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A framed exterior tube, 47-column core, composite floors, and roof-level hat truss shared gravity and lateral loads.

1. Closely spaced perimeter columns and spandrels formed the exterior framed tube.
2. Floor trusses connected the perimeter to the core and transferred gravity load.
3. The hat truss could redistribute some load between the core and exterior.

Stories above grade

110 MEASURED

Square floor plate

207 ft 2 in per side MEASURED

Perimeter columns

59 per face MEASURED

Closely spaced columns connected by spandrel beams formed the framed exterior tube.

Core columns

47 MEASURED

• NIST NCSTAR 1-1Design, Construction, and Maintenance of Structural and Life Safety Systems

08:46:30 EDT / NIST engineering clock / T+0

The aircraft opened a damage zone more than half the tower's width, damaging perimeter columns, floors, core columns, partitions, and fireproofing.

1. NIST's baseline impact used 443 mph, a 10.6° downward trajectory, and 25° left-wing-down roll.
2. The nose struck near the Floor 96 slab; the visible entry wound extended from Floor 93 through Floor 99.
3. The tower remained standing: impact damage was severe but did not by itself initiate global collapse.

Impact zone

Floors 93–99 OBSERVED

Nose level

Near Floor 96 OBSERVED

Downward trajectory

10.6° NIST MODEL

Roll

25° left-wing-down NIST MODEL

• NIST NCSTAR 1Final Report on the Collapses of the World Trade Center Towers
• NIST NCSTAR 1-2BAnalysis of Aircraft Impacts into the World Trade Center Towers
• NIST NCSTAR 1-5Reconstruction of the Fires in the World Trade Center Towers

First minutes

Surviving columns, floors, spandrels, and the hat truss accepted redistributed loads while the damaged tower retained substantial reserve capacity.

1. Local column loss changes load paths rather than causing immediate global failure.
2. Aircraft debris stripped fireproofing from structural steel over a broad internal region.
3. Impact damage set the initial conditions for the later fire-driven structural response.

• NIST NCSTAR 1Final Report on the Collapses of the World Trade Center Towers
• NIST NCSTAR 1-2BAnalysis of Aircraft Impacts into the World Trade Center Towers
• NIST NCSTAR 1-6 Vol. 1Structural Fire Response and Probable Collapse Sequence of the World Trade Center Towers

T+minutes to T+102 min

Jet fuel ignited large areas on multiple floors, but was mostly consumed in the first few minutes. Office and aircraft contents sustained the longer fires.

1. The dominant long-duration fuel was ordinary office combustible material, not a continuing pool of jet fuel.
2. NIST modelled maximum upper-layer air temperatures around 1,000°C; air temperature is not automatically the temperature of every steel member.
3. Steel did not need to melt: unprotected heated members weakened, expanded, sagged, yielded, and buckled below the steel melting point.

Principal fire floors

93–99 OBSERVED

Maximum modelled upper-layer air temperature

≈1,000°C / 1,800°F NIST MODEL

Peak air-layer result, not a uniform steel temperature.

Bare-steel strength at 1,000°C

≈10% of room-temperature value MEASURED

Material-property context, not a claim that every member reached 1,000°C.

• NIST NCSTAR 1-5Reconstruction of the Fires in the World Trade Center Towers
• NIST NCSTAR 1-6 Vol. 1Structural Fire Response and Probable Collapse Sequence of the World Trade Center Towers
• NIST Towers FAQQuestions and Answers About the NIST WTC Towers Investigation

Visible bowing documented near 10:23 EDT / Approximately T+97 min

Core weakening, heated-floor sagging, retained floor connections, and redistributed gravity load produced visible inward bowing of the south perimeter.

1. The vast majority of relevant floor connections remained able to transmit inward pull while floors sagged.
2. This pull-in mechanism is different from a claim that detached floors simply pancaked first.
3. The visibly bowed perimeter approached instability under combined axial load, heat, and geometric deformation.

Maximum visible south-face bowing

≈55 in ±6 in MEASURED

Near Floor 97, measured from photographic evidence shortly before initiation.

• NIST NCSTAR 1-6 Vol. 1Structural Fire Response and Probable Collapse Sequence of the World Trade Center Towers
• NIST NCSTAR 1-6 Vol. 2Probable Collapse Sequences and Supporting Analyses
• NIST Towers FAQQuestions and Answers About the NIST WTC Towers Investigation

10:28:22 EDT / NIST engineering clock / T+102 min

The inwardly bowed south wall lost stability, buckling spread rapidly across adjacent faces, and the upper building section tilted south before descending.

