NIST Technical Note 1856
 

Simulation of a Fire in a Hillside

Residential Structure ­

San Francisco, CA

Kristopher J. Overholt
 

Craig G. Weinschenk
 

Daniel Madrzykowski
 

This publication is available free of charge from: 

http://dx.doi.org/10.6028/NIST.TN.1856 

NIST Technical Note 1856

Simulation of a Fire in a Hillside

Residential Structure ­

San Francisco, CA

Kristopher J. Overholt
 

Craig G. Weinschenk
 

Daniel Madrzykowski
 

Fire Research Division 

Engineering Laboratory 

This publication is available free of charge from: 

http://dx.doi.org/10.6028/NIST.TN.1856 

November 2014 

UNITED STATES OF AMERICA

DEPARTMENT OF COMMERCE

U.S. Department of Commerce 

Penny Pritzker, Secretary 

National Institute of Standards and Technology 

Willie May, Under Secretary of Commerce for Standards and Technology and Acting Director 
Certain commercial entities, equipment, or materials may be identified in this 

document in order to describe an experimental procedure or concept adequately. 

Such identification is not intended to imply recommendation or endorsement by the 

National Institute of Standards and Technology, nor is it intended to imply that the 

entities, materials, or equipment are necessarily the best available for the purpose. 

National Institute of Standards and Technology Technical Note 1856

Natl. Inst. Stand. Technol. Tech. Note 1856, 48 pages (November 2014)

CODEN: NTNOEF

This publication is available free of charge from:

http://dx.doi.org/10.6028/NIST.TN.1856
Contents

Contents iii
 

List of Figures v
 

List of Tables vi
 

List of Acronyms vii
 

List of Symbols viii
 

1 Introduction 1
 

2 Summary of the Fire Incident 2
 

3 Model Description 4
 

3.1 Geometry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
 

3.2 Fires . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
 

3.3 Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
 

3.4 Ventilation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
 

3.5 Numerical Mesh . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
 

3.6 Summary of Model Input Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . 16
 

4 Simulation Results 17
 

4.1 Heat Release Rate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
 

4.2 Pressure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
 

4.3 Velocity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
 

4.4 Temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
 

5 Discussion of Simulation Results 31
 

5.1 Simulated Interior Stairwell Flow Path . . . . . . . . . . . . . . . . . . . . . . . . . 31
 

5.2 Assessing the Hazard . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
 

5.3 Tactical Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
 

6 Summary 39
 

References 41
 

iii
 
A Dimensioned Drawings 45

iv
 
List of Figures
 

3.1 Plan view of the second floor, first floor, and basement floor. . . . . . . . . . . . . . 6
 

3.2 Front and rear sides of the structure after the incident. . . . . . . . . . . . . . . . . 7
 

3.3 Location of fuel items in the basement. . . . . . . . . . . . . . . . . . . . . . . . . 8
 

3.4 Prescribed HRR vs. time for the simulation. . . . . . . . . . . . . . . . . . . . . . . 10
 

3.5 Front and rear sides of the structure with a 10 cm computational mesh. . . . . . . . 14
 

3.6 Rear-left side of the structure within the computational domain. . . . . . . . . . . . 15
 

4.1 Prescribed and calculated HRR vs. time from the simulation. . . . . . . . . . . . . 19
 

4.2 Snapshots of exterior combustion occurring on the rear side of the structure. . . . . 21
 

4.3 Snapshot of smoke flowing out of the doors on the front side of the structure. . . . . 22
 

4.4 Simulated pressures on the right interior side of the structure. . . . . . . . . . . . . 24
 

4.5 Simulated velocities on the right interior side of the structure. . . . . . . . . . . . . 26
 

4.6 Simulated velocities on the right interior side of the structure. . . . . . . . . . . . . 27
 

4.7 Simulated temperatures on the right interior side of the structure. . . . . . . . . . . 29
 

4.8 Simulated temperatures on the right interior side of the structure. . . . . . . . . . . 30
 

5.1 Top view of the simulated flow path in the interior stairwell. . . . . . . . . . . . . . 32
 

5.2 Simulated temperatures on the first floor before basement window failure. . . . . . . 33
 

5.3 Simulated temperatures on the first floor after basement window failure. . . . . . . . 34
 

5.4 Simulated temperatures in the garage after basement window failure. . . . . . . . . 35
 

A.1 Dimensioned drawing of the second floor. . . . . . . . . . . . . . . . . . . . . . . . 46
 

