Reconstruction Methodology and Evidence Standards

This document describes the evidence hierarchy, modeling choices and verification used in the World Trade Center reconstruction. Source consistency is distinct from validation of a structural simulation.

The project’s standard is to distinguish observed evidence, calculated results and illustrative geometry, and to record unresolved assumptions alongside the sources.

1. Evidence Hierarchy

Read the model and account using these three evidence tiers. A citation does not make every coordinate in the associated mesh a measured quantity:

Tier 1: Directly Observed Physical Evidence

Physical artifacts, verified photographic records, and instrumental measurements that exist independently of post-disaster numerical modeling.

• Pre-collapse high-resolution photography & video: Photographic records from dozens of vantage points establishing entry/exit gash geometry (NIST NCSTAR 1-2), visible flame and smoke migration across floor windows (NCSTAR 1-5A), window-by-window exterior column inward bowing measurements (NCSTAR 1-3, Fig. 6-18), and synchronized multi-angle video of collapse initiation (NCSTAR 1-6 §6.1).
• Physical steel recovery and metallurgical analysis: NIST cataloged 236 recovered steel elements, including 90 perimeter-panel samples and 55 core-column samples. Only a subset had known original locations. Recovery records, photographs and material examinations constrain some damage, but recovered failure modes alone do not establish when each connection failed (NIST NCSTAR 1-3 and NCSTAR 1-3C).
• Original architectural and structural contract drawings: Official Port Authority of New York & New Jersey (PANYNJ) contract sets, Leslie E. Robertson Associates (LERA) tenant and alteration records, Pacific Construction (PAC1) Tower A plans, and Foundation drawings R-60 through R-77 detailing the 3,100-foot perimeter slurry wall and bedrock tieback arrays.
• Instrumental records: Lamont-Doherty Earth Observatory seismic recordings of aircraft impacts and collapse ground waves; FAA radar transponder tracking.

Tier 2: Calibrated Engineering Calculations and Forensic Simulations

Engineering analyses reconstruct conditions that could not be directly observed. Agreement with exterior observations constrains their assumptions; it does not turn calculated interior damage into a measured survey.

• Impact dynamics (LS-DYNA): NIST calculated aircraft fragmentation and structural damage in separate base, less severe and more severe impact cases. Estimates of insulation dislodgment used that impact work with additional assumptions; the structural damage footprints are not insulation-loss maps (NCSTAR 1-2; NCSTAR 1 §6.14.5).
• Fire propagation (NIST FDS): Computational fluid dynamics (CFD) modeling of combustible fuel loads, ventilation-limited fire spread, and upper-layer compartment gas temperatures exceeding 1,000 °C (NIST NCSTAR 1-5).
• Thermal-structural response (ANSYS): NIST separately calculated heat transfer and structural response. The viewer samples named subsystem displacement contours, including preliminary Bᵢ and Dᵢ cases; those are distinct from final global B and D. Global models used imposed pull-in forces and disconnections informed by subsystem work and observations. No universal sag maximum or force per connection applies across these studies (NCSTAR 1-6C; NCSTAR 1-6D §2.5 and Chapter 5).
• Progressive collapse limit states: Academic conservation of momentum and energy analyses (Bažant & Zhou, 2002; Bažant & Verdure, 2007) evaluating kinetic energy versus plastic energy dissipation capacity during progressive downward failure.

Tier 3: Geometric, Interpolative, and Illustrative Representations

Visual modeling decisions made for interactive 3D WebGL rendering, clearly identified so they are not mistaken for historical surveying.

• Representative member schedules: Trusses outside the primary impact zones repeat standardized architectural panel modules (NIST NCSTAR 1-2A Appendix G typologies) rather than unique tenant configurations.
• Bounded early motion: The North model reaches a representative 8° south / 80 ft downward pose. The angle follows NIST’s reported lower-bound orientation; placement is a conditional archive-image estimate with uncertain camera and ejection-band calibration, not a synchronized measurement. The South model passes 7° east / 3° south and continues to a selected 20° east, holding 3° south. South downward placement is a minimum non-rising reference-cut convention, not a measured fall distance. Reference cuts near floors 98 and 82, nominal pivots, intermediate paths and timing are illustrative. The model does not solve instability, support resistance or subsequent collapse propagation.
• Display cutaways and annotations: Colors, arrows and transparent layers clarify source categories and structural relationships. They do not encode computed stress, temperature or connection capacity.