1. This is the end of NIST's high-detail tower collapse-initiation reconstruction.
2. Once downward motion imposed a sudden dynamic load, the structure below could not arrest the growing falling mass.
3. The first exterior panels reached the ground before all visible core remnants had fallen.

Time from impact

102 min OBSERVED

Observed upper-section tilt

At least ≈8° south OBSERVED

Before dust obscured the view.

First exterior panels to ground

≈11 s MEASURED

Not the total collapse duration.

Core remnants

Roughly 60 stories remained briefly OBSERVED

Significant portions remained for approximately 15–25 seconds after initiation.

Uncertainty boundary: NIST's detailed tower reconstruction followed events from aircraft impact through collapse initiation. NIST did not model the full post-initiation collapse. The post-initiation WebGL descent is therefore an evidence-aligned explanatory animation, not a member-by-member reproduction of a NIST collapse simulation.

• NIST NCSTAR 1-6 Vol. 1Structural Fire Response and Probable Collapse Sequence of the World Trade Center Towers
• NIST NCSTAR 1-6 Vol. 2Probable Collapse Sequences and Supporting Analyses
• NIST Towers FAQQuestions and Answers About the NIST WTC Towers Investigation

WTC 2 used the same basic tower system as WTC 1, with the rectangular core rotated ninety degrees.

1. The impact therefore encountered a different sequence of long-span and short-span floor regions.
2. Impact fifteen floors lower meant the affected columns carried larger accumulated gravity loads.
3. The structural system nevertheless retained reserve capacity immediately after impact.

Stories above grade

110 MEASURED

Square floor plate

207 ft 2 in per side MEASURED

Perimeter columns

59 per face MEASURED

Closely spaced columns connected by spandrel beams formed the framed exterior tube.

Core columns

47 MEASURED

• NIST NCSTAR 1-1Design, Construction, and Maintenance of Structural and Life Safety Systems
• NIST NCSTAR 1-2BAnalysis of Aircraft Impacts into the World Trade Center Towers

09:02:59 EDT / NIST engineering clock / T+0

The faster, shallower, off-centre impact produced an entry wound across nine floors and concentrated damage toward the east and southeast core region.

1. NIST's baseline impact used 546 mph, a 6° downward trajectory, and 38° left-wing-down roll.
2. The nose centre struck near the Floor 81 slab; the visible entry wound extended from Floor 77 through Floor 85.
3. WTC 2's baseline speed was about 23% greater than WTC 1's and produced about 50% more translational impact energy.

Impact zone

Floors 77–85 OBSERVED

Nose level

Near Floor 81 slab OBSERVED

Downward trajectory

6° NIST MODEL

Roll

38° left-wing-down NIST MODEL

• NIST NCSTAR 1Final Report on the Collapses of the World Trade Center Towers
• NIST NCSTAR 1-2BAnalysis of Aircraft Impacts into the World Trade Center Towers
• NIST NCSTAR 1-5Reconstruction of the Fires in the World Trade Center Towers

T+minutes to T+56 min

The principal fires on Floors 79–83 heated unprotected structural components after impact debris had damaged floors, partitions, and fireproofing.

1. Jet fuel caused near-simultaneous ignition across multiple floors but was mostly consumed in the first few minutes.
2. Office and aircraft contents became the dominant sustained fuel.
3. Extensive window breakage provided more ventilation than in much of WTC 1's fire zone.

Principal fire floors

79–83 OBSERVED

Maximum modelled upper-layer air temperature

≈1,000°C / 1,800°F NIST MODEL

Peak air-layer result, not a uniform steel temperature.

• NIST NCSTAR 1-5Reconstruction of the Fires in the World Trade Center Towers
• NIST NCSTAR 1-6 Vol. 1Structural Fire Response and Probable Collapse Sequence of the World Trade Center Towers
• NIST Towers FAQQuestions and Answers About the NIST WTC Towers Investigation

First observed about 09:21–09:23 EDT / Approximately T+20 min to T+50 min

Sagging floors retained enough connection to pull the east perimeter inward, and the visible deformation increased as fires and load redistribution evolved.

1. Floor sag converts part of the gravity-driven deformation into horizontal pull on the perimeter.
2. Geometric bowing reduces the wall's ability to carry compressive gravity load.
3. The observed deformation grew before collapse initiation, providing a visible precursor.