A.2 Dimensioned drawing of the first floor. . . . . . . . . . . . . . . . . . . . . . . . . 47
 

A.3 Dimensioned drawing of the basement floor. . . . . . . . . . . . . . . . . . . . . . 48
 

v
 
List of Tables

2.1 Abridged NIOSH approximate fire event timeline . . . . . . . . . . . . . . . . . . . 3
 

3.1 Timeline of ventilation events in the simulation . . . . . . . . . . . . . . . . . . . . 13
 

3.2 Fire model input parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
 

4.1 Fire incident and simulation event timeline . . . . . . . . . . . . . . . . . . . . . . 18
 

5.1 Flow path related LODD/LODI incidents . . . . . . . . . . . . . . . . . . . . . . . 38
 

vi
 
List of Acronyms
 

BC Battalion Chief 

E Engine 

FDS Fire Dynamics Simulator 

HGL Hot Gas Layer 

HRR Heat Release Rate 

HRRPUA Heat Release Rate per Unit Area 

IC Incident Command 

LODD Line of Duty Death 

LODI Line of Duty Injury 

NIOSH National Institute for Occupational Safety and Health 

NIST National Institute of Standards and Technology 

SFPE Society of Fire Protection Engineers 

UL Underwriters Laboratories 

vii
 
List of Symbols
 

ft foot 

m meter 

min minute 

Pa Pascal 

psi pound per square inch 

s second 

W Watt 

viii 
Abstract
 

Fire Dynamics Simulator (FDS), which is a fire model that is developed and maintained by the National Institute of Standards and Technology (NIST), was used to provide insight into the dynamics 

of a fire that occurred on June 2, 2011, within a multi-level, single-family residential structure in 

San Francisco, CA, that resulted in the death of two firefighters. The inputs for the FDS simulation 

are documented in this report and are based on the fire scenario, including the building geometry, 

interior furnishings, and ventilation conditions. The fire started in the basement and resulted in 

ventilation limited (fuel rich) fire conditions in the basement area. After the rear basement windows began to fail, the interior stairwell acted as a chimney for hot gases in the basement to flow 

towards regions of lower pressure and vent openings located on the front side of the structure. The 

temperature of the gases in the interior stairwell was estimated to be in excess of 700 ◦C (1300 ◦F). 

Two firefighters were located in the flow path between the basement and the doors on the front 

side of the structure, were exposed to these elevated temperatures, and later died as a result of their 

injuries. 
Section 1

Introduction

Part of the function of the Fire Research Division at the National Institute of Standards and Technology (NIST) is to develop and apply technology, measurements, and standards to improve the 

understanding of the behavior, prevention, and control of fires to enhance fire fighting operations 

and equipment, fire suppression, fire investigations, and disaster response. NIST has previously 

used Fire Dynamics Simulator (FDS) to provide insight into the fire development and thermal conditions of fires that have resulted in line of duty deaths (LODDs) [1–6]. The objective of these 

studies has been to improve the safety and effectiveness of firefighters. 

On June 2, 2011, a fire in a multi-level, wood-frame residential structure claimed the life of 

two firefighters of the San Francisco Fire Department. NIST examined the fire dynamics of this 

incident at the request of the National Institute for Occupational Safety and Health (NIOSH) and 

the San Francisco Fire Department. A computer simulation of the fire incident was conducted using 

Fire Dynamics Simulator [7] and Smokeview [8] to provide insight into the fire development and 

thermal conditions that likely existed in the residence during the fire. The specific objectives of the 

simulation detailed in this report are: 

1. To examine the effect of fire-induced flow paths (including temperature, pressure, and fire 

conditions) in this multi-level residential structure using physics-based calculations. 

2. To provide visualizations of the fire behavior that are representative of the conditions that 

members of the San Francisco Fire Department likely experienced during the course of their 

interior operations. 

This document describes the input and the results of the FDS (version 6.1.2) simulation. The simulation was developed using a combination of knowledge of the fire scenario and appropriate engineering approximations and assumptions. Analysis of the simulation results focuses on the hazardous conditions that developed following the development of a flow path inside of the structure. 

