NIST NCSTAR 1-5E

Federal Building and Fire Safety Investigation of the

World Trade Center Disaster

Experiments and Modeling of

IViultiple Workstations Burning in a

Compartment

Anthony Hamins

Alexander Maranghides

Kevin B. McGrattan

Thomas J. Ohiemilier

Robert L. Anieitner

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

NIST NCSTAR1-5E

Federal Building and Fire Safety Investigation of the

World Trade Center Disaster

Experiments and Modeling of

Multiple Workstations Burning in a

Compartment

Anthony Hamins

Alexander Maranghides

Kevin B. McGrattan

Thomas J. Ohienniller

Robert L. Anieitner

Building and Fire Research Laboratory

National Institute of Standards and Technology

September 2005

U.S. Department of Commerce

Carlos M. Gutierrez, Secretary

Technology Administration Michelle O'Neill, Acting Under Secretary for Technology

National Institute of Standards and Technology

William Jeffrey, Director
Disclaimer No. 1

Certain commercial entities, equipment, products, or materials are identified in this document in order to describe a procedure or concept adequately or to trace the history of the procedures and practices used. Such identification is not intended to imply recommendation, endorsement, or implication that the entities, products, materials, or equipment are necessarily the best available for the purpose. Nor does such identification imply a finding of fault or negligence by the National Institute of Standards and Technology.

Disclaimer No. 2 ^ The policy of NIST is to use the International System of Units (metric units) in all publications. In this document,

however, units are presented in metric units or the inch-pound system, whichever is prevalent in the discipline.

Disclaimer No. 3

'

Pursuant to section 7 of the National Construction Safety Team Act, the NIST Director has determined that certain evidence received by NIST in the course of this Investigation is "voluntarily provided safety-related information" that is "not directly related to the building failure being investigated" and that "disclosure of that information would inhibit the voluntary provision of that type of information" (15 (JSC 7306c).

In addition, a substantial portion of the evidence collected by NIST in the course of the Investigation has been

provided to NIST under nondisclosure agreements.

Disclaimer No. 4 NIST takes no position as to whether the design or construction of a WTC building was compliant with any code

since, due to the destruction of the WTC buildings, NIST could not verify the actual (or as-built) construction, the properties and condition of the materials used, or changes to the original construction made over the life of the

buildings. In addition, NIST could not verify the interpretations of codes used by applicable authorities in determining

compliance when implementing building codes. Where an Investigation report states whether a system was

designed or installed as required by a code provision, NIST has documentary or anecdotal evidence indicating

whether the requirement was met, or NIST has independently conducted tests or analyses indicating whether the requirement was met.

Use in Legal Proceedings

No part of any report resulting from a NIST investigation into a structural failure or from an investigation under the National Construction Safety Team Act may be used in any suit or action for damages arising out of any matter mentioned in such report (15 USC 281a; as amended by P.L. 107-231).

National institute of Standards and Technology National Construction Safety Team Act Report 1-5E

Natl. Inst. Stand. Technol. Natl. Constr. Sfty. Tm. Act Rpt. 1-5E, 158 pages (September 2005)

CODEN: NSPUE2

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 2005

For sale by the Superintendent of Documents, U.S. Government Printing Office

Internet: bookstore.gpo.gov — Phone: (202) 512-1800 — Fax: (202) 512-2250

Mail: Stop SSOP, Washington, DC 20402-0001
Abstract

A series of large-scale experiments were conducted in the National Institute of Standards and Technology

(NIST) Large Fire Laboratory from November 4 to December 10, 2003, to assess the accuracy with which

the NIST Fire Dynamics Simulator (FDS) predicts the thermal environment in a burning compartment. In addition, the experiments established a data set to validate prediction of the heat release rate associated

with the burning of office furnishings similar in type to those found in the World Trade Center (WTC)

towers. The experiments were designed to recreate aspects of the WTC fires including issues associated

with limited ventilation, fire spread and growth on real furnishings, and the effects of debris and jet fuel on the heat release rate of fires burning under conditions believed to be similar to those occurring on September 11, 2001.

Within a steel-frame compartment (nominally 3 m by 7 m by 4 m high) lined with calcium silicate boards were placed three computer workstations (office modules or workstations) composed of tables, desks,

fabric-lined partitions, carpeting, a task chair, paper-filled filing cabinets and bookshelves, as well as a personal computer, keyboard, and monitor. The same configuration of furnishings was used in all experiments except one in which the workstation components were rearranged into component pieces to represent a disrupted non-standard configuration, which may have occurred as aircraft entered the WTC

structure. In some of the experiments, several liters ofjet fuel were distributed about the workstation

components or ceiling tiles within the compartment until a large percentage of the horizontal surfaces of

the furnishings were covered. A 2 MW hydrocarbon fire generated by nozzles spraying onto a 1 m by

2 m pan was used to ignite the compartment furnishings. The fuel was a commercial blend of heptane

isomers.

Six fire experiments were conducted and nearly 70 instruments were used to measure a number of

important variables including the heat release rate of the fire and the vertical profiles of gas phase

temperatures. A 2 MW hydrocarbon spray fire burning for 10 min (or 2 min ifjet fuel was present) was

used to ignite the furniture. A fire spread through the workstations and was allowed to bum until the compartment contents became a charred jumble and the heat release rate was small, which was typically one hour after fire initiation. The measurements led to findings regarding the thermal behavior of the

compartment and the importance of the jet fuel and the presence of dislodged ceiling tiles.

The FDS software was used to simulate the heat release rate and thermal environment in the compartment

prior to the experiments. FDS predicted the peak upper layer gas temperature to within approximately

1 0 percent of the measurements on average, which was good agreement considering model sensitivity to input parameters such as the heat release rate and experimental uncertainties associated with the

measurement. A comparison of the predictions with other measurements was also good. The results of

the comparison of the FDS calculations with measurements provided confidence in the application of

FDS to the analysis of the thermal environment in compartments with multiple office workstations

burning, which is a key part of the WTC Investigation.

Keywords: fire model, heat release rate, World Trade Center.

NISTNCSTAR 1-5E, WTC Investigation in
Abstract

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iv NISTNCSTAR 1-5E, WTC Investigation
Table of Contents

Abstract iii

List of Figures ix

List of Tables xiii

List of Acronyms and Abbreviations xv

Preface xvii Acknowledgments xxvii

Executive Summary xxix

Chapter 1 Introduction 1

1 . 1 Modeling as an Investigative Tool 1

1 .2 Overview of Experiments 2

1 .3 Overview of the Fire Dynamics Simulator 3

1 .4 Model Validation 4

1.5 Organization of This Report 5

1.6 References 6

Chapter 2 Experimental Configuration, Apparatus, and Procedures 7

2.1 Test Facility 7

2.2 Experimental Configuration 7

2.3 Test Variables and Test Matrix 10

2.4 Experimental Configuration 1

1

2.4.1 Ventilation 13

2.4.2 Description of the Multi-Workstation Configuration 14

2.5 Fuel Flow, Burner, and Delivery System 20

2.6 Instrumentation 21

2.6.1 Load Cells 22

2.6.2 Oxygen Consumption Calorimetry 22

2.6.3 Thermocouple Trees - Compartment Temperatures 23

2.6.4 Thermocouples - Desk Surface Temperatures 23

2.6.5 Video Recording Cameras 23

2.6.6 Gas Analyzers 23

NISTNCSTAR 1-5E, WTC Investigation v
Table of Contents

2.6.7 Data Acquisition 24

2.7 Test Procedure 24

2.8 References 26

Chapters . Fire Behavior and Flame Spread 27

3 . 1 General Observations 27

3.2 Workstation Ignition and Full Fire Involvement 30

3.3 Flame Spread Observations During Tests 1 , 2, and 4 33

3.3.1 Testl 33

3.3.2 Test 2 34

3.3.3 Test 4 34

3.3.4 Tests 3, 5, and 6 35

3.3.5 Burner Location Effect on Ignition and Flame Spread 35

3.4 Flame Spread Video and Thermometry Comparison 36

3.5 Key Events in the Evolution of the Compartment Fires 36

3.6 Summary 42

3.7 References 43

Chapter 4

Mass Loss 45

4. 1 Instrumentation and Procedure 45

4.2 Results and Discussion 46

4.2.1 Total Mass Loss 50

4.2.2 Mass Loss Rate 50

4.3 Summary . 52

Chapter 5 Heat Release Rate 53

5.1 Oxygen Consumption Calorimetry 53

5.1.1 Description of the Heat Release Rate Measurement Facihty 53

5. 1 .2 Calibration and Measurement Uncertainty 54

5.2 Results and Discussion 55

5.3 References 58

vi NISTNCSTAR 1-5E, WTC Investigation
Table of Contents

Chapter 6

Gas Temperature 61

6. 1 Instrumentation 61

6.2 Uncertainty 62

6.3 Discussion and Results 66

6.4 References 73

Chapter 7 Upper Layer Concentrations of CO2, CO, and O2 75

7.1 Gas Sampling Procedure 75

7.2 Instrumentation 75

7.3 Discussion 75

7.3.1 Test Results 76

7.4 Summary 77

Chapter 8 Simulation of Compartment Fire Phenomena 81

8.1 Introduction 81

8.2 NIST Fire Dynamics Simulator 81

8.3 Calibration and Validation Experiments 82

8.3.1 Description of the Workstation Components 83

8.3.2 Description of the Simulations 85

8.3.3 Discussion 93

8.4 Summary 97

8.5 References 97

Chapter 9

Summary of Results 99

Appendix A

Location of Compartment Contents and Instrumentation 101

Appendix B

Gas Analyzer Data 103

Appendix C

Gas Phase Temperatures 105

Appendix D

Comparison of Simulations and Measurements 113

NISTNCSTAR 1-5E, WTC Investigation \ ii
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viii NISTNCSTAR 1-5E, WTC Investigation
List of Figures

Figure P-1. The eight projects in the federal building and fire safety investigation of the WTC

disaster xix

Figure 2-1. The workstation layout on the 9 1 st floor of WTC 1 . Locations with hand-drawn lines represent location where aircraft impact may have disrupted the original layout 7

Figure 2-2. Plan view of the experimental configuration 12

Figure 2-3. West side of experimental enclosure. Five ventilation openings are present. The

knee wall was removed for access to iiistmmentation 13

Figure 2-4. Elevation view of experimental configuration 14

Figure 2-5. Views of interior of test compartment 18

Figure 2-6. Image highlighting the arrangement of ceiling tiles used in Tests 3 and 4. The spray

burner and compartment openings are visible towards the rear 18

Figure 2-7. The arrangement of "rubblized" Workstations 1 (left) and 2 (right) with ceiling tiles before Test 5. Part of the spray burner is visible on the left 19

Figure 2-8. The arrangement of Workstation 3 before Test 5 19

Figure 2-9. Spray burner and fire pan abutting Workstation 1 before Test 2 (location Bl) 21

Figure 3-1 . View of the fire compartment Vi min after the start of Test 5 28

Figure 3-2. View of the fire compartment several minutes after the start of Test 5 28

Figure 3-3. View of the fire compartment with test windows before the start of Test 6 29

Figure 3^. View of fire compartment 2 min after the start of Test 6 29

Figure 3-5. Determining ignition and full-involvement times from thermocouple data 32

Figure 3-6. Temperature measurements as a function of time for thermocouples on the desks of

Workstation 1 during Tests 1 and 2 32

Figure 3-7. View external to the compartment 30 s before ignition of the spray burner in Test 6 37

Figure 3-8. View external to the compartment 5 s before ignition of the spray burner in Test 6 38

Figure 3-9. View external to the compartment 5 s after ignition of the spray burner in Test 6 38

Figure 3-10. View external to the compartment 30 s after ignition of the spray burner in Test 6 39

Figure 3-11. View external to the compartment 4 min 30 s after ignition of the spray burner in Test 6 39

Figure 3-12. View external to the compartment 9 min 30 s after ignition of the spray burner in Test 6 40

Figure 3-13. View external to the compartment 1 7 min after ignition of the spray burner in Test 6 40

NISTNCSTAR1-5E, WTC Investigation ix
List of Figures

Figure 3-14. Photograph of the north face of WTC 1 recorded at 8:53:15 a.m. on September 1 1, 2001. Labels showing floor and column numbers were added to the original

photograph 41

Figure 3-15. Photograph of the south face of WTC 1 taken at approximately 9:41:30 a.m. on September 1 1

, 200 1 . Column and floor numbers were added to the original

photograph 42

Figure 4-1 . The mass of Workstations 1 , 2, and 3 as a function of time during Test 1 46

Figure 4-2. The mass of Workstations 1, 2, and 3 as a function of time during Test 2 47

Figure 4-3. The mass of Workstations 1, 2, and 3 as a function of time during Test 3 47

Figure 4-4. The mass of Workstations 1, 2, and 3 as a function of time during Test 4 48

Figure 4-5. The mass of Workstations 1, 2, and 3 as a function of time during Test 5. The power

to the load cells under Workstations 1 and 2 failed for 2.5 min at 1 80 s 48

Figure 4-6. The mass of Workstations 1, 2, and 3 as a function of time during Test 6 49

Figure 4-7. The sum of the mass of Workstations 1, 2, and 3 for Tests 1 through 6. The power to the load cells under Workstations 1 and 2 failed for 2 min at 180 s in Test 5 .49

Figure 4-8. The mass of Workstations 1,2, and 3 as a function of time during Test 1, showing

straight line fits to each of the three burning periods for Workstation 1. The data is taken from Fig. 4-1 5

1

Figure 5-1 . The heat release rate as a function of time for Tests 1 through 6 55

Figure 5-2. The load cell mass loss and the heat release rate ( ^ ) as a function of time during

Test 1. The rapid burning (Atrb) and slow burning (Atj) periods are indicated 56

Figure 6-1 . Comparison of bare bead and aspirated thermocouple measurements as a function of

time at a location 213 cm below the ceiling on Tree 4 during Test 1 63

Figure 6-2. Comparison of bare bead and aspirated thermocouple measurements as a function of

time at a location 305 cm below the ceiling on Tree 4 during Test 1 ; 64

Figure 6-3. Comparison of bare bead and aspirated thermocouple measurements as a function of

time at a location 213 cm below the ceiling on Tree 4 during Test 2 64

Figure 6-4. Comparison of bare bead and aspirated thermocouple measurements as a function of

time at a location 305 cm below the ceihng on Tree 4 during Test 1 65

Figure 6-5. The difference between the bare bead and the aspirated thermocouple measurements

as a function of the bare bead temperature for the data presented in Figs. 6-1 through

6-4 66

Figure 6-6. Bare bead thermocouple measurements as a function of time at four locations (below

the ceiling) on Tree 1 during Test 1 67

Figure 6-7. Bare bead thermocouple measurements as a function of time at four locations (below

the ceiling) on Tree 1 during Test 2 68

X NISTNCSTAR 1-5E, WTC investigation
List of Figures

Figure 6-8. Bare bead thennocouple measurements as a function of time at four locations (below

the ceiling) on Tree 1 during Test 3 68

Figure 6-9. Bare bead thennocouple measurements as a function of time at four locations (below

the ceiling) on Tree 1 during Test 4 69

Figure 6-10. Bare bead thermocouple measurements as a function of time at four locations (below

the ceiling) on Tree 1 during Test 5 69

Figure 6-11. Bare bead thermocouple measurements as a function of time at four locations (below

the ceiling) on Tree 1 during Test 6 70

Figure 6-12. Bare bead thermocouple measurements as a function of time at four locations (below

the ceiling) on Tree 4 during Test 1 70

Figure 6-13. Bare bead thermocouple measurements as a function of time at four locations (below

the ceiling) on Tree 4 during Test 1 71

Figure 6-14. Bare bead thermocouple measurements as a function of time at four locations (below

the ceiling) on Tree 4 during Test 3 71

Figure 6-15. Bare bead thermocouple measurements as a function of time at four locations (below

the ceiling) on Tree 4 during Test 4 72

Figure 6-16. Bare bead thermocouple measurements as a function of time at four locations (below

the ceiling) on Tree 4 during Test 5 72

Figure 6-17. Bare bead thermocouple measurements as a function of time at four locations (below

the ceiling) on Tree 4 during Test 6 73

Figure 7-1. The volume fractions of O2, CO, and CO2 on a dry basis as a function of time during

Test 1 78

Figure 7-2. The volume fractions of O2, CO, and CO2 on a dry basis as a function of time during

Test 2 78

Figure 7-3. The volume fractions of O2, CO, and CO2 on a dry basis as a function of time during

Test 3 79

Figure 7^. The volume fractions of O2, CO, and CO2 on a dry basis as a function of time during

Test 4 79

Figure 7-5. The volume fractions of O2, CO, and CO2 on a dry basis as a function of time during

Test 5 80

Figure 7-6. The volume fractions of O2, CO, and CO2 on a dry basis as a function of time during

Test 6 80

Figure 8-1. Comparison of the measured and simulated Q for a single workstation 85

Figure 8-2. Geometry of the simulations 86

Figure 8-3. Comparison of the measured Q with the simulated Q using two grids sizes for Test 1 87

Figure 8^. Comparison of the measured and simulated temperatures at 2.5 cm and 274 cm

below the ceiling on Tree 4 (rear of the compartment) during Test 1 87

NISTNCSTAR 1-5E, WTC Investigation xi
List of Figures

Figure 8-5. Comparison of the measured and simulated temperatures at 2.5 cm and 274 cm

below the ceiling on Tree 3 (middle of the compartment) during Test 1 88

Figure 8-6. Comparison of the measured and simulated temperatures at 2.5 cm and 274 cm

below the ceiling on Tree 2 (middle of the compartment) during Test 1 88

Figure 8-7. Comparison of the measured and simulated temperatures at 2.5 cm and 274 cm

below the ceiling on Tree 1 (front of the compartment) during Test 1 89

Figure 8-8. Comparison of the measured and simulated Q for Test 1 89

Figure 8-9. Comparison of the measured and simulated Q for Test 2 90

Figure 8-10. Comparison of the measured and simulated Q for Test 3 , 90

Figure 8-11. Comparison of the measured and simulated Q for Test 4 91

Figure 8-12. Comparison of the measured and simulated Q for Test 5 91

Figure 8-13. Comparison of the measured and simulated Q for Test 6 92

Figure 8-14. Comparison of the measured and simulated O2 and CO2 volume fractions on a dry

basis for Test 1 92

xii NISTNCSTAR 1-5E, WTC Investigation
List of Tables

Table P-1. Federal building and fire safety investigation of the WTC disaster xviii

Table P-2. Public meetings and briefings of the WTC Investigation xxi

Table 2-1 . Test matrix 10

Table 2-2. Mass of workstation contents 15

Table 2-3. Mass of workstation materials by type 15

Table 2^. Liquid fuel spray burner 20

Table 2-5 . Measurements 21

Table 2-6. Instrument characterization 22

Table 2-7. Duration of data acquisition files 24

Table 3-1 . Ignition and full involvement times 3

1

Table 3-2. Description of key events in evolution of the compartment fires 36

Table 4-1 . Load cell specifications 45

Table 4-2. Total mass loss for the three workstations from t = 0 s until 3,900 s 50

Table 4-3. The mass loss rate for each of the workstations during the rapid and slow burning

periods for Tests 1 through 6 52

Table 5-1 . Instruments used in the calorimeter system 54

Table 5-2. Total heat release 57

Table 6-1 . Component and expanded temperature measurement uncertainty 62

Table 8-1 . Heat release rate, Q , when one-half of the energy was released 95

Table 8-2. Time when one-half of the energy was released.^ 95

Table 8-3. Fire duration.' 95

NISTNCSTAR 1-5E, WTC investigation ^iii
List of Tables

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XIV NISTNCSTAR 1-5E, WTC Investigation
List of Acronyms and Abbreviations

Mul vlliyillo

ASTM ASTM International

BFRL Building and Fire Research Laboratory

BPS Building Performance Study

CFD computational fluid dynamics

DAQ data acquisition

DTAP dissemination and technical assistance program

FDS Fire Dynamics Simulator

FEMA Federal Emergency Management Agency

FMRC Factory Mutual Research Corporation

FSI Fire-Structure Interface

FVM Finite Volume Method

LES Large Eddy Simulation

NIST National Institute of Standards and Technology

PANYNJ Port Authority of New York and New Jersey

P.L. Public Law

R&D research and development

SEAoNY Structural Engineers Association of New York

SLE special limits oi error use United States Code

WTC World Trade Center

WTC 1 World Trade Center 1 (North Tower)

WTC 2 World Trade Center 2 (South Tower)

WTC 7 World Trade Center 7

Abbreviations

°C degrees Celsius

°F degrees Fahrenheit

micrometer

NIST NCSTAR 1-5E, WTC Investigation XV
List of Acronyms and Abbreviations

cm centimeter

g - gram

in. inch

ft feet GJ gigajoule

GW gigawatt

K Kelvin

Kg kilogram

kJ kilojoule

kW kilowatt

L liter m meter

square meter

m^ cubic meter

mA milliamperes

MJ megaJoule

mm millimeter

MW megawatt

min minute

s second

W watt

XVI NISTNCSTAR 1-5E, WTC Investigation
Preface

Genesis of This Investigation

Immediately following the terrorist attack on the World Trade Center (WTC) on September 1 1

, 2001 , the

Federal Emergency Management Agency (FEMA) and the American Society of Civil Engineers began

planning a building performance study of the disaster. The week of October 7, as soon as the rescue and

search efforts ceased, the Building Performance Study Team went to the site and began its assessment. This was to be a brief effort, as the study team consisted of experts who largely volunteered their time

away from their other professional commitments. The Building Performance Study Team issued its report in May 2002, fulfilling its goal "to detennine probable failure mechanisms and to identify areas of

future investigation that could lead to practical measures for improving the damage resistance of buildings

against such unforeseen events."

On August 21, 2002, with funding from the U.S. Congress through FEMA, the National Institute of

Standards and Technology (NIST) announced its building and fire safety investigation of the WTC

disaster. On October 1, 2002, the National Construction Safety Team Act (Public Law 107-231), was

signed into law. The NIST WTC Investigation was conducted under the authority of the National

Construction Safety Team Act.

The goals of the investigation of the WTC disaster were:

• To investigate the building construction, the materials used, and the technical conditions that contributed to the outcome of the WTC disaster.

• To serve as the basis for: - Improvements in the way buildings are designed, constructed, maintained, and used; - Improved tools and guidance for industry and safety officials; - Recommended revisions to current codes, standards, and practices; and - Improved public safety.

The specific objectives were:

1 . Determine why and how WTC 1 and WTC 2 collapsed following the initial impacts of the

aircraft and why and how WTC 7 collapsed;

2. Determine why the injuries and fatalities were so high or low depending on location,

including all technical aspects of fire protection, occupant behavior, evacuation, and

emergency response;

3. Determine what procedures and practices were used in the design, construction, operation,

and maintenance of WTC 1,2, and 7; and

4. Identify, as specifically as possible, areas in current building and fire codes, standards, and

practices that warrant revision.

NISTNCSTAR 1-5E, WTC Investigation X\ 11
Preface

NIST is a nonregulatory agency of the U.S. Department of Commerce's Technology Administration. The

purpose of NIST investigations is to improve the safety and structural integrity of buildings in the United

States, and the focus is on fact finding. NIST investigative teams are authorized to assess building

performance and emergency response and evacuation procedures in the wake of any building failure that has resulted in substantial loss of life or that posed significant potential of substantial loss of Hfe. NIST

does not have the statutoiy authority to make findings of fault nor negligence by individuals or

organizations. Further, no part of any report resulting from a NIST investigation into a building failure or from an investigation under the National Construction Safety Team Act may be used in any suit or action

for damages arising out of any matter mentioned in such report (15 USC 281a, as amended by Public

Law 107-231).

Organization of the Investigation

The National Construction Safety Team for this Investigation, appointed by the then NIST Director, Dr. Arden L. Bement, Jr., was led by Dr. S. Shyam Sunder. Dr. William L. Grosshandler served as Associate Lead Investigator, Mr. Stephen A. Cauffman sei-ved as Program Manager for Administration,

and Mr. Harold E. Nelson served on the team as a private sector expert. The Investigation included eight

interdependent projects whose leaders comprised the remainder of the team. A detailed description of

each of these eight projects is available at http://wtc.nist.gov. The purpose of each project is summarized

in Table P-1, and the key interdependencies among the projects are illustrated in Fig. P-1.

Table P-1. Federal building and fire safety investigation of the WTC disaster.

Technical Area and Project Leader Project Purpose

Analysis of Building and Fire Codes and

Practices; Project Leaders: Dr. H. S. Lew

and Mr. Richard W. Bukowski

Document and analyze the code provisions, procedures, and

practices used in the design, construction, operation, and

maintenance of the structural, passive fire protection, and

emergency access and evacuation systems of WTC 1, 2, and 7. Baseline Structural Perfonnance and

Aircraft Impact Damage Analysis; Project

Leader: Dr. Fahim H. Sadek

Analyze the baseline performance of WTC 1 and WTC 2 under

design, service, and abnormal loads, and aircraft impact damage on

the structural, fire protection, and egress systems.

Mechanical and Metallurgical Analysis of

Structural Steel; Project Leader: Dr. Frank

W. Gayle

Determine and analyze the mechanical and metallurgical properties

and quality of steel, weldments, and connections from steel recovered from WTC 1,2, and 7. Investigation of Active Fire Protection

Systems; Project Leader: Dr. David

D. Evans; Dr. William Grosshandler

Investigate the performance of the active fire protection systems in WTC 1 , 2, and 7 and their role in fire control, emergency response, and fate of occupants and responders.

Reconstruction of Thermal and Tenability

Enviromnent; Project Leader: Dr. Richard

G. Gann

Reconstruct the time-evolving temperature, thermal environment,

and smoke movement in WTC 1, 2, and 7 for use in evaluating the

structural performance of the buildings and behavior and fate of

occupants and responders.

Structural Fire Response and Collapse

Analysis; Project Leaders: Dr. John

L. Gross and Dr. Therese P. McAlhster

Analyze the response of the WTC towers to fires with and without

aircraft damage, the response of WTC 7 in fires, the performance

of composite steel-trussed floor systems, and determine the most

probable structural collapse sequence for WTC 1, 2, and 7. Occupant Behavior, Egress, and Emergency

Communications; Project Leader: Mr. Jason D. Averill

Analyze the behavior and fate of occupants and responders, both

those who survived and those who did not, and the perfonnance of

the evacuation system.

Emergency Response Technologies and

Guidelines; Project Leader: Mr. J. Randall

Lawson

Document the activities of the emergency responders from the time

of the terrorist attacks on WTC 1 and WTC 2 until the collapse of

WTC 7, including practices followed and technologies used.

xvni NISTNCSTAR 1-5E, WTC Investigation
Preface

NIST WTC Investigation Projects

rsjisr

Figure P-1. The eight projects in the federal building and fire safety

investigation of the WTC disaster.

National Construction Safety Team Advisory Committee

The NIST Director also established an advisory committee as mandated under the National Construction

Safety Team Act. The initial members of the committee were appointed following a public solicitation. These were:

• Paul Fitzgerald, Executive Vice President (retired) FM Global, National Construction Safety

Team Advisory Committee Chair

• John Barsom, President, Barsom Consulting, Ltd.

• John Bryan, Professor Emeritus, University of Maryland

• David Collins, President, The Preview Group, Inc.

• Glenn Corbett, Professor, John Jay College of Criminal Justice

• Philip DiNemio, President, Hughes Associates, Inc.

NISTNCSTAR 1-5E, WTC Investigation XIX
Preface

• Robert Hanson, Professor Emeritus, University of Michigan

• Charles Thornton, Co-Chairman and Managing Principal, The Thomton-Tomasetti Group,

Inc.

• Kathleen Tiemey, Director, Natural Hazards Research and Applications Information Center,

University of Colorado at Boulder

• Foraian Williams, Director, Center for Energy Research, University of California at San

Diego

This National Construction Safety Team Advisory Committee provided technical advice during the

Investigation and commentary on drafts of the Investigation reports prior to their public release. NIST

has benefited from the work of many people in the preparation of these reports, including the National

Construction Safety Team Advisory Cormnittee. The content of the reports and recommendations,

however, are solely the responsibility of NIST.

Public Outreach

During the course of this Investigation, NIST held public briefings and meetings (listed in Table P-2) to

solicit input from the public, present preliminary findings, and obtain comments on the direction and

progress of the Investigation from the public and the Advisory Committee.

NIST maintained a publicly accessible Web site during this Investigation at http://wtc.nist.gov. The site contained extensive information on the background and progress of the Investigation.

NIST's WTC Public-Private Response Plan

The collapse of the WTC buildings has led to broad reexamination of how tall buildings are designed,

constructed, maintained, and used, especially with regard to major events such as fires, natural disasters,

and terrorist attacks. Reflecting the enhanced interest in effecting necessary change, NIST, with support

from Congress and the Administration, has put in place a program, the goal of which is to develop and

implement the standards, technology, and practices needed for cost-effective improvements to the safety

and security of buildings and building occupants, including evacuation, emergency response procedures,

and threat mitigation.

The strategy to meet this goal is a three-part NIST-led public-private response program that includes:

• A federal building and fire safety investigation to study the most probable factors that contributed to post-aircraft impact collapse of the WTC towers and the 47-story WTC 7 building, and the associated evacuation and emergency response experience.

• A research and development (R&D) program to (a) facihtate the implementation of

recommendations resulting from the WTC Investigation, and (b) provide the technical basis

for cost-effective improvements to national building and fire codes, standards, and practices

that enhance the safety of buildings, their occupants, and emergency responders.

XX NISTNCSTAR 1-5E, WTC Investigation
Preface

Table P-2. Public meetings and briefings of the WTC Investigation.

Date Location Principal Agenda

June 24, 2002 New York City, NY Public meeting: Public comments on the Drajl Flan for the pending WTC Investigation.

August 21. 2002 Gaithersburg. MD Media briefing announcing the formal start of the Investigation.

December 9, 2002 Washington, DC Media briefing on release of the Public Update and NIST request

for photographs and videos. April 8, 2003 New York City, NY Joint public forum with Columbia University on first-person interviews. April 29-30. 2003 Gaithersburg, MD NCST Advisory Committee meeting on plan for and progress on WTC Investigation with a public comment session. May 7. 2003 New York City, NY Media briefing on release of May 2003 Progress Report.

August 26-27, 2003 Gaithersburg, MD NCST Advisory Committee meeting on status of the WTC

investigation with a public comment session. September 17,2003 New York City, NY Media and public briefing on initiation of first-person data

collection projects.

December 2-3, 2003 Gaithersburg, MD NCST Advisory Committee meeting on status and initial results and release of the Public Update with a public comment session. February 12, 2004 New York City, NY Public meeting on progress and preliminary findings with public

comments on issues to be considered in fonnulating final recommendations.

June 18.2004 New York City, NY Media/public briefing on release of June 2004 Progress Report. June 22-23, 2004 Gaithersburg, MD NCST Advisory Committee meeting on the status of and

preliminary findings from the WTC Investigation with a public

comment session. August 24. 2004 Northbrook, IL Public viewing of standard fire resistance test of WTC floor system at Underwriters Laboratories, Inc. October 19-20. 2004 Gaithersburg, MD NCST Advisory Committee meeting on status and near complete

set of preliminary findings with a public comment session. November '''' ''004 r^?iithpr<;hiircT K/IF) Nr^^T Arlvi<;nrv r^nmmitlpp Hi<irii^;';ir»n nn Hraft ;^nnn?il rpnnrt to Congress, a public comment session, and a closed session to discuss pre-draft recommendations for WTC Investigation.

April 5. 2005 New York City, NY Media and public briefing on release of the probable collapse

sequence for the WTC towers and draft reports for the projects on

codes and practices, evacuation, and emergency response. June 23. 2005 New York City, NY Media and public briefing on release of all draft reports for the WTC towers and draft recommendations for public comment.

September 12-13,

2005

Gaithersburg, MD NCST Advisory Committee meeting on disposition of public

comments and update to draft reports for the WTC towers. September 13-15,

2005

Gaithersburg, MD WTC Technical Conference for stakeholders and technical community for dissemination of findings and recommendations

and opportunity for public to make technical comments.

• A dissemination and technical assistance program (DTAP) to (a) engage leaders of the

construction and building community in ensuring timely adoption and widespread use of

proposed changes to practices, standards, and codes resulting from the WTC Investigation

and the R&D program, and (b) provide practical guidance and tools to better prepare facility owners, contractors, architects, engineers, emergency responders, and regulatory authorities

to respond to future disasters. The desired outcomes are to make buildings, occupants, and first responders safer in future disaster events.