Initial measured bow

≈10 in ±1 in MEASURED

Near Floor 80 at approximately 20 minutes.

Later measured bow

≈20 in ±1 in MEASURED

Near Floor 80 at approximately 50 minutes.

• NIST NCSTAR 1-6 Vol. 1Structural Fire Response and Probable Collapse Sequence of the World Trade Center Towers
• NIST NCSTAR 1-6 Vol. 2Probable Collapse Sequences and Supporting Analyses
• NIST Towers FAQQuestions and Answers About the NIST WTC Towers Investigation

09:58:59 EDT / NIST engineering clock / T+56 min

The east wall buckled, instability spread, and the upper section tilted east and south before substantial downward motion.

1. The combined impact, fireproofing, fire, floor, core, and perimeter response reached global instability.
2. The upper section contained far more floors than an intact floor connection system could accept as a suddenly applied load.
3. Post-initiation debris motion is less certain than the documented initiation sequence.

Time from impact

56 min OBSERVED

Tilt before significant downward motion

≈7–8° east; ≈3–4° south OBSERVED

Conservative executive-summary observation.

First exterior panels to ground

≈9 s MEASURED

Not the total collapse duration.

Core remnants

Roughly 40 stories remained briefly OBSERVED

Significant portions remained for approximately 15–25 seconds after initiation.

Uncertainty boundary: NIST's detailed tower reconstruction followed events from aircraft impact through collapse initiation. NIST did not model the full post-initiation collapse. The post-initiation WebGL descent is therefore an evidence-aligned explanatory animation, not a member-by-member reproduction of a NIST collapse simulation.

• NIST NCSTAR 1-6 Vol. 1Structural Fire Response and Probable Collapse Sequence of the World Trade Center Towers
• NIST NCSTAR 1-6 Vol. 2Probable Collapse Sequences and Supporting Analyses
• NIST Towers FAQQuestions and Answers About the NIST WTC Towers Investigation

After WTC 1 collapsed at 10:28:22 EDT / Nearly seven hours before WTC 7 collapse

Debris from WTC 1 damaged WTC 7's southwest region and ignited fires on at least ten floors.

1. Seven exterior columns were severed in the southwest region, but NIST found that this structural damage did not initiate the eventual collapse.
2. The fires started by the debris—not an aircraft strike or fuel-oil system—became the initiating hazard.
3. The exterior damage played a secondary role late in the facade collapse.

Exterior columns severed by debris

7 NIST MODEL

Floors with debris-initiated fires

At least 10 OBSERVED

• NIST NCSTAR 1AFinal Report on the Collapse of World Trade Center Building 7
• NIST NCSTAR 1-9Structural Fire Response and Probable Collapse Sequence of World Trade Center Building 7
• NIST WTC 7 FAQQuestions and Answers About the NIST WTC 7 Investigation

Hours

Fires on Floors 7–9 and 11–13 burned out of control after damaged city mains deprived the lower-zone sprinkler system of its primary and backup water source.

1. The fuels were ordinary office combustibles at ordinary combustible-load levels.
2. Window breakage ventilated the fires and allowed continued spread and growth.
3. NIST reported no accelerants and found that diesel fuel systems did not contribute to collapse.

Uncontrolled fire floors

7–9 and 11–13 OBSERVED

Duration before collapse

Almost 7 hours OBSERVED

Measured from the WTC 1 collapse to 17:20:52.

• NIST NCSTAR 1AFinal Report on the Collapse of World Trade Center Building 7
• NIST NCSTAR 1-9Structural Fire Response and Probable Collapse Sequence of World Trade Center Building 7
• NIST WTC 7 FAQQuestions and Answers About the NIST WTC 7 Investigation

As fires reach the northeast floor system

Heating elongated long floor beams, distorted framing, and imposed horizontal forces on connections principally designed to carry vertical gravity loads.

1. Interior columns remained below about 200°C in the model; most exterior columns remained below 150°C, with east/south exterior columns on Floors 12–13 reaching about 300°C.
2. Some beam sections exceeded 600°C, but key connection, beam, and girder failures occurred primarily below approximately 400°C.
3. Long, restrained members can create large thermal forces even where steel has not lost most of its room-temperature strength.

Interior-column temperature

Below ≈200°C NIST MODEL

Calculated maximum range for interior columns including 79–81.

Highest exterior-column temperature

≈300°C / 570°F NIST MODEL

East and south exterior columns on Floors 12–13.