This document is organized as follows: Section 2 provides a summary of the fire incident, Section 3 

describes the relevant model input parameters and assumptions that were used to develop the simulation, Section 4 presents the simulation results, and Section 5 discusses the simulation results as 

they relate to firefighter safety and effectiveness. Appendix A contains dimensioned drawings of 

the basement, first, and second floors of the structure. 

1
 
Section 2

Summary of the Fire Incident

On the morning of June 02, 2011, the San Francisco Fire Department was dispatched to a residence 

based on a report of curtains on fire. The account of events for this incident was documented in 

NIOSH Fire Fighter Fatality Investigation and Prevention Program Report #F2011-13 [9]. For this 

analysis, the details regarding the timeline are considered to be approximate values. The following 

narrative of the incident was extracted from the NIOSH incident report [9]: 

On June 02, 2011, a 48 year-old career lieutenant and a 53 year-old fire fighter/paramedic 

died in a multi-level residential structure fire while searching for the seat of the fire. Note: 

The residential structure where the fatalities occurred was built on a significantly sloped 

hillside common throughout the city. The fire floor was one floor below street level. Six 

companies and three command chiefs were dispatched to a report of an electrical fire at a 

residential home. 

When Engine 26, staffed with a lieutenant, fire fighter/paramedic, and driver arrived at approximately 1048 hours, they noticed light smoke showing as they made entry through the 

front door, side A, street level, of the building. Minutes later, the incident commander (IC) 

tried contacting them over the radio, but received no response. A battalion chief (BC) assigned to “the fire attack group” followed the hoseline through the door and spoke to the 

[lieutenant and fire fighter/paramedic] on the street level floor. The lieutenant stated to the 

BC that the fire must be a floor below them. The BC stated they would attack the fire from 

the [left side] of the structure and exited the front door. The [lieutenant and fire fighter/­

paramedic] did not follow. A few minutes later the IC again tried to contact Engine 26 via 

radio with no response. 

The crew from Engine 24, assigned to back up Engine 26, and a split crew from Rescue 1 

tried to make entry through the door in the garage but could not advance due to the heat. 

The BC went to the [door on the left side of the structure], located one floor below street 

level, and forced the door with the Engine 11 crew on the hoseline. They immediately felt 

a blast of heat from the fully involved basement area. The Rescue 1 crew backed out of the 

garage and re-entered on [the left side of the structure] after the Engine 11 crew knocked 

down the large room and contents fire. At about the same time, the Engine 24 crew also 

backed out of the garage and followed the Engine 26 crew’s hoseline through the front 

door. In zero visibility conditions, separate members of the Engine 24 crew independently 

found a downed member of the Engine 26 crew. 

2
 
The Incident Commander was alerted of a downed fire fighter but, did not initially realize, 

until moments later that it was actually two downed fire fighters. Both [fire fighters] were 

removed from the structure and immediate medical treatment was provided. The [fire fighters] were transported to the local medical center where the lieutenant was pronounced dead 

and the fire fighter/paramedic died two days later. 

Table 2.1 shows an overview of the timeline of events. The following section includes a plan view 

of the structure, the location where the downed firefighters were located, and more details of the 

fire development. 

Table 2.1: Abridged NIOSH approximate fire event timeline [9] 

Incident Time Fire Behavior / Fireground Operation 

(hh:mm:ss) 

10:45:00	 Dispatch for a curtain fire due to an electrical short at a residential structure. 

10:48:00	 E26 arrives on scene, reports light smoke conditions, and makes entry into 

the front door with a 1 3⁄4 in hoseline. 

10:54:00	 E26 crew states that the fire is located below the first floor. 

10:56:00	 BC9 observes smoke but no fire at the left rear corner of the structure. 

10:58:42	 Fire self-vents from the rear side of the structure when a glass window in the 

basement fails. Additional glass windows on the rear side of the structure fail 

within the next two minutes. 

10:59:23	 BC9 forces open the basement door on the left side of the structure, 

reports heavy fire and smoke, and requests a second hoseline. 

11:00:00	 BC6 notices a severe change in conditions (heavy black smoke from garage). 