NiSTNCSTAR 1-5E, WTC Investigation xxi
Preface

National Construction Safety Team Reports on the WTC Investigation

A final report on the collapse of the WTC towers is being issued as NIST NCSTAR 1. A companion

report on the collapse of WTC 7 is being issued as NIST NCSTAR lA. The present report is one of a set that provides more detailed documentation of the Investigation findings and the means by which these

technical results were achieved. As such, it is part of the archival record of this Investigation. The titles of the full set of Investigation publications are:

NIST (National Institute of Standards and Technology). 2005. Federal Building and Fire Safety

Investigation ofthe World Trade Center Disaster: Final Report on the Collapse ofthe World Trade

Center Towers. NIST NCSTAR 1. Gaithersburg. MD, September.

NIST (National Institute of Standards and Technology). 2006. Federal Building and Fire Safety

Investigation ofthe World Trade Center Disaster: Final Report on the Collapse of World Trade Center 7. NIST NCSTAR 1 A. Gaithersburg, MD.

Lew, H. S., R. W. Bukowski, and N. J. Carino. 2005. Federal Building and Fire Safety Investigation of

the World Trade Center Disaster: Design, Construction, and Maintenance ofStructural and Life Safety

Systems. NIST NCSTAR 1-1. National Institute of Standards and Technology. Gaithersburg, MD,

September.

Fanella, D. A., A. T. Derecho, and S. K. Ghosh. 2005. Federal Building and Fire Safety

Investigation of the World Trade Center Disaster: Design and Construction ofStructural Systems.

NIST NCSTAR 1-1 A. National Institute of Standards and Technology. Gaithersburg, MD,

September. .

Ghosh, S. K., and X. Liang. 2005. Federal Building and Fire Safety Investigation ofthe World

Trade Center Disaster: Comparison ofBuilding Code Structural Requirements. NIST

NCSTAR 1-1 B. National Institute of Standards and Technology. Gaithersburg, MD, September.

Fanella, D. A., A. T. Derecho, and S. K. Ghosh. 2005. Federal Building and Fire Safety

Investigation of the World Trade Center Disaster: Maintenance and Modifications to Structural

Systems. NIST NCSTAR 1-lC. National Institute of Standards and Technology. Gaithersburg,

MD, September.

Grill, R. A., and D. A. Johnson. 2005. Federal Building and Fire Safety Investigation ofthe World

Trade Center Disaster: Fire Protection and Life Safety Provisions Applied to the Design and

Construction of World Trade Center 1, 2, and 7 and Post-Construction Provisions Applied after

Occupancy. NIST NCSTAR 1-lD. National Institute of Standards and Technology. Gaithersburg,

MD, September.

Razza, J. C, and R. A. Grill. 2005. Federal Building and Fire Safety Investigation ofthe World

Trade Center Disaster: Comparison of Codes, Standards, and Practices in Use at the Time ofthe

Design and Construction of World Trade Center I, 2, and 7. NIST NCSTAR 1-lE. National

Institute of Standards and Technology. Gaithersburg, MD, September.

Grill, R. A., D. A. Johnson, and D. A. Fanella. 2005. Federal Building and Fire Safety

Investigation ofthe World Trade Center Disaster: Comparison ofthe 1968 and Current (2003) New

xxii NIST NCSTAR 1-5E, WTC Investigation
Preface

York City Building Code Provisions. NIST NCSTAR 1 - 1 F. National Institute of Standards and

Technology. Gaithersburg, MD, September.

Grill, R. A., and D. A. Johnson. 2005. Federal Building and Fire Safety Investigation ofthe World

Trade Center Disaster: Amendments to the Fire Protection and Life Safety Provisions ofthe New

York City Building Code by Local Laws Adopted While World Trade Center 1, 2, and 7 Were in Use. NIST NCSTAR 1-lG. National Institute of Standards and Technology. Gaithersburg, MD,

September.

Grill, R. A., and D. A. Johnson. 2005. Federal Building and Fire Safety Investigation ofthe World

Trade Center Disaster: Post-Construction Modifications to Fire Protection and Life Safety Systems

of World Trade Center 1 and 2. NIST NCSTAR 1-lH. National Institute of Standards and

Technology. Gaithersburg, MD, September.

Grill, R. A., D. A. Johnson, and D. A. Fanella. 2005. Federal Building and Fire Safety Investigation

ofthe World Trade Center Disaster: Post-Construction Modifications to Fire Protection, Life

Safety, and Structural Systems of World Trade Center 7. NIST NCSTAR 1-1 1. National Institute of

Standards and Technology. Gaithersburg, MD, September.

Grill, R. A., and D. A. Johnson. 2005. Federal Building and Fire Safety Investigation ofthe World

Trade Center Disaster: Design, Installation, and Operation ofFuel System for Emergency Power in World Trade Center 7. NIST NCSTAR 1-1 J. National Institute of Standards and Technology.

Gaithersburg, MD, September.

Sadek, F. 2005. Federal Building and Fire Safety Investigation ofthe World Trade Center Disaster:

Baseline Structural Performance and Aircraft Impact Damage Analysis ofthe World Trade Center

Towers. NIST NCSTAR 1-2. National Institute of Standards and Technology. Gaithersburg, MD,

September.

Faschan, W. J., and R. B. Garlock. 2005. Federal Building and Fire Safety Investigation ofthe

World Trade Center Disaster: Reference Structural Models and Baseline Performance Analysis of

the World Trade Center Towers. NIST NCSTAR 1-2A. National Institute of Standards and

Technology. Gaithersburg, MD, September.

Kirkpatrick, S. W., R. T. Bocchieri, F. Sadek, R. A. MacNeill, S. Holmes, B. D. Peterson, R. W. Cilke, C. Navarro. 2005. Federal Building and Fire Safet}' Investigation ofthe World Trade

Center Disaster: Analysis ofAircraft Impacts into the World Trade Center Towers, NIST

NCSTAR 1-2B. National Institute of Standards and Technology. Gaithersburg, MD, September.

Gayle, F. W., R. J. Fields, W. E. Luecke, S. W. Banovic, T. Foecke, C. N. McCowan, T. A. Siewert, and

J. D. McColskey. 2005. Federal Building and Fire Safety Investigation ofthe World Trade Center

Disaster: Mechanical and Metallurgical Analysis ofStructural Steel. NIST NCSTAR 1-3. National

Institute of Standards and Technology. Gaithersburg, MD, September.

Luecke, W. E., T. A. Siewert, and F. W. Gayle. 2005. Federal Building and Fire Safety

Investigation ofthe World Trade Center Disaster: Contemporaneous Stmctural Steel

Specifications. NIST Special Publication 1-3A. National Institute of Standards and Technology.

Gaithersburg, MD, September.

NIST NCSTAR 1-5E, WTC Investigation XXlll
Preface

Banovic, S. W. 2005. Federal Building and Fire Safety Investigation ofthe World Trade Center

Disaster: Steel Inventoiy and Identification. NIST NCSTAR 1 -3B. National Institute of Standards

and Technology. Gaithersburg, MD, September.

Banovic, S. W., and T. Foecke. 2005. Federal Building and Fire Safety- Investigation ofthe World

Trade Center Disaster: Damage and Failure Modes ofStructural Steel Components. NIST

NCSTAR 1-3C. National Institute of Standards and Technology. Gaithersburg, MD, September.

Luecke, W. E., J. D. McColskey, C. N. McCowan, S. W. Banovic, R. J. Fields, T. Foecke,

T. A. Siewert, and F. W. Gayle. 2005. Federal Building and Fire Safety^ Investigation ofthe World

Trade Center Disaster: Mechanical Properties ofStructural Steels. NIST NCSTAR 1-3D.

National Institute of Standards and Technology. Gaithersburg, MD, September.

Banovic, S. W., C. N. McCowan, and W. E. Luecke. 2005. Federal Building and Fire Safety

Investigation ofthe World Trade Center Disaster: Physical Properties ofStructural Steels. NIST

NCSTAR 1-3E. National Institute of Standards and Technology. Gaithersburg, MD, September.

Evans, D. D., R. D. Peacock, E. D. Kuligowski. W. S. Dols, and W. L. Grosshandler. 2005. Federal

Building and Fire Safety Investigation ofthe World Trade Center Disaster: Active Fire Protection

Systems. NIST NCSTAR 1-4. National Institute of Standards and Technology. Gaithersburg, MD,

September. , ...

Kuligowski, E. D., D. D. Evans, and R. D. Peacock. 2005. Federal Building and Fire Safety

Investigation of the World Trade Center Disaster: Post-Construction Fires Prior to September 11,

2001. NIST NCSTAR 1-4A. National Institute of Standards and Technology. Gaithersburg, MD,

September.

Hopkins, M., J. Schoenrock, and E. Budnick. 2005. Federal Building and Fire Safety Investigation

ofthe World Trade Center Disaster: Fire Suppression Systems. NIST NCSTAR 1-4B. National

Institute of Standards and Technology. Gaithersburg, MD, September.

Keough, R. J., and R. A. Grill. 2005. Federal Building and Fire Safety Investigation ofthe World

Trade Center Disaster: Fire Alarm Systems. NIST NCSTAR 1-4C. National Institute of Standards

and Technology. Gaithersburg, MD, September.

Ferreira, M. J., and S. M. Strege. 2005. Federal Building and Fire Safety Investigation ofthe

World Trade Center Disaster: Smoke Management Systems. NIST NCSTAR 1-4D. National

Institute of Standards and Technology. Gaithersburg, MD, September.

Gann, R. G., A. Hamins, K. B. McGrattan, G. W. Mulholland, H. E. Nelson, T. J. Ohlemiller,

W. M. Pitts, and K. R. Prasad. 2005. Federal Building and Fire Safety Investigation ofthe World Trade

Center Disaster: Reconstruction ofthe Fires in the World Trade Center Towers. NIST NCSTAR 1-5. National Institute of Standards and Technology. Gaithersburg, MD, September.

Pitts, W. M., K. M. Butler, and V. Junker. 2005. Federal Building and Fire Safety Investigation of

the World Trade Center Disaster: Visual Evidence, Damage Estimates, and Timeline Analysis.

NIST NCSTAR 1-5 A. National Institute of Standards and Technology. Gaithersburg, MD,

September.

XXIV NIST NCSTAR 1-5E, WTC Investigation
Preface

Hamins, A., A. Maranghides, K. B. McGrattan, E. Johnsson, T. J. Ohlemiller, M. Donnelly,

J. Yang, G. Mulholland, K. R. Prasad, S. Kukuck, R. Anleitner and T. McAllister. 2005. Federal

Building and Fire Safety Investigation ofthe World Trade Center Disaster: Experiments and

Modeling ofStructural Steel Elements Exposed to Fire. NIST NCSTAR 1-5B. National Institute of

Standards and Technology. Gaithersburg, MD, September.

Ohlemiller, T. J., G. W. Mulholland, A. Maranghides, J. J. Filliben, and R. G. Gann. 2005. Federal

Building and Fire Safety Investigation ofthe World Trade Center Disaster: Fire Tests ofSingle

Office Workstations. NIST NCSTAR 1-5C. National Institute of Standards and Technology.

Gaithersburg, MD, September.

Gann, R. G., M. A. Riley, J. M. Repp, A. S. Whittaker, A. M. Reinhom, and P. A. Hough. 2005.

Federal Building and Fire Safety Investigation ofthe World Trade Center Disaster: Reaction of

Ceiling Tile Systems to Shocks. NIST NCSTAR 1-5D. National Institute of Standards and

Technology. Gaithersburg, MD, September.

Hamins, A., A. Maranghides, K. B. McGrattan, T. J. Ohlemiller, and R. Anleitner. 2005. Federal

Building and Fire Safety Investigation ofthe World Trade Center Disaster: Experiments and

Modeling ofMultiple Workstations Burning in a Compartment. NIST NCSTAR I-5E. National

Institute of Standards and Technology. Gaithersburg, MD, September.

McGrattan, K. B., C. Bouldin, and G. Forney. 2005. Federal Building and Fire Safety

Investigation ofthe World Trade Center Disaster: Computer Simulation ofthe Fires in the World

Trade Center Towers. NIST NCSTAR 1-5F. National Institute of Standards and Technology.

Gaithersburg, MD, September.

Prasad, K. R., and H. R. Baum. 2005. Federal Building and Fire Safety Investigation ofthe World

Trade Center Disaster: Fire Structure Interface and Thermal Response ofthe World Trade Center

Towers. NIST NCSTAR 1-5G. National Institute of Standards and Technology. Gaithersburg,

MD, September.

Gross, J. L., and T. McAllister. 2005. Federal Building and Fire Safety Investigation ofthe World Trade

Center Disaster: Structural Fire Response and Probable Collapse Sequence ofthe World Trade Center

Towers. NIST NCSTAR 1-6. National Institute of Standards and Technology. Gaithersburg, MD,

September.

Carino, N. J., M. A. Stames, J. L. Gross, J. C. Yang, S. Kukuck, K. R. Prasad, and R. W. Bukowski.

2005. Federal Building and Fire Safety Investigation ofthe World Trade Center Disaster: Passive

Fire Protection. NIST NCSTAR 1-6A. National Institute of Standards and Technology.

Gaithersburg, MD, September.

Gross, J., F. Hervey, M. Izydorek, J. Mammoser, and J. Treadway. 2005. Federal Building and

Fire Safety Investigation ofthe World Trade Center Disaster: Fire Resistance Tests ofFloor Truss

Systems. NIST NCSTAR 1-6B. National Institute of Standards and Technology. Gaithersburg,

MD, September.

Zarghamee, M. S., S. Bolourchi, D. W. Eggers, O. O. Erbay, F. W. Kan, Y. Kitane, A. A. Liepins,

M. Mudlock, W. I. Naguib, R. P. Ojdrovic, A. T. Sarawit, P. R Barrett, J. L. Gross, and

NIST NCSTAR 1-5E. WTC Investigation XXV
Preface

T. P. McAllister. 2005. Federal Building and Fire Safety Investigation ofthe World Trade Center

Disaster: Component, Connection, and Subsystem Structural Analysis. NIST NCSTAR 1-6C.

National Institute of Standards and Technology. Gaithersburg, MD, September.

Zarghamee, M. S., Y. Kitane, O. O. Erbay, T. P. McAllister, and J. L. Gross. 2005. Federal

Building and Fire Safety Investigation ofthe World Trade Center Disaster: Global Structural

Analysis ofthe Response ofthe World Trade Center Towers to Impact Damage and Fire. NIST

NCSTAR 1-6D. National Institute of Standards and Technology. Gaithersburg, MD, September.

McAlhster, T., R. W. Bukowski, R. G. Gann, J. L. Gross, K. B. McGrattan, H. E. Nelson, L. Phan,

W. M. Pitts, K. R. Prasad, F. Sadek. 2006. Federal Building and Fire Safety Investigation ofthe World

Trade Center Disaster: Structural Fire Response and Probable Collapse Sequence of World Trade

Center 7. (Provisional). NIST NCSTAR 1-6E. National Institute of Standards and Technology.

Gaithersburg, MD.

Gilsanz, R., V. Arbitrio, C. Anders, D. Chlebus, K. Ezzeldin, W. Guo, P. Moloney, A. Montalva,

J. Oh, K. Rubenacker. 2006. Federal Building and Fire Safety Investigation ofthe World Trade

Center Disaster: Structural Analysis ofthe Response of World Trade Center 7 to Debris Damage

and Fire. (Provisional). NIST NCSTAR 1-6F. National Institute of Standards and Technology.

Gaithersburg, MD.

Kim, W. 2006. Federal Building and Fire Safety Investigation ofthe World Trade Center

Disaster: Analysis ofSeptember 11, 2001, Seismogram Data. (Provisional). NIST NCSTAR 1-6G.

National Institute of Standards and Technology. Gaithersburg, MD.

Nelson, K. 2006. Federal Building and Fire Safety Investigation ofthe World Trade Center

Disaster: The Con Ed Substation in World Trade Center 7. (Provisional). NIST NCSTAR 1-6H.

National Institute of Standards and Technology. Gaithersburg, MD.

Averill, J. D., D. S. Mileti, R. D. Peacock, E. D. Kuligowski, N. Groner, G. Proulx, P. A. Reneke, and

H. E. Nelson. 2005. Federal Building and Fire Safety Investigation of the World Trade Center Disaster:

Occupant Behavior, Egress, and Emergency Communication. NIST NCSTAR 1-7. National Institute of

Standards and Technology. Gaithersburg, MD, September.

Fahy, R., and G. Proulx. 2005. Federal Building and Fire Safety Investigation ofthe World Trade

Center Disaster: Analysis ofPublished Accounts of the World Trade Center Evacuation. NIST

NCSTAR 1-7A. National Institute of Standards and Technology. Gaithersburg, MD, September.

Zmud, J. 2005. Federal Building and Fire Safety Investigation ofthe World Trade Center

Disaster: Technical Documentationfor Sm'vey Administration. NIST NCSTAR 1-7B. National

Institute of Standards and Technology. Gaithersburg, MD, September.

Lawson, J. R., and R. L. Vettori. 2005. Federal Building and Fire Safety Investigation ofthe World

Trade Center Disaster: The Emergency Response Operations. NIST NCSTAR 1-8. National Institute of

Standards and Technology. Gaithersburg, MD, September.

XXVI NIST NCSTAR 1-5E, WTC Investigation
Acknowledgments

The authors are grateful to other staff of the National Institute of Standards and Technology: Lauren

DeLauter, Edward Hnetkovsky, Jack Lee, Roy McLane, Jay McElroy, and Mike Selepak for their assistance conducting the experiments, to Tyler Smith for assistance with data analysis, to Drs. Richard

Gann, George Mulholland, and Rodney Bryant for helpful discussions regarding experimental design and

data analysis, and to Dr. Bill Pitts for assistance with selecting World Trade Center photographs for inclusion in this report.

NISTNCSTAR 1-5E, WTC Investigation XXVll
Acknowledgments

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xxvni NISTNCSTAR 1-5E, WTC Investigation
Executive Summary

This report represents part of the reconstruction of the thermal and tenability environment of the World

Trade Center (WTC) Investigation, which was used to evaluate the structural performance of the

buildings. As part of this endeavor, the National Institute of Standards and Technology (NIST) Fire

Dynamic Simulator (FDS), a computational model of fire and its effects, was used to reconstruct the timeevolving temperature, thermal environment, and smoke movement in WTC 1, 2, and 7. The FDS is one

of the first large-domain computational fluid dynainics (CFD) fire codes that predicts and visualizes the spread, growth and suppression of a fire based on the underlying scientific principles governing fluid

motion. The code numerically solves the modeled conservation equations of mass, momentum and

energy that govern low-speed, thermally driven flows with an emphasis on smoke and heat transport from

fires. The companion software package, called Smokeview, graphically presents the results of the FDS

three-dimensional time-dependent simulation. Smokeview animates in three dimensions the smoke

particulates, heat fluxes, temperatures and fluid velocities within a building. The FDS/Smokeview

software package allows viewing of the simulated results from any angle and from inside or outside the

computational boundaries.

For application to the WTC Investigation, FDS needed experimental data to guide the adaptation and

development of the model for this specific purpose, and to ascertain the accuracy of the model

predictions. Ideally, the uncertainty in the agreement with experiments would be much smaller than the

effect of varying the unknowns in the actual conditions that occurred on September 11, 2001. As part of

the NIST-led Investigation, the results of the FDS calculations were used as input to the NIST Fire- Structure Interface (FSI) software, which was then used to calculate the heat transfer from the fire to steel members that were part of the WTC buildings. The FSI used the information generated by FDS on the

thermal environment and coupled that information to a transient, three-dimensional finite element model

with a different grid structure. The coupling was used to help determine the thermal response of simple

and complex steel members that were part of the WTC buildings.

To gain confidence in the accuracy of these models as applied to the WTC Investigation, two large-scale

test series were conducted to test the numerical computations, plus various small-scale experiments were

conducted to provide FDS with input data. The two large-scale test series involved fires in compartments

with approximately the same height as a floor in the WTC towers, and both series of experiments were

conducted in the NIST Large Fire Laboratory. The results of the first series of tests are described in detail

in NIST NCSTAR 1-5B' The objective of the first series of experiments was to assess the accuracy with

which the FDS predicted the thermal environment in a burning compartment and to establish a data set to

test the prediction of the temperature rise in the structural steel components. A liquid fuel spray burner was used to generate a fixed amount of energy in a compartment that was fitted with various targets and

obstructions, including steel structural members such as columns, trusses and rods. The heat release rate drives compartment fire effects, including radiative and convective heat transfer within the compartment

and the temperature rise of the gases and the contents of the compartment and its surfaces. The fire sizes tested were between 1.9 MW and 3.4 MW to assure that the stmctural components were immersed in

This reference is to one of the companion documents from this Investigation. A list of these documents appears in the Preface of this report.

NIST NCSTAR 1-5E, WTC Investigation XXIX
Executive Summary

flames and hot gases. The fuels used were well-characterized hydrocarbons or a mixture of

hydrocarbons, selected to cover the expected range of soot loading in fires generated by typical office

furnishings. The fire conditions were over-ventilated, that is the rate of burning was fuel limited and not

limited by the availability of oxygen.

The second series of large-scale experiments described in this report were quite different from the first series of experiments. These experiments were also conducted in the NIST Large Fire Laboratory. The

objective of the second study was to assess the accuracy with which the FDS predicted the thermal

environment in a burning compartment and to establish a data set to test predictions of the heat release

rate associated with the burning of office furnishings similar to those found in the WTC towers. The

experiments were designed to recreate aspects of the WTC fires, including issues associated with limited

ventilation, fire spread and growth on real furnishings, and the effects of debris and jet fuel on the heat

release rate of the fires. A large experimental compartment was fabricated in which office workstations

representative of typical office furnishings, not unlike those found in WTC 1, 2, and 7, were burned under

conditions believed to be similar to those occurring on September 11, 2001. This series of experiments

investigated the accuracy of the fire model for under-ventilated conditions in which burning was limited

by the availability of oxygen.

Three computer workstations (office modules or workstations) composed of desks, fabric-lined partitions,

carpeting, a task chair, paper-filled filing cabinets and bookshelves, as well as a personal computer,

keyboard, and monitor were placed within a steel-frame compartment (nominally 3 m by 7 m by 4 m

high) lined with calcium silicate boards. The same configuration of furnishings was used in all experiments except one, in which the workstation components were rearranged or broken into component

pieces to represent a disrupted, non-standard configuration, which may have occurred as aircraft entered

the WTC structures. In some of the experiments, several liters ofjet fuel were poured over the

workstation components, hi some experiments, ceiling tiles were distributed about the compartment,

simulating collapse of the drop ceiling. These tiles covered a large percentage of the horizontal surfaces

of the furnishings. A 2 MW hydrocarbon fire generated by four nozzles spraying onto a 1 m by 2 m pan was used to ignite the compartment furnishings. A commercial blend of heptane isomers was used as the fuel.

Six fire experiments were conducted, and nearly 70 instruments were used to measure a number of

important variables, including the heat release rate of the fire and the vertical profiles of gas phase • temperatures. The overall fire behavior was documented using video cameras, viewing the fire from

different orientations. Following the establishment of baseline signals from all the measurement devices,

the spray burner, and when present, the jet fuel were ignited. The spray fire continued to bum at a steady

rate for 10 min (or 2 min if jet fuel was present). The fire spread through the workstations and was

allowed to bum until the compartment contents had become an unrecognizable charred jumble and the

heat release rate was small, which was typically 1 hour after fire initiation. The measurements led to findings regarding the thermal behavior of the compartment and the importance of the jet fiiel and the

presence of the ceiling tiles.

• In the presence ofjet fuel, the fire spread rapidly, and the heat release rate reached a maximum within a few minutes.

• The intact ceiling tiles had little apparent impact on the rate of fire spread or the heat release

rate.

XXX NISTNCSTAR 1-5E, WTC Investigation
Executive Summary

• Upper layer temperatures obtained values as high as 800 "C to 1,200 "C after 20 min to 30 min of exposure, depending on the exact location within the compartment and the test conditions.

• The measured mass loss burning rate of workstations reconfigured into "rubble" to represent

what may have occurred when aircraft entered the WTC structure was about 40 percent

smaller than that of workstations burning in a standard configuration.

• The value of the measured heat release rate suggests that burning conditions during the second series of experiments were underventilated or oxygen limited.

An independent series of reduced-scale experiments was performed on the burning of a single office

workstation in the open, with and without the presence of ceiling tiles and jet fuel (refer to NIST

NCSTAR 1-5C), in an effort to calibrate the FDS calculation for the simulations described in this report.

The results of the reduced-scale experiments were used as input for FDS. which was then used to simulate

the fire spread and growth, heat release rate, and the thermal environment associated with the burning of

multiple workstations in a compartment. The numerical accuracy of the FDS predictions was determined

through quantitative comparison with the measurements. The comparison of predictions with

measurements showed that:

• FDS was able to accurately predict the general shape and magnitude of the time dependent

heat release rate.

• FDS predicted the time at which one-half of the energy was released to within approximately

3 min or 22 percent of the measurements.

• FDS predicted the value of the heat release rate when one-half of the energy was released to within 1 . 1 MW or 9 percent of the measurements on average, which was accurate considering

the measurement uncertainty.

• FDS predicted the duration of significant heating of the fires to within approximately 6 min

or 15 percent of the measurements on average, although the long tail in the heat release rate curve was under-predicted.

• FDS predicted the peak upper layer gas temperature to within approximately 10 percent of

the measurements on average, which was good agreement considering model sensitivity to input parameters such as the heat release rate and uncertainties associated with the

measurement.

This study provided a check on the accuracy of the FDS, and the results provided confidence in the

application of FDS to the analysis of the thermal environment in compartments with multiple office

workstations burning, which is a key part of the investigation of the WTC fires and collapse.

NIST NCSTAR 1-5E, WTC Investigation XXXI
Executive Summary

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xxxu NISTNCSTAR 1-5E, WTC Investigation
Chapter 1 Introduction

1 .1 MODELING AS AN INVESTIGATIVE TOOL

Reconstructing the fires and their impact on structural components in the World Trade Center (WTC)

buildings on September 11, 2001, requires extensive use of computational models. WTC buildings 1,2,

and 7 each survived for a different length of time from the start of the disaster. The duration of their survival resulted from complex interactions among the features of the building construction, the initial damage to the buildings, and the ensuing fires. Accurate computational modeling is critical to the

Investigation because of this complexity and because so little direct evidence is available. While the

Investigation Team has compiled and catalogued an extensive array of photographs and video records,

these are limited to perspectives external to the building. They cast little light on the physical damage and

the progress of the fires inside the immediate vicinity of the windows and other openings. Analysis of the recovered WTC steel provides limited indication of the maximum temperatures experienced. However,

this analysis cannot identify the times at which these peak temperatures were reached and the timevarying history of the thermal environment throughout the buildings. This type of information, combined

with characterization of the building damage from the incident aircraft (WTC 1 and WTC 2) or from

debris from the collapsing towers (WTC 7), is central to assessing the plausibility of various possible

explanations for the collapse of the three buildings. Computer simulation of the fires includes

consideration of such factors as:

• The initial building damage (exterior and interior);

• The nature, quantity, and distribution of the combustibles and debris; and

• Changes in the ventilation as the fires progressed.

Additional modeling of the transfer of heat from the fire to the structural components complements these

modeling considerations. Combined, the simulations then produce a complete thermal history, filling in the gaps left by the paucity of information. Simulations that capture the essence of the photographic

evidence and the occupant accounts are then presumed to be the most probable replications of the actual events.

For the use of such models to be a viable investigative tool, it is essential to know the accuracy with

which the models capture the physical phenomena of the fires and the concurrent heat transfer to the

building structure. Two large-scale test series were conducted to ascertain the accuracy of the numerical

model, and various reduced-scale experiments were conducted to provide the model with input data for

different materials. The two large-scale test series involved fires in compartments with approximately the same height as a floor in the WTC towers. Both series of experiments were conducted at the National

Institute of Standards and Technology (NIST) Large Fire Laboratory. The results of the first series of

NISTNCSTAR 1-5E, WTC Investigation 1
Chapter 1

tests are described in detail in NIST NCSTAR 1-5B.' The objective of that series of experiments was to assess the accuracy with which the NIST Fire Dynamics Simulator (FDS) predicted the thermal

environment in a burning compartment and to establish a data set to evaluate the prediction of the

temperature rise of structural steel components. In those experiments, a liquid fuel spray burner was used

to generate a fixed amount of energy in a compartment that was fitted with various targets and

obstructions, including steel objects such as columns, trusses, and rods. The fire sizes tested were between 1.9 MW and 3.4 MW to assure that the structural components were immersed in flames and hot

gases. The fuels used were well-characterized hydrocarbons or a mixture of hydrocarbons, selected to cover the expected range of soot loading in fires generated by typical office furnishings. The fire conditions were over-ventilated, that is, the rate of burning was fuel limited and not limited by the

availability of oxygen. . ^

The second series of experiments described in this report was quite different. The objective of the second

study was to assess the accuracy with which the NIST FDS predicted the thermal environment in a burning compartment and to establish a data set to evaluate prediction of the heat release rate associated

with the burning of typical office furnishings. The experiments were designed to recreate aspects of the

WTC fires including complex phenomena such as limited ventilation, fire spread and growth on real furnishings, and the effects of debris and jet fuel on the heat release rate of the fires. A large

compartment was used in which office workstations comparable to those in WTC 1, 2, and 7 were burned

under conditions believed to be similar to those that occurred on September 11, 2001. This report

documents this series of experiments and the comparison of the FDS simulation with the measurements in an effort to determine the accuracy of the model for use in the WTC Investigation.

An independent series of reduced-scale experiments was performed on the burning of a single office

workstation in the open, with and without the presence of ceiling tiles and jet fuel (NIST NCSTAR 1-5C)

in an effort to calibrate the FDS calculation for the simulations described in this report. The results of the

reduced-scale experiments were used as input for FDS, which was then used to simulate the fire spread

and growth, heat release rate, and the thermal environment associated with the burning of multiple

workstations in a compartment. The numerical accuracy of the FDS predictions was determined through

quantitative comparison with the measurements.

1.2 OVERVIEW OF EXPERIMENTS

A series of large-scale experiments was conducted in the NIST Large Fire Laboratory from November 4

to December 10, 2003. Workstations, arranged in a manner that is typical of a modem office were placed

within a steel-frame compartment (3 m by 7 m by 4 m) lined with calcium silicate board (referred to as Marinite). The workstations were composed of several components including desks, a chair, a computer,

etc. The fire was initiated by introducing liquid heptane through a four-nozzle spray bumer onto a I m by

2 m pan that was adjacent to one of the workstations and, when present, by igniting jet fuel that had been

distributed throughout each of the workstations. The experimental configuration and fire scenarios were

selected to examine the effects of (1) the presence of ceiling tiles, (2) the presence ofjet fuel, (3) the

effect of changing the fuel configuration by breaking the furniture into "piles of rubble."

This reference is to one of the companion documents from this Investigation. A list of these documents appears in the Preface of this report.

2 NIST NCSTAR 1-5E, WTC Investigation
Introduction

Six experiments were designed to assess the accuracy with which FDS predicts the fire spread, heat

release rate, and thermal environment in a compartment burning multiple workstations in a configuration

representative of that found in the WTC buildings. In each of these experiments, sets of three

workstations, identical to the generic workstations tested previously in the open (NIST NCSTAR 1-5C), were burned in a large compartment with a limited number of openings.

The experimental configuration included natural ventilation flowing through windows along one portion

of one wall of the compartment. Fresh air entered and combustion products were exhausted through the

openings. Such a configuration may have occurred in the WTC fires as ventilation was restricted to flow

in and out through open windows. The experimental configuration and fire scenarios were selected to examine the effects of underventilated burning of realistic furnishings, the presence ofjet fuel, and the presence of ceiling tiles. The under-ventilated fire scenario is a challenging test of the fire model. The

efficacy of the model has recently been investigated under over-ventilated burning conditions in an independent series of experiments (Hamins et al. 2004).

In summary, the objectives of this study were to assess the accuracy of the FDS predictions of the thermal

environment in a burning compartment that recreated salient aspects of the WTC tower fires through

comparison with a series of measurements.

1.3 OVERVIEW OF THE FIRE DYNAMICS SIMULATOR

The WTC fires were modeled using the NIST Fire Dynamics Simulator/Smokeview (McGrattan 2004)

software package. The FDS is one of the first large-domain computational fluid dynamics (CFD) fire models that predicts the spread, growth, and suppression of a fire based on the underlying scientific principles governing fluid motion. Smokeview is an OpenGL graphics based software for visualizing

information from large calculations. The purpose of computing is insight, not numbers. Focused on this objective, Smokeview facilitates an understanding of calculation output through visualization of the

results. Smokeview animates FDS results in a variety of user selected modes, including the trajectory of

smoke, and slices of the temperature and velocity fields within the three-dimensional domain.

