towers were classified as Class- IB

noncombustible, fire-protected, and retrofitted with sprinklers consistent with Local Law 5/1973.

NISTNCSTAR 1-6, WTC Investigation 329
Chapter 10

10.2.1 Selection of Fire Resistive Materials

Finding 2: The passive fire protection for the floor trusses was specified to be 0.5 in. of BLAZESHIELD Type D, ahhough the technical basis for the selection of this product and required thickness

value is not known. After applying the Type D sprayed fire resistive materials to the lower 40 floors of

WTC 1, the BLAZE-SHIELD insulating material was switched to Type D/CF (reported to meet or exceed

the insulating properties of Type D) which did not contain asbestos. In 1995, the Port Authority

conducted a study to establish the fireproofing requirements for the floor trusses in areas undergoing

major tenant renovation. The thickness required to achieve a 2 h fire rating was determined to be 1.5 in. using the BLAZE-SHIELD II product. At the time of the WTC disaster, fireproofing had been upgraded on a number of floors in the WTC towers: 18 floors in WTC 1, including all of the floors affected by the

aircraft impact and fires, and 13 floors in WTC 2, although none that were directly affected by the aircraft impact and fires.

10.2.2 Equivalent thickness of SFRM

Finding 3: Based on analyses of SFRM thickness measurements and interpretation of photographs

showing the condition of the originally applied material, the average thickness of the original thermal

insulation on the floor trusses was estimated to be 0.75 in. with a standard deviation of 0.3 in. (coefficient

of variation of 0.40). The average thickness of the upgraded thermal insulation was estimated to be

2.5 in. with a standard deviation of 0.6 in. (coefficient of variation of 0.24). Based on finite-element

simulations, it was concluded that the original passive fire protection on the floor trusses was thermally

equivalent to a uniform thickness of 0.6 in., and the upgraded insulation was thermally equivalent to a unifomi thickness of 2.2 in. These values were used in the thermal analyses for determining temperature

histories of structural components.

Finding 4: No information was available on in-place conditions of the thermal protection on the exterior

columns and spandrel beams, and little information was available on the conditions of fire resistive material on core beams and columns. For thermal analyses of the towers, the SFRM on these elements was taken to have uniform thicknesses equal to the specified thickness. This assumption was supported

by the observation that measured average thickness tended to be greater than the specified thickness

while, due to variability, the effective thickness tended to be less than the average uniform thickness. The

specified thickness values were 0.5 in. for beams and spandrels, 2.06 in. (2 1/16 in.) for columns lighter

than 14WF228, and 1.19 in. (1 3/16 in.) for columns equal to or heavier than 14WF228.

Finding 5: The adhesive strength of BLAZE-SHIELD DC/F to primed steel was found to be a third to a half of the adhesive strength to steel that had not been coated with primer paint. The SFRM products used

in the WTC towers were applied to steel components with primer paint.

10.3 FIRE RESISTANCE TESTS

Four Standard Fire Tests (ASTM E 1 19) were conducted on floor assemblies constructed to duplicate, as closely as practical, the floor system used in the WTC towers. Full scale tests with a 35 ft span and

having Va in. thick SFRM were tested, one in the restrained test condition and the other in the unrestrained

test condition. Tests of half-scale specimens, which spanned approximately 17 ft, were conducted using

SFRM conditions simulating the "as specified" condition (0.5 in. thick SFRM) and the "as-applied"

330 NISTNCSTAR 1-6, WTC Investigation
Findings

condition (0.75 in. thick SFRM). The following findings are based on this series of four tests and a comparison of their results.

10.3.1 Structural Performance

Finding 6: Test assemblies, representative of the WTC floor system, exposed to the Standard Fire Test (ASTM E 1 19) conditions resulted in extensive spalling on the underside of the floor slab, thermal

damage to the bridging trusses, and buckling of compression diagonals and vertical struts of the main

trusses.

Finding 7: All four tests demonstrated that the floor assemblies were capable of sagging without failure. The unrestrained test, which had two 0.875 in. bolts fastening the main truss to the truss seats, did not sag

sufficiently to bear on the bolts. In the three restrained tests the main truss ends were welded to the truss seats to provide the required restraint. The magnitude of the sagging observed in the tests was consistent

with that computed from finite element structural analyses. No evidence of knuckle failure was seen in the tests.

Finding 8: All four test assemblies supported their full design load under standard fire conditions for two

hours without failure.