2. Primary Source Datums and Geometry Verification

Floor Elevations and Story Heights

The model uses the LERA story-height schedule (`floor_heights.txt`), which references Port Authority drawings A-AB-201 and A-AB-301, for both towers. Typical tenant stories are 12 feet floor to floor; special floors have different spacing. The schedule establishes relative story elevations, with floor 1 (concourse) as the viewer’s vertical origin. It is not a surveyed street-grade or sea-level datum. Story spacing accounts for:

• 12-foot standard tenant office stories.
• Two-story mechanical equipment rooms (Floors 7/8, 41/42, 75/76, 108/109) with heavy perimeter double-spandrel girders.
• The lower concourse, plaza and storage levels, each with its own scheduled height.
• Distinct upper-floor, sloping-roof, roof-panel and penthouse references. NIST’s reported tower heights of 1,368 and 1,362 feet must not be relabeled as drawing top-of-steel elevations. Local roof and penthouse surfaces retain their own source datums and unresolved interfaces. NCSTAR 1-1, p. 7.

Substructure and Slurry Wall ("The Bathtub")

The subterranean enclosure is modeled directly from foundation contract sheets R-60 through R-77:

• Perimeter Slurry Wall: 3,100 linear feet of 3-foot-thick reinforced concrete wall descending 65 to 80 feet to solid schist bedrock.
• Tieback Array: 1,548 illustrative multi-tier rock tendons representing the temporary drill anchors installed at 10-foot nominal spacing through Hudson River silt into bedrock sockets prior to floor slab installation.
• Sublevel Datums: Service level B1 (EL 294′) through lower pump room B6 (EL 242′), including PATH railway track tunnels and platform elevations from drawing R-65.

3. Forensic Case Studies in Reconciling Contradictions

Case Study A: Antenna Motion and Upper-Section Tilt

An apparent early antenna descent in a view from the north is not direct proof that the core dropped independently. NIST compared other views and explained that appearance through southward tilt of the upper building section. Its probable sequence includes core weakening and shortening, but the footage does not establish an antenna plunging through the roof. The viewer must not turn that inference into a measured independent core-drop trajectory. NCSTAR 1-6, §6.1, p. 153; NCSTAR 1, pp. 150–151.

Case Study B: Connected Floors and Wall Bowing

NIST did not support successive floor detachment as the mechanism that initiated collapse. Its probable sequence includes sagging floors, some failed connections, some remaining paths for inward pull, and column instability. Observed inward displacement reached 55 ± 6 inches on WTC 1’s south face. WTC 2’s east-face record distinguishes an early 7–9-inch observation from a later 20-inch maximum. These observations measure displacement, not force or the condition of every seat. NIST investigation FAQ, collapse mechanism and connected-floor explanations; NCSTAR 1-3, Figs. 6-15 and 6-18.

Recovered connections show several failure modes. Their final condition may include impact, fire, collapse and recovery damage; it does not by itself prove a pre-collapse tensile failure at each location (NCSTAR 1-3C, Chapter 3).

4. Source Review and Geometry Verification

The September 12, 2026 damage audit re-extracted all 24 tower/case/floor records from the final NCSTAR 1-2 floor-damage figures. Eight source pages contain 81 distinct solid or dotted annotation paths. The review preserves the source PDF hash, raw subpaths, page registration and visual overlays; disconnected shapes remain separate. Nominal core registration and small orthogonalization adjustments introduce roughly two feet of graphic uncertainty. This is a working tolerance for the diagram fit, not a NIST measurement-error estimate.

Solid outlines represent severe structural damage; dotted outlines represent openings. NIST excluded both from its subsequent structural models. The viewer leaves severe regions amber for inspection, which does not assign them surviving load capacity. The source figures do not identify every damaged truss or fracture elevation. NCSTAR 1-2, pp. 187–189, 210–211, 232–234 and 255–256.

Verification compares core table records with the source PDF, checks floor openings against actual slab/deck intersections, checks retained pieces and source identities, and exercises damage-case changes and view restoration. Such checks establish consistency between sources, data and rendered geometry; they do not validate a finite-element response or prove complete historical accuracy. AI assistance is part of the authoring process and is not independent evidence.

5. Explicit Limitations and Boundaries

1. Not an Aerodynamic or Hydrodynamic Solver: The browser model renders historical and calculated forensic states; it does not compute real-time structural stress tensors, wind turbulence, or fluid dynamics.
2. Post-Impact Interior Obscurement: Entry-face geometry combines FEMA diagrams and localized NIST observations; obscured areas, approximate registration and uncertain fracture depths remain. Interior impact damage is calculated. Source-traced retained pieces do not establish complete three-dimensional fracture geometry or load continuity.
3. Tenant Alterations: The model reflects original design and structural alteration drawings on file with the Port Authority; minor non-structural architectural partition shifts made by individual commercial tenants between 1973 and 2001 are not comprehensively reconstructed.
4. Non-Sensational Educational Mandate: The explainer is designed strictly as an engineering record. All representations prioritize sober structural physics, educational utility, and respect for historical fact.
5. Combining separate response studies: North Tower illustration mode adds a sampled subsystem sag field to representative impact deformations without a verified common nodal reference. South Tower mode registers its sampled total floor displacement to the impact baseline, but that registration also remains approximate. Neither animation is a single coupled NIST simulation.