The model numerically solves the conservation equations of mass, momentum, and energy that govern

low-speed, thermally driven flows with an emphasis on smoke and heat transport from fires. The

formulation of the equations and the numerical algorithm are contained in a companion document, called

Fire Dynamics Simulator (Version 4) - Technical Reference Guide (McGrattan 2004). The models and

their documentation are freely available to the public on the World Wide Web at the NIST site http://www.fire.nist.gov/fds. This site provides links to a setup program for PC installation. It also

contains documentation for Smokeview and FDS, sample calculations, software updates, and links for providing feedback about the software.

The core algorithm is an explicit predictor-corrector scheme, second order accurate in space and time.

Turbulence is treated by means of the Smagorinsky form of Large Eddy Simulation (LES). For most

applications, FDS uses a mixture fraction combustion model. Radiative heat transfer is included in the

model via the solution of the radiation transport equation for a non-scattering gray gas, and in some

limited cases using a wide-band model. The equation is solved using a technique similar to finite-volume

methods for convective transport and uses approximately 100 discrete angles. All solid surfaces are assigned thermal boundary conditions, and information about the burning behavior of the material. More

NIST NCSTAR 1-5E, WTC Investigation 3
Chapter 1

detailed infomiation regarding the FDS software is discussed in Chapter 10 of this report and in the

technical reference guide (McGrattan 2004) and references therein.

The companion software package, called Smokeview, graphically presents the results of the FDS threedimensional time-dependent simulation. Smokeview is a software tool designed to visualize numerical

predictions generated by FDS. FDS and Smokeview are used in concert to respectively model and

visualize fire phenomena. Smokeview visualizes both dynamic and static data. Dynamic data is visualized by animating particle flow (showing location and "values" of tracer particles), 2D contour

slices (both within the domain and on solid surfaces), and 3D level surfaces. 2D contour slices can also

be drawn with colored vectors that use velocity data to show flow direction, speed, and value. Static data

are visualized similarly by drawing 2D contours, vector plots, and 3D level surfaces anywhere within a simulation scene at a fixed time.

Normally Smokeview is used in a post-processing step to visualize FDS data after a calculation has been

completed. Smokeview may also be used during a calculation to monitor a simulation's progress and

before a calculation to visualize the calculation set-up, which may include walls, vents, sprinklers, smoke

detectors, etc. In order to set up correct FDS input files more quickly, Smokeview is typically used to

edit or create blockages by specifying the size, location, and/or material properties. More detailed

information regarding Smokeview can be found in the Smokeview User's Guide (Forney and

McGrattan 2004).

The accuracy of the FDS calculation was tested, and the FDS computation was calibrated and modified

based on a series of fire experiments reported in NIST NCSTAR 1-5C. Their report describes a series of

fire experiments burning individual workstations or workstations. The report also discusses measurement

uncertainty, properties of the fires that are most important to the calculation, the initial FDS combustion

module, and how the input data for the combustion simulation were obtained. The report concludes with

a description of the changes made to the FDS combustion module to better describe the workstation fires. The FDS code, as modified to describe the fuels in the workstation in a simplified manner, was able to provide good estimates of the peak heat release rate (NIST NCSTAR 1-5C). It was not, however, able to accurately predict the time of the peak heat release rate. The present study is different in configuration,

objectives, and complexity from the single workstation experiments reported in NIST NCSTAR 1-5C. In

this study, the workstations were burned in a compartment rather than in the open (under a ceiling),

leading to underventilated or oxygen-limited burning, which undoubtedly affected the rates of fire heat

release and flame spread. In this study, the use of multiple workstations arranged about a carpeted

passageway led to a variety of possible flame spread time-lines.

1.4 MODEL VALIDATION

Sufficient evaluation of fire models is necessary to establish acceptable uses and limitations of models, to ensure the adequacy of their technical basis, appropriateness of their desired use, and confidence level of

their predictions. ASTM E 1355 (ASTM International 1997) defines model evaluation as "the process of

quantifying the accuracy of chosen results from a model when applied for a specific use." The model

evaluation process consists of two main components: verification and validation. Verification is "the

process of determining the correctness of the solution of a system of governing equations in a model."

Verification does not imply that the governing equations are appropriate; only that the equations are being

solved correctly. Validation is defined as "the process of determining the correctness of the assumptions

4 NIST NCSTAR 1-5E, WTC Investigation
Introduction

and governing equations implemented in a model when applied to the entire class of problems addressed

by the model." Typically, validation involves comparing model results with experimental measurement.

Differences that cannot be explained in terms of numerical error are attributed to the assumptions and

simphfications of the physical model.

This part of the NIST Investigation was motivated by the need to validate the models for the WTC

application. Throughout its development, FDS has undergone various forms of evaluation, at NIST and

elsewhere. McGrattan (2004) provides a sunmiary of the work performed to date to verify and validate

FDS for many applications. This document provides the basis for validation of the FDS as applied to investigation of the WTC fires. The intent of these experiments was to test rather than to calibrate the

FDS calculations. An FDS simulation of each experiment was carried out before the tests were conducted

and model predictions were then compared to the measurements.

The experiments described in this report were designed to provide the data necessary for \'alidation of the

fire simulation as applied to modeling the WTC disaster. The primary goal of the experiments was to compare the observed fire progression and heat release rate behavior with that predicted by the FDS

software in an effort to obtain an understanding of how the fire might develop and spread from one

workstation to another. The experiments allowed comparison of measurements and predictions of a number of critical parameters, which facilitated a check of the accuracy of the predictions and assured

that the model was capturing properly the physics of fire dynamics and thermal-structural behavior.

The compartment was instrumented to measure transient fire dynamic effects such as the heat release rate and temperature profiles within the compartment. The instrumentation for the tests measured nearly

150 channels of data, including over 50 thermocouples. Approximately 40 measurements were made to determine the global heat release rate ( ^ ) of the fire using oxygen consumption calorimetry. The

measurements reported here tested the accuracy of the heat release rate and the thermal behavior of the

compartment as predicted by FDS. Other important measurements were also made during the

experiments. The concentrations of oxygen, carbon monoxide, and carbon dioxide were measured in the

upper layer of the compartment. These measurements are an important consistency check on the efficacy

of the fire model. Since the calculation of heat release rate is dependent on the burning of oxygen,

measurement of the oxygen concentration in the upper layer is an important check on model consistency.

The measurement of temperature on the surface of the workstation components allowed tracking of fire spread from one workstation to the next. A video record of the experiments was completed to qualitatively compare model simulations with fire behavior. Tracking the heat release rate of the fire is one of the most important operations of the fire model because it impacts the overall thermal environment

of the compartment. It is particularly important in the case of the NIST-led WTC Investigation because

the fire model is being applied to simulate the time varying boundary conditions on the structural components in the building. Inaccuracy in the fire model calculations will propagate as inaccuracies in the calculation of thermal load on the building structure. The measurements reported here facilitate a check of the accuracy of the heat release rates predicted by FDS.

1 .5 ORGANIZATION OF THIS REPORT

A detailed description of the instrumentation, procedure, uncertainties, and results are given in detail below for each of the measurements. Chapter 2 of this report describes the experimental configuration,

including the experimental conditions, the fuel flow and details of the test compartment. Chapters 3

NIST NCSTAR 1-5E, WTC Investigation 5
Chapter 1

through 7 describe observation and measurements of the fire and flame spread behavior (Chapter 3), the mass loss (Chapter 4), the heat release rate (Chapter 5), the volume fraction of gas species in the upper

layer (Chapter 6), and the gas-phase temperature (Chapter 7). Chapter 8 provides a description of the

model simulations and a comparison of the measured and simulated thennal environment in the

compartment. Chapter 9 summarizes the results of this study. Appendix A contains a detailed list of the

location of key compartment contents and instrumentation. Appendix B lists the specific analyzers

employed for calorimetry and for measurement of the concentrations of the upper layer gases. Appendix C presents results not shown in Chapter 6 on the measured gas phase temperatures recorded by

the bare bead thermocouples. Appendix D presents results not shown in Chapter 8 that compare the

measurements to the simulations.

1.6 REFERENCES

ASTM International. 1997. Standard Guide for Evaluating the Predictive Capabilities of Deterministic

Fire Models, ASTM E 1355-97, West Conshohocken, PA.

Forney, G. P., and McGrattan, K. 2004 User 's Guidefor Smokeview Version 4 - A Tool for Visualizing

Fire Dynamics Simulation Data. NIST Special Publication 1017, National Institute of Standards and

Technology, Gaithersburg, MD, August."

Hamins, A., Maranghides, A., and Mulholland, G. 2003. The Global Combustion Behavior of I MWto 3

MH^ Hydrocarbon Spray Fires Burning in an Open Environment, NISTIR 7013, National Institute of

Standards and Technology, Gaithersburg, MD, June.

McGrattan. K. B., ed. 2004. Fire Dynamics Simulator (Version 4), Technical Reference Guide. NIST

Special Pubhcation 1018. National Institute of Standards and Technology, Gaithersburg, MD, July.

' All NIST publications cited here and elsewhere in this report are available for download from the World Wide Web:

http://t'ire. nist.gov/bfrlpubs/

6 NISTNCSTAR 1-5E, WTC Investigation
Chapter 2

Experimental Configuration, Apparatus, and Procedures

2.1 TEST FACILITY

Experiments were conducted in the National Institute of Standards and Technology (NIST) Large Fire

Laboratory, which is 27 m (90 ft) by 37 m (120 ft) in size. Fire products flowed into the large exhaust

hood for measurement of the heat release rate and were then exhausted from the building. The exhaust

hood used in these experiments was approximately 9 m (30 ft) by 12 m (40 ft) in size and had a maximum

capacity of approximately 15 MW. A computerized data acquisition system was used to record the instrumentation signals.

A common furnishing arrangement on several floors of World Trade Center (WTC) 1 was a continuous

open space occupied by a large array of workstations (NIST NCSTAR 1-5C). Each office workstation (or

workstation or module) was defined by privacy panels on four sides with an opening on one side only.

Such an arrangement was used, for example, by the firm that occupied floors 93 to 100 of WTC 1 (NIST

NCSTAR 1-5C). More than 950 workstations were used by this finn alone. Figure 2-1 below, which is a reproduction of Fig. 2-17 from the World Trade Center Building Performance Study (McAllister 2002),

shows an example of the workstation layout on the 91st floor of WTC 1. The workstations were clustered

in sets of six or eight stations sharing at least one common privacy panel with the clusters separated by an

aisle. A fire propagating among the stations would have to either pass over (or through) a privacy panel

or pass across an aisle. Each station contained an estimated 400 kg of combustible materials of various

types, assuming that all of the file cabinets were full of papers. The hand-drawn lines in Fig. 2-1

represent locations where the aircraft impact may have disrupted the original workstation layout

Figure 2-1. The workstation layout on the 91st floor of WTC 1. Locations with

hand-drawn lines represent location where aircraft impact may have disrupted the

original layout.

2.2 EXPERIMENTAL CONFIGURATION

(McAllister 2002).

-NNIST NCSTAR 1-5E, WTC Investigation 7
Chapter 2

Each experiment described in this report investigated the character of a set of three workstations burning

in a compartment. The individual workstations were identical to the generic workstations burned in the

open (NIST NCSTAR 1-5C). The workstations were an analog (not a replica) of one type of station used on floors 93 to 100 of WTC 1. There were at least four different workstation layouts used by this firm.

From the limited infomiation available, it appears that the fire behavior of the workstation most

extensively characterized here would not differ greatly from that of the other layouts. Once validated, it

is possible that the fire model could be used to test this assumption. The experiments conducted by NIST

(NIST NCSTAR 1-5C) emphasized the global burning behavior of the workstation, as well as details of

the fire spread through its various components. The scenario that was emulated in the experiments was

fire propagation over and through a privacy panel. The use of a large ignition source was employed to simulate the burning of an adjacent station. There are several relevant categories of parameters pertinent

to the unique circumstances of the WTC fires:

• Presence and Quantity of Jet Fuel (Jet A). The impact of the airplanes likely left varying

amounts of Jet A distributed throughout the building, including on various surfaces in the

workstations. Some areas were probably heavily engulfed with fuel, and others, farther from

the impact zone, were probably dry. Those workstations closest to the impact would be

expected to be destroyed into rubble, as well as soaked in Jet A. The focus of the NIST

experiments was on the propagation and duration of the fire that threatened the steel structure. Because of its high volatility and heat of combustion, even a limited amount of Jet A has the

potential to alter the course of a fire. The presence or absence of Jet A was, thus, an

important parameter to be examined because propagation away from the impact area could

involve both conditions. NIST chose 4 L ofjet fuel per workstation. In these experiments,

the same quantities were used for each workstation. In addition, 1 L was used for the

carpeted hall. This was sufficient to lead to simultaneous ignition throughout combustible

surfaces in the compartment.' The quantity chosen, 4 L per workstation, was sufficient to form a continuous layer on the major horizontal surfaces and thus was sufficient to alter the

fire growth process substantially. The jet fiiel was expected to cause rapid ignition of the

combustibles throughout the compartment, but it was not expected to contribute significantly

to the measured heat release rate on its own.

• Presence and Quantity of Inert Rubble on Surfaces. The airplanes caused localized

destruction of materials in and around the impact zones, and they also caused collapse of the

false ceilings on non-impact floors. The ceiling tiles are a nearly inert material (as

characterized by measurements in a micro-calorimeter) with a thermal insulating character

and are capable of retarding or even preventing the fire involvement of surfaces onto which

they fall. This is a difficult effect to predict since it can interact with fire spread on

workstation surfaces. The fractional ceiling tile area coverage was 70 percent for the tests that included ceilmg tiles. This value was more than twice the value used by NIST (NIST

NSTAR 1-5C), which conducted two single workstation bum experiments with the tiles covering about 30 percent of the exposed area. For the single workstations, this resulted in

In a designed experiment in which one is examining the potential roles of more than one factor, one attempts to vary each

factor by an amount sufficient to be clearly measured but not so much as to cause the factors to interact strongly. A pool of

Jet A with an area equal to that of the workstation foot print is itself capable of yielding a fire of greater than 1 3 MW heat

release rate (NIST NCSTAR 1-5C). The combination ofjet fuel and other materials would then exceed the capacity of the NIST calorimeter facility. This was an incentive to limit the quantity of jet fuel.

8 NIST NCSTAR 1-5E, WTC Investigation
Experimental Configuration, Apparatus, and Procedures

about a 20 percent reduction in the peak heat release rate. Increasing the coverage to 70 percent is expected to further reduce the peak heat release rate since less of the surface

area would be accessible for burning.

• Extent of Rubble. The airplane impact zone could be expected to involve extensively

fragmented workstation components along with fragmented materials from the airplane itself This type of fragmentation would likely result in what amounts to "packing" of the various

fuel surfaces, leading to decreased ease of air access to those surfaces and, thus, a reduced

burning rate. This type of burning is difficult to model, given only the fuel characterizations

based on Cone Calorimeter measurements reported in NIST NCSTAR 1-5C. This issue was

addressed, in a partial manner, in one of the experiments (Test 5) in which the workstations were configured into a "pile of rubble," or disassembled into a non-standard configuration.

In the other five experiments, the workstations were tested in an undisturbed configuration.

Results from these five experiments only apply to the extensive areas beyond the immediate

impact zones and to WTC 7.

• Fuel Content of File Cabinets. The three file cabinets in the generic workstation were

capable of holding a combined load of up to about 150 kg of paper. Such paper content is normally included in the fire load estimates of office occupancy. When enclosed in steel, however, this effect is partially discounted (NIST NCSTAR 1-5C). It was anticipated that such paper would not contribute appreciably to the fire intensity until the peak heat release

rate was past. Small amounts of paper (two reams) were placed in file cabinets on opposite

sides of the workstation. The fraction burned was measured in two of the single workstation

experiments conducted by NIST (NIST NCSTAR 1-5C). The paper loading elsewhere in the

workstations was guided by information about the typical layout of workstations on floors 93

to 100 of WTC 1, following NIST NCSTAR 1-5C.

• Ignition Scenario. The heat release rate history for a workstation subsequent to ignition, like any complex assemblage of fuels, can be expected to depend significantly on the mode and

location of the ignition event. This is particularly true for small, localized ignition versus a large area ignition. In the WTC towers, the airplane impact provided an overwhelming

ignition event in the area of impact. This intense fire then spread to other, less disrupted

areas, providing a continuing series of large ignition sources. This suggested the scenario

used here, which emulates the involvement of multiple workstations in a large-scale spread

process. The ignition source chosen was thus quite large, simulating the burning of a workstation adjacent to the workstations of interest.

• Relative location of the fuel and source of ventilation. Fires in the towers were observed to move about the different sides of the building (NIST NCSTAR 1-6A). As the fires spread

through the building, it is likely that various ventilation conditions occurred including both

underventilated and overventilated burning conditions. Fire Dynamics Simulator (FDS)

validation under overventilated conditions was tested previously (Hamins et al. 2003).

Among the possible underventilated configurations that occurred, it is likely that fire either

propagated through workstations toward open windows or fire propagated through

workstations away from open windows.

NIST NCSTAR 1-5E, WTC Investigation 9
Chapter 2

From the list of relevant parameters given above, the first three and the last one (the presence of Jet A

fuel, ceiling tiles and "rubblized" fuel load, relative location of the fuel and the ventilation source) were

selected as being most pertinent for testing the FDS calculation results and were considered part of the

experimental design.

There has been some amount of previous work examining the burning behavior of office furnishings.

Madrzykowski (1996, 1998) measured the heat release rate behavior of workstations representative of

those found in some U. S. Government office buildings. The workstations considered here were somewhat larger than those studied by Madrzykowki and they had a smaller paper load (50 kg rather than

98 kg). Peak heat release rates (Q) as high as 7 MW were measured as the number of workstation

privacy panels varied from two to four. One of the experiments suggested a potential sensitivity to small

details of the events - a thermoplastic shelf support failed early leading to a shelf collapse that spewed

loose papers onto the fire, which immediately enhanced it substantially. For the generic workstations

tested here, NIST NCSTAR 1-5C reported that shelf collapses did not have a noticeable effect, perhaps

because the papers on them were relatively contained. , .

2.3 TEST VARIABLES AND TEST MATRIX

Six experiments were conducted. The test matrix was designed based on appreciation of the plausible

WTC fire scenarios and the results of the single workstation experiments (NIST NCSTAR 1-5C). The

experiments reported here investigated the impact of several parameters on fire behavior, including the

location of the burner, the apphcation ofjet fuel distributed about each workstation, and the presence of

fallen ceiling tiles with the tile distribution determined from an independent series of reduced scale

experiments. A statistical analysis was conducted to determine which combination of experimental

parameters should be selected for testing. The resulting test matrix is listed in Table 2-1. The Table hsts the test number, the presence or absence of ceiling tiles and jet fuel, the burner location, the state of the

fuel load, and the presence or absence of glass windows".

Table 2-1. Test matrix.

Test Ceiling Tiles Jet Fuel Burner Location Workstations Windows Notes

1 None None Bl (Front) Intact No

2 None None Bl (Front) Intact No Test 1 repeat

3 Present Present Bl (Front) Intact No

4 Present None B2 (Rear) Intact No

5 Present Present B2 (Rear) Organized rubble No

6 None Present B2 (Rear) Intact Yes

The burner location was selected to challenge the FDS calculation results with different gas flow patterns

and to cover the extremes of possible ventilation configurations. The burner was placed either abutting

the west end of workstation 1 near the enclosure openings (Bl location) or abutting the east end of

workstation 2 close to the east wall of the enclosure (B2 location). Figure 2-1 shows the two locations of

the burner relative to the room contents. These two sites resulted in significantly different access to the

In Test 6, an assembly containing four glass windows was positioned on the west wall of the enclosure, which was the same

wall in which ventilation openings were present.

10 NIST NCSTAR 1-5E, WTC Investigation
Experimental Configuration, Apparatus, and Procedures

air needed for combustion. In the Bl ("front") location, much of the oxygen in the air initially entering

the enclosure was consumed by the burner and the burning workstation 1 , with the result that limited

oxygen was available for combustion in the middle and rear of the compartment. With the burner in the B2 ("rear") location, the fresh air passed directly to the rear of the compartment.

Four liters ofjet fuel were evenly distributed about each workstation, using the same procedure that was

used in the single workstation bum experiments (NIST NCSTAR 1-5C). One liter was distributed in the

corridor. In the experiments with Jet A present, ignition of the three workstations was designed to occur

simultaneously, therefore, challenging the ability of the model to handle rapid flame spread.

The effect of the presence of fallen ceiling tiles was investigated to determine their effect on flame spread

and maximum heat release rate. In tliree tests, nearly 70 percent of the top surfaces were covered. The

effect of lesser coverage (30 percent) was investigated in the previous test series for single workstations

(NIST NCSTAR 1-5C). Measurement of the heat release rate of the ceiling tiles showed that they were

quasi-inert (Lyon 2004).

One experiment (Test 5) investigated the effect of disassembling each of the workstations into pieces in what is referred to in Table 2-1 as "organized rubble." The pieces were placed on top of each other,

simulating the effect of an aircraft causing furniture to break-up and collapse into piles. During Test 5, the workstation pieces in workstation 1 were placed unassembled on top of each other, occupying the same footprint as the assembled workstation. The same mass of combustibles was present as in the fully

assembled workstation tests. No steel filling cabinets were used in these tests. Ceiling tiles and broken

up drywall were intermixed with the rubble in Workstation 1 . Workstation 2 was identical to Workstation 1, except without the drywall. Workstation 3 was partially assembled as seen in Fig. 2-8.

The same mass of ceiling tile and drywall as in workstation 1 were intermixed with the components in Workstation 3.

Test 6 had a 2.4 m by 2.4 m assembly containing four commercially available glass windows mounted on

the north end of the west wall. The assembly was fitted with two different types of glass. Two sections

of double-pane tempered glass (with a gel insulation sandwiched between the glass panes) were installed

high and low on the right (as seen from outside the compartment) and two sections of single-pane

tempered glass were installed high and low on the left. Manzello et al. (2005) describe the glass assembly

in detail. No openings or columns were present adjacent to the glass assembly. During the course of the

fires, heat broke some of the windows. This resulted in a change in both the degree and pattern of

ventilation. The FDS fire model does not predict window breakage. Therefore, for this test, FDS was re- run after experimental determination of the time at which window breakage occurred.

2.4 EXPERIMENTAL CONFIGURATION

A schematic of the test configuration is shown in Figs. 2-2 and 2-4. The test series involved fires in compartments with the same height as that in WTC 1 or WTC 2. The steel-frame experimental enclosure was 10.81 m long by 7.02 m wide by 3.36 m tall (35.5 ft by 23 ft by 11 ft). The walls and ceiling were

covered with two or three layers of Marinite I (calcium silicate board) each with a thickness of 1.25 cm. A sub-floor 0.48 m in height was used to house instrumentation and was not included in the above

dimensions. The floor was also covered with Marinite I.

NIST NCSTAR 1-5E, WTC Investigation 11
Chapter 2

7.0 m

10.7 m

Figure 2-2. Plan view of the experimental configuration.

The compartment dimensions were selected to represent a section of the WTC North tower. The

compartment was configured to resemble a section of the Marsh & McLennan office on the 96th floor

where the first plane struck on September 1 1 . The total compartment volume was 255 m\ The

compartment was built inside an exoskeleton of steel tubing and beams. The enclosure was equipped

with an eave extending down 0.92 m from the ceiling. The compartment was located so that the exhaust

vents were under the NIST 9 m by 12 m exhaust hood. Figure 2-3 shows the west side of the

experimental enclosure with the knee wall removed for access. The liquid fuel spray burner is evident

just behind the ventilation openings between the columns.

12 NISTNCSTAR 1-5E, WTC Investigation
Experimental Configuration, Apparatus, and Procedures

Source: NIST. Figure 2-3. West side of experimental enclosure. Five ventilation openings are present.

The knee wall was removed for access to instrumentation.

Each of three workstations was placed on an isolated platform made of calcium silicate board. The top

surface of each platform was flush with the floor of the compartment. Each platform was supported on

water-cooled load cells, located in the subfloor, to monitor the mass of the workstation during the test. Two of the workstations were contiguous, exemplifying a part of the type of cluster that existed in many

large office spaces. The third workstation was separated from the other two by an aisle, representing a

part of a second cluster. This array enabled an assessment of FDS's ability to replicate two different

modes of workstation-to-workstation fire spread: direct flame impingement and radiative ignition from

the hot ceiling layer. Figure 2-4 shows aspects of the configuration corresponding to Fig. 2-3.

2.4.1 Ventilation

The enclosure had five openings and four columns on the front (west wall), mimicking window openings

through which fresh air entered, and heat and combustion products were emitted. The openings were

2.12 m high, started 0.32 m above the floor, and were each 0.45 m wide. The narrow openings limited the

amount of fresh air that entered the burning enclosure. In one test, (Test 6), the ventilation changed

during the experiment as window glass, placed to the left of the ventilation openings, cracked and fell out approximately 3 min after ignition.

NISTNCSTAR 1-5E, WTC Investigation 13
Chapter 2

0.9 m

APPROXIMATE

DIMENSIONS

OPENINGS

2.1 m tall - 0.45 m wide

COLUMNS

0.5 m wide

0.3 m n

INSTRUMENTATION SU B-FLOOR (external to experimental enclosure)

CUBICLE 3

FIRE PAN AND

SPRAY BURNER CUBICLE 1 LOCATION 1

2.4.2

Figure 2-4. Elevation view of experimental configuration.

Description of the Multi-Workstation Configuration

Figure 2-2 shows the multi-workstation experimental configuration. The layout was composed of

three generic workstations positioned in an "L" shape. Each of the workstation is labeled with a number

in Fig. 2-2. The spray burner was located either to the east of Workstation 2 or to the west of

Workstation 1. Compartment openings were located on the west side of the compartment. A 0.5 m wide

carpeted passageway separated Workstation 3 from Workstations 1 and 2.

Workstations

The generic workstation is described in detail in NIST NCSTAR 1-5C. A brief description is given here. The layout, including the placement of the various non-stationary items, was suggested by personnel from

the company that supplied office furnishings to the occupants of WTC I. A frequent visitor to the WTC

offices provided observations on the distribution of paper and office clutter (NIST NCSTAR 1-5C).

The generic workstation was a 2.44 m by 2.44 m (8 ft by 8 ft) workstation. Table 2-2 lists the contents of

the workstation, which included carpeting, privacy wall panels, a chair, bookcases, three filing cabinets,

four work surfaces, reams of copier paper, a thermoplastic waste basket, a computer, a monitor, and a keyboard. Table 2-2 also lists the total and the fraction of the mass that was combustible. Each

workstation weighed about 557 kg. Approximately 45 percent of the mass of the contents (249 kg) was

combustible. Table 2-3 lumps the workstation contents into four categories. The largest type of material

14 NIST NCSTAR 1-5E, WTC Investigation
Experimental Configuration, Apparatus, and Procedures

was wood/laminate (45 percent by mass), followed by paper (25 percent), plastics (16 percent) and carpet

(14 percent). The heats of combustion of these materials, based on the Cone Calorimeter results, varied

by nearly a factor of three (NIST NCSTAR 1-5C).

Table 2-2. Mass oF workstation contents.

Component

Ivl(kg)

tvi hiicl'iKlA Fractioni'^f\m ^ KiivtiKI<i \/1 Ulll U U^llUltr ItImS!) (kg)

Work "siirfarp — inplnminp laminjifp over mpdiiim

density fiberboard; 4 pieces, 6.1 m total length x

0.61 m wide x 28 mm thick

82.8 1.0 82.8

27 reams of paper and 14 document boxes 63.7 1.0 63.7

Plastic kick plates and trim (base of walls, inside and outside)

7.1 1.0 7.1

Computer keyboard 1.2 1.0 1.2

Plastic waste basket 0.7 1.0 0.7 36 nylon carpet tiles with rubber backing 38.0 0.9 34.2

Shelf ends - particle board or dense foam 3.8 0.9 3.4 (assumed plastic)

Bulldog chair - fabric, foam, thermoplastic shell and base19.4 0.8 15.5

Computer monitor - ABS 17.6 0.3 5.3 Computer processor 12.3 0.3 3.7

9 wall panels with aluminum angle, wood frames,

fiberglass, and metal mesh, 3 sizes

168.2 0.18 30.3 (25.3 kg wood,

5.0 kg fabric)

Book shelf 8.3 0.1 0.8

3 two-drawer steel filing cabinets, 0.91 m long x

0.5 1 m deep x 0.76 m high

142.5 0 0.0

Total 557.1 0.45 248.7

Table 2-3. Mass of workstation materials by type.

Mass Effective Heat of Combustion

Material (kg) Fraction of Total (MJ/kg)

Wood/laminate 111.5 0.45 14

Paper 63.7 0.25 14

Plastics" 39.3 0.16 16-38

Carpet 34.2 0.14 22

a. Includes computer monitor shell (16 MJ/kg), wall fabric (30 MJ/kg), and chair composite (38 MJ/kg).

The privacy wall panels were mid-height (1.22 m high, except for one 1.52 m high panel section

supporting the bookcase) and enclosed all four sides of the workstation except for a 1.22 m (4 ft) opening.

The panels were covered on both sides with a thermoplastic fabric. The interior structure of the panels was composed of layers of fiberglass and perforated steel. Each panel section had a 38 mm softwood

frame around its periphery, but this was covered in part by fabric and in part by a steel outer frame. The

work or desk surfaces (top 0.75 m, 29.5 in., above the floor) were formed from four unequal sections of

laminated, medium density fiberboard, supported by brackets from the wall panels. The office chair was

NIST NCSTAR 1-5E, WTC Investigation 15
Chapter 2

upholstered with a non-thermoplastic fabric over polyurethane foam (seat and seat back), supported by a one-piece thermoplastic shell; its five-legged base was also thermoplastic, though there were steel framing and support elements. .

The three steel filing cabinets (0.91 m wide, 0.51 m deep, 0.68 m high, 36 in. by 20 in. by 27 in.) with

two horizontal drawers each rested directly on the carpet tiles. The bookcase (1.22 m, 48 in. long) had a

steel shelf and top, but these were supported only on their ends by combustible end panels; the steel front

closure panel was fabric-covered steel, and it was open (on top of the bookcase). The carpet tiles were

nylon fiber-faced over a dense foam rubber backing. A square area 2.74 m by 2.74 m (9 ft by 9 ft) was

covered with 36 carpet tiles. The entire assembly rested on top of a double layer of Marinite 1(13 mm,

Vi in. thick) sheets. The support structure beneath this was in turn placed atop a set of four weighing cells, one at each comer.

The computer monitor was a nominally a 1 7 in. CRT-based unit. Its front face was taped with fiberglass

tape, and it was pointed toward the wall panel opposite the workstation opening in the event of an

implosion. The keyboard was placed in its normal location, parallel to the angled segment of the work

surface. The computer processor (tower-type container with plastic only on the front face of the

container) was placed on the floor next to a waste paper basket (both on the north side of the workstation,

opposite the opening).

The wastebasket was thermoplastic, and it contained one ream of copier paper atop five balled-up paper ream wrappers. Copier paper was chosen to substitute for all types of paper-based products expected in a workstation (loose leaf, catalogs, books, etc.), since it is reproducible and similar in general fire behavior

to all such products. It was positioned in both horizontal and vertical arrays, arrangements which

potentially may behave quite differently, but this did not appear to be crucial in any of the single

workstation experiments (NIST NCSTAR 1-5C).

Copier paper was also distributed along the desks and bookshelves. The paper in the bookcase and the

paper in the southwest comer (to the left of the workstation opening) were placed vertically in open, thin

cardboard "document boxes", each with an 89 mm (3.5 in.) wide interior dimension. These boxes had a sloped top edge such that they equaled the long dimension of the paper in the rear but were only 100 mm

(4 in.) tall in front. In the bookcase, each document box contained 1 .33 reams of copier paper (a rather

loose degree of packing). The ten boxes were grouped in sets of two with equal spacing (approximately

3 cm, 1.2 in.) between each pair. The four boxes on the desk surface contained one ream each of copier

paper (an even looser degree of packing) and were grouped as a set of four units in a hne, in contact. Other paper was laid as open horizontal stacks on the desk surface. A few sheets were tacked (as groups

of three to five sheets) to the workstation walls on three sides.

Two of the three file cabinets contained a very limited amount of copier paper. Two piles were laid horizontally in two file drawers. Each pile consisted of one ream laid in the drawer comer with the long

dimension of the paper aligned with the depth dimension of the drawer topped by a second ream aligned

at 90 degrees to the lower ream. Both placements were worst case in terms of heat exposure, i.e., they were against the file cabinets, immediately adjacent to an area subject to intense buming (open area under

the desk surface). It was anticipated that the contribution of the copier paper to the heat release rate would be rather small and tend to occur past the peak Q for the workstation.