11:01:00	 Heavy fire and black smoke observed at rear of structure. Incident command 

(IC) attempts to contact E26 several times via radio with no reply. 

11:02:00	 E32 crew makes entry into the basement with a second hoseline and begins 

suppression operations. 

11:08:00	 Two downed E26 firefighters are found on the first floor. 

11:09:00	 The downed E26 firefighters are carried out of the structure. 

3
 
Section 3

Model Description

Fire Dynamics Simulator [7] is a computational fluid dynamics (CFD) model developed and maintained by NIST that solves a form of the Navier-Stokes equations appropriate for low-speed, thermally driven flow with an emphasis on smoke and heat transport from fires. Within a CFD model, 

the room or building is divided into small three-dimensional rectangular control volumes or computational cells. The cells are contained together within a larger volume known as the computational 

domain. The CFD model computes the density, velocity, temperature, pressure, and gas concentrations in each cell. Based on the laws of conservation of mass, momentum, and energy, the model 

tracks the generation and movement of fire gases. One of the most important aspects of FDS is 

that it is mathematically verified [10] and validated against fire test data [11] to ensure that it is 

accurate and provides the expected results, given appropriate input data. A complete description of 

the FDS model is provided in the FDS Technical Reference Guide [12]. 

Smokeview is a software tool designed to visualize simulation results from FDS [8]. Smokeview visualizes smoke and other attributes of the fire simulation by showing tracer particle flow 

and two dimensional (2D) or three dimensional (3D) shaded contours of gas flow data, including 

temperature and flow vectors showing flow direction and magnitude. Smokeview has the ability 

to visualize fire and smoke by displaying a series of partially transparent planes where the transparencies in each plane (at each grid node) are determined from soot densities computed by FDS. 

Smokeview can also visualize data at particular snapshots in time using 2D or 3D contours of data 

that show temperature, flow direction, and flow magnitude. 

Input data from various sources must be collected and documented to simulate a fire using 

FDS or any other fire model. For the simulation results presented in this document, information 

was obtained from two primary sources. The following information was gathered from the fire 

scene: geometry of the building and the compartments being modeled, size and location of exterior 

and interior ventilation openings, and fire damage to the building. The following information was 

gathered from witnesses, first responders, reports from the San Francisco Fire Department and 

NIOSH, and recorded media such as fire ground radios or photographs: information on the fire 

development timeline, sequence and approximate timing of ventilation openings to the outside, 

and weather conditions. 

4
 
The analysis of the simulation results is focused on the interior conditions on the first floor 

where the downed firefighters were located and the conditions that resulted in the establishment of 

a flow path in the interior stairwell leading into the basement. Based on the goals of the analysis 

and the information collected, several assumptions were made. For example, the fire was in the 

early growth stage when firefighters arrived [9]; however, the exact ignition time, fire growth rate, 

and transient heat release rate (HRR) of the fire were not known. The potential fuel load within the 

structure was estimated from post-incident photographs and the timeline from the NIOSH incident 

report. 

3.1 Geometry

The structure involved in this fire incident was a four-story residential building with a flat roof. 

The exterior was primarily stucco with large windows on the top three stories of the rear side of 

the structure. The structure had exterior dimensions of 8.5 m (28 ft) by 17.7 m (58 ft) and a height 

of 5.9 m (19 ft) above street level, as shown in Fig. 3.1. The right side of the structure shared a 

common fire wall with another residential structure. 

The structure was built on a sloped landscape such that, from the front side of the structure, 

two stories were located at and above street level, and two stories were located below street level. 

The first floor and second floor (i.e., the two stories at and above street level) each had an interior 

floor area of 136 m2 (1470 ft2). The interior walls were composed of gypsum board over insulated 

wood framing. The entrance stairs located on the front side of the structure led to the front door 

and an interior landing, which allowed access to the first floor (ten stairs down) or the second floor 

(five stairs up). 