10.3.2 Fire Resistance Ratings

Finding 9: The 1968 New York City (NYC) building code—the code that the WTC towers were

intended but not required to meet when they were built—required a 2 h fire rating for the floor system.

Finding 10: The restrained duplicated floor system obtained a fire resistance rating of 1.5 h while the

unrestrained floor system achieved a 2 h rating. This finding was unexpected since the unrestrained

rating is typically less than the restrained rating.

Finding 11: The test of the 17 ft specimen with as-applied SFRM did not produce the same rating as the

35 ft test specimen, giving 2 h and 1 .5 h, respectively. In both cases, the rating was established on the

basis of temperatures of the unexposed surface (top of concrete slab) and not on the ability of the specimen to support the load.

Finding 12: The 45 min rating for the standard 17 ft test with the specified 0.5 in. SFRM did not meet

the 2 h requirement of the 1968 NYC Building Code. This test had no SFRM on the bridging trusses nor on the underside of the metal deck.

Finding 13: The 2 h rating for the standard 17 ft test with the as-applied average 0.75 in. SFRM met the 2 h requirement of the 1968 NYC Building Code. This test had half the SFRM thickness on the bridging

trusses (0.375 in.) and overspray on the underside of the metal deck.

Finding 14: The difference in test results for the two 17 ft specimens is due primarily to the concrete slab performance (spalling and cracking) and the presence or lack of SFRM overspray on the metal deck and

not due to the SFRM thickness on the trusses. Differences in the degree of concrete spalling were

possibly due to differences in moisture content and the slab cracking.

NISTNCSTAR 1-6, WTC Investigation 331
Chapter 10

10.4 RESPONSE OF STRUCTURAL COMPONENTS

The response of the structural components and their connections for the tenant floors and exterior walls was examined with detailed structural models. Results of the floor and exterior wall component and

connection analyses identified structural behaviors and failure modes that were required for inclusion in the global analyses.

10.4.1 Floor System

Finding 15: The interior truss seats had a greater vertical shear capacity than the exterior truss seats. The

controlling failure mode for vertical shear was weld fracture. However, the vertical load at the truss connection of approximately 16 kip had to increase by a factor of two to six to reach failure (weld

fracture) for temperatures near 600 °C to 700 °C.

Finding 16: Detailed structural analysis of a single truss section of the composite floor system subjected

to elevated uniform temperatures was found to initially push out on the exterior columns as a result of the concrete slab thermal expansion and then pull inward as the web diagonals buckled and the truss sag

increased. The magnitude of the pull-in force was found to depend highly on the stiffness of the exterior

box column which, in turn, depended on expansion of floors above and below.

Finding 17: Detailed analysis of the knuckles (shear connectors in the floor system for composite action)

tlirough test simulation and detailed truss analysis found that failure of the knuckles in the floor system was unlikely. This finding was also supported by the lack of any knuckle failures in the four standard fire resistance tests (ASTM E 1 19) of the floor truss assemblies with twice the floor load that was on the

WTC floors.

10.4.2 Exterior Wall System

Finding 18: Large inelastic deformations and buckling of the spandrels at elevated temperatures were

predicted, but were found not to significantly affect the stabihty of the exterior columns. Partial

separations of the spandrel splices were also predicted at elevated temperatures, but were found not to significantly affect the stability of the exterior columns.

Finding 19: Analyses of bolted splices in the exterior columns found that the splice may slide or open

when the exterior columns are bowing and subject to large lateral deflections. No column splice bolts were predicted to have failed.

Finding 20: An exterior wall section (9 columns wide and 9 floors high) was found to bow inward when

the floor connections applied an inward pull force. For the condition where three sequential floors were

disconnected, there was no bowing of the columns for five different elevated temperature conditions.

When the column section with three disconnected floors was subjected to increased axial column loads,

the wall section bowed outward over the unsupported column length.

10.5 FIRE PROTECTION AND PARTITION DAMAGE DUE TO AIRCRAFT

IMPACT

332 NISTNCSTAR 1-6, WTC Investigation
Findings

The aircraft impact of the WTC towers caused extensive damage to the buildings' exteriors, penetrated

into the interiors causing further damage to the structural systems, dislodged insulation, and ignited multifloor fu-es. The strucmral damage to each tower resulting from the aircraft impact was estimated using a

transient finite element analysis. Results of this analysis were used to predict damage to the structure,

fireproofmg, and partition walls in the path of the debris field.