16 NIST NCSTAR 1-5E, WTC Investigation
Experimental Configuration, Apparatus, and Procedures

The chosen ignition scenario mimicked that used for the single workstation (NIST NCSTAR 1-5C) for the isolated workstations. The concept was to create a scenario in which fire spread was proceeding

linearly through a large array of workstations. A large ignition source (a 2 MW, four nozzle spray burner over a 2 m by 1 m pan) was placed immediately adjacent to the exterior of one wall panel of the test station, simulating the burning of the adjacent workstation. The size and placement (pan bottom 0.81 m

above the floor) of this ignition source were guided by preliminary FDS predictions. Of course, a real workstation would yield a transient heat release rate and, thus, a time-varying heat input to its neighbor.

A constant heat release rate for the spray burner was used for consistency without sacrificing realism.

The entire assembly was placed beneath the NIST 9 m by 12m exhaust hood and calorimeter system to capture the combustion products and measure the total heat release rate during the burning of the

workstations. Unlike the experiments on the single workstations (NIST NCSTAR 1-5C), the air supply to the fire was restricted through the compartment openings. The burning behavior was, therefore,

ventilation limited during a large portion of the experiments.

Ceiling Tile Distribution in Multi-Workstation Tests

The distribution of ceiling tiles mimicked that used by for the single workstation (NIST NCSTAR 1-5C),

which was based on a series of ambient temperature full-scale and reduced-scale experiments. In the fire

tests, the tiles were laid out with their edges parallel to the privacy panels to facilitate reproducible

placement in successive tests. Forty-nine tiles were used, and the percentage of the area covered by tiles in the fire experiments was as follows: central floor area, 67 percent; floor area under desk surfaces,

10 percent; desk surface remote from bookcase, 50 percent; two desk surfaces near bookcase, 32 percent;

chair seat, 25 percent. All of the tiles were laid flat on horizontal surfaces, and some overlap of tiles was

used. Figure 2-5 is a photograph taken before Test 3, which highlights the arrangement of the ceiling

tiles used in Tests 3 and 4 for each of the workstations. The spray burner and compartment openings are

visible toward the rear of the photograph shown in Fig. 2-6. In Test 5, the ceihng tiles were placed on top

and within the "rubblized" workstations as shown in Figs. 2-7 and 2-8.

Analysis using the Federal Aviation Administration micro-calorimeter (Walters and Lyons 2000) showed

that under aerobic pyrolysis conditions, the total heat release rate for a virgin ceiling tile sample was

1.9 kJ/g ±5 percent with a char mass fraction of 83 percent remaining after pyrolysis. Each ceiling tile weighed approximately 0.4 kg so that the total mass of tiles per workstation was approximately 20 kg.

This would be expected to yield 40 kJ of energy. If averaged over a 10 min period, the heat release rate would be expected to yield approximately 0.07 kW, a negligible contribution to the total heat release rate of the burning workstations, which was on the order of 10 MW.

NIST NCSTAR 1-5E, WTC Investigation 17
Source: NIST. Figure 2-5. Views of interior of test compartment.

Source: NIST. Figure 2-6. Image highlighting the arrangement of ceiling tiles used in Tests 3 and 4. The spray burner and compartment openings are visible towards the rear.

18 NISTNCSTAR 1-5E, WTC Investigation
Experimental Configuration, Apparatus, and Procedures

Source: NIST. Figure 2-7. The arrangement of "rubblized" Workstations 1 (left) and 2 (right) with

ceiling tiles before Test 5. Part of the spray burner is visible on the left.

Source: NIST. Figure 2-8. The arrangement of Workstation 3 before Test 5.

NISTNCSTAR 1-5E, WTC Investigation 19
Chapter 2

2.5 FUEL FLOW, BURNER, AND DELIVERY SYSTEM

The liquid spray burner, pan, and fuel (mixture of heptanes) were the same as those used for the single

workstation (NIST NCSTAR 1-5C). The spray burner was used to cause repeatable rapid ignition on one

side of the adjacent workstation. The fuel used was a commercial blend of heptane isomers. Depending

on the test, the burner was located abutting the top of a workstation partition at the east end of

Workstation 1 or the west end of Workstation 2. The ignition fire intensity was a nominal 2 MW fire. The spray burner was operated for the first few minutes of the tests, for either 2 min or 10 min depending on the test scenario. Table 2-4 lists the burner location, the fuel flow rate, and the duration of the spray.

Table 2-4. Liquid fuel spray burner.

Flow Rate Duration

Test Location Initiation (L/min) (min)

1 Bl Time = 0 s 4.0 10

2 Bl Time = 0 s 4.0 10

3 Bl After Jet A ignition 4.0 2 4 B2 Time = 0 s 4.0 10

5 B2 After .let A ignition 4.0 2

6 B2 After Jet A ignition 4.0 2

A blend of heptane isomers (designated here as "heptane") was used as the spray burner fuel. The

density, the heat of combustion, and the carbon to hydrogen ratio for the fuels were measured for the

commercial heptane and n-heptane. The heat of combustion and the carbon to hydrogen ratio for the

commercial heptane was measured to be within 1 percent of the value for n-heptane (Galbraith

Labs 2003), which was within experimental uncertainty for the measurements. The fuel flow was

4.0 L/min, which was approximately equivalent to a 2 MW fire. The fuel flow to the hydrocarbon spray

burner was calibrated before each of the experiments. The fuel was delivered through four 90 degree

spray angle WL Vi type stainless steel nozzles spraying downward. The BETE nozzles were spaced

50 cm apart and located 47 cm above the fire pan. The 1 m by 2 m stainless steel pan was positioned

85 cm above the floor of the enclosure. Figure 2-9 is a photograph of the spray burner and fire pan

abutting workstation 1 before Test 2. In the photograph, the burner is positioned at Location Bl, which was near the ventilation openings on the west side of the compartment. The photograph shows that the

pan was mounted on a table 80 cm above the floor.

The ignition of the spray burner initiated the tests, and was defined as the zero reference time for data

collection. For the cases when jet fuel was present, the fuel spray was initiated after the jet fuel in the

compartment was fully involved. The fuel spray was somewhat coarse and some droplets made it to the

pan though there was no significant liquid accumulation there. The duration of the fuel flow was 2 min

for the cases when jet fuel was present and 10 min in the absence ofjet hael. The fuel flow was

monitored using an ExactFlow dual rotor turbine flow meter during the experiments.

20 NIST NCSTAR 1-5E, WTC Investigation
Experimental Configuration, Apparatus, and Procedures

Source: NIST. Figure 2-9. Spray burner and fire pan abutting Workstation 1 before Test 2 (location B1).

2.6 INSTRUMENTATION

More than 150 channels of data were acquired for each of the six experiments. Table 2-5 provides an overview of the instrumentation, measurement technique, and measurement locations used in the

experiments. Appendix A lists the location of each instrument.

Table 2-5. Measuremenits. (Chapter) Measurement Instrument/Method Location

(3) Spray burner fuel flow Turbine flow meter Fuel piping outside enclosure

(3) Flame Spread Bare-bead thermocouples; digital

video cameras Thennocouples attached to combustiblesurfaces; video cameras with inside &

outside views.

(4) Mass loss rate 3 load cell systems Sub-floor under each workstation

(5) Heat release rate Oxygen consumption calorimetry Exhaust duct

(6) Gas temperature 4 trees each with 4 bare-bead

thermocouples

Four thermocouple trees

(7)0., CO. and CO volume

fractions (dry basis)

Gas sampling with paramagnetic

and non-dispersive IR Upper layer

Video record Digital video cameras Inside and outside views

Chapters 3 through 7 of this report describe each of the measurements in detail, including the instrumentation that was used, an estimate of uncertainty, and the experimental results. Table 2-6

summarizes some of that information and lists the character of the instrumentation in terms of important

NISTNCSTAR 1-5E, WTC Investigation 21
Chapter 2

measurement characteristics, namely, typical values of the expanded measurement uncertainty (with an

expansion factor of 2), the measurement time response, and estimates of the spatial resolution of the measurement. These properties characterize and limit the utility of a measurement and are important

considerations in ternis of comparison of model predictions with measurements. The time response was

typically 1 s, varying from 1 s for the thermocouples to 15 s for the heat release rate measurement. The

spatial resolution of the measurements was typically 1 cm to 5 cm, varying from sub-centimeter to

1 ,000 cm for the mass loss rate measurement of the load cells.

Table 2-6. Instrument characterization.

Instrumentation Typical Expanded Uncertainty

Nominal Time

Response (s)

Spatial Resolution

(cm)

Load cell 5 % 10 1.000 Heat release rate via oxygen

consumption calorimetry

15 %

(for Q > 0.8 MW)

15 Global measure

Gas temperature:

• Bare bead thermocouple

• Aspirated thermocouple

< 120 °C (dependent on temperature)

7T

1

1

0.5

1 CO via non-dispersive IR^ 10% 10 5 COi via non-dispersive IR"^ 10% 10 5 O2 via paramagnetic"" 5 % 10 5

a. Gas volume percentages are reported on a dry basis.

2.6.1 Load Cells

The rate of burning of combustible items in the compartment controlled the fire heat release rate and,

thereby, the rate of fire spread. Load cells were employed to measure the mass loss of each of the

workstations during each of the tests. The mass loss rate determined from the load cell data provided a measure of the fuel gasification rate and computation of the effective heat of combustion of the

workstations. A detailed description of the load cells and mass loss rate measurement results is given in Chapter 4.

2.6.2 Oxygen Consumption Calorimetry

The west end of the enclosure was located under the NIST 9 m by 12 m exhaust hood for collection of the effluent and measurement of the heat release rate using oxygen consumption calorimetry. The plume of

gases generated during the tests exited through the west wall of the compartment and were subsequently

captured and swept into the exhaust duct. Data collected from an array of differential pressure

transducers and thermocouples in the exhaust duct were employed to compute the mass flow rate of the

gases in the exhaust duct. A representative gas sample was continuously removed from the exhaust duct

by a gas pump. The gaseous sample was pumped to the control room. In the control room, the gas

sample was filtered to remove particulate and cooled in a dry ice trap to condense water from the gas. The resulting conditioned dry gas was analyzed using gas analyzers to quantify oxygen (O2), carbon

monoxide (CO), and carbon dioxide (CO2). A detailed description of the heat release rate measurement

technique and the measurement results is given in Chapter 5.

22 NISTNCSTAR 1-5E, WTC Investigation
Experimental Configuration, Apparatus, and Procedures

2.6.3 Thermocouple Trees - Compartment Temperatures

Thermocouple trees were constructed by NIST personnel and located at four locations inside the

compartment. See Table A-1 in the Appendix for the exact locations. Thermocouples were placed at four positions on the tree, namely, 2.5 cm, 91 cm, 83 cm, and 274 cm below the ceiling. During Tests 1 and 2, a pair of aspirated thermocouples were positioned close to Tree 4 to assess the effect of radiative

exchange on the temperature measurements by the nearby bare bead thermocouples.

2.6.4 Thermocouples - Desk Surface Temperatures

Thermocouples were employed during the test series to measure the surface temperatures of the desk

surfaces to provide an indication of flame spread from workstation to workstation. During Test 1 , each

workstation was equipped with three desk surface thermocouples. In Tests 2, 3, 4, and 6, only two

thermocouples per desk were used. In Test 5, no thermocouples were located in the workstations. Exact

locations are listed in Appendix A. The temperature record was used to assist in the analysis of flame

spread from workstation to workstation, which is discussed in Chapter 3.

2.6.5 Video Recording Cameras

Two video cameras recorded the events within the compartment during each test. On the east wall, a video camera imaged the eastern walls of Workstations 2 and 3. A portion of the carpet between

Workstations 2 and 3 was also visible. A second camera on the western portion of the north wall was

aimed to capture events at Workstations 1 and 3. Specifically, the northern partitions of Workstation 1. as well as some of the contents were observed. This camera also recorded the western wall of Workstation 3 and a small section of the northern wall of Workstation 2.

Two video cameras were positioned outside the compartment west wall. One camera recorded the inside

of the compartment through the columns. The second camera recorded events at the test wall portion on

the western wall. In combination with the thermocouple data, the video record was used to assist in the

analysis of flame spread from workstation to workstation, which is discussed in Chapter 3.

2.6.6 Gas Analyzers

The O2, CO, and CO2 were monitored continuously at one location during each test. The compartment

hot layer was sampled to provide information on the tenability of the compartment. The sampling

location was at (x,y,z) = (970 cm, 351 cm, 270 cm). The exhaust stack was also sampled to provide data

for calorimetry computations. The specific analyzers employed during testing are listed in Appendix B.

In each case, gas was pulled from the sampling location using a pump. Oxygen was quantified using

magneto-dynamic, paramagnetic oxygen balances. Carbon monoxide and carbon dioxide were quantified

using gas analyzers employing non-dispersive infrared technology. In all cases, gas concentrations were

reported on a dry basis. A detailed description of the gas analyzers and the measurement results is given

in Chapter 7.

NISTNCSTAR 1-5E, WTC Investigation 23
Chapter 2

2.6.7 Data Acquisition

The NIST Large Fire Laboratory data acquisition (DAQ) system is a modular system acquired from

National Instruments. DAQ boards installed in a standard desktop computer were interfaced with two

DAQ chassis. Each chassis, in turn, was connected to multiple modules capable of digitally multiplexing

input data signals and supplying the multiplexed signal to the DAQ boards. Tenninal blocks capable of

up to 32 data channels of input were connected to each module. Each terminal block contained a

thermistor used to provide cold junction compensation to each thennocouple measurement.

The DAQ desktop computer runs National Instruments LabView software. The software was customized

for calculation of real-time fire behavior parameters, graphical display of real-time data, and data storage.

Data from each test was stored in its own data file. The first 2,000 s to 3,000 s contained data recorded

during calibration of the calorimeter. Once the calorimeter was "on-line", all test equipment was readied,

and test personnel were prepared. Background data was then recorded for use in the calorimetry

computations. The background data was typically not recorded until all test preparations were complete.

The test director determined termination of data collection. Table 2-7 lists the amount of time

background was acquired, the time data acquisition was stopped, which happened when fire suppression

occurred or when the calorimeter instrumentation was taken off-line. In some cases, raw data was

recorded past the data acquisition stop time listed in the table below. Data was acquired with a sampling

rate of 1 Hz. The test duration was approximately 1 hour for each of the Tests, yielding on the order of one million discrete pieces of data.

Ta Die 2-7. Duration of data acquisition files.

Background (s) Test Start (s) Data Acquisition Stopped (s) Suppression (s)

Test 1 -150 0 4,867 Test 2 -328 0 5,360 -5,122

Test 3 -522 0 6,524 -9,609

Test 4 -327 0 3,521

Tests -686 0 5,755 4,656 Test 6 -300 0 5,044

2.7 TEST PROCEDURE

The procedure was similar to that followed in the experiments reported in NIST NCSTAR I-5C, except

that the ignition burner was turned off after approximately 2 min for Tests 3, 5, and 6 (when jet fuel was

present) and after 10 min for Tests 1, 2, and 4. The tests continued until the measured heat release rate

fell below 0.5 MW, which was typically 65 min after ignition.

The following list describes the sequence of events in preparation for an experiment. The list includes the

major steps associated with setup and running of the experiment. A detailed list of test procedures and

safety checks was considered before each experiment including the following:

• Checked all connected instrumentation for proper functioning

• Checked load cell platforms for interference

24 NIST NCSTAR 1-5E, WTC Investigation
Experimental Configuration, Apparatus, and Procedures

• Prepared enclosure flooring and installed carpeting

• Assembled workstations

• Loaded workstations

• Installed thermocouples in workstations

• Covered workstations and corridor with drop ceiling tiles when dictated by test matrix

• Photographically documented experimental setup

• Installed column and kneewall

• Checked liquid spray burner fuel flow

• Conducted final check of compartment instrumentation

• Calibrated Heat Release Rate Oxygen Calorimeter and continuous sampling gaseous loop

• Conducted safety brief

• Collected background/baseline data

• Poured Jet A fuel when dictated by test matrix

• Evacuated enclosure

• Ignited spray burner

• Stopped fuel flow to spray burner

• Allowed compartment to cool to ambient

• Tested that compartment was empty of combustion products

• Removed column and kneewall

• Removed burned materials

• Refurbished enclosure as necessary

After each experiment, at least two days were required to remove the debris, inspect the compartment,

patch cracks in the marinite, check instrumentation, and install the new workstations.

NISTNCSTAR 1-5E, WTC Investigation 25
Chapter 2

2.8 REFERENCES

Galbraith Labs. 2003. Report of Test Results. Knoxville, TN, March, p. 20.

Hamins, A., Maranghides, A., and Mulholland, G. 2003. The Global Combustion Behavior of 1 MWto 3

MWHydrocarbon Spray Fires Burning in an Open Environment, NISTIR 7013, National Institute of

Standards and Technology, Gaithersburg, MD, June.

Lyon, R. E. 2004. Report ofMicro-Calorimeter Tests Results, Federal Aviation Administration, William

J. Hughes Technical Center, NJ. .

Madryzkowski, D. 1996. "Office Work Station Heat Release Rate Study: Full Scale vs. Bench Scale,"

Proceedings oflnterjlam '96, Interscience Communications, Ltd., London, pp. 47-55.

Madryzkowski, D. 1998. "Office Building Fire Research Program: An Engineering Based Approach to Fire Safety Design," Proceedings ofthe 5th Fire and Materials Conference, San Antonio, Texas,

Interscience Communications, Ltd., London, pp. 23-33.

Manzello, S. L., Gann, R. G., Kukuck, S. R., Prasad, K. R., and Jones, W. W., 2005. Fire Performance

ofa Non-Load Bearing Glass Wall Assembly, to be submitted to Fire and Materials.

McAllister, T., ed. 2002. World Trade Center Building Performance Study: Data Collection,

Preliminaiy Obsen'ations, and Recommendations. FEMA 403. Federal Emergency Management

Agency. Washington, DC, May.

Walters, R. N., and R. E. Lyon. 1997. Proc. 42'"^ Int. SAMPE Symposium and Exhibition, 42(2),

1335-1344.

26 NISTNCSTAR 1-5E, WTC Investigation
Chapter 3

Fire Behavior and Flame Spread

3.1 general observations

Chapter 2 of this report describes the experimental configuration in detail, including the experimental

conditions, the fuel flow, ventilation openings, and details of the test compartment. This chapter

describes the behavior of the fire, including the flame spread observed during each of the tests. The

general appearance of the fires was observed through video recordings from a perspective external to the

compartment, as well as from two views within the compartment. Thermocouples were used in conjunction with the video recordings to evaluate the fire behavior, including the mechanisms of fire spread through the workstations and the time of ignition of each workstation.

There were many similarities in the general appearance of the fires as they evolved during the six tests. In the early stages of the fire, an upper layer formed at the ceiling and grew in size, descending to below the

openings. Ignition occurred most quickly in Tests 3, 5, and 6 because Jet A aviation fuel was poured onto

the workstation surfaces and other materials, creating a quasi-sirnultaneous ignition and flaming burning

everywhere. In the presence of the jet fuel, a thick black upper layer formed quickly, and copious

amounts of smoke were emitted as seen in Fig. 3-1 . Figure 3-2 shows the compartment several minutes

into Test 5, when much less black smoke was emitted.

Figures 3-3 and 3^ show the west view of the compartment before and during Test 6, respectively. The

four windows on the left side of the compartment in Test 6 are seen in Fig. 3-3. During the rapid burning

phase of the tests, copious amounts of black smoke were emitted from the compartment. Figure 3^ is a view of the fire compartment 2 min after the start of Test 6. The windows broke after approximately

3 min in this test. At the end of the tests, little smoke was emitted from the compartment as the charred

remains of the workstation contents continued to slowly bum and release heat.

Examination of the video record of the six experiments demonstrated a number of general noteworthy

findings regarding the burning behavior of the fires:

• The burner in the back of the enclosure seemed to accelerate the rate of flame spread (times

to ignition and full involvement). It was not clear from the video record that the presence of

the ceiling tiles had any significant effect on the burning behavior of the workstations. Any

impact by the ceiling tiles may have been overridden by the effects associated with the burner

location.

• The presence of Jet A resulted in the enclosure being fully involved in fire in a little over one

min. In these tests, rapid fiiU involvement was followed by the formation of a large cloud of

black smoke that billowed from the compartment for a few minutes. The smoke layer then

dissipated, and the burning continued with less smoke emission.

NISTNCSTAR 1-5E, WTC Investigation 27
Chapter 3

28 NISTNCSTAR 1-5E, WTC Investigation
Fire Behavior and Flame Spread

Source: NIST. Figure 3-3. View of the fire compartment with test windows before the start of Test 6.

Source: NIST. Figure 3-4. View of fire compartment 2 min after the start of Test 6.

NISTNCSTAR 1-5E, WTC Investigation 29
Chapter 3

. • The video record was helpful in documenting the sequence of events that led to flame spread

from one workstation to the next and from one side of the compartment to the other.

• The video record suggests that radiation from the hot upper layer of the compartment, rather

than flame spread from a contiguous burning item, led to ignition of Workstation 3 in Test 4.

. This mode of ignition could not be verified for the workstations in the other tests, due to smoke obscuration of the video record, and camera views that limited observation.

• Flame spread can proceed through several means. A common mechanism involves burning

brands that are lofted by the convective forces associated with the fire and fall onto

combustible materials. Such a mechanism has been observed in warehouse fires and

wildland fires, for example, both of which involve cellulosic materials. There is no evidence

from the video record that flying brands played a role in flame spread.

• The passageway separating Workstations 1 and 2 from Workstation 3 was carpeted. Burning

carpet spread the fire from Workstation 1 to Workstation 3 for both Tests 1 and 2. Of course,

the hot upper layer undoubtedly played a role in the rate at which this occurred, as the

workstation contents were preheated by radiative heat transfer.

3.2 WORKSTATION IGNITION AND FULL FIRE INVOLVEMENT

Thermocouples were used in conjunction with video recordings to evaluate the time of ignition and full

fire involvement during Tests 1, 2, and 4. Such data were not reviewed for Tests 3, 5, and 6, because

Jet A aviation fuel was poured on the workstation surfaces and other materials prior to ignition, creating a quasi-simultaneous ignition scenario. Table 3-1 lists the estimated times to full involvement for each

workstation, determined by desk surface temperature measurements and video review in which time zero was taken as the time the spray burner fire was initiated. Table 3-1 also lists the time to ignition based

upon video review. Estimating the ignition time and time to full involvement from the thermocouple and

video records is subjective. The values in the table should be thought of as estimates that are intended to provide qualitative infonnation only.

During Tests 1, 2, 3, 4, and 6, two thermocouples were used to measure desk surface temperatures, one

thermocouple on the top desk surface and one thermocouple attached to the bottom desk surface. Test 5 did not employ thermocouples mounted to the desk surfaces because the workstations and desks were not assembled in the standard configuration. Examination of the thermocouple temperatures versus time

typically shows a period of rapid temperature increase to a value above 400 °C. A tangent line was

extrapolated through this temperature ramp and the slope of the temperature-timecurve prior to the ramp.

The intersection of the lines was taken as the ignition time. The full involvement time estimate is reported in Table 3-1 for each desk thermocouple. The thermocouple with the later response is indicative

of full fire involvement of the workstation. Figure 3-5 shows an example of the estimation approach used

to determine ignition and full fire involvement from the desk thermocouple measurements. The

uncertainty in the ignition time was esfimated as 10 s.

30 NISTNCSTAR 1-5E, WTC Investigation
Fire Behavior and Flame Spread

Table 3-1. Ignition and full involvement times.

Test

Ipnition/Riill Involvpnipnt

(Thermocouples) (s)"

lonitinn (C^iAfn(^m\

Video (s)"

1Riill Ull lllTl/lvdll^-Ili 1 n vol vpmont (\^ C^skmpra^ \_- cllllvl c4 1 Video (s)'

s

Test 1 405/460 18 (north) 416 (north)

_oR

Test 2 390/420 14 (north) 434 (north)

Test 3 31/38 (b) 32 (north)' o

Test 4 420/470 381 (north) 420 (north)

Test 5 (d) (b) 73 (north)

Test 6 70/77 (b) 55 (north)

Test 1 725/750 539 (north) 727 (north)'

c

Test 2 730/758 500 (north) 738 (north)

"S Test 3 24/36 (b) 25 (north)

Test 4 265/370 13 (north) 270 (north)' o

Test 5 (d) (b) 64 (east)

Test 6 47/47 (b) 22 (north)

Test 1 730/802 555 (northV 809 (north)*^

c

Test 2 810/810 534 (north)' 768 (north)-'

«

Test 3 110/150 (b) 75 (north)'

Test 4 455/465 273 (east) 410 (east) O

Test 5 (d) (b) 57 (east)

Test 6 56/60 (b) 69 (east)

a. t = 0 at spray burner ignition.

b. Electric match ignited prior to spray burner ignition.

c. Video image obscured by smoke.

d. Workstations and desks broken up. No thermocouples used.

e. Extensive visible burning inside workstation used as full involvement criteria.

f. Workstation partition ignited from bummg carpet.

g. Burning all around west partition was used as full involvement criteria.

During replicate Tests 1 and 2, the full involvement times of Workstations 2 and 3 estimated from the

thermocouples were within 1 0 s of each other. The largest difference in the time to full involvement for Workstation 1, based upon data from the desk thermocouples, was approximately 40 s. Examination of

Workstation 1 thermocouple data from Tests 1 and 2 demonstrates that the response of the desk surface

thermocouples during Tests 1 and 2 were very similar. The 40 s difference is the sole result of the

thermocouples attached to the bottom desk surface. Figure 3-6 shows the temperatures of the desk

mounted thermocouples on Workstation 1 as a function of time during Tests 1 and 2. Full fire involvement occurred at 460 s and 420 s in Tests 1 and 2, respectively.

Ignition of parts of Workstation 1 was rapid in Tests 1 and 2 as seen in the video record and noted in Table 3-1 . The rapid ignition of Workstation 1 in these tests was due to the proximity of the spray burner

to the workstation. The thermocouple data indicated ignition on the desktops at a later time (see

Table 3-1) as the fire spread from the partition. The analogous situation occurred in Test 4 for Workstation 2.

NISTNCSTAR 1-5E, WTC Investigation 31
Chapter 3

Oo

CD

1—

CDE

CD H

700

600

500

400

300

200

100

0

-100

1 1 1 1

1

1 1 1 1

1 1 1 1 11

1 1 1 1

1

1 1 1 1

1 1 1 1 1

j 1 /A /

- -

: —o— Test 4, Cubicle 1 A

: Desktop Temperature

- —^— Test 4, Gub^cte 1

/ V

: Desk Bott.om Temperature -

/' _/ :

"

1 1

i 1 1 1 1 1 1 1 1 1 1 1 , , , , i ,,

1

, , 1 i 150 200 250 300 350 400 450 500 550

Time (s)

Figure 3-5. Determining ignition and full-involvement times from thermocouple data.

Oo

CD

i_

0)Q.

E

0)

200 300

Time (s)

400 500 600

Temperature measurements as a function of time for thermocouples on the

desks of Workstation 1 during Tests 1 and 2.

32 NISTNCSTAR 1-5E, WTC Investigation
Fire Behavior and Flame Spread

3.3 FLAME SPREAD OBSERVATIONS DURING TESTS 1, 2, AND 4

The video record was examined to evaluate the mechanisms of flame spread within the workstations and

from workstation to workstation during Tests 1, 2, and 4. During Tests 1 and 2, the heptane spray burner was located adjacent to Workstation 1. During Test 4, the spray burner was located east of

Workstation 2, immediately adjacent to the workstation wall.

3.3.1 Test 1

Workstation 1

The north camera recorded the fabric on the outer portion of the west wall burning and melting in roughly

1 8 s. Flames were not definitively located within the workstation walls until 120 s after spray burner

ignition. After 259 s, flames began to spread across the desk surface in the southern portion of

Workstation 1. From 365 s to 375 s, the flames spread to the chair arm and quickly involved the entire

chair and all visible desk surfaces. Analysis of the north camera video indicate that full involvement

occurred at approximately 386 s following spray burner ignition.

Workstation 2

The top of Workstation 2 was obscured after approximately 450 s. During Test 1 . the north video camera showed flames beginning to spread along the bottom of the western partition on the north wall of

Workstation 2 at 539 s after ignition. It is uncertain if the burning material was carpet, the melted

baseboard from the partition, or both. By 572 s, flames had spread and clearly reached the entrance to Workstation 2. Due to visible obscuration, however, it is not clear if the flame spread to the interior of

Workstation 2 was the result of flame spread along the hallway carpet or by another pathway. The video

record of the camera in the east wall of the compartment was obscured during the ignition of

Workstation 2.

Workstation 3

The video record of the north camera showed flame spread along the carpet, igniting the southwest comer

of Workstation 3 at 555 s. The resulting flames were small. At 599 s, the small flames crept up the

southwest comer between the workstation wall and a piece of thermoplastic trim. By 715 s, flames had

spread along the bottom of the west wall of Workstation 3, although no determination can be made if the

spread came from within the workstation or from along the outer wall. Based upon the north camera

video, full involvement of Workstation 3 occurred at 809 s.

During Test 1, the video record of the east camera looking at Workstation 3 became obscured by 544 s. At 7 1 7 s, flame activity was seen on the wall segment west of the entrance to Workstation 3 although it is not clear if the fiiel was carpeting or a wall segment. By 738 s, the wall segment east of the entrance of

Workstation 3 was ignited. Within 1 5 s, flame activity either ceased or was obscured by compartment

smoke. Due to visible obscuration, no further activity was observed by the east camera video until 886 s

at which point a vortex of flames and smoke was observed to occupy the eastem portion of the test compartment.

NISTNCSTAR 1-5E, WTC Investigation 33
Chapter 3

3.3.2 Test 2

Workstation 1

The north camera recorded the fabric on the outer portion of west wall burning and melting in roughly

14 s. Flames were not definitively located within the workstation walls until 146 s after spray burner

ignition. After 263 s, flames began to spread across the desk surface in the southern portion of

Workstation 1 . At 644 s, the chair ignited followed by the remainder of the visible desk surface igniting

within 5 s. Analysis of the north camera video indicates that full involvement occurred at approximately

434 s after spray burner ignition.

Workstation 2

^

Video recorded by the north camera shows ignition of the northwest comer of Workstation 2 at 500 s. Flame spread along the bottom of the west partition of Workstation 2 entrance. It is not clear if the

burning material is carpet or melted baseboard. Flame spread along this partition reached the workstation

entrance within approximately 18 s. However, the top of Workstation 2 became obscured after 420 s. The origin of the flame spread along the partition was not clear. The east camera image was obscured

during the Workstation 2 ignition event. Full involvement of Workstation 2 at 738 s was observed on the

north camera video.

Workstation 3

Video from the north camera clearly shows the ignition of the west wall of Workstation 3 at 534 s. Within 15 s, the mehed baseboard of the partitions was burning. The image from the east camera location was obscured.

3.3.3 Test 4

Workstation 1

The desk surface of Workstation 1 ignited at 381 s. By 420 s, the entire desk surface and chair were fully

involved, and flaming material dripped to the floor igniting the carpeting.

Workstation 2

Ignition of Workstation 2 was somewhat obscured by the glare from the spray burner, but appeared to occur at roughly 13 s on the video from the north camera. Though partially obscured by Workstation 3

and the glare from the spray burner, the north camera video showed complete involvement of

Workstation 2 by approximately 270 s.

Workstation 3

The video recorded by the east camera suggests that Workstation 3 ignited at 273 s as the result of radiant

impingement, not direct flame spread. At 362 and 371 s, flames burst through the partition wall seams,

first on the east wall and then the southeast comer. By the time flames began to propagate across the

carpet from Workstation 2 to 3 at 386 s, the partition walls of Workstation 3 were fully involved. The

34 NISTNCSTAR 1-5E, WTC Investigation
Fire Behavior and Flame Spread

north camera captured the ignition of the exterior west partition from fire activity within the workstation

at 418 s.

3.3.4 Tests 3, 5, and 6

Jet fuel was distributed about the three workstations and the carpeted passageway in Tests 3, 5, and 6. Fire involvement occurred almost simultaneously throughout the furnishings, typically in less than 1 min.

3.3.5 Burner Location Effect on Ignition and Flame Spread

The data in Table 3-1 indicate that burner location had little impact on the ignition times of the

workstation adjacent to the burner. In Tests 1 , 2, and 4, the partition nearest the spray burner ignited

within 1 3 s to 1 8 s. The data suggest that the burner location had a significant impact upon the time to

full involvement of the adjacent workstation. The discussion below addresses this issue for each

workstation.