The basement (i.e., the first story below street level) had a total floor area of 121 m2 (1300 ft2) 

and consisted of a finished living area and a large utility area. A laundry area was located on a 

landing on the interior stairwell between the first floor and basement. The basement was accessible 

via this interior set of stairs or via an exterior door on the left side of the structure. The basement 

had an exterior balcony on the rear side of the structure. The sub-basement level (i.e., the second 

story below street level) was not connected to the other three levels of the house, contained no 

windows, and was only accessible via an exterior door of the left side of the structure. The exterior 

entrances to the basement and sub-basement levels were connected via exterior wooden stairs on 

the left side of the structure. 

The fire, which is described in Section 3.2, originated in the finished living room on the basement level (near the rear side of the structure), which had a floor area of 72 m2 (775 ft2). More 

detailed discussion on the impact of the fire on the interior conditions is provided in Section 4. 

Figure 3.2 shows the exterior of the front and rear sides of the structure after the incident. Note 

that the majority of the exterior damage occurred above the openings located on the basement floor 

(where the wooden balcony was located). Fully dimensioned drawings of the second floor, first 

floor, and basement are shown in Appendix A. 

5
 
8.53 m

17.73 m

Roof Deck

Living Room

Kitchen Dining Room

Family

Room

Balcony

Second Floor

Front Side Rear Side

Up to Second Floor

Garage

Bedroom Bedroom

Bedroom

Locations

of downed

reghters

First Floor

Front Door Up to First Floor

Living Room

Laundry

Bedroom

Crawl

Space

Utility Room

Balcony

Basement Floor

Figure 3.1: Plan view of the second floor (top), first floor (middle), and basement floor (bottom). 

The fire originated in the living room on the basement floor. Stairs and rooms are identified using 

information collected by NIST. The sub-basement level was not connected to the other three levels 

of the house and is not shown. 

6
 
Figure 3.2: Front side (top) and rear side (bottom) of the structure after the incident. Photographs 

courtesy of the San Francisco Fire Department. 

7 
3.2 Fires

The fire originated in the basement living room and was initiated by a handheld electrical appliance 

that was located near the rear basement windows [9]. The closest significant fuel package was 

the couch located in the middle of the basement living room. On-scene photographs and postincident reports were used to estimate the type of fuel, fire size, and ventilation openings on the 

structure. Based on these sources of information, the simulation included three source fires: 1) an 

initial couch fire in the basement, 2) a flashover fire that involved additional furniture items in 

the basement, and 3) a fire that occurred on the balcony located on the rear exterior side of the 

basement. 

The basement contained upholstered furniture composed primarily of polyurethane foam, and 

the balcony was constructed of wood. The upholstered furniture items located in the middle of 

the basement living room were very large and flat, similar to the size and geometry of a mattress. 

Therefore, the HRR of the initial couch fire was based on an experimentally measured HRR for a 

mattress fire [13] to represent the approximate fire growth rate and magnitude of a similar household furniture item. The HRRs of the additional furniture items were estimated based on previous 

experimental work that characterized the HRR of geometrically similar upholstered furniture fuel 

items [13–15]. Other combustible items and furnishings such as tables, stools, bookshelves, and 

carpet were present in the basement living room. However, to simplify the analysis and specification of the design fire, only the large upholstered furniture fuel items and balcony were included in 

the simulation because they were considered to have contributed the largest amount of energy in 

this fire scenario. 

The primary fuel packages were located in the basement as shown in Fig. 3.3. In this figure, 

the numbered labels indicate the order in which the fuel items were burning: 1) the initial couch 

fire, 2) the secondary furniture fires, and 3) the balcony fire. 

Up to First Floor

Living Room

Laundry

Bedroom

Crawl

Space

Utility Room

Balcony

Basement Floor

1

2

2

2

2

3

Front Side Rear Side

Figure 3.3: Location of fuel items in the basement. The numbered labels indicate the order in which 

the fuel items were burning: Item 1 is associated with the initial couch fire in the basement, Items 2 

are associated with the flashover fire that involved additional furniture items in the basement, and 

Item 3 is associated with a fire that occurred on the exterior basement balcony. 

8
 
Since the exact time at which the fire started was not known relative to the notification of a fire 

or how quickly the fire spread, several simplifying assumptions were made. One assumption was 

that the initial couch fire in the basement was specified to start at the beginning of the simulation. 