Finding 21: For WTC 1, partitions were damaged and fireproofing was dislodged by direct debris impact

over five floors (Floors 94, 95, 96, 97, and 98) and included most of the north floor areas in front of the

core, the core, and central regions of the south floor areas, and on some floors, extended to the south wall. For WTC 2. partitions were damaged and fireproofmg was dislodged by direct debris impact over six floors (Floors 78, 79, 80, 81, 82, and 83) and included the south floor area in front of the core, the central

and east regions of the core, and most of the east floor area, and extended to the north wall.

Finding 22: The fireproofing damage estimates were conservative as they ignored possibly damaged and

dislodged fireproofing in a much larger region that was not in the direct path of the debris but was subject

to strong vibrations during and after the aircraft impact. A robust criteria to generate a coherent pattern of

vibration-induced dislodging could not be established to estimate the larger region of damaged

fireproofing.

10.6 OBSERVATIONS AND TIMELINE

Thousands of photographs and hours of video records were reviewed for insights into the structural performance of the towers. A timeline of significant events that characterized the weakening and

eventual collapse of the WTC towers was developed with the photographs and videos that were timestamped. Quantitative information, such as the amount of inward bowing observed on the exterior walls

of the buildings, was extracted fi^om key photographs through image enhancement and scaled

measurements. Key observ ations and the timehnes were used to guide the global collapse analyses.

10.6.1 WTC 1

Finding 23: Inward bowing of the south exterior wall was first observed at 10:23 a.m. The bowing

appeared to extend between Floors 94 and 100 and Columns 305 and 359. The maximum bowing was

estimated from images to be 55 in. ±6 in. at Floor 97 on the east side of the south face of WTC 1. The

central area in available images was obscured by smoke. The extent of fires observed on all faces of

WTC 1 was similar, although somewhat more extensive on the east and west faces (where short span

floors were located) and similarly extensive on the north and south faces (where long span floors were

located). Inward bowing was observed only on the south face. The north face had extensive aircraft impact damage, and the damaged floors were not capable of imposing inward pull forces on the north

face.

Finding 24: The time to collapse initiation was 102 minutes from the aircraft impact (8:46:30 a.m. until 10:28:22 a.m.).

Finding 25: From exterior observations, filting of the building secfion appeared to take place near

Floor 98. Column buckling was then observed to progress rapidly across the east and west faces.

NISTNCSTAR 1-6, WTC Investigation 333
Chapter 10

Finding 26: The WTC 1 building section above the impact and fire area tilted to the south as the

structural collapse initiated. The tilt was toward the side of the building that had the long span floors. Video records taken from east and west viewpoints showed that the upper building section tilted to the

south. Video records taken from a north viewpoint showed no discemable east or west component in the

tilt. A tilt to the south of at least 8 degrees occurred before dust clouds obscured the view and the building

section began to fall downward.

10.6.2 WTC 2

Finding 27: On the east face and north face of WTC 2, draped objects were observed through the

windows of floor 82 on the east face and floors 8 1 to 83 on the north face near the northeast comer. The

draped objects appeared to be hanging floors. The drape of these objects was observed to increase with

time and extend across approximately half of the east face.

Finding 28: Inward bowing of the east wall was first observed at 9:21 a.m. The inward bowing was

approximately 10 in.±l in. at Floor 80 and extended between Floors 78 and 83 and Columns 304 and 344.

The remaining portion of the face to the south of Column 344 was not included in the image. The bowing

appeared to extend over a large fraction of the east face and to be greatest near the center of the face. Fires were more extensive along the east face (where long span floors were located) and at the east side of the

north and south faces (where short span floors were located). Fires were not observed on the west face

(where long span floors were located). Inward bowing was observed only on the east face. The south face

had extensive aircraft impact damage, and the damaged floors were not capable of imposing inward pull

forces on the south face. There was no impact damage or fire on the west floors to cause pull-in forces on

the west face.

Finding 29: An increase of the inward bowing of the east wall was observed at 9:53 a.m. The inward

bowing appeared to extend between Floors 78 and 84 and Columns 305 and 341. The remaining portion

of the face to the south of Column 344 was not included in the image. The maximum bowing was

estimated from images to be 20 in.±l in. at Floor 80 on the east face of WTC 1

Finding 30: The time to collapse initiation was 56 minutes after aircraft impact (9:02:59 a.m. to 9:58:59 a.m.).