The data in Table 3-1 demonstrate the significance of burner location with respect to flame spread.

During Tests 1 and 2, flames spread from Workstation 1 to Workstation 2 first. Workstation 3 was the

last workstation to ignite. Recall that Test 4 differed from Tests 1 and 2 in that the burner was positioned

in the east portion of the room, immediately adjacent to Workstation 2. During Test 4, Workstation 2 was

the first to ignite with subsequent ignition of Workstation 3 followed by Workstation 1.

Effect of Burner Location on Workstation 1

The spray burner position had a large effect on the ignition of Workstation 1, but a minor effect only on

full involvement. The video record indicates that Workstation 1 required roughly 6 more minutes to ignite during Test 4 than during Tests 1 and 2. Full involvement in all three tests occurred at 7 min ±

1 min, according to both thermocouple data and the video record.

Effect of Burner Location on Workstation 2

The proximity of the spray burner to Workstation 2 in Test 4 resulted in both rapid ignition and rapid full

fire involvement. Ignition required only 1 3 s as recorded by the north video camera. The data in Table 3-1 indicate that Workstation 2 was fully involved either 6 min (thermocouple data) or 4.5 min

(video record) after the spray burner was ignited. During Tests 1 and 2, the average value for time to full involvement of Workstation 2 from both the thermocouple data and video record was over 12 min.

Effect of Burner Location on Workstation 3

The position of the spray burner in Test 4 caused rapid ignition and full involvement of Workstation 3. Ignition of Workstation 3 during Test 4 occurred after approximately 4.5 min, while the same event took

an average of 9 min during Tests 1 and 2. Averaging data from both the video record and thermocouples.

Test 4 full involvement of Workstation 3 occurred at approximately 7.3 min compared to an average of

over 13 min during Tests 1 and 2.

NISTNCSTAR 1-5E, WTC Investigation 35
Chapter 3

3.4 FLAME SPREAD VIDEO AND THERMOMETRY COMPARISON

Table 3-1 lists the estimated workstation ignition and fliU involvement time based upon thermocouple

measurements and the video record. Both desk surface thermocouples and video review were employed

to estimate the time to full involvement, except during Test 5 in which thermocouples were not used.

The two techniques were in good agreement. The average difference of the results for the two techniques was less than 30 s. Of the 15 comparable data pairs, 5 of the differences were greater than 30 s, and 3 of

these 5 "outliers" were associated with Workstation 3. This is not unexpected given the room setup and

difficulty associated with obtaining clear video images during the test series. The north video camera

only imaged the outer section of the west wall of Workstation 3. The east camera view of Workstation 3 was obscured by Workstation 2 and also frequently by smoke within the compartment.

In 1 2 of the 1 5 data pairs, the estimated time to full involvement from the video was less than that estimated from the thermocouple data. This could again be due to the difficulty associated with trying to image all fire activity occurring within a workstation. For example, review of the Test 1 video record

indicates that Workstation 1 becomes fully involved at 416 s, while review of the thermocouple data

suggests 460 s as the appropriate measure. At 416 s, the video clearly shows most of the top desk surface

involved with fire activity on both the chair and the carpeting near the workstation entrance. The carpet

fire near the entrance does obscure the rest of the workstation floor. At 460 s, the intensity of the fire within Workstation 1 does increase markedly as observed on the video.

3.5 KEY EVENTS IN THE EVOLUTION OF THE COMPARTMENT FIRES

The development of the fires in the tests without jet fuel present (Tests 1, 2, and 4) was much slower than

when jet fuel was present (Tests 3, 5, and 6). Key events in the appearance of the fires are summarized in Table 3-2, as observed from a perspective external to the compartment. Table 3-2 compares the

timehnes in Tests 4 and 6, which highlights the differences in the fire timehne when jet fuel was present

as compared to when it was not. The table highlights the ignition, development of a smoke layer and its transport from the compartment, the transition from emission of smoke to the emission of flames through

the ventilation openings, and progression to a time when no flames exited the compartment.

Table 3-2. Description of key events in evolution of the compaii ment fires.

Event"

Test 4

(min:sec)

Test 6

(inin:sec)

Jet A ignition NA - 0:30^ Smoke starts exiting compartment 0:10 -0:05 Heavy black smoke starts exiting compartment 3:30 0:05

Start of transition to flaming (very small flames) 7:00 0:30

Start of flaming only (very little smoke) 10:30 4:30

Glowing within, but no flames exiting the compartment 19:45 9:30

Distributed limited burning 21:00 17:00

a. Time zero (0:00) was defined as the moment when the spray burner was ignited.

b. A negative number implies that the event occurred before ignition of the spray burner. Key: NA, not applicable, jet fuel was not used in Test 4.

36 NISTNCSTAR 1-5E, WTC Investigation
Fire Behavior and Flame Spread

The difference in time to particular events between Tests 4 and 6 was large. There was a SVi min

difference in Tests 4 and 6, for example, to the time when heavy black smoke started to exit the compartment. In the tests with jet fuel present, a smoky upper layer rapidly developed. This is documented in Figs. 3-7 through 3-13 for Test 6, which show the evolution of the fire corresponding to the events listed in Table 3-2 from a camera perspective external to the compartment and facing the compartment openings. A smoke laden upper layer began to fill the compartment within seconds after ignition of the jet ftiel (see Fig. 3-7), and smoke began to exit the compartment as seen in Fig. 3-8.

Figure 3-9 shows that the upper layer continued to drop and smoke rapidly flowed through the openings.

Figure 3-10 shows that, soon thereafter, flames were observed to exit the compartment. Within a short

period of time, flames filled the ventilation openings as shown in Fig. 3-11. The bottom and top single

pane windows on the left edge of the window assembly (as seen from outside of the compartment)

cracked and fell out approximately 185 s and 212 s, respectively, after ignition of the spray burner.

Figure 3-11 shows flames exiting the area where the top single-pane window had been located. The

video record shows that the lower left window was also broken at this time. The double-pane windows

on the right in Fig. 3-1 1 survived over the entire test duration. Manzello et al. (2005) provides a detailed

description of the timeline associated with window breakage during this test. As the test proceeded, the

smoke cleared, and only flames could be seen exiting the openings as documented in Fig. 3-12. Finally,

limited burning and charring took place at distributed locations about the compartment, and little smoke

or flames exited the openings as seen in Fig. 3-13.

Source: NIST. Figure 3-7. View external to the compartment 30 s before ignition

of the spray burner in Test 6.

NISTNCSTAR 1-5E. WTC Investigation 37
Chapter 3

Source: NIST. Figure 3-8. View external to the compartment 5 s before ignition

of the spray burner in Test 6.

Source: NIST. Figure 3-9. View external to the compartment 5 s after ignition

of the spray burner in Test 6.

38 NISTNCSTAR 1-5E, WTC Investigation
Fire Behavior and Flame Spread

Source: NIST. Figure 3-10. View external to the compartment 30 s after ignition

of the spray burner in Test 6.

Source: NIST. Figure 3-11. View external to the compartment 4 min 30 s after

ignition of the spray burner in Test 6.

NISTNCSTAR 1-5E. WTC Investigation 39
Source: NIST. Figure 3-12. View external to the compartment 9 min 30 s after

ignition of the spray burner in Test 6.

Source: NIST. Figure 3-13. View external to the compartment 17 min after

ignition of the spray burner in Test 6.

40 NISTNCSTAR 1-5E, WTC Investigation
Fire Behavior and Flame Spread

It is of interest to compare the experimental observations with the extensive photograph record of the

World Trade Center (WTC) fires in an attempt to identify common features or differences. NIST

compiled, catalogued, and time-stamped an extensive array of photograph and video records of the WTC

fires (NIST NCSTAR 1-6A). Naturally, the images are limited to perspectives external to the building.

The experiments reported on here appear to recreate aspects of the actual WTC fires. Based on available

photographic evidence, NIST reported that the first area in WTC 1 where substantial fire spread and

growth was observed was on the north face of the 97th floor; just to the west (to the right in the

photograph of the aircraft strike point (NIST NCSTAR 1-6A). Figure 3-14 shows that extensive fire was

present at windows 97-1 10 to 97-1 16 around 8:50 a.m., which was almost 3'/2 min after the aircraft impact with the building. NIST found that windows were broken during the aircraft impact and

subsequent fireballs. The fire would subsequently spread from this location to the west face.

Figure 3-14 reveals that at any one time, different locations on WTC 1 exhibited different smoke and

flame yields as they were at different points in the burning process. Just a few minutes after the aircraft collision, the north face of WTC 1 (see Fig. 3-14) resembles early times during the experiments reported on here, when copious amounts of smoke (see Fig. 3-9) and flames (see Fig. 3-10) were emitted from the

compartment. Almost an hour later. Fig. 3-15 shows that similar behavior was displayed on the south

face of WTC 1 . In this sense, the experiments conducted in this study recreated aspects of the WTC fires that can be thought of as typical of fires burning in compartments.

Figure 3-13 shows smoke markings due to deposits of soot-laden gases flowing from the compartment.

The markings take the form of carets (" ^ " shaped black marks) on the outer columns that define the

compartment "windows'" or openings. Smoke deposits in the shape of straight lines on the wall adjacent

to the glass windows was also observed (see Fig. 3-13). The same type of markings were obser\'ed in the

WTC fires. Figure 3-15, for example, shows the caret-like markings near windows on the 95th and 96th

floors of the south face of WTC 1

.

Figure 3-14. Photograph of the north face of WTC 1 recorded at 8:53:15 a.m. on

September 11, 2001. Labels showing floor and column numbers were added to the

original photograph.

NIST NCSTAR 1-5E. WTC Investigation 41
Chapter 3

Figure 3-1 5. Photograph of the south face of WTC 1 taken at approximately 9:41 :30 a.m. on September 11, 2001. Column and floor numbers were added to the original

photograph.

3.6 SUMMARY

During Tests 1 and 2, with the burner next to Workstation 1, all three workstations exhibited reproducible

times to full involvement. In both experiments, Workstation 1 reached full involvement in a little over 400 s. Full involvement for Workstation 2 was achieved 300 s later. Workstation 3 reached full involvement less than 100 s after Workstation 2. In Test 4, with the burner next to Workstation 2,

Workstation 2 reached full involvement in approximately 300 s. Workstations 1 and 3 were fully

involved 150 s later, with Workstation 3 being ignited from radiation.

The time from ignition to full involvement was also significantly affected by burner placement. For

Tests 1 and 2, full involvement occurred 400 s after ignition of Workstation 1, 200 s after Workstation 2

ignition and 250 s after Workstation 3 ignition. In comparison, during Test 4, despite the presence of

ceiling tiles, Workstation 2 was fully involved 250 s after its ignition; with Workstations 1 and 3 being

fully involved 50 and 150 s, respectively, after their ignitions.

The presence of Jet A in Tests 3, 5, and 6 resulted in all workstations being fully involved in a little over 60 s.

42 NISTNCSTAR 1-5E, WTC Investigation
Fire Behavior and Flame Spread

3.7 REFERENCES

Manzello, S. L., Gann, R. G., Kukuck, S. R., Prasad, K. R., and Jones, W. W. 2005. Fire Performance of a Non-Load Bearing Glass Wall Assembly, to be submitted to Fire and Materials.

NISTNCSTAR 1-5E, WTC Investigation 43
Chapter 3

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44 NISTNCSTAR 1-5E, WTC Investigation
Chapter 4

Mass Loss

The rate of burning of combustible items in the compartment controlled the fire heat release rate and,

thereby, the rate of fire spread. The combustibles in the compartment consisted of the workstation

components, jet fuel added to the workstations, and the hydrocarbon spray burner. The rate of mass loss of each of the workstations was measured during each of the tests using several load cells.

4.1 INSTRUMENTATION AND PROCEDURE

Load cells were employed to measure workstation mass loss during each test. Each workstation, its contents, and flooring rested on a platform mechanically isolated from the adjacent floor. Each platform was supported by a network of four load cells, located in the subfloor. Load cells generate an electrical signal proportional to the load acting upon them. Each load cell network had a total weight capacity of

907 kg (2,000 lb). The load cells in the subfloor were thermally protected with both insulation and a

single-pass, water cooling system. Temperature inside each enclosure was monitored to insure that the

thermal environment did not jeopardize data integrity. Nitrogen was pumped at a low flow into each load

cell enclosure to prevent infiltration of corrosive gases. Workstations 1 and 2 employed load cell networks manufactured by Mettler Toledo. Workstation 3 employed a load cell network supplied by The

Scale People. Specifics of each system are hsted in Table 4-1

.

The signals generated by the individual load cells were measured by a load cell terminal for intermediate

display and retransmitted to the data acquisition (DAQ) system. The mass loss rate determined from the

load cell data provides a measure of the fuel gasification rate and allows for computation of the effective

heat of combustion of the workstations and associated materials.

During test preparation, the output from each load cell terminal was monitored to insure that the load cells were not overloaded. The signal outputs from the individual load cells of Workstation 3 were recorded,

while the cumulative load cell outputs were recorded for the network of loadcells associated with

Workstations 1 and 2. Prior to each test, calibration was accomplished by adding a 25 kg weight to each

load cell, which demonstrated proper output of the load cell electronics.

Table 4-1. Load cell specifications.

Type Mettler The Scale People

Load cell model 0958-1-4-X5-2 CW-500

Terminal model JXGA-3080-000 665 Digital Indicator (GSE)

Capacity Four X 227 kg load cells Four X 227 kg load cells Output signal (terminal) 0-10 Voh 0-20 mA

Accuracy 5 % 5 %

NISTNCSTAR 1-5E, WTC Investigation 45
Chapter 4

The mass loss rate determined from the load cell data provided a measure of the fuel gasification rate. In concert with the measurement of the heat release rate, the mass loss data provided a global measure of the

effective heat of combustion of the workstations, which is discussed in Chapter 5.

4.2 RESULTS AND DISCUSSION

Figures 4-1 through 4-6 show the load cell mass measurements for each workstation during each of the

six tests. For purpose of comparison, the output of each load cell was normalized such that each load cell reported 900 kg prior to test initiation. In the following charts, mass data greater than 900 kg prior to time

zero represent artifacts of the experimental procedure, such as personnel moving through the

compartment, arniing the ignition system, or perfonning final checks. For all plots, sudden decreases or increases in mass, as evidenced by nearly vertical data, are the result of material falling onto or off of the

load cell platform. During Test 5, a circuit breaker tripped three minutes after ignition on the circuit powering load cells under Workstations 1 and 2. It took approximately 2.5 min for the power to be

restored and the load cells to return to service.

For further comparison. Fig. 4-7 shows the sum of all tliree normalized load cells for Tests 1 through 6. The curves for the various tests were quite similar with the exception of Test 5, which was unique in terms of the experimental configuration. The mass loss rate or mass burning rate for Test 5 was much

slower than for the other tests. Most rapid mass loss occurred earlier during Tests 3, 5, and 6, when jet fuel was present. When jet fuel was not present, the fastest period of mass loss occurred almost 500 s

later. The power to the load cells under Workstations 1 and 2 failed for 2.5 min at 180 s during Test 5.

05

0) O

03O

950

900

850

800

750

700

650

600

V,

' '

I

'

I

' ' ' ' I

' o Load Cell Cubicle 1 Test 1

- Load Gels Cubicie 2 Test 1 Load Ceii Cubicle 3 Test 1

0 1000 2000 3000 4000 5000

Time (s)

Figure 4-1. The mass of Workstations 1, 2, and 3 as a function of time during Test 1.

46 NISTNCSTAR 1-5E, WTC Investigation
Mass Loss

950

O

TSO

T

1 1 1

1

1 1 1 1

1 1 1 1 1

1

Load Cell Cubicle 1 Test 3 ~ Load Cell Cubicie 2 Test 3

Load Cell Cubicle 3 Test 3

'-H

1000 2000 3000 4000 5000 6000

Time (s)

Figure 4-2. The mass of Workstations 1, 2, and 3 as a function of time during Test 2.

0) O

T3

CCO

950

900

850

800

750

700

650

600

h

^4

o Load Cell Cubicle 1 Test 3

L03Ci C&l'i CuDsCi© Z I ©St S & Load Cell Cubicle 3 Test 3

0 1000 2000 3000 4000 5000 6000

Time (s)

Figure 4-3. The mass of Workstations 1, 2, and 3 as a function of time during Test 3.

NISTNCSTAR 1-5E, WTC Investigation 47
Chapter 4

0) O

t3

CDO

950

900

850

800

750

700

650

0

Load Cell Cubicle 1 Test 4

Load Ceil Cubicle 2 Test 4 Load Ceil Cubicle 3 Test 4

=f^r:~"—

---^—

1000 2000 3000

Time (s)

Figure 4-4. The mass of Workstations 1, 2, and 3 as a function of time during Test 4.

0) O

aCOo

950

900

850 -

800 ^

750 h

700 h

650 ^

600 ^

'

i

' - ' '

I

' ' ' '

I—

—o— Load Cell Cubicle 1 Test 5 —u— Load Celi Cubicle 2 Test 5

Load Celi Cubicle 3 Test 5

-->:>•

_J I I L 1000 2000 3000

Time (s)

4000

Figure 4-5. The mass of Workstations 1, 2, and 3 as a function of time during Test 5. The

power to the load cells under Workstations 1 and 2 failed for 2.5 min at 180 s.

48 NISTNCSTAR 1-5E, WTC Investigation
/Ifass Loss

950

oO

o

o

650

-| 1 r

Load Cell Cubicle 1 Teste

Load Cell Cubscie 2 Test 6

Load Cell Cubicle 3 Test 6

1000 2000

Time (s)

3000 4000

Figure 4-6. The mass of Workstations 1, 2, and 3 as a function of time during Test 6.

0 600 1200 1800 2400 3000 3600 4200 4800

Time (s)

Figure 4-7. The sum of the mass of Workstations 1, 2, and 3 for Tests 1 through 6. The

power to the load cells under Workstations 1 and 2 failed for 2 min at 180 s in Test 5.

NISTNCSTAR 1-5E, WTC Investigation 49
Chapter 4

4.2.1 Total Mass Loss

Table 4-2 shows the total mass loss for each of the load cells from the time of ignition until 3,900 s after ignition during each of the six experiments. The mass loss was small after 3,900 s, and suppression of the

fire occurred shortly thereafter in some tests. The mean total mass loss for the tests, excluding Test 5, was 680 kg with a standard deviation of 3 percent. The total combustible mass for each workstation was

248.7 kg (see Table 2-2) or 251.9 kg in Tests 3, 5, and 6 when jet fuel was present, equivalent to 746.1 kg

for the three workstations or 755.7 kg when jet fuel was present. The mass loss of Workstation 2 in Tests 1 and 6 was very close or exceeded this value, possibly due to shifting of load onto Load Cell 2

from the adjoining workstation as it burned.

In Test 5, the total mass loss was approximately 25 percent less when compared to the mean mass loss of

the other tests. The total mass loss for Load Cell 3 in Test 5 was greater than the total mass loss of Load

Cells 1 and 2. This was because the configuration of the workstations in Test 5 was different from the

other tests. While Workstation 3 was configured in a non-standard marmer, the combustibles in Workstations 1 and 2 were "shielded" from the fire, as the panels and desk surfaces were laying

horizontally on top of each other. The mass loss in Test 5 for Workstations 1 and 2 were about 40 percent

less than the workstations burning in the standard configuration.

Table 4-2 suggests that there was no discemable effect of the jet fuel (Tests 3, 5, and 6), the ceiling tiles (Tests 3, 4, and 5), or ignition burner location on the total mass loss of the combustible portion of the

workstations. From the data in Table 4-2, the mean total mass loss (excluding Test 5) was 681 kg for the

tliree workstations or 227 kg per workstation. This was about 14 percent larger than that measured during

the single workstation experiments (NIST NCSTAR 1-5C). The mass lost represents about 90 percent of

the total combustible mass.

Table 4-2. Total mass loss for the three workstations from t = 0 s until 3,900 s. Mass Loss (kg)

Test 1 Test 2 Test 3 Test 4 Tests Test 6

Load 1 196 226 214 243 147 205

Load 2 265 232 241 230 121 248

Load 3 212 216 252 220 237 207

Sum" 673 + 34 674 ± 34 707 + 35 693 + 35 505 + 20 660 ±33

a. And calculated standard deviation.

4.2.2 Mass Loss Rate

Figure 4-8 shows the time varying mass for each of the three load cells during Test 1. The mass loss rate

is the temporal derivative of the time varying mass taken from the load cell data. The mass loss rate measurements could be naturally partitioned into three distinct time periods: a warai-up period (At^), a period of rapid burning (Atr), and a longer, slow burning period (Ats). Figure 4-8 shows the three burning

periods for the mass loss of Workstation 1 during Test 1 . The transient mass loss rate during each period was approximated as a straight line. This approach minimized noise in the mass loss rate determination,

while adequately representing its value. It was possible to apply this method to determine the mass loss

rate for each of the load cells for each of the tests except for Workstation 3 in Test 5 in which the mass

loss rate behavior during the rapid burning period was atypical and not easily represented by a single

50 NIST NCSTAR 1-5E, WTC Investigation
Mass Loss

straight line fit. In that case, the mass loss rate was estimated as the average rate of mass loss over a 1,000 s period. Table 4-3 presents the mass loss rates during the rapid burning and the slow burning

periods for each of the load cells for Tests 1 through 6. The mean mass loss rates during the rapid

burning period for Workstations 1 through 3, excluding Test 5, was -0.19 kg/s, -0.21 kg/s, and -0.19 kg/s,

respectively, yielding a mean value of -0.20 kg/s. The mean mass loss rates during the slow burning

period for Workstations 1 through 3, excluding Test 5, was -0.009 kg/s, -0.010 kg/s, and -0.01 1 kg/s,

respectively, yielding a mean value of -0.010 kg/s. Test 5 was excluded from the calculation of the mean

value due to the atypical configuration used in that test. The standard relative uncertainty in the mass loss

rate was estimated as 15 percent and 30 percent for the rapid and slow burning periods, respectively. The

mean mass loss rate during the rapid burning period was nearly 20 times larger than during the slow

burning period. The variation in the mass loss rate during the slow burning period was larger than during

the rapid burning period. The mass loss rate results are nearly the same for each of the three workstations

during both the rapid and the slow burning periods.

950

CD

Q) O

D

o

-3— Load Cell Cubicle 1 Test 1 ^li"— Load Cell Cubicle 2 Test 1 A Load Cell Cubicle 3 Test 1

600 _l I L_ I I L 1000 2000 3000

Time (s)

4000 5000

Figure 4-8. The mass of Workstations 1, 2, and 3 as a function of time during Test 1,

showing straight line fits to each of the three burning periods for Workstation 1. The

data is taken from Fig. 4-1.

NISTNCSTAR 1-5E, WTC Investigation 51
Chapter 4

Table 4-3. The mass loss rate for each of the workstations during the rapid and slow

burning periods for Tests 1 through 6. Mass Loss Rate during the rapid burning

period, Atr (kg/s)"

Mass Loss Rate during tiie slow burning period,

At. (kg/s)"

Test Workstation

1 Workstation2 Workstation

3 Workstation1 Workstation

2 Workstation

3

1 -0.17 -0.24 -0.11 -0.0087 -0.013 -0.021

2 -0.18 -0.21 -0.17 -0.010 -0.0080 -0.012

3 -0.17 -0.22 -0.20 -0.0054 -0.0021 -0.0038

4 -0.21 -0.18 -0.22 -0.011 -0.017 -0.0098

5 -0.085 -0.055 -0.17 -0.022 -0.019 -0.012

6 -0.21 -0.22 -0.23 -0.010 -0.011 -0.0076

Average'' -0.19+ 0.03 -0.2 1± 0.04 -0.19+ 0.06 -0.00901 0.005 -0.010+0.007 -0.01 1± 0.009

a. The standard relative uncertainty in the mass loss rate was estimated as 15 percent and 50 percent for the rapid and slow

burning periods, respectively,

b. Excludes Test 5.

The mass loss rate results for Test 5 were atypical, particularly for Workstations 1 and 2, which were

comprised of unassembled workstations that were configured (in an identical manner except for the

addition of drywall to Workstation 1 ) to simulate piles of rubble (see Figs. 2-6 and 2-7). The mass loss

rates for Workstation 3 in Test 5 were similar to the mean values for Workstation 3 in the other tests. This is attributed to the fact that the configuration used for Workstation 3 in Test 5 was much more

similar to the configuration used for the workstations in the other tests, except that the furniture was

displaced and pushed from its standard configuration as seen in Fig. 2-8. The mass loss rates during the

rapid burning period for Workstations I and 2 in Test 5 were 26 percent and 45 percent, respectively, of

the mean values of the mass loss rates in the other five tests. The mass loss rates during the slow burning

period for Workstations 1 and 2 in Test 5 were 240 percent and 190 percent, respectively, of the mean

values of the mass loss rates in the other five tests. The rapid burning period, however, dominated the mass loss rate behavior, and the total mass loss in Test 5 was significantly smaller than in the other tests as seen in Table 4-3.

4.3 SUMMARY

Measurements of the total mass loss and the mass loss rate are indicators of the fire intensity as materials

bum and release heat. The results showed that there was no discemable effect of the jet fuel (Tests 3, 5, and 6), the ceiling tiles (Tests 3, 4, and 5), or ignition burner location on the total mass loss of the

combustible portion of the workstations. The mass loss burning rate for workstations configured such

that the combustibles were "shielded" from the fire was about 40 percent less than the mass loss rate for

workstations burning in the standard configuration.

The next section of this report describes the measured heat release rate and its relationship to the mass

loss rate data. Whereas the mass loss rate data was independently measured for Workstations 1, 2, and 3, the heat release rate measurement represents the heat generated from all of the combustibles within the

compartment and exemplifies the global fire behavior.

52 NISTNCSTAR 1-5E, WTC Investigation
Chapter 5

Heat Release Rate

The heat release rate of a fire is a critical experimental parameter, as it represents the amount of energy

per unit time that is generated by the fire. The value of the heat release rate drives the thermal

environment, including the radiative and convective heat transfer, the fluid flow, the thermal insult onto secondary items such as structural members, the rate of fire spread and growth, the volume of smoke and

toxic products, and uhimately, the hazard associated with a fire. This chapter is divided into two parts.

First, the heat release rate determination from oxygen consumption calorimetry is discussed. Then, the mass loss rate information from Chapter 4 is used to determine the effective heat release rate per unit mass of combustibles. Measurement uncertainty is discussed for both measurements.

5.1 OXYGEN CONSUMPTION CALORIMETRY

5.1.1 Description of the Heat Release Rate Measurement Facility

The fire heat release rate ( Q) was measured using oxygen consumption calorimetry in the standard manner through measurement of exhaust products in the 6 m by 6 m hood. This form of fire calorimetry was used by Huggett (1980), who showed that the oxygen deficit in the duct flow (relative to ambient air)

is a measure of the heat release rate of a fire. Parker (1982) and Janssens and Parker (1995) discuss the

details of the heat release rate calculation.

Bryant et al. (2003) give a comprehensive description of the experimental methodology, hardware,

instrumentation, calibration, experimental procedures, and uncertainty. Much of this discussion borrows

from that reference. The facility was designed to provide quantitative results with minimum uncertainty.

A large number of instruments were employed in the heat release rate measurement (see Table 5-1),

including the measurement of temperature and pressure in the exhaust duct.

The combustion products from the compartment flowed into the 9 m by 12 m exhaust duct where the gas

flow was characterized. The exhaust gas was sampled in the exhaust duct and transported to the

instruments in a control room for measurements of the oxygen, carbon dioxide, and carbon monoxide

concentrations. The relative humidity was measured in the test bay area, and a correction was made for ambient humidity as an estimate of the water concentration in the duct flow. The computation of heat

release rate (Q) was made following Bryant et al. (2003):

2 l + ^{a-\) ^ 02

(5-1)

with

(1 — ^02 ~ ^C02 ~ ^CO )^02

NISTNCSTAR 1-5E, WTC Investigation 53
Chapter 5

Ehc ^ heat of combustion of hydrocarbon fuel

Eco - heat of combustion of carbon monoxide

^ = oxygen depletion factor

= mass flow rate in exhaust duct

a

M,

X,

mass flow rate of air

combustion products expansion factor

molecular weight of species i .

exhaust-gas concentration measurement of species i

X" = ambient-gas concentration measurement of species i

Tab e 5-1. Instruments used in the calorimeter system.

Parameter

Measured Instrument Type Make and Model Range

Oxygen Paramagnetic Servomex 540 0%-21 % v/v

Carbon dioxide Infrared extinction Siemens Ultramat 5E 0 %-5 % v/v

Carbon monoxide Infrared extinction Siemens Ultramat 5E 0 %-3 % v/v

Relative humidity Thin film capacitance Dickson THDx 0 %-95 %

Temperature Thermocouple Omega K-type 0 T-1,350"C

Exhaust velocity Bi-directional probe MKS model 220 differential pressure transducer OPa-lOPa

5.1.2 Calibration and Measurement Uncertainty

The relative expanded uncertainty of the measured heat release rate in the 9 m by 12m calorimetry

system was 0.15 (NIST NCSTAR 1-5C). A propagation of error analysis showed that the dominant

source of uncertainty was measurement of the oxygen concentration, the exhaust mass flux, and the

assumed value of the heat of combustion of the fuel (Bryant et al. 2003). The expanded uncertainty (with

a coverage factor of 2) corresponds to a level of confidence of 94.5 percent. The term "relative" means

that the uncertainty is expressed as a ratio of the uncertainty to the measured value, and the term

"expanded" means that the uncertainty represents two times the standard deviation of the uncertainty.

Calibration bums using natural gas were conducted several times during the test series to assure accurate measurement of the heat release rate ( Q ). The calibration burner used natural gas flowing at a measured

and controlled rate. This provided an independent measurement of the heat release rate to compare to the

measurement by oxygen consumption calorimetry. The heat output of the burner was held constant for

3 min to 5 min at each setting. The measured Q was typically within 10 percent of the expected heat

release rate based on the natural gas mass flow rate.

54 NIST NCSTAR 1-5E, WTC Investigation
Heat Release Rate

The calorimetry measurement response time was on the order of 15 s (Bryant et al. 2003). The volume of

the test compartment also affected the time response of the measurement, as filling and mixing caused

time-averaging and a lag in the calorimetry results relative to the other measurements.

5.2 RESULTS AND DISCUSSION

Figure 5-1 shows the heat release rates as a function of time for Tests 1 through 6. The fuel spray burner was shut after 120 s when jet fuel was present (Tests 3. 5, and 6); otherwise, it was shut after 600 s. The

results show that the heat release rate ( Q) obtained peak values that varied from nearly 1 1 MW to 16 MW. The peak value of Q was obtained earlier in those tests when jet ftjel was present (Tests 3, 5, and 6). The noise or range of scatter in the data was on the order of 200 kW for all of the tests. The

relative expanded uncertainty of the heat release measurement was estimated as 15 percent (see NIST

NCSTAR 1-5C).

CD (T

0

{/) CD

a:

"cD <U X

16000

14000

12000

10000

8000

6000

4000

2000

0

T-T I II I I I I I I

I

I I I I

I

I I -©—Test 1 HRR (kW)

-Test 3HRR (kW)

-Test 4 HRR (kW)

—Test 5 HRR (kW)

_j I I Lj I I I Li I i i i i i i 'i I l _l i _i ... 0 500 1000 1500 2000 2500 3000 3500

Time (s)

Figure 5-1. The heat release rate as a function of time for Tests 1 through 6.

The largest value of the heat release rate was almost 1 6 MW, which was measured in Test 6. This test was configured with four glass window panels located to the left of the vents on the west wall of the

enclosure. The lower left single-pane glass window cracked and fell out of the frame approximately 185 s after ignition, and the upper did the same about 212 s after ignition. Manzello (2005) discusses the

window behavior during this test in detail. Breakage of the windows pro\'ided relatively higher amounts

of ventilation than in the other tests and was probably responsible for the higher peak heat release rate observed during this test. The double-pane glass windows on the right never fell out.

The heat release rate measurements complemented the mass loss measurement results. The load cell data

for all of the tests, presented in Chapter 4, show that the largest mass loss occurred simultaneously in all three workstations, during the fast burning period. The peak heat release rate also occurred during this

NIST NCSTAR 1-5E, WTC Investigation 55
Chapter 5

period. Typical results are shown in Fig. 5-2, in which the mass loss and heat release rate ( ^ ) are plotted as a function of time during Test 1. Measurement during the single open workstation bums

yielded peak values of Q equal to 8 MW to 9 MW, depending on the test conditions (NIST

NCSTAR 1-5C), with the spray burner contributing 2 MW. The peak Q values for Tests 1 through 6 were small relative to what might be expected if each workstation simultaneously produced 8 MW to 9 MW. Over-ventilated fires simultaneously involving all three workstations undergoing the fast burning

period, should have yielded a total of 24 MW to 27 MW. The relatively small value of the measured peak

heat release rate seen in Fig. 5-1 as compared to this reference value suggests that the compartment fire was ventilation limited.

