3.1 STATED PURPOSE AND METHODS OF THE NIST REPORTS

The first and foremost stated purpose of the NIST reports on the WTC towers is to discover how and why each of the 3 buildings collapsed. This purpose is stated in the introductions of each report. It is also clearly stated on the opening page, in the opening paragraph of their website featuring their collective work on the collapses. It is also clearly stated multiple times within the Executive Summary of findings (in NCSTAR 1-6).




From the home page of the NIST World Trade Center Disaster website:

The goals of the investigation of the WTC disaster were:

The specific objectives were:

Note that every listed goal or objective depends upon determining how and why the buildings collapsed.



The first sentence of the Executive summary reproduced:

"One of the four objectives of the National Institute of Standards and Technology (NIST) investigation of the collapse of the World Trade Center (WTC) towers was to determine why and how the two towers (WTC 1 and WTC 2) collapsed following the initial impacts of the aircraft."


The method in which the NIST went about determining a collapse sequence is reproduced from the Executive summary. The condensed form of the NIST conclusions is given in the final reports, in the opening pages of NCSTAR 1-6.

The structural outline of the NIST executive summary is reproduced below in order to show the specific logical sequence of the methodology and reasoning used within the NIST reports. The purpose, scope, methodology used and final conclusions drawn are clearly outlined within the summary. In the summary the NIST once again states the purpose and scope of the reports, which is to determine how and why each building collapsed.



The executive summary contains a flow chart of the steps taken by the NIST to arrive at the determination of probable collapse sequences for each building The chart serves to show the general methodology used throughout the reports:



Fig 2.21, Reproduced from NIST NCSTAR 1-6 Executive Summary figure E.1 describing, in detail, the process the NSIT used to determine how and why each building fell as it did.



The end result, not surprisingly, is the determination of the collapse sequence of each building.

The 11 specific steps used by the NIST to reach their conclusions are listed from the executive summary:

Note that section E.2.1, in the executive summary specifies 11 steps of the NIST approach in its most explicit form, listed as A. through K.


A) Develop finite element models based on reference models
B) Develop the constitutive relationships for the materials used in the contruction of the towers
C) Characterize the passive fire protection applied to the structural steel
D) Conduct standard fire resistance tests of composite floor systems
E) Establish the damage to the structure, passive fire protection, and partition walls as a result of aircraft impact
F) Document observations and data related to structural events
G) Compute temperature histories for structural components subjected to fires
H) Conduct component and subsystem analysis
I) Conduct analysis of major subsystems
J) Conduct a separate global analysis for each tower
K) Determine the probable collapse sequence for each tower. A probable collapse sequence for each tower was determined. The collapse sequences were evaluated against key observables."


Note that the end result is, not surprisingly, step K.

Also note that there are only 3 steps which depend on careful observation of the visual record: steps E, F and K.

E requires careful observation of aircraft impact
F requires careful observation of fire progression
K requires careful observation of the initial collapse sequence






In the broadest sense the executive summary consists of 5 parts:


E.1 PURPOSE AND SCOPE
E.2 METHODOLOGY AND ANALYSIS RESULTS
E.3 PROBABLE COLLAPSE SEQUENCE OF WTC 1 AND WTC 2
E.4 FACTORS THAT AFFECTED PERFORMANCE
E.5 FINDINGS


E.1 defines the purpose of the report as determining the how and why of each collapse, E.2 describes the methods used to determine the collapse initiation sequence and E.3, .4 and .5 are their conclusions.

The detailed methodology used by the NIST can also be seen within the specific layout of the executive summary, expanded below to include all sections in order:


E.1 PURPOSE AND SCOPE

To determine how and why the buildings collapsed following the initial impacts of the aircraft.


E.2 METHODOLOGY AND ANALYSIS RESULTS

E.2.1 Overview and approach: Specifies 11 steps of the NIST approach, listed as A. through K.


E.2.2 Structural response:

Passive fire protection for structural components

Test of truss floor components and subsystem

Structural response of components and detailed subsystems to assumed damage and fire:

Material properties and failure criteria
Floor subsystem analysis
Core column and exterior column and panel analysis


Aircraft impact damage

Observations and timeline of structural events:

WTC 1 key observations
WTC 2 key observations

Structural response of major tower subsystems:

Core subsystem
Full floor subsystem
Exterior wall subsystem

Structural response of the WTC towers:

WTC 1 global analysis results
WTC 2 global analysis results
Structural response of the WTC towers to fire without impact damage

Probable collapse sequence:

Possible collapse hypothesis (May 2003)
Working collapse hypothesis (June 2004)
Leading collapse hypothesis (October 2004)
Probable collapse sequence (May 2005)


E.3 PROBABLE COLLAPSE SEQUENCE OF WTC 1 AND WTC 2


E.4 FACTORS THAT AFFECTED PERFORMANCE


E.5 FINDINGS


E.5.1 Passive fire protection

E.5.2 Fire resistance tests

E.5.3 Structural response of components:

Floor system
Exterior wall system

E.5.4 Fireproofing and partition damage due to aircraft impacts

E.5.5 Observations and timeline:

WTC 1
WTC 2

E.5.6 Structural response of major tower subsystems:

Isolated core subsystem

Full floor subsystem

Isolated exterior wall subsystem

E.5.7 Structural response to aircraft impact damage and fire

E.5.8 Probable collapse sequence:

Role of building core

Role of the building floors

Role of exterior frame-tube

Probable collapse sequence



Notice once again that the end result is the determination of a probable collapse sequence for each building. This is stated as the purpose and scope of the reports and is clearly the end result as seen in the outline and the summary chart shown above.






THE NIST DEPENDENCY ON THE VISUAL RECORD


In much more detail than the Executive Summary, NCSTAR 1-6 section 9.2 clearly describes how the NIST was reliant upon the visual record to establish a collapse hypothesis for each building:

my bold

NCSTAR 1-6

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 personnel and emergency communication records (NIST NCSTAR 1-8)

The NIST obtained input information through the visual record and through interviews.

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.


Observations were used to develop timelines just as this book uses features to develop mappings. The mappings developed allows the NIST obserations to be checked for accuracy.

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 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.



The NIST seems very aware their collapse hypothesis has to match key events.


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).


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 columns 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.



In May, 2003, according to this description two leading hypotheses were core-led or perimeter-led failure.

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 1 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.


They are claiming their structural sequence is consistent with key observables.





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 11, 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 aircraft impact analysis determined damage to the exterior and the interior of the building and included
the structural system, insulation, partition walls, and furnishings 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.




Figure 9�"1, a critical analysis inter-dependencies, is the same flowchart shown earlier in this section as fig 3.12

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 temperature-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.



my bold.

The relation between observation and measurement and the developement of a collapse hypothesis is clear. One must be put in a correct and detailed timeline, or set of mappings, in order to formulate an accurate hypothesis.







Continue to part 3.2: NIST WTC1 Misrepresentations



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