Finding 31 : From exterior observations, tilt of the building section above the impact and fire area appeared to take place near Floor 82. Column buckling was then seen to progress across the north face.

Finding 32: The building section above the impact and fire area tilted to the east and south at the onset of structural collapse. The tilt occurred toward the east side with the long span floors. Estimates made

from photographs indicate that there was approximately a 3 degree to 4 degree tilt to the south and a 7 degree to 8 degree tilt to the east prior to significant downward movement of the upper portion of the

building. The tilt to the south did not increase any further as the upper building section began to fall, but

the tilt to the east continued, reaching 20 degrees to 25 degrees before dust clouds obscured the view.

334 NISTNCSTAR 1-6, WTC Investigation
Findings

1 0.7 STRUCTURAL RESPONSE OF MAJOR TOWER SUBSYSTEMS

Prior to conducting the analysis of the global structural response of each tower, major structural subsystems were analyzed to provide insight into their behavior within the WTC global system. The

three major structural subsystems, the core framing, a single exterior wall, and full tenant floors, v/ere

analyzed separately for their response to impact damage and fire. The hat truss was not analyzed

separately as its structural behavior did not require significant simplification in the global analysis. The

component analyses provided a foundation for these large, nonlinear analyses with highly redundant load

paths, and they enabled a significant reduction in fmite element model complexity and size. The major

subsystem models used final estimates of impact damage and elevated temperatures determined from the

aircraft impact analysis and the fire dynamics and thermal analyses.

10.7.1 Isolated Core Subsystem

Finding 33: The WTC 1 isolated core subsystem analysis found that the core structure was most

weakened from impact and thermal effects at the center of the south side of the core. Smaller

displacements occurred in the global model due to the constraints of the hat truss and floors.

Finding 34: The WTC 2 isolated core subsystem analysis found that the core structure was unstable for the estimated structural damage to core columns. The core was most weakened from impact and thermal

effects at the southeast comer and along the east side of the core. Larger displacements occurred in the

global model as the isolated core model had lateral restraints imposed that were somewhat stiffer than in the global model.

10.7.2 Full Floor Subsystem

Finding 35: Floor sagging was caused primarily by either buckling of truss web diagonals or disconnection of truss seats at the exterior wall or the core perimeter. Except for the truss seat failures near the southeast comer of the core in WTC 2 following the aircraft impact, web buckling or tmss seat

failure was caused primarily by elevated temperatures of the structural components.

Finding 36: Analysis results from both the detailed truss model and the full floor models found that the

floors began to exert inward pull forces when floor sagging exceeded approximately 25 in. for the 60 ft floor span.

Finding 37: Sagging at the floor edge was due to loss of vertical support at the tmss seats. The loss of

vertical support was caused in most cases by the reduction in vertical shear capacity of the tmss seats due

to elevated steel temperatures.

10.7.3 Isolated Exterior Wall Subsystem

Finding 38: Inward pull forces were required to produce inward bowing that was consistent with

displacements measured from photographs. The inward pull was caused by sagging of the floors. Heating

of the inside face of the exterior columns also contributed to inward bowing.

NISTNCSTAR 1-6, WTC Investigation 335
Chapter 10

Finding 39: The observed inward bowing of the exterior wall indicated that most of the floor

connections were intact to cause the observed bowing.

Finding 40: The floors that were identified through analysis to be affected by the fires and the dislodged

insulation matched well with the floors that were observed to have participated in the inward bowing of

the exterior walls.

Finding 41: The extent of floor sagging required at each floor was greater than that predicted by the full floor models. The estimates of the extent of sagging at each floor were governed by the combined effects of insulation damage and fire; insulation damage estimates were limited to areas subject to direct debris

impact. Other sources of floor and insulation damage from the aircraft impact and fires (e.g., insulation

damage due to shock and subsequent vibrations as a result of aircraft impact or concrete slab cracking and

spalling as a resuh of thermal effects) were not included in the floor models.

10.8 STRUCTURAL RESPONSE TO AIRCRAFT IMPACT DAMAGE AND FIRE

Global analysis of WTC 1 and WTC 2 used final estimates of impact damage and elevated temperatures

to determine the structural response and sequence of component and subsystem failures that led to collapse initiation.

10.8.1 General Findings

Finding 42: The structural analyses of WTC I and WTC 2 found that the collapse of the towers was due

to the combined effects of structural and insulation damage from aircraft impact and the subsequent fires on the core, floor systems, and exterior walls. The towers collapsed when the weakened core and exterior

columns could no longer redistribute or support the building loads with their reduced load carr>'ing

capacity.