Table 5-2 lists the total heat release, which represents integration of the measured time dependent heat

release rate from the time of ignition until 3,900 s in each of the six experiments. For most of the tests, the heat release rate was relatively small after 3,900 s. Table 5-2 shows that the average of the total heat

release for the tests, excluding Test 5, was 15.8 GJ with a standard deviation of 13 percent. A portion of

the total heat release was associated with items other than the workstations, including the heptane spray

fire and the wall lining material. The contributions by the jet fiael and the ceiling tiles are also discussed.

cn

CO

CO

CD

0O

T3m

o

CD2-

C0

CD

0)

(/> CD

s.

CO

2000 3000

Time (s)

Figure 5-2. The load cell mass loss and the heat release rate (^) as a function of time

during Test 1. The rapid burning (Atrb) and slow burning (Ats) periods are indicated.

56 NIST NCSTAR 1-5E, WTC Investigation
Heat Release Rate

Table 5-2. Total heat release.

Test (GJ)

^^orrecieu lur Spray Fire (GJ)i^orretieii lor opr^y rirc, .iti Fuel, Tiles & Wall Lininjj (C.I)

1 16.5 15.3 1 1.7

2 12.8 11.6 13.4

3 14.6 14.3 12.7

4 14.0 12.8 12.7

5 8.8 8.6 8.2

6 11.3 11.1 10.8 Average (Tests 1-4, 6) 13.8 (+ 14 %) 13.0 (+ 14%) 12.3 (± 8 %)

a. Using the definition that the total energy is released between the time of ignition and the time when the HRR fails to 2 MW.

The heptane spray burner provided 2 MW of power for varying amounts of time, depending on the test: 120 s in Tests 5 and 6, 150 s in Test 3, and 600 s in Tests 1, 2, and 4, yielding 0.24 GJ, 0.30 GJ, and

1.2 GJ of energy during the tests, respectively. Table 5-2 shows the value of the total heat release

measurement after correction for the energy associated with the heptane spray fire.

During Tests 1 and 3, the compartment lining material may have contributed to the measured heat release

rate. In Test 1, the three layers of the wall and ceiling lining material were exposed to heat for the first time. Before Test 3, the exposed layer of lining material was replaced because it was cracked and

breaking. The lining material was composed of calcium silicate sheets, which contained 4 percent to 8 percent organic fibers that release energy upon heating. Lyon (2004) used a micro-calorimeter to measure the heat content of a sample of the lining material. Their measurements showed that under

aerobic conditions the total heat release for a virgin calcium silicate sample was 0.68 kJ/g ±5 percent.

Walters and Lyons also measured a fire-exposed sample extracted from the compartment after Test 6,

which was measured to have a value nearly equal to 0 kJ/g. The mass of calcium silicate for each layer in the compartment was estimated as 1,750 kg, which had the potential to yield 1.2 GJ of energy. The

hypothesis that the lining material contributed to the heat release rate was supported by the video record

made during the tests, which clearly showed that during Test 1 the walls glowed red hot. This type of

visible evidence was not present in the video record for Test 2, in which the organic fiber content of the

lining materials presumably would have been reduced due to previous fire exposure. This hypothesis may

explain the difference in the heat release rate (and its integrated value) between Tests 1 and 2 as noted in Fig. 5-1 and Table 5-2. If all of the calcium silicate lining material underwent pyrolysis over a 20 min

(1,200 s) period (during the fast burning period, for example), then the lining material would have been

responsible for 3 MW of the measured heat release rate observed during that period and 3.6 GJ of energy

over the duration of the test.

During Tests 3 through 5, 138 ceiling tiles (per test) were distributed throughout the workstations and the

carpeted hallway. In addifion to the wall lining material discussed above, Lyon (2004) measured the heat content of a ceiling tile sample using the micro-calorimeter. Their measurements showed that under

aerobic conditions, the total heat release for an unexposed ceiling tile sample was 1.07 kJ/g ± 5 percent.

Each tile weighed 0.51 kg, indicating that approximately 0.13 GJ of potential energy was associated with

the tiles.

NISTNCSTAR 1-5E, WTC Investigation 57
Chapter 5

The contribution of the jet fuel spilled before Tests 4 through 6 was a negligible contribution to the total heat release. The heat of combustion of the jet fuel was 42.6 kJ/g (Hamins et al. 2003), so the 13 L

(10 kg) ofjet fuel would be expected to contribute 430 MJ to the fires. If the jet fuel burned in a steady manner for about 1 min, then the heat release rate associated with jet fuel in all three workstations would

be expected to be about 7 MW. This number is consistent with the measurements shown in Fig. 5-1, in which the initial heat release rate is on the order of 10 MW for the tests with the dispersed jet fuel. Approximately 2 MW of the heat release rate for early times in Tests 3,5, and 6, as shown in Fig. 5-1, can be attributed to the spray burner.

Tests 1 and 3 had the largest total heat release values (from Fig. 5-1) as seen in Table 5-2. In Test 5, the

total heat release was 1 1.6 GJ or approximately 73 percent of the mean of the other tests. Although the mass of combustibles in Test 5 was the same as the other tests, the configuration in Test 5 was arranged

such that the combustibles were shielded from the fire to simulate "rubble." The results in Table 5-2

complement the mass loss data shown in Table 4-2. It was not surprising, therefore, that the total heat

release was significantly lower in Test 5 than in the other tests.

As mentioned above, the total heat release was impacted by the heptane spray fire and to a lesser extent by the presence of the jet fuel, the wall hning material, and the ceiling tiles, depending on the test conditions. Table 5-2 lists corrected values for the total heat release, in which the energy associated with

the spray burner has been subtracted. In a separate column, the total heat release has been corrected for the energy associated with the spray burner, the jet fuel, the ceiling tiles and the lining material. The

correction for the lining material assumes that all previously unexposed lining material completely

pyrolyzed, contributing energy to the fire. Table 5-2 shows that the average value of the total heat release

for Tests 1 through 4 and 6 decreased from 15.8 GJ to 13.8 GJ once the corrections were applied. In

principle, the corrected value represents the measured heat release from the furnishings only, for all of the

tests. This notion is supported by the fact that, after the corrections were applied, the standard deviation

among the tests reduced from 1 3 percent to 8 percent and the total heat release measured in Tests 1 and 2 were nearly equal.

The total heat release and the mass loss measured during the single open workstation bums yielded

approximately 3.9 GJ and 200 kg on-average, respectively, which varied somewhat depending on the test conditions (NIST NCSTAR 1-5C). For the three workstations burning in the experiments reported here,

the measured total heat release and mass loss per workstation were equal to 4.6 GJ and 227 kg,

respectively, as derived from Tables 5-2 and 4-2. These somewhat larger values are consistent with the

idea that radiation from a well-developed hot upper layer and the walls of the compartment radiated onto

the fuel, leading to faster burning and more complete combustion.

5.3 REFERENCES

Bryant, R., Ohlemiller, T., Johnsson, E., Hamins, A., Grove, B., Guthrie, W. F., Maranghides, A., and

Mulholland, G. 2003. The NIST 3 Megawatt Quantitative Heat Release Rate Facility - Procedures

and Guidance, NIST Special Report 1007, National Institute of Standards and Technology,

Gaithersburg, MD, December.

Galbraith Labs. 2003. Report of Test Results. Knoxville, TN, March.

58 NIST NCSTAR 1-5E, WTC Investigation
Heat Release Rate

Hamins, A., Maranghides, A., and Mulholland, G. .2003. The Global Combustion Behavior of 1 MW to 3 MWHydrocarbon Spray Fires Burning in an Open Environment, NISTIR 7013, National Institute of Standards and Technology, Gaithersburg, MD, June.

Huggett, C. 1980. Estimation ofthe Rate ofHeat Release by Means ofOxygen Consumption, Journal of

Fire and Materials, 12, 61-65.

Janssens, M., Parker, W. J. .1995. "Oxygen Consumption Calorimetry," in Heat Release in Fires,

(Babrauskas, V., Grayson, S. J., Eds., E & FN Spon), pp. 31-59, London, UK.

Manzello, S. L., Gann, R. G., Kukuck, S. R., Prasad, K. R., and Jones, W. W. 2005. Fire Performance of

a Non-Load Bearing Glass Wall Assembly, to be submitted to Fire and Materials.

Parker, W. J. 1982. Calculations ofthe Heat Release Rate by Oxygen Consumptionfor Various

Applications, NBSIR 8 1 -2427, National Bureau of Standards.

Lyon, R. E. 2004. Report ofMicro-Calorimeter Tests, Federal Aviation Administration, William J. Hughes Technical Center, NJ.

Walters, R. N., and Lyon, R. E. 1997. Proc. 42"'' Int. SAMPE Symposium and Exhibition, 42(2),

1335-1344.

NISTNCSTAR 1-5E, WTC Investigation 59
Chapter 5

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60 NIST NCSTAR 1-5E, WTC Investigation
Chapter 6

Gas Temperature

6.1 INSTRUMENTATION

Thermocouple trees were constructed by National Institute of Standards and Technology (NIST)

personnel and located at four locations inside the compartment. Appendix A lists the coordinate locations

of each of the thermocouples. Thermocouples were placed at four positions, namely, 2.5 cm, 91 cm,

83 cm and 274 cm below the ceiling. The thermocouple beads were located on individual horizontal

"branches" approximately 6 cm from the vertical tree support. Each tree was suspended from the ceiling

and attached to the floor with a screw to provide tension for maintaining vertical alignment. During

Tests 1 and 2, a number of aspirated thermocouples were positioned near Tree 4 to assess the effect of

radiative exchange on the bare bead thermocouple measurements. For these tests (Tests 1 and 2),

thermocouples were placed on Tree 4 near the aspirated probes, 213 cm and 305 cm below the ceiling.

The intense temperatures encountered during the test series, as well as mechanical damage during

compartment preparation, occasionally caused a thermocouple to fail. After each test, the thermocouple

extensions were visually inspected and were observed to be undamaged. Applying heat to the

thermocouple junctions while monitoring the data acquisition system output assured proper function of

the thermocouples. This insured that theimocouples were functioning, located properly, wired correctly

and located on the proper channel of the data acquisition file. Thermocouples that did not function

properly were replaced.

Thermocouples used in Test 1 employed high-temperature glass fiber insulation rated to 704 °C. Except

for the thermocouple junction and minimal thermocouple insulation, thermocouples within the

compartment were wrapped with mineral wool insulation with embedded, single pass water-cooling lines. Temperatures at all four thermocouple trees during Test 1 exceeded 700 °C, and the exposed portions of

the thermocouples showed severe themial damage. As a result, for Tests 2 through 6, thermocouples

insulated with ceramic insulation (Nextel) were installed along the outside of the existing thermocouple

trees. In all tests, the thermocouple conductors were type K, 24 gage chromel and alumel junctions of

"special limits of error" (SLE) wire. Chromel is an alloy of primarily nickel and chromium, whereas

alumel is comprised primarily of nickel, aluminum, manganese and silicon.

Two aspirated thermocouples were installed adjacent to two tree thermocouples during Tests 1 and 2. The aspirated thermocouples were useful for assessment of the thermocouple measurement error due to radiative exchange. Aspirated thermocouples are constructed such that the thermocouple junction is within two concentric steel tubes. Air was drawn through the tubes to engulf the sensing junction with

sample gas. Concurrently, the concentric tubes protected the thermocouple sensing junction from

impinging thermal radiation providing a "true" gas temperature. Further details can be found in Blevins ( 1 999) and Blevins and Pitts ( 1 999).

NISTNCSTAR 1-5E, WTC Investigation 61
Chapter 6

6.2 UNCERTAINTY

Several sources of error must be considered in order to understand the behavior of a thermocouple signal

and to estimate the thennocouple measurement accuracy. Thermocouple measurement errors can be due

to breakdown of the thermocouple insulation at high temperatures, decalibration at high temperatures,

inherent measurement accuracy limited by materials effects, and error due to radiative exchange.

High temperatures decrease the resistance of insulation materials. A shunt can occur when the electrical resistance of the insulation approaches that of the thermocouple circuit. Shunt impedance can occur,

especially in environments where long, small conductor thermocouples are installed within extreme

temperature environments (Omega 2004). The resulting shunt completes the thermocouple circuit at a point other than the thennocouple junction, resulting in erroneous temperature measurements.

A number of reports list decalibration as a source of thermocouple measurement error due to high

temperature oxidizing conditions (Bums et al. 1993; Omega 2004). At elevated temperatures, the crystal

lattice of the conductors can undergo physical change. These uncertainty changes facilitate permeation of

chemical species from the air into the conductors ultimately changing the chemical composition of the

conductors. The end result is a deviation from the reference function used to convert the voltage to temperature. Multiple manufacturers report techniques and equipment that can be employed to eliminate, or at a minimum quantify, the decalibration. An alternative technique to address this issue is frequent

replacement of the thermocouples, which was employed during this test series.

The manufacturer reported accuracy for thermocouples employing SLE wire is 1.1 °C for temperatures

between 0 °C and 293 °C. and 0.4 percent between 293 °C and 1,250 °C. Type K thermocouples are capable of measuring temperatures as high as 1,350 °C for brief periods of time, although this may lead to changes in the thermocouple calibration (Bums et al. 1993). These uncertainty values are interpreted as the 95 percent confidence interval or an expansion factor of two for expression of the uncertainty. At

temperatures greater than 870 °C, the rating of the thermocouple conductors is exceeded, but the

thennocouples are still capable of measuring temperature albeit subject to more rapid deterioration.

Given that the exposures to these temperatures were brief, the uncertainty specifications were multiplied

by a factor of two at temperatures greater than 870 °C as a conservative estimate of accuracy degradation.

In this study, data from thermocouples that experienced temperatures above 1,350 °C were considered

inaccurate and are not reported. The standard uncertainty values are listed in Table 6-1 as |lIw

Table 6-1. Component and expanded temperature measurement uncertainty.

Combined Expanded

Temperature Mr Ma Uncertainty }ic Uncertainty (°C)

(°C) (°C) (°C) (°C) (°C)

' (k=2)

0 1.1 5.6 0 6 11 120 l.I 5.6 1 6 12 150 1.1 30 1 30 60

870 1.8 30 5 30 61 900 3.6 61 5 61 122

1,350 5.4 61 7 61 122

62 NISTNCSTAR 1-5E, WTC Investigation
Gas Temperature

Blevins (1999) reported on the development of a model to compute radiation effects on thermocouple

error. Radiation exchange between the thermocouple and walls, flame gases, soot, and ambient

environment being the predominant sources reported. In the same document, report of soot accumulation on the themiocouple bead or sensing junction and changing its thermo-physical properties is also credited

with contributing to error. Furthermore, variability in the convective heat transfer between the sample gas

and the thermo-junction is cited as a source of error. To estimate the magnitude of the measurement error due to radiative exchange, a number of thermocouple measurements were compared to measurements by

nearby aspirated themiocouples. Figures 6-1 through 6-4 compare the aspirated thermocouples and

adjacent bare bead type K thermocouples in Tests 1 and 2. Figure 6-1 shows that during Test 1 , the bare

bead thermocouple 213 cm below the ceiling on Tree 4 recorded a sharp decrease in temperature starting

979 s after ignition, as compared to the aspirated thermocouple. The thermocouple appears to recover and

match the aspirated thermocouple measurement during the period from 1,250 s to 1,350 s. Other

thermocouples on Tree 4 do not exhibit similar behavior to the bare bead thermocouple at 1.250 s. Figures 6-2 through 6^ show that the other pairs of aspirated thermocouples and bare bead

thermocouples experienced some amount of deviation from each other, but generally tracked each other

to within 70 °C. This led to the conclusion that this thermocouple experienced some type of failure -

perhaps chromium oxidation in the positive leg of the thennocouple circuit or a failure of the insulation,

at least for the 280 s of data following the 979 s mark. For this reason, that data were not used in subsequent analysis.

1200

CE 1000

CD ^

o

CD O

o. 00

Q)

800

600

400

200

~i 1 r T 1 1 r

Test 1 Bare Bead

Test 1 Aspirated

2000

Time (s)

4000

Figure 6-1. Comparison of bare bead and aspirated thermocouple measurements as a function of time at a location 213 cm below the ceiling on Tree 4 during Test 1.

NISTNCSTAR 1-5E, WTC Investigation 63
Chapter 6

1200

CO

•E 1000

oO

CDO

=5 ^

2 E

a5 "

a. in

E o

q3 ^

^ ^'

CD

800

600

400

200

0

T i 1 T "I 1 r "1

i r

Test 1 Bare Bead

Test 1 Aspirated

1000 2000

Time (s)

3000 4000

Figure 6-2. Comparison of bare bead and aspirated thernnocouple measurements as a function of time at a location 305 cm below the ceiling on Tree 4 during Test 1.

Oo

1000

CD = 800

<DO

2? 0

=5 JQ

CD O

CL CO

CD

CD

600

400

200 -

J i I L_ 1000 2000

Time (s)

3000 4000

Figure 6-3. Comparison of bare bead and aspirated thermocouple measurements as a

function of time at a location 213 cm below the ceiling on Tree 4 during Test 2.

64 NISTNCSTAR 1-5E, WTC Investigation
Gas Temperature

Oo

1000

1 800

O

=5 ^

2 E

CD O

Q. to E o

a5 <^0

0)

600 -

400

200

"T 1 1 T "T 1 1 1 1

1 1 1 1 1

1 1 1 1 [

1—I

I—

I—I—I—I—I—

I

\—I

'—

r

Test 2 Bare Bead

Test 2 Aspirated

J 1 1 1

1 1 1 I I I I I 1 I I I I I I I I 1 I I I I I I I I I 1 I I L 0 500 1000 1500 2000 2500 3000 3500 4000

Time (s)

Figure 6-4. Comparison of bare bead and aspirated thermocouple measurements as a function of time at a location 305 cm below the ceiling on Tree 4 during Test 1.

To estimate the standard thermocouple error due to radiation, the temperatures recorded by the aspirated

thermocouples were subtracted from the temperatures recorded by the adjacent bare bead thermocouples.

The differences calculated for each of the two thermocouples for both Test 1 and Test 2 were combined

and plotted as a function of the bare bead thermocouple reading and are shown in Fig. 6-5. The data

spread was evaluated statistically in three distinct ranges to estimate the thermocouple radiation error. Below 130 °C, approximately 9,300 data points have an average value of 9 °C and a sample standard

deviation of 6 °C. Between 130 °C and 870 °C, the rated maximum of the thermocouple conductors, over 15,000 data points average -2 °C with a sample standard deviation of 30 °C. For temperatures greater

than 870 °C, the computed average of roughly 1 ,600 data points equals -26 °C with a sample standard

deviation of 60.6 °C. These results are used as approximations of the standard error due to radiation

within the temperature ranges of interest and are reported in Table 6-1 below as \Xr. An additional

uncertainty associated with the aspirated probe measurement itself is listed in the table as |ia with a value

taken as approximately 7 at 1,350 "C, which varied inversely with temperature (Blevins 1999).

Table 6-1 lists individual component uncertainties for the bare bead thermocouple measurement. The

component uncertainties in Table 6-1 were added in quadrature to compute the combined uncertainty, jic. The expanded uncertainty, equivalent to two standard deviations, is also listed in the table. The majority

of the combined thermocouple measurement uncertainty is due to radiation.

NISTNCSTAR 1-5E, WTC Investigation 65
Chapter 6

Cl3

O

u

oE

o

'-a

(U CD ^ -a. ^ <

b '

E 8

CD O

E ^

CD

(D 00

CU CO

150

100

50

0

-50

-100

-150

-200

~i I r T 1 r

J I L J I L_ 200 400 600 800 1000 1200

Temperature (°C) Bare Bead Thermocouple

Figure 6-5. The difference between the bare bead and the aspirated thermocouple

measurements as a function of the bare bead temperature for the data presented in

Figs. 6-1 through 6-4.

6.3 DISCUSSION AND RESULTS

Figures 6-6 through 6-1 1 and Figs. 6-12 through 6-17 show the bare bead thermocouple measurements

on Trees 1 and 4, respectively, during Tests 1 through 6. Tree 1 was located in the middle of the front of

the compartment at (x, y) = (285 cm, 351 cm), in the passageway adjacent to Workstation 1, whereas

Tree 4 was located at the rear of the compartment at (x, y) = (969 cm, 575 cm), almost 3 m from

Workstation 3, which was the nearest workstation. Figure 2-2 provides a plan view of the compartment

and the instrumentation, and Appendix A presents the coordinates of the thermocouple trees and other

important instrumentation.

Figures 6-6 through 6-1 1 show that early in the tests, the highest temperatures were measured at locations closer to the ceiling. As time continued, the upper layer descended, and the readings of the

lower themiocouples were not unlike the upper thermocouples. At early times in the tests, the

temperatures increased at a faster rate when jet fuel was dispersed throughout the compartment (Tests 3,

5, and 6). At later times, this pattern broke down. A number of the thermocouples failed during the tests.

During Test 2 for example, the bare bead thermocouple 2.5 cm below the ceiling on Tree 1 recorded a sharp increase in temperature starting 1,000 s after ignition. The temperature reading exceeded 1,400 °C,

a sign of possible failure for a type K thermocouple. The temperatures recorded by the other

thermocouples on Tree 4 during Test 1 show that the behavior was unique to the thermocouple located

2.5 cm from the ceiling. For this reason, the data following the 1,000 s mark are not plotted in the figure.

Other failures occurred during the other tests, and the data are handled in a similar fashion.

66 NISTNCSTAR 1-5E, WTC Investigation
Gas Temperature

Test 2 was a repeat of Test 1. The peak temperatures measured in Test 1 were similar to those in Test 2. A comparison of Figs. 6-1 and 6-2 to Figs. 6-3 and 6-4 show that the peak aspirated thermocouple

temperatures at 213 cm and 303 cm from the ceiling on Tree 4 were about 50 °C higher in Test 1 than Test 2. On the other hand, a comparison of Figs. 6-6 and 6-7 show that the peak bare bead thermocouple

measurements in Test 1 were 30 °C to 100 °C lower than those in Test 2 at Tree 1 . With the measurement

uncertainty of the bare bead thermocouples nearly 120 °C (Table 6-1), these differences in the peak

temperatures cannot be considered significant.

Figures 6-8 through 6-1 1 show the bare bead thermocouple measurements on Tree 1 during Tests 3 through 6. The three thermocouples closest to the ceiling typically obtained peak temperatures over

1,000 °C to 1,100 °C except in Test 5 in which the peak temperatures were lower likely due to lower heat

release rate obtained during that test. The temperature measurements in the rear of the compartment were

generally smaller as seen in Figs. 6-12 through 6-17, which represent bare bead thermocouple

measurements on Tree 4 during Tests 1 through 6. These results indicate that the compartment

temperatures closer to the windows obtained the largest peak temperatures, a result that was not unexpected. Appendix C contains plots of the thermocouple measurements on Trees 2 and 3 during

Tests 1 through 6.

The time dependent temperature data reported here will be useful for comparison to that predicted by the

Fire Dynamics Simulator and will be discussed in Chapter 8.

Time (s)

Figure 6-6. Bare bead thermocouple measurements as a function of time at four

locations (below the ceiling) on Tree 1 during Test 1.

NISTNCSTAR 1-5E. WTC Investigation 67
Chapter 6

oo

CDQ.

E

CD

1200

1000

800

600

400

200

0

I [[ I II

I I I I I

! I

I I

I

I I I I

I

I I I I

I I

I I I I

I

I I I

I I I I I I I 'i I I I I I I I

'

I I

'

I I

' I I I I 1 I I I I I I I I I I 500 1 000 1 500 2000 2500 3000 3500 4000

Time (s)

Figure 6-7. Bare bead thermocouple measurements as a function of time at four

locations (below the ceiling) on Tree 1 during Test 2.

O 1000

o

CD

i_

ZJ

-t—> cc0

a.E

CD

—0—-2.5 cm

-91 cm

- 1 83 cm — 274 cm

I I 'I I I I I I I I I I I I I I I I ' I I' I I I '

I I I I' 0 500 1000 1500 2000 2500 3000 3500 4000

Time (s)

Figure 6-8. Bare bead thermocouple measurements as a function of time at four

locations (below the ceiling) on Tree 1 during Test 3.

68 NISTNCSTAR 1-5E, WTC Investigation
Gas Temperature

I

I I I I I

I I I I I

I I I I I

I I I I

I I I I II r I I I

I

I I I I

I I I I I

Time (s)

Figure 6-9. Bare bead thermocouple measurements as a function of time at four

locations (below the ceiling) on Tree 1 during Test 4.

Time (s)

Figure 6-10. Bare bead thermocouple measurements as a function of time at four

locations (below the ceiling) on Tree 1 during Test 5.

NISTNCSTAR 1-5E, WTC Investigation 69
Chapter 6

0 I I I I I I I I I I I I I I I I I I I I I I I i I I I I I I [ I I I I I I I I 0 500 1 000 1 50 0 2 000 2500 3000 3500 4000

Time (s)

Figure 6-11. Bare bead thermocouple measurements as a function of time at four

locations (below the ceiling) on Tree 1 during Test 6.

1 200 |-|—I—I—I—I—

j—I—I—1—I—I—1—I—I—I—

I—1—1—I—I—j—I—I—1—

T

Time (s)

Figure 6-12. Bare bead thermocouple measurements as a function of time at four

locations (below the ceiling) on Tree 4 during Test 1.

70 NISTNCSTAR 1-5E. WTC Investigation
Gas Temperature

I I I II r —o—2.5 cm —n~91 crn 183 cm

274 cm

J I I I I I I I I I I I I I I I I I 1 I I I I L 1000 2000 3000

Time (s)

4000 5000

Figure 6-13. Bare bead thermocouple measurements as a function of time at four

locations (below the ceiling) on Tree 4 during Test 1.

Oo

15

CDQ.

E

(D

1000

800

600

400

200

Tests

- Tree 4

T—I—T—I—

I—I—I—I—

r

2000 3000

Time (s)

5000

Figure 6-14. Bare bead thermocouple measurements as a function of time at four

locations (below the ceiling) on Tree 4 during Test 3.

NISTNCSTAR 1-5E, WTC Investigation 71
Chapter 6

Oo

0

Q.

£

0

1000

800 -

600

400

200

I ' I ' 'I I I I I I 'I I I I ' ' 1I I I ' ' '

I ' ' 'I I ' ' ' 0 500 1000 1500 2000 2500 3000 3500 4000

Time (s)

Figure 6-15. Bare bead thermocouple measurements as a function of time at four

locations (below the ceiling) on Tree 4 during Test 4.

1000

800 - oo2 600

S. 400

E

CD

200 -

0 500 1000 1500 2000 2500 3000 3500 4000

Time (s)

Figure 6-16. Bare bead thermocouple measurements as a function of time at four

locations (below the ceiling) on Tree 4 during Test 5.

72 NISTNCSTAR 1-5E, WTC Investigation
Gas Temperature

800

I- I

' '

' I I

[ I I I I

I

I I I I

[ T I I

I

I I i I [

O

0

0 500 1000 1500 2000 2500 3000 3500 4000

Time (s)

Figure 6-17. Bare bead thermocouple measurements as a function of time at four

locations (below the ceiling) on Tree 4 during Test 6.

Blevins, L. G. 1999. "Behavior of Bare and Aspirated Thermocouples in Compartment Fires,"

Proceedings ofthe 33"' National Heat Transfer Conference, August 15-17, Albuquerque, NM.

Blevins, L. G., and Pitts, W. M. 1999. Fire Safety Journal 33, 239-259; NISTIR 6310, National Institute of Standards and Technology, Gaithersburg, MD, April.

Bums, G. W., Scroger, M. G., Strouse, G. F., Croarkin, M. C, and Guthrie, W. F. 1993. TemperatureElectromotive Force Reference Functions and Tables for the Letter-Designated Thermocouple Type

Based on the ITS-90, NIST Monograph 175, National Institute of Standards and Technology,

Gaithersburg, MD, April.

Hamins, A., Maranghides, A., Johnsson, R., Donnelly, M., Yang, J., Mulholland, G., Anleitner, R. L. 2003. Report ofExperimental Results for the International Fire Model Benchmarking and Validation

Exercise #3, NIST Special Publication 1017, National Institute of Standards and Technology,

Gaithersburg, MD, November.

He, Y. P., Fernando, A., Luo, M. C. 1998. Fire Safety Journal, 31, 19-38.

McCaffrey, B. J., and Heskestad, G. 1976. Combust. Flame, 26, 125-127.

Omega Engineering, Inc. 2004. Tempej-ature Handbook, ""Using Thermocouple.'''' (see

www.omega.eom/temperature/Z/pdf/z02 1 -032.pdf).

NISTNCSTAR 1-5E, WTC Investigation 73

6.4 REFERENCES
Chapter 6

Pitts, W. M., Braun, E., Peacock, R. D., Mitler, H. E., Johnsson, L., Reneke, P. A., and Blevins, L. G.

2002. "Temperature Uncertainties for Bare-Bead and Aspirated themiocouple Measurements in Fire

Environments," Thermal Measurements: The Foundation ofFire Standards, ASTM STP 1427, L. A.

Gritzo and N. Alvares, eds., ASTM International, West Conshohocken, PA.

Pitts, W. M., Braun, E., Peacock, R. D., Mitler, H. E., Johnsson, E. L., Reneke, P. A., and Blevins, L. G.

1998. Temperature Uncertainties for Bare-Bead and Aspirated Thermocouple Measurements in Fire

Environments: National Institute of Standards and Technology, Gaithersburg, MD, NISTIR 6242,

October.

74 NISTNCSTAR 1-5E, WTC Investigation
Chapter 7

Upper Layer Concentrations of CO2, CO, and O2

7.1 GAS SAMPLING PROCEDURE

Gas samples were pulled from the sampling location near the middle of the compartment (x,y,z = 970 cm,

351 cm, 276 cm) using a pump. The gas sample was filtered through a glass microfiber filter to remove

particulate larger than 1.5 |im diameter and routed through a glass trap immersed in a dry ice bath to condense water vapor prior to analysis. In all cases, gas concentrations are reported on a dry basis. The

gas analyzers in the calorimeter differ only in that a sampling pump near the exhaust duct pulled the

sample from the duct, through a particulate filter and pumped it to the control room. This gas was then

filtered, conditioned, and pumped to the gas analyzers.

Instrumentation was the same as that used for the calorimetry measurements described in Chapter 5. Oxygen was quantified using an analyzer that takes advantage of the paramagnetic electron configuration

of molecular oxygen in which unpaired electrons are susceptible to magnetic fields. The analyzer is sensitive and nearly free of interferences. Sample gas is passed through a cell subject to a strong

magnetic field. The susceptibility of oxygen to the magnetic field causes a gas-filled dumbbell suspended

in the cell to torque from an equilibrium position. The electric current required to maintain the

dumbbell's equilibrium position is proportional to the oxygen concentration.

Carbon monoxide and carbon dioxide were quantified using gas analyzers employing non-dispersive

infrared technology. An infrared source was alternately directed through one cell containing nitrogen,

which does not absorb infrared radiation, and a second cell containing the flowing sample gas. Each

beam of infrared light impinged on a double layer detector connected pneumatically and filled with the

analyte of interest. Differences in absorption between the two layers results in a flow between the layers

that is sensed by a microflow detector in proportion to the analyte concentration in the sample cell.

As water was removed prior to quantification, the experimental results are reported on a diy basis in terms of volume. The units employed in describing the magnitude of the species concentrations are either

volume fraction or volume percent. The uncertainties in the volume fraction measurements were

5 percent, 10 percent, and 10 percent for the O2, CO2, and CO volume fractions, respectively.

The gas in the hot upper layer of the compartment was sampled to quantify the volume fi-actions of

oxygen, carbon monoxide, and carbon dioxide. The sample was collected from the eastern half of the

compartment, approximately 2.8 m (9 ft) above the floor. The oxygen volume fraction minima and the

carbon monoxide and carbon dioxide volume fraction maximum are described below. Figures 7-1

through 7-6 depict the gas concentrations measured in the compartment follow the text discussion. For

7.2 instrumentation

7.3 DISCUSSION

NISTNCSTAR 1-5E, WTC Investigation 75
Chapter 7

the sake of clarity, only 5 min (300 s) of data prior to ignition and 20 min (1,200 s) after ignition are displayed. -

7.3.1 Test Results

Before ignition in all of the tests, the data shows that the gas species volume fractions were steady

ambient values. Within 50 s after ignition, the oxygen volume fraction began to decline, while the carbon

monoxide and carbon dioxide volume fractions began to increase. The largest rate of change of the gas

species volume fraction occurred during the tests in which jet fuel was present (Tests 3, 5, and 6). In those tests, the oxygen volume fraction typically dropped from its ambient value to 0.02 within 200 s, while the CO and CO2 volume fractions increased from ambient values to 0.15 and 0.03, respectively,

within the same time frame. The species volume fractions began to return to ambient values after the

initial rapid change, when combustion of the jet fuel was nearly complete. Later in the tests, the data

exhibit several maxima and minima, reflecting the complexity of the compartment burning process.