Some east-side floor beams

>600°C / 1,100°F NIST MODEL

Principal connection/floor failures

Primarily below ≈400°C / 750°F NIST MODEL

Thermal expansion dominated this part of the mechanism.

• NIST NCSTAR 1AFinal Report on the Collapse of World Trade Center Building 7
• NIST NCSTAR 1-9Structural Fire Response and Probable Collapse Sequence of World Trade Center Building 7
• NIST WTC 7 FAQQuestions and Answers About the NIST WTC 7 Investigation

Collapse initiation sequence

Expanded northeast floor beams pushed a key Floor 13 girder away from its seat at Column 79; local floor failures then cascaded down to Floor 5.

1. The Floor 13 failure and other fire-induced damage removed lateral support around Column 79.
2. Column 79 became unsupported in the east-west direction across nine stories and buckled between Floors 5 and 14.
3. This Column 79 buckling was NIST's initiating event for WTC 7's global collapse.

Floor cascade

Floor 13 down to Floor 5 NIST MODEL

Unsupported Column 79 length

9 stories NIST MODEL

• NIST NCSTAR 1AFinal Report on the Collapse of World Trade Center Building 7
• NIST NCSTAR 1-9Structural Fire Response and Probable Collapse Sequence of World Trade Center Building 7
• NIST WTC 7 FAQQuestions and Answers About the NIST WTC 7 Investigation

Approximately 12.9 s from initial floor failures to facade motion

Column 79's descent produced the east-penthouse kink and drop. Failures advanced through Columns 80 and 81 and westward across the core before the exterior frame descended.

1. Falling floors removed lateral support and imposed debris loads on neighbouring columns.
2. The comparatively stiff exterior frame temporarily remained as an increasingly unsupported shell.
3. This interior-first sequence explains why a later exterior view can look comparatively uniform.

Initial floor failures to north-facade motion

12.9 s MEASURED

Measured video interval and NIST analytical prediction both reported as 12.9 seconds.

• NIST NCSTAR 1AFinal Report on the Collapse of World Trade Center Building 7
• NIST NCSTAR 1-9Structural Fire Response and Probable Collapse Sequence of World Trade Center Building 7
• NIST WTC 7 FAQQuestions and Answers About the NIST WTC 7 Investigation

17:20:52 EDT / 5.4 s visible 18-story roofline descent

Video analysis found an initial resisted interval, approximately 2.25 seconds at gravitational acceleration after support had been lost, and a final interval of renewed resistance.

1. Stage 1 corresponds to exterior-column buckling and acceleration below gravity.
2. Stage 2 is a measured free-fall interval, consistent with negligible remaining support beneath the facade after interior failure.
3. Stage 3 slows as the descending upper facade meets the collapsed structure and debris below.

Visible descent

18 stories in 5.4 s MEASURED

Stage 1

0–1.75 s · acceleration below g MEASURED

Stage 2

1.75–4.0 s · gravitational acceleration MEASURED

Approximately 2.25 seconds of free fall after support below had become negligible.

Stage 3

4.0–5.4 s · acceleration below g MEASURED

Uncertainty boundary: NIST notes that uncertainty grows substantially once global collapse and random debris interaction begin; later deformation should remain evidence-labelled rather than presented as exact.

• NIST NCSTAR 1AFinal Report on the Collapse of World Trade Center Building 7
• NIST NCSTAR 1-9Structural Fire Response and Probable Collapse Sequence of World Trade Center Building 7
• NIST WTC 7 FAQQuestions and Answers About the NIST WTC 7 Investigation

Seconds before 09:37:46 EDT

The fixed published approach brings N644AA toward the Pentagon's west facade from the southwest, nearly level and only a few feet above the ground in the final fraction of a second.

1. The NTSB reconstructed a descending right turn before the final southwest approach; the Commission reported an impact speed of approximately 530 mph.
2. ASCE reported that at about 320 ft from the facade—roughly 0.42 seconds before impact—the aircraft was nearly level and only a few feet above the ground.
3. By the final tenth of a second the aircraft had rolled slightly left, leaving the right wing elevated; the right wing struck construction equipment and the left engine contacted the ground near impact.

Distance at near-level observation

≈320 ft / 0.42 s MEASURED

Impact speed

≈530 mph MEASURED

WebGL aircraft motion

Illustrative interpolation between published anchors ILLUSTRATIVE

Uncertainty boundary: The browser interpolates between published approach anchors. It is not an exact aerodynamic or impact-dynamics simulation.