Finding 43: Impact damage alone did not cause collapse of the towers, as they were stable after the

aircraft impact. Global analyses showed that both towers had substantial reserve capacity after the

aircraft impact.

Finding 44: The multi-floor fires alone did not cause collapse of the towers. Without impact damage to the insulation, the structural steel temperatures would have been generally less than 200 °C to 300 °C,

with a few isolated locations of structural steel temperatures exceeding 400 °C in WTC 1 floors and 500

°C in WTC 2 floors. The core would not have weakened, the floor sag would have been insufficient to pull inward on the exterior columns, and the exterior walls would not have bowed inward.

Finding 45: The towers would likely not have collapsed under the combined effects of aircraft impact

and the subsequent multi-floor fires if the insulation had not been dislodged or had been only minimally

dislodged by aircraft impact. Had insulation not been dislodged by the debris field, temperature rise of

structural components would likely have been insufficient to induce global collapse. Structural

components that became thermally weakened were generally determined by impact of the debris field. The existing condition of the insulation prior to aircraft impact and the insulation thickness on the WTC

floor system did not play a role in initiating collapse of the towers.

336 NISTNCSTAR 1-6, WTC Investigation
Findings

Finding 46: Creep strain was significant in the core and exterior columns over the 56 min to 102 min

period of fire exposure in columns with temperatures greater than 500 °C to 600 °C and high stress. Columns with creep strains of sufficient magnitude to cause column shortening played a significant role

in the collapse initiation.

Finding 47: The faces of the buildings that exhibited inward bowing were associated with the long span

direction of the floor system. The primary direction of tilting at collapse initiation for WTC 1 and WTC 2 was in the direction of the bowed faces.

10.8.2 Performance with Intact Fire Protection

Finding 48: A detailed thermal-structural analysis, which did not include slab delamination/spalling

effects, showed that a full collapse of the WTC floor system would not occur even with a number of

failed trusses or connections.

Finding 49: Most of the horizontal and vertical capacity of the floor connections to the exterior and core columns significantly exceeded the demand under design load conditions.

10.9 PROBABLE COLLAPSE SEQUENCES

The results of structural analyses conducted in this study on components, subsystems, isolated exterior

walls and cores, and global models of WTC 1 and WTC 2 showed that the collapses of the towers were

initiated due to the combined effects of the structural and insulation damage from aircraft impact and the

subsequent intense fires. The probable collapse sequences for WTC 1 and WTC 2 are based upon the

collective consideration of structural analyses, statistical based methods, observations, and laboratory

testing.

10.9.1 Role of the Building Core

Finding 50: The core columns were weakened significantly by the aircraft impact damage and thermal

effects. Thermal effects dominated the weakening of WTC 1 . As the fires moved from the north to the

south side of the core, following the debris damage path, the core was weakened over time by significant

creep strains on the south side of the core. Aircraft impact damage dominated the weakening of WTC 2. Immediately after impact, the vertical displacement at the southeast comer of the core increased 6 in. (from 4 in. to 10 in.). With the impact damage, the core subsystem leaned to the southeast and was

supported by the south and east floors and exterior walls.

Finding 51: As the core was weakened from aircraft impact and thermal effects, it redistributed loads to the exterior walls primarily through the hat truss. Additional axial loads redistributed to the exterior

columns from the core were not significant (only about 20 percent to 25 percent on average) as the

exterior columns were loaded to approximately 20 percent of their capacity before the aircraft impact.

10.9.2 Role of the Building Floors

Finding 52: The primary role of the floors in the collapse of the towers was to provide inward pull forces

that induced inward bowing of exterior columns (south face of WTC 1; east face of WTC 2).

NISTNCSTAR 1-6, WTC Investigation 337
Chapter 10

Finding 53: Sagging floors continued to support floor loads as they pulled inward on the exterior

columns. There would have been no inward pull forces if the floors truss seats had failed and

disconnected.

10.9.3 Role of Exterior Frame-Tube

Finding 54: Column instability over an extended region of the exterior face ultimately triggered the global system failure as the loads could not be redistributed through the hat truss to the already weakened

building core. In the area of exterior column buckling, load transferred through the spandrels to adjacent

columns and adjacent exterior walls. As the exterior wall buckled (south face for WTC 1 and east face

for WTC 2), the colunm instability propagated to adjacent faces and caused the initiation of the building

collapse.