During the tests in which jet fuel was not present (Tests 1, 2, and 4), the changes in gas species volume

fraction were much more gradual. The O2 volume fraction in Tests 1, 2, and 4, for example, decreased to values of less than 0.05 only after 500 s to 900 s of burning.

Analysis of the measurement results showed that the location of the spray burner or the presence of

ceiling tiles had no discemable effects on the measured O2, CO, and CO2 volume fractions. A detailed

description of the measurement results for each of the tests follows.

Test 1

During Test 1, between 1,400 and 1,650 s after ignition, oxygen concentrations reached a minimum,

while the carbon monoxide and carbon dioxide concentrations obtained maximum values. Oxygen

decreased to below 0. 1 percent, carbon dioxide exceeded 20 percent and carbon monoxide concentrations

peaked above 2 percent.

Test 2

Test 2 was a repeat of Test 1. During Test 2, maximum carbon monoxide and carbon dioxide with' coiresponding minimum oxygen concentration occurred between 800-1,000 s following ignition. Carbon

monoxide and carbon dioxide peaked at approximately 1.1 percent and 16 percent, respectively. The

minimum oxygen concentration was just below 2 percent. The measurable differences between Tests 1 and 2, as mentioned earlier, can in part be attributed to the organic fuel contributions from Marinite

during Test 1

.

Test 3

Test 3 gas concentration extremes occurred over a prolonged period of time from 100 s to 1,000 s following ignition. Oxygen concentrations frequently decreased below 1 percent while carbon dioxide

and carbon monoxide peaked above 24 percent and near 4 percent, respectively.

76 NISTNCSTAR 1-5E, WTC Investigation
Upper Layer Concentrations of CO2, CO, and O2

Test 4

In Test 4, minimum oxygen concentrations and maximum carbon monoxide and carbon dioxide

concentrations occurred in a relatively narrow time frame ranging from 400 s to 550 s following fire ignition. The minimum oxygen concentration during this time was near 3 percent while carbon dioxide

concentrations peaked above 13 percent and carbon monoxide peaked just below 0.5 percent.

Test 5

During Test 5, approximately 3 min after ignition, a circuit breaker tripped causing a power failure to the

compartment hot layer gas analyzers and two of the load cell networks. It took approximately 400 s to reset the circuit breaker and return the instruments to service. During Test 5, it is possible that the

minimum oxygen concentration and maximum carbon dioxide and carbon monoxide concentration

occurred while power to the gas analyzers was interrupted. Prior to the power failure, the oxygen was as low as 4 percent by volume, while carbon monoxide and dioxide peaked at 0.6 percent and 12 percent,

respectively.

Test 6

During Test 6, peak carbon monoxide and carbon dioxide, as well as minimum oxygen concentrations

occurred during the first 240 s after ignition. Oxygen decreased to below 0.2 percent while carbon

monoxide and carbon dioxide peaked near 16 percent and carbon monoxide peaked near 3.6 percent.

7.4 SUMMARY

Gas samples were extracted and analyzed to determine the volume fractions of O2, CO, and CO2. The

instrumentation required removing water from the samples, and the results are reported here on a dry

basis. The measurements results showed that:

• Changes in the volume fraction of gas species occurred most rapidly for the tests in which jet fuel was present.

• Analysis of the measurement results showed that there was no discemable effect of the

location of the spray burner or the presence of ceiling tiles.

The species volume fractions are sensitive to a variety of fire dynamic conditions. These measurement

results provide a database that represents a challenge for model validation as seen in the next Chapter.

NISTNCSTAR 1-5E, WTC Investigation 77
Chapter 7

0.25

0.20

I

0.15

0.10

o>

o O

o

0.05

0.00

1—I—I

\—I—

I—I—I—1—I—

[—I—I—1—I—I—I—I—I—I—

I—i—I—1—1—

I—I—I—I—

r

Test 1

0.050

0.040

O

O

0.030 g

c

3

CD 0.020

0.010

1500 2 000 2 500

Time (s)

0.000

3000

Figure 7-1 . The volume fractions of O2, CO, and CO2 on a dry basis as a function of time

during Test 1.

c

o

o

TO

E

o>

r o

o

"Dc

TO

0.25

0.20

0.15

0.10

0.05

0.00

1—I—I—I—I—

I—I—i—I—I—I—I—I—I—I—I—I—i—I—I—I—I—1—I—I—I—I—I—I—

r

Test 2

0.050

0.040

O

O

0.030 g

c

3

CD 0.020

0.010

0.000

05O

500 1000 1500 2000 2500 3000

Time (s)

Figure 7-2. The volume fractions of O2, CO, and CO2 on a dry basis as a function of time

during Test 2.

78 NISTNCSTAR 1-5E, WTC Investigation
Upper Layer Concentrations of CO2, CO, and O2

0.25

c

o

o

CDE

_^o>

f O

O

T3C

CD

0.20 -

0.15

0.10

0.05

0.00

"I

I I I I I

I I I I—

I—I—I—I—I—

I—I—I—I—I—

I—I—I—I—I—I—I—I—I—

r

"^^St 3

— CO 1 •

62'

J I I L ^1 r I

I I 1 " I I

0.05

0.04

0.03

- 0.01

0.00

o

o

<

o_c

3

ft) \4 0.02 :^

03O

500 1000 1500 2000 2500 3000

Time (s)

Figure 7-3. The volume fractions of O2, CO, and CO2 on a dry basis as a function of time

during Test 3.

0.25 1—I—I—1—

I—I—I—I—I—

I—I—I—I—I—

I—I—I—I—I—I—I—1—I—I—I

I—I—I—

r

Test 4 I.

0.004

1- 4002 1 02

1 —•

0.00

0.005

O

O

-\ 0.003 I

c

3

CD 0.002 7

o_o' 0.001 ^

J I I I I i 2 1 ) - I - I t- If- -1 I 1 I I 1 I 1 I I I I 1 L 500 1000 1500 2000 2500 3000

Time (s)

0.000

Figure 7-4. The volume fractions of O2, CO, and CO2 on a dry basis as a function of time

during Test 4.

NISTNCSTAR 1-5E, WTC Investigation 79
Chapter 7

o

CO

CDE

_5o>

r O

o

D

c

CO

0.25

0.20 -

0.15

0.10

0.05

0.00 J I I I 1 I I 1 I I I I i_

0.005

0.004

O

O

0.003 ^

0.002

o

- 0.001

1000 1500 2000

Time (s)

0.000

3000

Figure 7-5. The volume fractions of O2, CO, and C02on a dry basis as a function of time

during Test 5.

0.25

g 0.20 o

COo 0.15

O

>

0.10

o

o

D

0.05

0.050

0.00 0.000

500 1000 1500 2000 2500 3000

Time (s)

Figure 7-6. The volume fractions of O2, CO, and CO2 on a dry basis as a function of time

during Test 6.

80 NISTNCSTAR 1-5E, WTC Investigation
Chapter 8

Simulation of Compartment Fire Phenomena

8.1 introduction

In the months following the attacks on the World Trade Center (WTC) and the Pentagon, there was an

active debate in the fire protection engineering community about the fires that erupted following the

impact of the aircraft on the buildings. Because fires of this magnimde in these t>'pes of buildings were

rare, there was a wide spectrum of opinion about the fire temperatures and their effect on the structural

steel. Much of the fire literature consists of empirical correlations derived from experiments ranging from

bench scale to room scale. Extrapolating these well-known correlations to the WTC and the Pentagon

requires a re-examination of the underlying physics. Many of these correlations are appropriate for a narrow range of fire sizes and building geometries and cannot be applied to the WTC.

Over the past 25 years, the National Institute of Standards and Technology (NIST) has developed several

computer models, widely used by fire protection engineers to assess the impact of fire on a building.

These models have become more and more detailed as computers have become faster and less expensive.

The earliest models were essentially correlations of existing fire test data, followed later by simple

computer programs that would solve mass and energy conservation equations for entire rooms or compartments. These models came to be known as "zone models," because they divide buildings into rooms and each room into well-mixed upper and lower zones. The concept of consening mass and

energy in "zones" has a natural extension where each room is divided into thousands or potentially

millions of zones or cells. Adding conservation of momentum to mass and energy yields what is known

as a computational fluid dynamics (CFD) model. Such models are used to design jet aircraft and predict

the weather by solving a modeled set of partial differential equations known as the Navier-Stokes

equations. The results yield a very detailed picture of the movement of fluids (either gases or liquids) as a function of time. A CFD fire model predicts the movement of hot gases from a fire as they spread

throughout a building. In the case of the WTC, these calculations can provide estimates of the

temperamre of the smoke and hot gases surrounding the structural steel, information needed to assess the

potential weakening of the steel over the course of 100 min.

8.2 NIST FIRE DYNAMICS SIMULATOR

The Fire Dynamics Simulator (FDS) is a CFD model of fire-driven fluid flow. Version 1 of FDS was

publicly released in February 2000. Versions 2 and 3 were publicly released in December 2001 and

November 2002, respectively. To date, about half of the applications of the model have been for design

of smoke handUng systems and sprinkler/detector activation smdies. The other half has consisted of

residential and industrial fire reconstructions. FDS has been de\'eloped with the goal of soh'ing practical

fire problems in fire protection engineering, while at the same time pro\ iding a tool to smdy fundamental

fire dynamics and combustion. Throughout its development, the model has been compared with a \ ariety

of fire experiments to ensure that its predictions are accurate. Most often, the experiments are not

necessarily designed solely for the purpose of model validation, and as a result, there are often se\ eral experimental feamres that are difficult to model. It is then difficult to assess the overall accuracy of the

NISTNCSTAR 1-5E, WTC Investigation 81
Chapter 8

model because the uncertainty in modeling the difficult features dominates the error. The large-scale

experiments described in this report were designed specifically to extend the validation of FDS to the

WTC fire scenarios.

FDS solves numerically a modeled fomi of the Navier-Stokes equations appropriate for low-speed,

thermally-driven flow with an emphasis on smoke and heat transport from fires. The formulation of the

equations and the numerical algorithm are contained in a FDS companion document, called the Fire

Dynamics Simulator (Version 4) - Technical Reference Guide (McGrattan 2004). The core algorithm is an explicit predictor-corrector scheme, second order accurate in space and time. Turbulence is treated by means of the Smagorinsky form of Large Eddy Simulation (LES). For most applications, FDS uses a mixture fraction combustion model. The mixture fraction is a conserved scalar quantity that is defined as the fraction of gas at a given point in the flow field that originated as fuel. The model assumes that combustion is mixing-controlled and that the reaction of fuel and oxygen is infinitely fast. The mass

fractions of all of the major reactants and products can be derived from the mixture fraction by means of

"state relations," empirical expressions arrived at by a combination of simplified analysis and

measurement.

Radiative heat transfer is included in the model via the solution of the radiation transport equation for a

non-scattering gray gas and in some limited cases using a wide band model. The equation is solved using

a technique similar to finite volume methods for convective transport, thus the name given to it is the

Finite Volume Method (FVM). Using approximately 100 discrete angles, the finite volume solver

requires about 1 5 percent of the total CPU time of a calculation, a modest cost given the complexity of

radiation heat transfer.

FDS approximates the governing equations on a rectilinear grid. The user-prescribed rectangular

obstructions are forced to conform with the underlying grid. All solid surfaces are assigned thermal

boundary conditions and information about the burning behavior of the material. Usually, material

properties are stored in a database and invoked by name by the user. Heat and mass transfer to and from

solid surfaces is usually handled with empirical correlations.

8.3 CALIBRATION AND VALIDATION EXPERIMENTS

The experimental program concentrated on the thermal properties of the office furnishings that constituted the bulk of the combustible fuel within the WTC buildings under study. Several types of

office workstations typical of those used in WTC 1 and WTC 2 were purchased. The thermal properties

of the major materials making up the workstations were derived from cone calorimeter experiments

(NIST NCSTAR 1-5C). These properties were input into FDS, which was used to simulate the burning

behavior of a single workstation burning under a 2.5 m ceiling with baffles to contain a hot layer of

smoke above the burning workstation. Other than the baffled ceiling, no walls surrounded the

workstation other than its own privacy panels. The thermal properties of the workstation components

were adjusted slightly so that the FDS prediction of the heat release rate would match the experiment.

Then, the model was used to predict the heat release rate of three workstations (workstations) burning

within a large enclosure. The purpose of this exercise was to determine if FDS could simulate the

dynamics of a fire in a setting similar to WTC 1, 2, and 7.

82 NIST NCSTAR 1-5E, WTC Investigation
Simulation of Compartment Fire Phenomena

A spray flame burning a 2 MW fire was used as a source of ignition for the fiimished compartment. The

properties of the heptane fuel were obtained from measurements made on a series of unconfincd burns

(Hamins et al. 2003). The fuel was a blend of heptane isomers, taken to be CyHu,. The soot yield was set at a constant value equal to 1.5 percent. The present version of FDS does not adjust the soot yield based on compartment ventilation or combustion efficiency. The radiative fraction for a spray fire burning the heptane blend is 0.44. FDS computes the radiative loss from the fire based on the temperature and gas

composition. This calculation is subject to considerable uncertainty because of the fourth-power

dependence on temperature. As a consequence, FDS uses a user-prescribed lower bound for the radiative

fraction, which in this case is 0.35. This means that the simulated fire radiates at least 35 percent of its energy. For the simulations reported here, the radiative fraction was computed to be about 0.40. The

FDS Technical Reference Guide (McGrattan 2004) provides a detailed discussion of how the output

parameters are interpreted in the simulations.

The heat release rate of the simulated burner was set to the idealized heat release rate based on the fuel flow, which was monitored during the experiment. No attempt was made to model the spray burner. The

heat release rate was assumed to be unifomily distributed over the 2 m by 1 m area in the fuel pan,

consistent with observations of the hydrocarbon spray fire.

8.3.1 Description of the Workstation Components

A detailed description of the various materials making up the office workstation is given in NIST

NCSTAR 1-5C and in Chapter 2 of this report. Table 2-2 is a detailed list of the combustible contents of

each workstation. Cone calorimeter experiments at three different heat fluxes were performed for the

carpet, desk (wood), computer monitor, chair, privacy panel, and stacked paper. For the simulations of

the WTC fires, only the carpet, desk, and privacy panel data was used directly. The carpet and privacy

panel were modeled as thermoplastics, with the burning rate assumed to be proportional to the heat flux from the surrounding gases. The desk was modeled as a charring solid, in which a pyrolysis front propagates through the material leaving a layer of char behind that insulates the material and reduces the

burning rate. Details of the pyrolysis models can be found in the FDS Technical Reference Guide

(McGrattan 2004). The thermal properties of the wood and its char were taken from both the calorimeter

experiments and the work of Ritchie et al. (1997). The desk was 2.8 cm thick with density 450 kg/m^,

specific heat 1.2 kJ/kg/K at 20 °C and 1.6 kJ/kg/K at 900 °C, conductivity 0.13 W/m/K at 20 °C and

0.16 W/m/K at 900 °C. The ignition temperature was determined to be 360 °C, and the heat of

combusfion was 14,000 kJ/kg ± 800 kJ/kg. Its total available energy content was 210 MJ/m' ± 50 MJ/m".

The carpet was modeled as a thermoplastic, with the burning rate taken as proportional to the heat flux

from the surrounding gases. Its density was 750 kg/m"\ specific heat 4.5 kJ/kg/K, conductivity

0.16 W/m/K, ignition temperature 290 °C, thickness 6 mm, heat of vaporization 2,000 kJ/kg, heat of

combustion 22,300 kJ/kg ± 600 kJ/kg. Its total available energy content was 61 MJ/m" ± 2 MJ/m".

The privacy panel was modeled as a thermally-thin thermoplastic. The product of specific heat, thickness

and density was 0.73 kJ/m"/K. Its surface density was 0.25 kg/m", ignition temperature 380 °C, heat of

vaporization 6,000 kJ/kg, heat of combustion 30,000 kJ/kg ± 500 kJ/kg. Its total available energy content was 6.0 MJ/m" ± 1.3 MJ/ m".

NIST NCSTAR 1-5E, WTC Investigation 83
Chapter 8

The compartment walls and ceiling were made of three layers of 1.27 cm (0.5 in) thick Marinite I, a product of BNZ Materials, Inc. The manufacturer provided the thermal properties of the material used in the calculation. The density was 737 kg/m"\ conductivity 0.12 W/m/K. The specific heat ranged from

1.2 kJ/kg/K at 93 °C to 1 .4 kJ/kg/K at 425 °C. These values differed somewhat from the values measured

by Taylor et al. (2003), but the calculation results were not sensitive to these differences.

In the simulations of the fires within the WTC, the chair, computer, paper, and other miscellaneous items

within the workstation were modeled as a single material by lumping the mass together into large boxes

and distributing them throughout the workstation. It is common practice in fire protection engineering to use surrogate materials for fire experiments, and this practice has been extended to numerical modeling.

Over the years, various items have been developed that are representative of various types of

commodities. For example, wood cribs are often used to represent ordinary combustibles found in residential or light industrial settings. Paper cartons with various amounts of plastic within are also used

as surrogates for a wide range of retail commodities. One in particular is called the FMRC (Factory

Mutual Research Corp.) Standard Plastic Commodity, or more commonly. Group A Plastic. This test fuel

is often used in sprinkler approval testing at Factory Mutual and Underwriters Laboratories in the United

States, and similar test fuels have been developed in Europe. In the late 1990s, FDS was used to simulate

large-scale rack storage fires to determine the effectiveness of the combined use of sprinklers, roof vents,

and draft curtains (curtain boards). As part of this effort, a considerable amount of work was done to characterize the themial properties of Group A Plastic (Hamins and McGrattan, 2003). Because Group A

Plastic has been shown to be fairly representative of fires fueled by a mixture of paper (cellulosic

materials) and plastic, and because it has been used in numerous FDS simulations, it was decided to model the contents of the office workstations with a fuel similar to Group A Plastic. Blind predictions of

the single open workstation bums were made using the material properties obtained during the

sprinkler/roof vent study (McGrattan et al. 1998), and then these properties were adjusted to match the

results of the experiments. Thus, the single workstation bums served to calibrate the model. They were

not intended to be validation experiments.

The surrogate fiiel was modeled as a homogenous solid with a density of 172 kg/m". The paper carton was treated as a thermally-thin material whose density x specific heat x thickness was equal to

1.0 kJ/m"/K. Its ignition temperature was 370 "C, and the heat of combustion was taken as 30 kJ/g. The

heat release rate of the boxes was set to 450 kW/m". This fuel package was similar to, but not the same

as. Group A Plastic. The density was increased to account for all the miscellaneous items within the

workstation. Unlike the desk, partition, and carpet, the boxes were simply given a buming rate rather

than a heat of vaporization, meaning that the simulated boxes bum at the given rate regardless of the heat

flux upon them. The reason for this was that it is not possible to predict the buming rate using a heat

feedback approach because the geometry of the scattered fuel was too complex to directly predict the

response of the material to the thermal insult. By collecting all the scattered items into boxes, the

geometry of the combustibles was greatly simplified, and as a result, the buming behavior required

simplification as well.

Several preliminary bums of single workstations in the open were performed to test FDS for a simplified

configuration. These bums served to check that the assumptions made in modeling the workstation with

the simplified fuel packages would produce reasonable results. A comparison of the heat release rate (Q)

of a single workstation fire and that of the simulation is shown in Fig. 8-1. Note that the peak Q in the

simulation occurred sooner than in the experiment. In the experiment, the time to peak Q was strongly

84 NISTNCSTAR 1-5E. WTC Investigation
Simulation of Compartment Fire Phenomena

influenced by the melting of the chair plastic onto the carpet. This level of detail is not captured in the

numerical model, especially given the fact that the chair has been lumped together with various other

combustible items. What is important is the fact that the model and experimental ^curves have the same

basic form, peak Q , and duration. When inputting the simulated workstations into the calculations for WTC 1, 2, and 7, the discrepancies in Q seen in the experiments will be outweighed by the differences in workstation content of the building tenants.

«3

OS

PC

12000

10000 -

8000

6000 -

4000 -

2000

0

Heat Release Rate, Single Workstation

Experiment

FDS

0 500 1000 1500 2000 2500

Time (s)

Figure 8-1. Comparison of the measured and simulated Q for a single workstation.

8.3.2 Description of the Simulations

Six experiments were performed within the compartment, of which five were simulated because Tests 1 and 2 were replicate tests. The geometry of the compartment used for the validation experiments was

relatively simple. The overall enclosure was rectangular, as were the vents and most of the obstructions.

Figure 8-2 is a snapshot of a simulation showing the fire and the major geometric features of the

compartment for the simulations. Note that the surrogate fuel packages were placed roughly where the

computer monitor, chair, and paper were located. In the figure, the 2 MW heptane burner was placed near

the openings of the compartment abutting workstation 1 as was the case in Tests 1, 2, and 3. For

simulation of Tests 4, 5, and 6, the burner was placed at the rear of the compartment abutting

workstation 2, mimicking the experimental configuration.

NISTNCSTAR 1-5E. WTC Investigation 85
Chapter 8

Figure 8-2. Geometry of the simulations.

In Tests 3, 5, and 6, the jet fuel was poured over the workstations and surrounding carpet. To simulate

this in the model, spray nozzles were positioned over the center of each workstation, 2 m above the floor. These nozzles are normally used to simulate water sprinklers, but in this case, the water was replaced by a

liquid having similar properties to Jet A. The nozzles were activated for 2 s, in which time the equivalent

amount of liquid as in the tests was ejected and spread over the furnishings. The measured and calculated

heat release rate ( Q ), the gas-phase compartment temperature, and the upper layer gas concentrations were compared.

For enclosures of the size considered in this report, it would be practical to simulate the fire using 10 cm

grid cells. For the simulations of WTC 1, 2, and 7, however, a grid 50 cm by 50 cm by 40 cm in height was used due to computational requirements (McGrattan et al. 2005). A grid sensitivity study was

conducted to test the sensitivity of the calculation results to the grid cell size. Figure 8-3 compares the

measured Q with the simulated Q (in which two grids sizes were used) for Test 1 . By simulating the

experiments at 25 cm and 50 cm grid spacing, the model was tested to determine whether it produced

significantly different results with grid cells of different size. Figure 8-3 shows that the two simulations

differed somewhat. The calculation results using the 50 cm grid showed a 20 percent larger Q peak than

the 25 cm grid results at about 600 s, whereas the calculation results using the 25 cm grid showed a 20 percent larger Q peak than the 50 cm grid results at about 1,100 s. Neither simulation exactly

' matched the measurement results, but both calculations accurately reproduced the dominant trends of the

measurements. In the WTC calculations, the workstation fires were at various stages of their combustion

and the aggregate burning of a large group of workstations was expected to smear any over-predictions or under-predictions of the transient heat release rate (McGrattan et al. 2005). Furthermore, by design, the

integrated heat release rate over time was equal for the two simulations. All of the simulations in this report were calculated using the same grid (50 cm) as used in the calculations of the WTC fires (McGrattan et al. 2005).

Figures 8-4 through 8-7 compare the measured and simulated gas-phase temperatures at thermocouples

2.5 cm and 270 cm from the ceiling on all four thermocouple trees during Test 1. Figures 8-8 tlirough

8-13 compare the measured and simulated Q , using the same grid spacing as used in the WTC fire simulations. Figure 8-14 compares the measured and the simulated volume fraction of O2 and CO2

sampled at the rear of the compartment in Test 1

.

86 NISTNCSTAR 1-5E, WTC Investigation
Simulation of Compartment Fire Phenomena

It should be noted that the predicted heat release rate represents the instantaneous heat release and does

not account for compartment effects, which tended to delay and time average the measured value as the products filled the upper layer of the compartment, became mixed, and only then flowed into the exhaust

hood. During the simulation, values of the temperature, heat flux, and gas species volume fractions were

reported as 5 s averages, which was more in-line with the time response of the various instruments (see

Table 2-6). Linear interpolation was used to approximate values between the grid cells.

Time (s)

Figure 8-3. Comparison of the measured Q with the simulated Q using

two grids sizes for Test 1.

1200

0 , , ,

1 0 Um 2000 30{K) 4000

Time (s)

Figure 8-4. Comparison of the measured and simulated temperatures at 2.5 cm and

274 cm below the ceiling on Tree 4 (rear of the compartment) during Test 1.

NISTNCSTAR 1-5E, WTC Investigation 87
Chapter 8

s

3'

i20()

1000

800 -

600 -

400

200 -

Test 1 , Tree 3 2,5 cm (Exp)

274 cm (Exp)

2.5 cm (FDS)

274 cm (FDS)

iOOO 2000 3000 4000

Time (s)

Figure 8-5. Comparison of the measured and simulated temperatures at 2.5 cm and

274 cm below the ceiling on Tree 3 (middle of the compartment) during Test 1.

1200

0 1000 2000 3000 4000

Time (s)

Figure 8-6. Comparison of the measured and simulated temperatures at 2.5 cm and

274 cm below the ceiling on Tree 2 (middle of the compartment) during Test 1.

88 NISTNCSTAR 1-5E, WTC Investigation
Simulation of Compartment Fire Phenomena

1200

Test 1 , Tree 1 2.5 cm (Exp)

274 cm (Exp)

2.5 cm(FDS)

274 cm(FDS)

4(M)()

Time (s)

Figure 8-7. Comparison of the measured and simulated temperatures at 2.5 cm and

274 cm below the ceiling on Tree 1 (front of the compartment) during Test 1.

16000

o

O

14000 -

12000 -

10000

8000

6000 -

4000

2000 -

0

0

E.Kpcrinieni 1 FDS

500 1000 1500 2000 2500

Time (s)

Figure 8-8. Comparison of the measured and simulated Q for Test 1.

3000

NISTNCSTAR 1-5E, WTC Investigation 89
Chapter 8

16000

1500

Time{s)

Figure 8-9. Comparison of the measured and simulated Q for Test 2.

16000

14000

P 12000

4> 1 0000

re

8000 -

6000 -

4000 -

2000 -

0 500 1000 1500 2000 2500

Time (s)

Figure 8-10. Comparison of the measured and simulated Q for Test 3

3000

90 NISTNCSTAR 1-5E, WTC Investigation
Simulation of Compartment Fire Plienomena

16000

Time (s)

Figure 8-1 1 . Comparison of the measured and simulated Q for Test 4.

16000

14000 -

12000 -

iOOOO -

8000 -

6000 -

4000 -

2000

0 500 1000 1500 2000 2500

Time (s)

Figure 8-12. Comparison of the measured and simulated Q for Test 5

3000

NISTNCSTAR 1-5E, WTC Investigation 91
Chapter 8

0 500 1000 1500 2000 2500 3000

Time(s)

Figure 8-13, Comparison of the measured and simulated O for Test 6.

0.25

0.20

ts 0 .15 - u

?s u.

liE^ 0.10 o>

0.05 -

0.00

Figure 8-14. Comparison of the measured and simulated O2 and CO2 volume fractions

on a dry basis for Test 1.

92 NISTNCSTAR 1-5E, WTC Investigation
Simulation of Compartment Fire Phenomena

8.3.3 Discussion

From a modeling perspective, the objective of the simulations of these experiments was to demonstrate

that a simplified model of a single office workstation can be used to predict the burning behavior of a group of workstations in an enclosure with features similar to WTC 1, 2, and 7. Due to the computational

time required to simulate the WTC building fires, the model of the workstation must be fairly simple.

However, because of the many uncertainties in the initial conditions of those fire simulations, the lack of

detail in the model was not considered a drawback.

The model captured many of the major features of the fire behavior measured during the experiments.

This is seen for the measured upper layer temperatures (2.5 cm from the ceiling) in Figs. 8-4 through 8-7

and for the heat release rates in Figs. 8-8 through 8-13. Additional data analogous to Figs. 8^ through

8-7 for temperatures measured at thermocouple Trees 2 and 3 are presented in Appendix D.

Figures 8-8 through 8-13 compare the simulated and measured heat release rates for the six experiments.

There were clearly similarities and differences between the sets of curves. FDS was able to accurately

predict the general shape and magnitude of the time dependent heat release rates. The areas under the curves were equal, guaranteed through model calibration based on the heat release rate measurements

made during the single office workstation bum experiments (NIST NCSTAR 1-5C). FDS tended to under-predict the late heat release rate behavior of the slow burning period of the fire when the heat

release rate dropped below 1 MW to 2 MW. When this long "tail" in the heat release rate curve was

observed, FDS predicted that the fiiel had already been completely consumed. Due to the relatively low

heat release rate during this period, however, this behavior was considered to be of secondary significance

in terms of the temperature of the upper gas layer and resulting thermal load on a structure.

The simulated Q for Tests 3,5, and 6 showed a large spike at the start of the simulations, probably

associated with the Jet A fuel that had been distributed over the workstations and surrounding carpet.

This was due to the fact that FDS uses a mixture fraction or a "mixed is burnf combustion model in which vaporized fuel bums immediately upon mixing with the oxygen in the air. This spike

(e.g.. Figs. 8-1 1 and 8-12) was not observed in the experimental data since the fuel was absorbed more

readily by the furnishings and thus required more time to burn. The jet fuel does significantly decrease

the time to obtain the Q peak value, and this effect was captured by the model following the initial spurious spike.

In Test 5, Workstations 1 and 2 were disassembled prior to the bum and the contents placed on top of the

respective load cells, and Workstation 3 was configured in a non-standard manner as described in Chapter 2 of this report. In this sense, Test 5 was a challenging experiment to model. No free bums of

workstations in a similar configuration had been performed, and this was the only test in which the

simulated fuel packages were modified from the free-bum configuration. To model this scenario, the

burning rate of the collective fuel packages was reduced by one-half to account for the decreased surface

area of the fuel package. The choice of one-half was somewhat ad hoc. In this regard. Test 5 can be

considered a calibration rather than a validation of the model. Reducing the buming rate of the fuel by

one-half while maintaining the same fuel mass produced reasonable results as seen in Fig. 8-12. The

model adequately simulated the reduced duration of vigorous buming in the experiment subject to the

general over-prediction of the slow buming period and early fuel bum-out.

NIST NCSTAR 1-5E, WTC Investigation 93
Chapter 8

Tests 1 and 4 (Figs. 8-8 and 8-1 1, respectively) were similar in design except for the burner location. In Test 1, the burner was near the window openings; in Test 4 it was near the rear of the compartment. The

peak heat release rate was measured to occur about 1 5 min after ignition in Test 1 , whereas it occurred at about 10 min in Test 4. The model showed a similar trend. The faster growth seen in Test 4 was

probably due to the fact that the compartment heated up more quickly with the fire deep inside rather than near the vents, leading to more rapid spread of the fire across the pre-heated furnishings. Even though

ceiling tiles were distributed over the desk and carpet in Test 4, this did not seem to have a noticeable

effect on the fire growth, or at the very least the burner position seemed to have a far greater role in explaining the difference between Tests 1 and 4.

Quantitative comparison of the heat release rate curves was performed in the context of assessing the

accuracy of FDS for use in simulating the fires on a full floor of workstations in the WTC. As estimated

in NIST NCSTAR 1-5C, in excess of 40 workstations would have been burning concurrently in the WTC,

but at various stages in their combustion "lifetimes." As the fires evolved, the key characteristics were how quickly a workstation became fully involved in the overall fire, the magnitude of the heat release rate

at that time, and the time when it was no longer a significant contributor to the total heat release rate. The

fire behavior was, thereby, represented by the time at which half of the total heat was released, the magnitude of the heat release rate at that time, and the time at which Q fell below 2 MW. This value was

selected because below approximately 2 MW combustion of the workstations was in its "slow" burning

period (see the discussion in Chapters 4 and 5, and Figs. 4-8 and 5-2), when the mass loss rate was

relatively small and the fires smoldered for long periods of time, typically for thousands of seconds. In

this regard, the fire duration in this study was defined as the time of ignition to the time when Q fell below 2 MW. The value of the total heat release was determined over the same period.