• NTSB AA77 studyFlight Path Study: American Airlines Flight 77
• 9/11 CommissionThe 9/11 Commission Report
• ASCE Pentagon studyThe Pentagon Building Performance Report
• DoD Pentagon historyPentagon 9/11

09:37:46 EDT / Impact

The nose and fuselage struck the west wall at or slightly below the second-floor slab while the wings crossed the slab line at a shallow left roll.

1. The fuselage impact was at or slightly below the second-floor slab; the left wing remained below that slab while the right wing crossed from below to above it.
2. The left engine struck the ground at nearly the same time as the nose struck the facade.
3. Impact, fuel deflagration, and fragment motion damaged exterior columns and distributed damage inward; the renderer separates those evidence categories rather than presenting one exact fracture surface.

NTSB recorder-study time

09:37:45 EDT MEASURED

The NTSB study and Commission chronology differ by one second; both official labels are retained.

Approach angle to facade

≈42° MEASURED

Aircraft attitude

Nearly level / slight left roll OBSERVED

Uncertainty boundary: Impact flash, flame shape, and individual fragment paths are illustrative and do not reproduce an exact fragment-by-fragment impact calculation.

• 9/11 CommissionThe 9/11 Commission Report
• ASCE Pentagon studyThe Pentagon Building Performance Report
• DoD Pentagon historyPentagon 9/11

Immediate post-impact condition

The oblique damage path crossed the outer E ring and continued through the D and C rings while much of the surrounding redundant frame remained standing.

1. The published damage path was approximately 270 ft along its angled route and reached roughly 210 ft into the building through the E, D, and C rings.
2. ASCE reported approximately 30 first-floor columns destroyed or losing capacity, about 20 more significantly impaired, and possibly about six second-floor columns destroyed.
3. Continuity, redundancy, and energy-absorbing reinforced-concrete framing redistributed loads around the damaged region and limited immediate collapse.

Angled damage path

≈270 ft MEASURED

Depth into building

≈210 ft MEASURED

First-floor columns losing capacity

≈30 MEASURED

Additional first-floor columns impaired

≈20 MEASURED

Possible second-floor columns destroyed

≈6 UNCERTAIN

Published as a possible count rather than a precise inventory.

Uncertainty boundary: The amber corridor is an evidence diagram, not a solid aircraft tunnel or exact map of every fragment and failed member.

• ASCE Pentagon studyThe Pentagon Building Performance Report
• DoD Pentagon historyPentagon 9/11

Minutes after impact

Aircraft fuel, aircraft contents, and building contents sustained fire across a broad first-floor area and smaller upper-floor regions while the surviving frame carried redistributed loads.

1. ASCE found fire damage over a very large area of the first story, a significant area of the second, a smaller part of the third, and stairwells above.
2. The impact had removed or impaired many columns, but beam-and-girder continuity and alternative load paths delayed and limited collapse.
3. Fire weakened additional structural elements locally; the page does not infer unknown temperatures or hidden member-by-member histories.

Immediate global collapse

Did not occur OBSERVED

Fire distribution

Broadest on Floor 1 MEASURED

Uncertainty boundary: Fire and smoke volumes are explanatory indicators, not an exact computational fire reconstruction.

• ASCE Pentagon studyThe Pentagon Building Performance Report
• DoD Pentagon historyPentagon 9/11

Official accounts differ: ≈09:57 or 10:15 EDT / Delayed partial collapse

A localized section of the E ring above the west impact zone collapsed after the immediate impact, while the great majority of the Pentagon remained standing.

1. The affected exterior section was approximately 95 ft wide and about 50 ft deep, spanning four floors above the ground level.
2. ASCE describes the local collapse as approximately 20 minutes after impact; the DoD historical narrative places the visible E-ring collapse at 10:15. Both official timings are shown.
3. The limited footprint is evidence of the structure's continuity, redundancy, and capacity to bridge damaged columns—not an absence of severe local damage.

Collapsed width

≈95 ft MEASURED

Collapsed depth

≈50 ft MEASURED

Vertical extent

Four E-ring floors MEASURED

ASCE interval

≈20 min after impact MEASURED

DoD narrative time

10:15 EDT OBSERVED

Uncertainty boundary: Illustrative WebGL debris and dust are not an exact forensic reproduction. The official collapse-time discrepancy is preserved rather than resolved by the animation.

• ASCE Pentagon studyThe Pentagon Building Performance Report
• DoD Pentagon historyPentagon 9/11