Finding 55: The exterior wall instability was induced by a combination of thermal weakening of the

columns, inward pull forces from sagging floors, and to a lesser degree, additional axial loads

redistributed from the core.

10.9.4 Probable Collapse Sequences

Finding 56: Although the north face of WTC 1 had extensive impact damage, thermal weakening of the core columns on the south side of the core and inward bowing of the south face caused the building to tilt to the south at collapse initiation. The extent of fires observed on all faces of WTC 1 was similar,

although somewhat more extensive on the east and west faces (where short span floors were located) and

somewhat less extensive on the north and south faces (where long span floors were located). Thermal

weakening of exterior columns with floor sagging (which induced inward pull and occurred on the south

side) caused inward bowing of the south face and tilting in the south direction.

Finding 57: Although the south face of WTC 2 had extensive impact damage, thermal weakening of the core columns on the east side of the core and inward bowing of the east face caused the building to tilt more to the east and less to the south at collapse initiation. Fires were more extensive along the east face

and at the east side of the north and south faces. Thermal weakening of exterior columns with floor

sagging (which induced inward pull and occurred on the east side) caused inward bowing of the east face

and primary tilting in that direction (with additional southward tilting due to the aircraft impact damage).

Finding 58: The time it took for each WTC tower to collapse was due primarily to the differences in structural damage, the time it took the fires to travel from the impact area across the floors and core to

critical locations, and the time it took to weaken the core and exterior columns. WTC 2 had asymmetric

structural damage to the core, including the severing of a comer core column, and WTC 1 had more

symmetrical damage. The fires in WTC 2 reached the east side of the building more quickly, within 10

min to 20 min, than the 50 min to 60 min it took the fires in WTC 1 to reach the south side.

Finding 59: NIST found no corroborating evidence for alternative hypotheses suggesting that the WTC

towers were brought down by controlled demolition using explosives planted prior to September 1 1, 2001. NIST also did not find any evidence that missiles were fired at or hit the towers. Instead,

photographs and videos from several angles clearly showed that the collapse initiated at the fire and

338 NISTNCSTAR 1-6, WTC Investigation
impact floors and that the collapse progressed from the initiating floors downward, until the dust clouds

obscured the view.

NISTNCSTAR 1-6, WTC Investigation 339
Chapter 10

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340 NISTNCSTAR 1-6, WTC Investigation
Appendix A

Summary of Aircraft Impact Damage for Initial Cases 1 to 4

WTC 1 case Ai - thermal insulation and partition damage for

OCCUPANCY floor

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NISTNCSTAR 1-6. WTC Investigation 341
Appendix A

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342 NISTNCSTAR 1-6, WTC Investigation
Summary of Aircraft Impact Damage for Initial Cases 1 to 4

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NISTNCSTAR 1-6. WTC Investigation 343
Appendix A

WTC 1 CASE Ai - DAMAGE FOR STRUCTURAL FLOOR

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Figure A-4. WTC 1 Case Aj aircraft impact damage to Structural Floor 93 and 94.

344 NISTNCSTAR 1-6, WTC Investigation
Summary of Aircraft Impact Damage for Initial Cases 1 to 4

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Figure A-5. WTC 1 Case Ai aircraft impact damage to Structural Floors 95 and 96.

NISTNCSTAR 1-6. WTC Investigation 345
Appendix A

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Figure A-6. WTC 1 Case Ai aircraft impact damage to Structural Floors 97 and 98.

346 NISTNCSTAR 1-6, WTC Investigation
Summary of Aircraft Impact Damage for Initial Cases 1 to 4

WTC 1 CASE B| - THERMAL INSULATION AND PARTITION DAMAGE FOR

OCCUPANCY FLOOR

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NISTNCSTAR 1-6, WTC Investigation 347
Appendix A

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348 NISTNCSTAR 1-6, WTC Investigation
Summary of Aircraft Impact Damage for Initial Cases 1 to 4

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NISTNCSTAR 1-6. WTC Investigation 349
Appendix A

WTC 1 CASE Bi - DAMAGE FOR STRUCTURAL FLOOR

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350 NISTNCSTAR 1-6. WTC Investigation
Summary of Aircraft Impact Damage for Initial Cases 1 to 4

[ I Structural damage to concrete slab. con beams and trusses

Figure A-11. WTC 1 Case Bj aircraft impact damage to Structural Floors 95 and 96.