Tables 8-1 through 8-3 summarize the degree of agreement between the simulated and measured Q

profiles. Table 8-1 compares the simulated and measured Q when half of the total energy of the fire was

released. On average, the value of Q was approximately 1 3 MW for both the measurements and the

simulations. The simulations over-predicted the measurements for one-half of the tests. The difference

between the predictions and the measurements was l.l MW (or 9 percent) on average for Tests 1 through

4 and 6, which was within the 15 percent experimental uncertainty that is discussed in Chapter 5. As

described above. Test 5 was a special case in which the experiments were in a non-standard configuration

and those experiments should be regarded as a calibration rather than a validation. The reported Q for Tests 1 and 3 were not corrected for the contributions by the wall lining material (see Chapter 5), because

the value of the period upon which the correction should be based is uncertain.

Table 8-2 compares the simulated and measured time when half of the total energy of the fire was

released. The value of this time was approximately 900 s and 1 ,090 s for the measurements and

simulations, respectively, and the difference was 190 s (or 22 percent) on average for Tests 1 through 4,

and 6. The simulations were larger than the measurements for all of the tests. The largest differences were for Tests 5 and 6. In Test 6, additional ventilation was available to the fire several minutes into the

test as the glass windows cracked and broke through. The model was somewhat low in the prediction of

the early burning behavior of the fire. Table 8-3 compares the simulated and measured fire duration. The

difference in the predictions and the measurements was 310 s (or 15 percent) on average for Tests 1 through 4 and 6 when the fire duration was on the order of 2,000 s. The simulations were larger than the

measurements for all of the tests.

94 NIST NCSTAR 1-5E, WTC Investigation
Simulation of Compartment Fire Ptienomena

Table 8-1. Heat release rate, Q, when one-half of the energy was released.

Test Measured Q (IMW)" Simulated O (MW)"

Difference (MW)'' and

^1 clCCIIla^C Lf 11 1 i. 1 L IILL ^

1 1 9 1 1 1 A -U. / (-0 /o)

2 Qy .yQ 1 1 d 1 . J ^ 1 H /o)

3 1 1.5 13.7 1.8 (14 %)

4 13.9 13.2 -0.7 (-5 %)

5 11.7 8.3 -3.4 (-34 o/„)

6 15.6 14.7 0.9 (6 "„)

Average'' 12.6 ± 18% 12.9+ 11 % 1.1 (9 %)

a. Using the definition that the total energy is released between the time of ignition and the time when the HRR fails to 2 MW.

b. Difference = (Simulated - Measured).

c. Percentage Difference = 100 x 2 x (Simuiated-Measured)/(Simulated + Measured).

d. The value represents the average of the absolute value of the differences for Tests 1 through 4 and 6.

Table 8-2. Time when one-half of the energy was released.^

Test Measured Time (s) Simulated Time (s)

Difference (s)''and

(Percentage Difference)'

1 1210 1,300 90 (7 %)

2 1120 1,300 180(14%,)

3 750 970 230 (26 %)

4 920 1,040 120(12 %)

5 540 1.110 570 (78 %)

6 510 830 320 (49 %)

Average** 900 + 31 % 1,090 ± 19% 190(22 %)

a. Using the definition that the total energy is released between the time of ignition to the time when Q falls to 2 MW.

b. Difference = (Simulated - Measured).

c. Percentage Difference = 100 x 2 x (Simulated - Measured)/( Simulated + Measured).

d. The value represents the average of the absolute value of the differences for Tests 1 through 4 and 6.

Table 8-3. Fire duration.^

Test Measured Duration (s) Simulated Duration (s)

Difference (s)'' and (Percentage

Difference)'

1 2,540 2,610 70 (3 %)

2 2,340 2,610 270(11 %)

3 2,110 2,320 210(9%)

4 2,190 2,490 310(13 %)

5 1,720 2.850 1,120 (47%)

6 1,450 2,150 700 (39 %)

Average** 2,130+ 19% 2,440 + 8 % 310(15%)

a. The duration is defined as the time from ignition to the time when O falls to 2 MW.

b. Difference = (Simulated - Measured).

c. Percentage Difference = 100 x 2 x (Simulated - Measured)/( Simulated + Measured).

d. The value represents the average of the absolute value of the differences for Tests 1 through 4 and 6.

NISTNCSTAR 1-5E, WTC Investigation 95
Chapter 8

Figures 8-4 through 8-7 show the measured and simulated temperatures on two thennocouples for each

of the four trees in Test 1. The simulations show trends similar to the measurements with the simulated

and measured profiles increasing, obtaining one or more peaks, and then decreasing later in the

experiments. The peak values were in general agreement although there were some differences in the

time at which they occurred. The agreement between the curves is driven to a large extent by the

agreement in the simulated and measured heat release rates. Accurate prediction of the upper layer of the

compartment temperature depends on an accurate prescription of the fire Q . According to an empirical

correlation by McCaffrey, Quintiere and Harkleroad (Walton and Thomas 2003), the rise in the upper

layer gas temperature AT^ in a compartment is related to the overall Q by the relation:

AT, = 6.85

,1/3

(8-1)

where:

Ao = area of openings (m")

Ho = height of openings (m)

At - total area of compartment surfaces (m"^)

A' = thermal conductivity of walls (kW/m/K)

v/all thickness (m)

Equation 8-1 shows that the temperature rise in a compartment is proportional to Q to the 2/3 power. At

the time when half of the energy was released, for example, the difference between the measured and

simulated peak values of Q were about 9 percent (see Table 8-1). According to Eq. 8-1, a 9 percent

discrepancy in Q corresponds to a 6 percent discrepancy in the temperature rise (= 2/3 x 9 %). For

temperatures of approximately 1,000 °C, this translates to roughly a ±90 °C temperature difference

between measurements and simulations due solely to differences in the measured and simulated values of

Q . The comparison of the measured and simulated temperatures considered the values of the peak

temperature, which were considered representative of the level of agreement. The average difference

between the simulated and measured peak temperatures shown in Figs. 8-4 through 8-7 for Test 1 and in Figures D-1 through D-20 (in Appendix D) for Tests 2 through 6 was 90 °C, which was in accord with the

sensitivity to differences in Q . The thermocouple measurement uncertainty of approximately 1 00 °C for

the peak temperatures was also considered, and the agreement between measurements and simulations,

therefore, was regarded as good.

Figure 8-14 shows that the measured and the simulated Ot and CO2 volume fractions in Test 1 were in good agreement while the fire was rapidly burning. The model under-predicted the O2 volume fraction at the rear of the compartment for a few hundred seconds as the heat release rate began to rapidly decrease.

This is simply a result of not accurately predicting the exact duration of the fire. Comparison of the CO

96 NISTNCSTAR 1-5E, WTC Investigation
Simulation of Compartment Fire Phenomena

concentration is not presented, as the mixture fraction chemistry model in FDS does not adequately

predict combustion intermediates such as CO.

8.4 SUMMARY

In an assessment of the model, it is important to maintain perspective on the accuracy required to reconstruct the actual WTC fires or fires that were sufficiently severe that they threatened the structural integrity of the building, many workstations bumed concurrently. The workstations were at various

stages of their combustion, and the aggregate burning of a large group of workstations is expected to average any over-predictions or under-predictions of the heat release rate, which will improve model

accuracy. The sign of errant prediction of the WTC fires, if any occurred, would be the prediction of too

short a burning time. Thus, in the simulations of the WTC fires, this output feature should be checked

against observations.

In summary, the peak Q and temperature were predicted within measurement uncertainty. The duration

of significant heating by the fires was also well represented by the model, although the long tail in the

heat release rate curve was under-predicted. Both the peak values and the duration of the heat release rate are important for the WTC simulations because it is not only important to predict the temperatures that the structural steel was exposed to. but also the duration of the exposure. In the actual WTC fires, the

degree of damage to the workstations by aircraft is unknown, which introduces an additional degree of

complexity and uncertainty to the analysis results. The resuhs of this study demonstrate that FDS

provides an adequate representation of the thermal environment during the WTC fires.

8.5 REFERENCES

Hamins, A., Maranghides, A., and Mulholland, G. 2003. The Global Combustion Behavior of 1 MWto 3

MWHydrocarbon Spray Fires Burning in an Open Environment, NISTIR 7013, National Institute of

Standards and Technology, Gaithersburg, MD, June.

Hamins, A., and McGrattan, K. B. 2003. "Reduced-Scale Experiments on the Water Suppression of a Rack-Storage Commodity Fire for Calibration of a CFD Fire Model." Fire Safety Science:

Proceedings ofthe Seventh International Symposium. International Association for Fire Safety

Science, 457-468.

McGrattan, K. B., ed. 2004. Fire Dynamics Simulator (Version 4) , Technical Reference Guide. NIST

Special Publication 1018. National Institute of Standards and Technology, Gaithersburg, MD, July.

McGrattan, K. B., Hamins, A., and Stroup, D. W. 1998. Sprinkler, Smoke and Heat Vent, Draft Curtain

Interaction: Large Scale Experiments and Model Development. NISTIR 6196-1, National Institute of

Standards and Technology, Gaithersburg, MD, September.

Ritchie, S. J.. Steckler, K. D., Hamins, A., Cleary, T., Yang J., and Kashiwagi, T. 1997. "The Effect of

Sample Size on the Heat Release Rate of Charring Materials," Proceedings ofthe Fifth International

Symposium on Fire Safety Science, 177-1 88.

Taylor, R. E., Groot, H., and Terrier, J. 2003. Thermophysical Properties ofPVC, PE, and Marinite,

Report No. TPRL 2958, Thermophysical Properties Research Laboratory, Inc., April.

NISTNCSTAR 1-5E, WTC Investigation 97
Chapter 8

Walton, W. D., and P. H. Thomas. 2003. "Estimating Temperatures in Compartment Fires." Fire

Protection Handbook, 3"^ Ed. National Fire Protection Association, Quincy MA.

98 NIST NCSTAR 1-5E, WTC Investigation
Chapter 9

Summary of Results

A series of large-scale compartment fires experiments was conducted in an effort to validate aspects of

model calculations conducted using the National Institute of Standards and Technology (NIST) Fire

Dynamic Simulator (FDS). This report describes measurements that were conducted to assess the

accuracy of the FDS fire model, which was used to simulate the fires and their thermal impact on

structural components in the investigation of the World Trade Center (WTC) fire and collapse. This

study involved a series of well-controlled experiments that were designed to recreate aspects of the WTC

fire, including issues associated with limited ventilation, fire spread and growth on real furnishings, the

effect of inert ceiling tiles, and the effects of debris and jet fuel on the heat release rate of the fires. The

experimental compartment was instmmented so that several aspects of the model were tested. Model

calculations were performed before testing began to provide a baseline set of "blind" predictions. The

objective of the study was to assess the accuracy with which the NIST FDS predicted the thermal

environment in a burning compartment and to establish a data set to validate the prediction of the heat

release rate associated with the burning of the furnishings. The accuracy of the model was assessed with

consideration of experimental uncertainty.

Using a large compartment, office workstations were burned under conditions that recreated aspects

similar to those that occurred on September 11, 2001. Three computer workstations that were composed

of tables, desks, fabric-lined partitions, carpeting, a task chair, paper-filled filing cabinets and

bookshelves, a personal computer, keyboard, and monitor, were placed within a steel-frame compartment

(nominally 3 m by 7 m by 4 m high) lined with calcium silicate boards. The workstations were

representative of typical office furnishings, not unlike those found in WTC 1, 2, and 7. The same

configuration of furnishings was used in all of the experiments, except one test in which the workstation

components were rearranged and broken into component pieces to represent a disrupted (non-standard)

configuration, which may have occurred after aircraft entered the WTC structure. In some experiments,

several liters ofjet fuel were distributed about the workstation components, and in some experiments,

ceiling tiles were distributed about the compartment. A 2 MW hydrocarbon fire, burning for as long as 10 min, was used to ignite the compartment furnishings. The fire was generated by a liquid fuel spray, which was directed onto a 1 m by 2 m pan from four nozzles. The fuel used was a commercial blend of

heptane isomers.

Six fire experiments were conducted, and nearly 50 instnuTients were used, to measure a number of

important variables, including the heat release rate of the fire, and the vertical profiles of gas phase

temperature. The overall fire behavior was documented using video cameras viewing the fire from

different orientations. Following the establishment of baseline signals from all the measurement devices,

the spray burner, and the jet fuel, when present, were ignited. The spray fire continued to bum at a steady

rate for 10 min (or 2 min ifjet fuel was present). The fire itself spread through the workstations and was

allowed to bum until the compartment contents had become an unrecognizable charred jumble and the

NISTNCSTAR 1-5E, WTC Investigation 99
Chapter 9

heat release rate was small, which was typically 1 hour after fire initiation. The measurements led to findings regarding the thermal behavior of the compartment and the importance of the jet fuel and the

presence of the ceiling tiles:

• In the presence ofjet fuel, fire spread rapidly, and the heat release rate rapidly increased,

reaching a maximum within a few minutes. ,

'

• The ceiling tiles had little apparent impact on the rate of fire spread or the heat release rate.

• Upper layer temperatures obtained values as high as 800 to 1 ,200 "C after 20 min to 30 min of exposure, depending on the test conditions and the location within the

compartment.

• The measured mass loss burning rate of workstations reconfigured into "rubble" to represent

what may have occurred after aircraft entered the WTC structure was about 40 percent

smaller than workstations burning in a standard configuration.

• Comparison of the measured heat release rates to analogous measurements for single

workstations burning in the open suggested that the burning conditions were underventilated

or oxygen limited.

The FDS software was used to simulate the heat release rate and thermal environment in a complex fire environment involving limited ventilation, realistic furnishings, and a compartment configuration that mimicked many aspects of the WTC buildings. The numerical accuracy of the FDS predictions was

quantitatively compared with the measurements. The findings can be summarized as follows:

• FDS was able to accurately predict the general shape and magnitude of the time dependent

heat release rate.

• FDS predicted the time at which one-half of the energy was released to within approximately

3 min or 22 percent of the measurements.

• FDS predicted the value of the heat release rate when one-half of the energy was released to within 1 . 1 MW or 9 percent of the measurements on average, which was good agreement

considering the measurement uncertainty.

• FDS predicted the duration of significant heating of the fires to within approximately 6 min

or 1 5 percent of the measurements on average, although the long tail in the heat release rate curve was under-predicted.

• FDS predicted the peak upper layer gas temperature to within approximately 10 percent of

the measurements on average, which was accurate considering model sensitivity to input

parameters such as the heat release rate and uncertainties associated with the measurement.

The results of the comparison of the FDS calculations with measurements yielded confidence in the

application of FDS to the analysis of the thermal environment in compartments with office workstations

experiencing a fire, which is a key part of the WTC hivestigation.

100 NISTNCSTAR 1-5E, WTC Investigation
Appendix A

Location of Compartment Contents and Instrumentation

The detailed coordinates of essential compartment contents and the instrumentation are listed in Tables A-1 and A-2, respectively. The origin (0,0,0) is located in the southwest comer of the compartment, inside the external pillars, just outside the kneewall and on the floor as shown in Fig. 2-1.

All measurements are relative to the origin.

Table A-1. Location o\ compartment contents.

Location (cm)

Compartment Item x-east y-north z-up

Fire pan SW comer 38 67 96

Fire pan NW comer 1 19 64 95

Fire pan SE comer 42 267 95

Fire pan NE comer 124 264 95

Workstation 1 SW comer 161 46 155 Workstation 1 NW comer 164 103 121 Workstation 1 SE comer 417 48 122 Workstation 1 NE comer 419 305 121 Workstation 2 SW comer 410 49 157

Workstation 2 NW comer 420 306 121 Workstation 2 SE comer 669 46 122

Workstation 2 NE comer 678 303 122 Workstation 3 SW comer 420 397 1 22

Workstation 3 NW comer 420 655 157

Workstation 3 SE comer 678 393 122

Workstation 3 NE comer 678 655 1 ''2

NISTNCSTAR 1-5E, WTC Investigation 101
Appendix A

Table A-2. Location of instrumentation.

Location (cm)

Instrument x-east y-north z-up Thermocouple Tree 1 285 351 62, 153,245,333'

Thermocouple Tree 2 564 351 62, 153,245,333'

Themiocouple Tree 3 821 351 62, 153,245,333'

Thennocouple Tree 4 969 575

32, 122,62, 153,245,

T T -^a. b J J J Asnirated thermocounles J / o ^7 1 77 jZ, IZZ

I Inner laver pas samnlinc y 1 yj 1 Z77^^/ D Workstation 1 thermocouple, underside west oU

Workstation TV V-fl IVO LUi LI WXl 11 thermocoiinle 111^ 1 1. llV-f^ V-F 1.1 L/l W ^ deskton VJWl31W\-'L/ ^ 1 ^ 80 77

NVorkststion 1 thermocouple, underside es^st J> / H 80 74

Workstation 2, thermocouple, underside west AIQ H / y 87o / lA

Wnrk<st?itinn ^ thprmoroiinlp 7 rlesVton JO / 0 / /o

VV wl J\.J> LallUll Z- IIICIIIUJL'UUUIC, UllVJClolUC CaoL 618 87 74

Workstation 3 thermocouple, underside west 461 610 74

Workstation 3 thermocouple, desktop 519 610 76

Workstation 3 thennocouple, underside east 611 610 74

Video cameras:

1. external: looking from outside through vents 350 ^-1.300 «60

2. internal: 125 700 9

3. internal: 1.089 160 9

Calorimetry system Exhaust hood sampling

a. Thermocouple trees had four thermocouples, located at different heights.

b. Two additional thermocouples (32 cm and 122 cm above the floor) were added to Tree 4 during Tests 1 and 2.

102 NISTNCSTAR 1-5E, WTC Investigation
Appendix B

Gas Analyzer Data

The gas analyzer data sheet shown below provides information on the gas analyzers used for calorimetry

and for the determination of gas concentrations from sampling within the compartment.

CONTINUOUS GAS SAMPLING DATA SHEET

Date: Test: DAQ File Name: DAQ VI name:

Calorimeter Analyzers

Servomex 540A O2 Analyzer

0-25% = 0-1 Volt Zero gas: N2

NIST ID: 549709 S/N: Flow: Bypass 2.4, Sample 4 (no units) Span gas: Breathing Grade Compressed Air Siemens Ultramat 5E CO Analyzer

0-20 mA Output, 249Q 0.1% resistor Zero Gas: N2

Ranges: 0-3/5/10/20%

NIST ID: 559934 S/N: D8-562

Flow: 11pm

Span Gas: Matheson 4.004% CO, 9.017%

CO2, N2 balance, Lot# 102-36-00269, Cyl. #

17453P, Exp. 01/21/06, NIST Cert: 194474-1

Range Constants: 0.620/1.034/2.069/4.0

Siemens Ultramat 5E CO2 Analyzer

0-20 mA Output, 249Q 0.1% resistor Zero Gas: N2

Ranges: 0-5/10/30/50%

NIST ID: 561170 S/N: DN-683

Flow: 11pm

Span Gas: Matheson 4.004% CO, 9.017%

CO2, N2 balance, Lot# 102-36-00269, Cyl. #

17453P. Exp. 01/21/06, NIST Cert: 194474-1

Range Constants: 1.033/2.026/6.203/4.0

Ceiling Sample - Rack #2

Columbus Instruments Paramax 101 0-25% = 4-20 mA, 249Q 0.1% resistor Zero gas: N2NIST ID: 605206 S/N: 02470-1 -w/Display

Flow: 2 SCFH (0.94 LPM)

Span gas: Breathing Grade Compressed Air Servomex 4100 Gas Purity 02 analyzer NIST ID: 609438

0-25% = 0-20 mA, 249Q 0.1% resistor Flow: 200 mL per minute Zero gas: N2 Span gas: Breathing Grade Compressed Air Siemens Ultramat 5E CO Analyzer

0-20 mA Output, 2490 0.1% resistor Zero Gas: N2

Ranges: 0-3/5/10/20%

NIST ID: 566024 S/N: E9-955

Flow: 11pm

Span Gas: Matheson 7.0018% CO,

18.0012% CO2, N2 balance, Lot# 102-26- 2403, Cyl. # AH019996, Exp06/19/05, NIST

Cert: 107-304

Range Constants: 0.623/1.033/2.066/4.0

Siemens Ultramat 5E CO2 Analyzer

0-20 mA Output, 2490 0.1% resistor Zero Gas: N2

Ranges: 0-5/10/30/50%

NIST ID: 566026 S/N: E9-959

Flow: 11pm

Span Gas: Matheson 7.0018% CO,

18.0012% CO2, N2 balance, Lot# 102-26- 2403, Cyl. #AH01 9996, Exp06/19/05, NIST

Cert: 107-304

Range Constants: 1.036/2.069/6.217/4.0

NISTNCSTAR 1-5E, WTC Investigation 103
Appendix B

\

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104 NISTNCSTAR 1-5E, WTC Investigation
Appendix C

Gas Phase Temperatures

Chapter 6 presents representative measurements of the gas phase temperature. This appendix presents the

measured time-temperature histories (Figs. C-1 through C-12) recorded by the bare bead thermocouples

located on Thermocouple Trees 2 and 3 during Tests 1 tlirough 6.

T 1 1—

r

T—I 1—

r

Q.

E

hO 1000

o

15

—o--2.5 cm

-91 cm

-183 cm — -274 cm

2000

Time (s)

3000 4000

Figure C-1 . Bare bead thermocouple measurements as a function of time at four

locations (below the ceiling) on Tree 2 during Test 1.

NISTNCSTAR 1-5E, WTC Investigation 105
Appendix C

ooE

Z5

CDQ.

E

T 1 1 r I I I

I

I I —O—2.5 cm —^—183 cm

# 274 cm

J I I L J L. 1000 2000

Time (s)

3000 4000

Figure C-2. Bare bead thermocouple measurements as a function of time at four

locations (below the ceiling) on Tree 2 during Test 2. After approximately 1,000 s, the

thermocouples failed in this test.

1200

Oo

Z5

Q.

E

CD

1000 -

800 -

600 -

400

200 -

Tests

Tree 2

2.5 cm

• Q 1 Qft]

183 cm

274 cm

J I L J 1 I 1 I I L 1000 2000

Time (s)

3000 4000

Figure C-3. Bare bead thermocouple measurements as a function of time at four

locations (below the celling) on Tree 2 during Test 3. After approximately 800 s, the

thermocouples failed in this test.

106 NISTNCSTAR 1-5E, WTC Investigation
Gas Phase Temperatures

J L J I 1000

J L ± J L I ' L 2000

Time (s)

3000 4000

Figure C-4. Bare bead thermocouple measurements as a function of time at four

locations (below the ceiling) on Tree 2 during Test 4. After approximately 750 s, the

thermocouples failed in this test.

1000

> —2.5 cm

"-CI— 91 cm

183 cm

274 cm

2000

Time (s)

4000

Figure C-5. Bare bead thermocouple measurements as a function of time at four

locations (below the ceiling) on Tree 2 during Test 5.

NISTNCSTAR 1-5E, WTC Investigation 107
Appendix C

Oo

CD

•13

. TO

CDQ.

E

(D

1200

1000

800

600

400

200

0

ji |! Test 6

i44 Tree 2

2.5 cm

91 cm

183 cm

274 cm

H V500 1000 1500 2000 2500 3000 3500 4000

Time (s)

Figure C-6. Bare bead thermocouple measurements as a function of time at four

locations (below the ceiling) on Tree 2 during Test 6.

Oo

ZJ0

Q.

E

0

I1200

1000

800

600

400

200

0

]—I—i—I—[—

I—I—I—I—

r

- Test 1 - Tree 3

T—I—I—I—

I

1—I—I—I—

I—

r

1—

r

—o--2.5 cm

1^ -91 cm

-183 cm — -274 cm

I I I I

I I ! I I I I L

2000 3000

Time (s)

5000

Figure C-7. Bare bead thermocouple measurements as a function of time at four

locations (below the ceiling) on Tree 3 during Test 1.

108 NISTNCSTAR 1-5E, WTC Investigation
Gas Phase Temperatures

1

I I I I

I

I I I I

I

I I I I I

I I I I I

I I I r

Time (s)

Figure C-8. Bare bead thermocouple measurements as a function of time at three

locations (below the ceiling) on Tree 3 during Test 2.

1200

1000

oo

ZDB

Q.

E

CD I0 tC—J I I I I I I I I I I I I I I I 1 1 1 1 1 1 1 L 0 1000 2000 3000 4000 5000

Time (s)

Figure C-9. Bare bead thermocouple measurements as a function of time at four

locations (below the ceiling) on Tree 3 during Test 3.

NISTNCSTAR 1-5E, WTC Investigation 109
Appendix C

ZJ

CDCL

E0

1400

1200 - O 1000

o

—O—2.5 cm —^— 183 cm • 274 cm

0 Hp '

I I

'

I I

' ' I ' '

I I I I I ' ' 'I

' '

I I

' 0 500 1000 1500 2000 2500 3000 3500 4000

Time (s)

Figure C-10. Bare bead thermocouple measurements as a function of time at four

locations (below the ceiling) on Tree 3 during Test 4.

Oo

CDQ.

E

(D

1000

800

600

400

200

Q I 1 I I I I I I I I I I I I I I I I I I I I I I I I I I I I I 1 I I I I I I 0 500 1000 1500 2000 2500 3000 3500 4000

Time (s)

Figure C-11. Bare bead thermocouple measurements as a function of time at four

locations (below the ceiling) on Tree 3 during Test 5.

110 NISTNCSTAR 1-5E, WTC Investigation
Gas Phase Temperatures

1000 I

I I I I I I I I I

[

I I

I I I I

I

I I I ]—n—I—1—

I—I—rn—i—|—i—i~i—

r

'Test 6

-Tree 3 —o—2.5 cm

Oo

E

CD

0

0 500 1000 1500 2000 2500 3000 3500 4000

Time (s)

Figure C-12. Bare bead thermocouple measurements as a function of time at four

locations (below the ceiling) on Tree 3 during Test 6.

NISTNCSTAR 1-5E, WTC Investigation
Appendix C

\

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112 NISTNCSTAR 1-5E, WTC Investigation
Appendix D

Comparison of Simulations and Measurements

Chapter 8 presents representative comparisons of the fire simulations and the measurements of the gas

phase temperatures and the volume fraction of the gas phase species. This appendix presents data plots

(Figs. D-1 through D-20) not shown in Chapter 8 that are analogous to Figs. 8-3 through 8-6 for temperature.

1200

4000

Time (s)

Figure D-1. Comparison of the measured and simulated temperatures at 2.5 cm and

274 cm below the ceiling on Tree 1 (rear of the compartment) during Test 2.

NISTNCSTAR 1-5E, WTC Investigation 113
Appendix D

1200

2.5 cm (Exp) —- 274 cm (Exp)

2,5crn(FDS)

...... 274CI1UFDS)

iUOO 3000 4000

Time (s)

Figure D-2. Comparison of the measured and simulated temperatures at 2.5 cm and

274 cm below the ceiling on Tree 2 (middle of the compartment) during Test 2. After

approximately 1,000 s, the thermocouples failed.

1200

1000 -

Q 800

f 600

i- 400 -

200

Test 2, Tree 3 2.5 cm (Exp)

274 cm (Ex-p)

2,5cm(FDS)

274 cm (FDS)

iooo 2000 5000 4000

Time (s)

Figure D-3. Comparison of the measured and simulated temperatures at 2.5 cm and

274 cm below the ceiling on Tree 3 (middle of the compartment) during Test 2.

114 NISTNCSTAR 1-5E, WTC Investigation
Comparison of Simulations and Measurements

g

£

1200

1000 -

800 -

600 -

400

200 -

Test 2, Tree 4 2.5 cm (Exp)

274 cm (Exp)

2.5 cm (FDS)

4000

Time (s)

Figure D-4. Comparison of the measured and simulated temperatures at 2.5 cm and

274 cm below the ceiling on Tree 4 (rear of the compartment) during Test 2.

0 1000 2000 3000 4000

Time (s)

Figure D-5. Comparison of the measured and simulated temperatures at 2.5 cm and

274 cm below the ceiling on Tree 1 (front of the compartment) during Test 3.

NISTNCSTAR 1-5E, WTC Investigation 115
Appendix D

1200

1000 -

800 -

600 - I

400 -

200 -

1000 ^000 3000 4000

Time (s)

Figure D-6. Comparison of the measured and simulated temperatures at 2.5 cm and

274 cm below the ceiling on Tree 2 (middle of the compartment) during Test 3. After

approximately 800 s, the thermocouples failed in this test.

¥

3

1200

1000 -

800 -

600

400 -

200

Test 3, 1 ree 3 2.5 ctn (Exp)

274 cm (Exp)

2,5ciTi(FDS)

274 cm (FDS)

1000 2000

Time (s)

3000 4000

Figure D-7. Comparison of the measured and simulated temperatures at 2.5 cm and

274 cm below the ceiling on Tree 3 (middle of the compartment) during Test 3.

116 NISTNCSTAR 1-5E, WTC Investigation
Comparison of Simulations and Measurements

1200

4(K)() Time (s)

Figure D-8. Comparison of the measured and simulated temperatures at 2.5 cm and

274 cm below the ceiling on Tree 4 (rear of the compartment) during Test 3.

0 iOOO 2000 3CK)0 4000

Time (s)

Figure D-9. Comparison of the measured and simulated temperatures at 2.5 cm and

274 cm below the ceiling on Tree 1 (front of the compartment) during Test 4.

NISTNCSTAR 1-5E, WTC Investigation 117
Appendix D

£

1200

1000 -

800 -

3^ 600 -

400

200

Test 4, Tree 12

iOOO 2{K)()

Time (s)

2.5 cm (Exp)

274 cm (Exp)

2.5 cm (FDS)

274 cm (FDS)

lOOO 4000

Figure D-10. Comparison of the measured and simulated temperatures at 2.5 cm and

274 cm below the ceiling on Tree 2 (middle of the compartment) during Test 4. After

approximately 750 s, the thermocouples failed.

1200

1000

g 800

3

£ 600 H o

Q.

E

h~ 4(>()

200

Test 4. Tree 3

louo 200(1

Time (s)

2.5 cm (Exp) 274 cm (Exp)

2.5 cm (FDS)

274 cm (FDS)

3000 4000

Figure D-11. Comparison of the measured and simulated temperatures at 2.5 cm and

274 cm below the ceiling on Tree 3 (middle of the compartment) during Test 4.

118 NISTNCSTAR 1-5E, WTC Investigation
Comparison of Simulations and Measurements

mo

1000 -

800 -

600 -

400 -

200 -

4000

Figure D-12. Comparison of the measured and simulated temperatures at 2.5 cm and

274 cm below the ceiling on Tree 4 (rear of the compartment) during Test 4.

Time (s)

Figure D-13. Comparison of the measured and simulated temperatures at 2.5 cm and

274 cm below the ceiling on Tree 1 (front of the compartment) during Test 5.

NISTNCSTAR 1-5E, WTC Investigation 119
Appendix D

1200

4000

Time (s)

Figure D-14. Comparison of the measured and simulated temperatures at 2.5 cm and

274 cm below the ceiling on Tree 2 (middle of the compartment) during Test 5.

1200

1000

800 -

c« 600 -

a.

400 -

200 -

1000 2000 5000 4000

Time (s)

Figure D-15. Comparison of the measured and simulated temperatures at 2.5 cm and

274 cm below the ceiling on Tree 3 (middle of the compartment) during Test 5.

120 NISTNCSTAR 1-5E, WTC Investigation
Comparison of Simulations and Measurements

1000 2000 3000 4000

Time (s)

Figure D-16. Comparison of the measured and simulated temperatures at 2.5 cm and

274 cm below the ceiling on Tree 4 (rear of the compartment) during Test 5.

0 1000 2000 3000 4000

Time (s)

Figure D-17. Comparison of the measured and simulated temperatures at 2.5 cm and

274 cm below the ceiling on Tree 1 (front of the compartment) during Test 6.

NISTNCSTAR 1-5E. WTC Investigation 121
Appendix D

1200

0 KlUU 2000 3000 4000

Time (s)

Figure D-18. Comparison of the measured and simulated temperatures at 2.5 cm and

274 cm below the ceiling on Tree 2 (middle of the compartment) during Test 6.

1200

4000

Time (s)

Figure D-19. Comparison of the measured and simulated temperatures at 2.5 cm and

274 cm below the ceiling on Tree 3 (middle of the compartment) during Test 6.

122 NISTNCSTAR 1-5E, WTC Investigation
Comparison of Simulations and Measurements

1200

1000

800 -

§ 600

£

400 -

200 -

Test 6 Tree 4 2,5 cm (Exp)

274 cm (Exp)

2.5cm(FDS)

274 cm(FDS)

i 1 T f 1000 2000 3000 4000

Time (s)

Figure D-20. Comparison of the measured and simulated temperatures at 2.5 cm and

274 cm below the ceiling on Tree 4 (rear of the compartment) during Test 6.

NISTNCSTAR 1-5E, WTC Investigation 123
Appendix D

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124 NISTNCSTAR 1-5E, WTC Investigation