NISTNCSTAR 1-6. WTC Investigation 351
Appendix A

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Figure A-12. WTC 1 Case B; aircraft impact damage to Structural Floors 97 and 98.

352 NISTNCSTAR 1-6, WTC Investigation
Summary of Aircraft Impact Damage for Initial Cases 1 to 4

WTC 2 CASE Ci - THERMAL INSULATION AND OCCUPANCY DAMAGE FOR

OCCUPANCY FLOOR

Floor 78

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Figure A-13. WTC 2 Case d aircraft impact damage to Structural Floors 78 and 79.

NISTNCSTAR 1-6, WTC Investigation 353
Appendix A

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Figure A-14. WTC 2 Case Ci aircraft impact damage to Structural Floors 80 and 81.

354 NISTNCSTAR 1-6, WTC Investigation
Summary of Aircraft Impact Damage for Initial Cases 1 to 4

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NISTNCSTAR 1-6, WTC Investigation 355
Appendix A

WTC 2 CASE Ci - DAMAGE FOR STRUCTURAL FLOOR

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Figure A-16. WTC 2 Case Cj aircraft impact damage to Structural Floors 78 and 79.

356 NISTNCSTAR 1-6, WTC Investigation
Summary of Aircraft Impact Damage for Initial Cases 1 to 4

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Figure A-17. WTC 2 Case Ci aircraft impact damage to Structural Floors 80 and 81.

NISTNCSTAR 1-6, WTC Investigation 357
Appendix A

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Figure A-18. WTC 2 Case d aircraft impact damage to Structural Floors 82 and 83.

358 NISTNCSTAR 1-6, WTC Investigation
Summary of Aircraft Impact Damage for Initial Cases 1 to 4

WTC 2 CASE D, - THERMAL INSULATION AND PARTITION DAMAGE FOR

OCCUPANCY FLOOR

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Figure A-19. WTC 2 Case Di aircraft impact damage to Structural Floors 78 and 79.

NISTNCSTAR 1-6, WTC Investigation 359
Appendix A

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Figure A-20. WTC 2 Case Di aircraft impact damage to Structural Floors 80 and 81 =

360 NISTNCSTAR 1-6, WTC Investigation
Summary of Aircraft Impact Damage for Initial Cases 1 to 4

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Figure A-21. WTC 2 Case Dj aircraft impact damage to Structural Floors 82 and 83.

NISTNCSTAR 1-6. WTC Investigation 361
Appendix A

WTC 2 CASE Di - DAMAGE FOR STRUCTURAL FLOOR

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Figure A-22. WTC 2 Case Dj aircraft impact damage to Structural Floors 78 and 79.

362 NISTNCSTAR 1-6, WTC Investigation
Summary of Aircraft Impact Damage for Initial Cases 1 to 4

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Figure A-23. WTC 2 Case Dj aircraft impact damage to Structural Floors 80 and 81.

NISTNCSTAR 1-6, WTC Investigation 363
Appendix A

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Figure A-24. WTC 2 Case Dj aircraft impact damage to Structural Floors 82 and 83.

364 NISTNCSTAR 1-6, WTC Investigation
Appendix B

Summary of Aircraft Impact Damage for final Cases A to D

WT0 1 case a - occupancy floor graphics

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Figure B-1. WTC 1 Case A aircraft impact damage to Occupancy Floors 93 and 94.

NISTNCSTAR 1-6, WTC Investigation 365
Appendix B

Sewre Floor Daniay

Floor insulation

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Figure B-2. WTC 1 Case A aircraft impact damage to Occupancy Floors 95 and 96.

366 NISTNCSTAR 1-6, WTC Investigation
Summary of Aircraft Impact Damage for Final Cases A to D

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Figure B-3. WTC 1 Case A aircraft impact damage to Occupancy Floors 97 and 98.

NISTNCSTAR 1-6, WTC Investigation 367
Appendix B

WTC 1 CASE A - STRUCTURAL FLOOR GRAPHICS

Severe Floor Damage

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Figure B-4. WTC 1 Case A aircraft impact damage to Structural Floors 93 and 94.

368 NISTNCSTAR 1-6, WTC Investigation
Summary of Aircraft Impact Damage for Final Cases A to D

Sevtrt Floor Damage

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NISTNCSTAR 1-6, WTC Investigation 369
Appendix B

Severe Floor Damage

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Figure B-6. WTC 1 Case A aircraft impact damage to Structural Floors 97 and 98.

370 NISTNCSTAR 1-6, WTC Investigation
Summary of Aircraft Impact Damage for Final Cases A to D

WTC 1 CASE B - OCCUPANCY FLOOR GRAPHICS

Severe Floor Damage

Floor insulation

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NISTNCSTAR 1-6, WTC Investigation 371
Appendix B

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Figure B-8. WTC 1 Case B aircraft impact damage to Occupancy Floors 95 and 96.

372 NISTNCSTAR 1-6, WTC Investigation
Summary of Aircraft Impact Damage for Final Cases A to D

Severe Floor Damay

Floor insulation

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Figure B-9. WTC 1 Case B aircraft impact damage to Occupancy Floors 97 and 98.

NISTNCSTAR 1-6. WTC Investigation 373
Appendix B

WTC 1 CASE B - STRUCTURAL FLOOR GRAPHICS

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374 NISTNCSTAR 1-6, WTC Investigation
Summary of Aircraft Impact Damage for Final Cases AtoD

SCTigre Floor Damage

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Figure B-11. WTC 1 Case B aircraft impact damage to Structural Floors 95 and 96.

NISTNCSTAR 1-6, WTC Investigation 375
Appendix B

Sex'ere Floor Damage

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376 NISTNCSTAR 1-6, WTC Investigation
Summary of Aircraft Impact Damage for Final Cases A to D

WTC 2 CASE C - OCCUPANCY FLOOR GRAPHICS

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Figure B-13. WTC 2 Case C aircraft impact damage to Occupancy Floors 78 and 79.

NISTNCSTAR 1-6, WTC Investigation 377
Appendix B

Floor insulation

| |

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Figure B-14. WTC 2 Case C aircraft impact damage to Occupancy Floors 80 and 81.

378 NISTNCSTAR 1-6, WTC Investigation
Summary of Aircraft Impact Damage for Final Cases A to D

Severe Floor Dajivay

Floor insulation

| |

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Figure B-15. WTC 2 Case C aircraft impact damage to Occupancy Floors 82 and 83.

NISTNCSTAR 1-6, WTC Investigation 379
Appendix B

WTC 2 CASE C - STRUCTURAL FLOOR GRAPHICS

S^rere Floor Damage

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Figure B-16. WTC 2 Case C aircraft impact damage to Structural Floors 78 and 79.

380 NISTNCSTAR 1-6, WTC Investigation
Summary of Aircraft Impact Damage for Final Cases A to D

Severe Floor Damage

Floor system

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VFigure B-17. WTC 2 Case C aircraft impact damage to Structural Floors 80 and 81.

NISTNCSTAR 1-6. WTC Investigation 381
Appendix B

Sewi^ Floor Damage

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Figure B-18. WTC 2 Case C aircraft impact damage to Structural Floors 82 and 83.

382 NISTNCSTAR 1-6, WTC Investigation
Summary of Aircraft Impact Damage for Final Cases A to D

WTC 2 CASE D - OCCUPANCY FLOOR GRAPHICS

Severe Floor Dantay

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Figure B-19. WTC 2 Case D aircraft impact damage to Occupancy Floors 78 and 79.

NISTNCSTAR 1-6, WTC Investigation 383
Appendix B

Sewre Floor Damas

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Figure B-20. WTC 2 Case D aircraft impact damage to Occupancy Floors 80 and 81.

384 NISTNCSTAR 1-6, WTC Investigation
Summary of Aircraft Impact Damage for Final Cases A to D

SCTiere Floor Damay

Floor insulation

| |

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Figure B-21. WTC 2 Case D aircraft impact damage to Occupancy Floors 82 and 83.

NISTNCSTAR 1-6, WTC Investigation 385
Appendix B

WTC 2 CASE D - STRUCTURAL FLOOR GRAPHICS

Severe Floor Damage

Floor BJ'Stem

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Figure B-22. WTC 2 Case D aircraft impact damage to Structural Floors 78 and 79.

386 NISTNCSTAR 1-6, WTC Investigation
Summary of Aircraft Impact Damage for Final Cases A to D

Severe Floor Damage

Floor sj'stem

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Figure B-23. WTC 2 Case D aircraft impact damage to Structural Floors 80 and 81.

NISTNCSTAR 1-6, WTC Investigation 387
Appendix B

Sewre Floor Damage

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Figure B-24. WTC 2 Case D aircraft impact damage to Structural Floors 82 and 83.

388 NISTNCSTAR 1-6, WTC Investigation