The duration of the fire simulation was 9 min (540 s). The beginning of the simulation (0 s) was 

selected to correspond to an on-scene time of 10:53:00, which was approximately five minutes 

after the fire department arrived on scene and approximately one minute before the E26 firefighting 

crew stated that the fire was located below the first floor. The end of the simulation (540 s) was 

selected to correspond to an on-scene time of 11:02:00, which was the time at which substantial 

fire suppression operations were initiated (see Table 2.1). 

The HRRs for the three primary source fires were prescribed in the simulation as follows. 

1. For the initial couch fire in the basement, the HRR from 0 s to 327 s followed the experimentally 

measured mattress HRR [13]. The HRR at 300 s was approximately 3 MW, and the HRR at 

327 s was approximately 5 MW. From 327 s until 540 s (the end of the simulation), the HRR 

of the initial couch fire was specified as a constant 5 MW. 

2. The secondary fires in the basement were divided among four additional upholstered furniture 

items (two couches, a lounge chair, and a chair). The HRRs of these additional furniture items 

were estimated based on previous experimental work that characterized the HRR of geometrically similar upholstered furniture fuel items, which reported HRRs of upholstered chairs and 

couches ranging from 2 MW to 5 MW [13–16]. The two large couches in the middle of the 

basement were specified with a peak HRR of 5 MW for each item. The lounge and chair located at the perimeter of the basement were specified with a peak HRR of 2 MW for each item. 

Therefore, the peak HRR of all four furniture items was a total of 14 MW. 

Based on post-incident reports and photographs, the HRR of these fuels was specified as follows. From 0 s to 300 s, these items were not burning. From 300 s to 342 s (the time at which 

the first basement window failed), the secondary fuel items were ignited, and the total HRR 

of the four furniture items increased linearly from 0 MW to 14 MW. From 342 s to 540 s (the 

end of the simulation), the total HRR of the four furniture items was specified as a constant 

14 MW. 

3. The third fire involved the wood on the rear basement balcony and exterior siding. To estimate 

the heat release rate per unit area (HRRPUA) of wood, Babrauskas and Grayson [17] conducted 

experiments in a cone calorimeter1 to determine the 5-min average of the HRRPUA for several 

different types of wood over a range of radiant heat fluxes. The results of that study indicate 

that the HRRPUA for different types of wood is approximately 50 kW/m2 . 

Based on post-incident reports and photographs, the HRR of these fuels was specified as follows. From 0 s to 440 s, the wooden items were not burning. From 440 s to 450 s, the exterior 

basement balcony and siding were ignited, and the total HRR increased linearly from 0 MW to 

1 MW (based on the surface area of the burning wood). From 450 s until 540 s (the end of the 

simulation), the HRR of the wooden balcony and siding was specified as a constant 1 MW. 

1The cone calorimeter is an experimental apparatus used to gather data about the ignition time, mass loss, combustion products, and heat release rate among other properties associated with burning small samples of materials [18]. 

9
 
Figure 3.4 shows the overall prescribed design fire for this scenario, which is the sum of the 

HRRs of the three source fires. In this figure, the text labels indicate the three stages of the fire based 

on the contribution of the fuel packages. Therefore, the estimated maximum specified fire size for 

this structure was approximately 20 MW. Note that the prescribed HRR can be different than the 

simulated HRR calculated by FDS based on the amount of available fuel, oxygen, and ventilation. 

A comparison of the prescribed HRR and the simulated HRR is discussed in Section 4.1. 

0 100 200 300 400 500

Time(s)

0

5

10

15

20

25

HRR (MW)

Initial couch fire Secondary fuels Wood patio

Figure 3.4: Prescribed HRR vs. time for the simulation. The text labels indicate the three stages of 

the fire based on the contribution of the fuel packages. 

Polyurethane foam was specified for the interior basement fuel items, which can be represented 

by the chemical formula, CH1.8O0.30N0.05, with specified product yields of soot (ys = 0.131 kg/kg) 

and carbon monoxide (yCO = 0.010 kg/kg) [19]. The product yields are constant and are expressed 

in terms of the amount soot or carbon monoxide emitted per unit mass of fuel consumed (kg/kg) 

and can be found in the Society of Fire Protection Engineers (SFPE) Handbook [19]. A balanced 

chemical reaction for the combustion of polyurethane foam in ambient air at 40 % relative humidity 

can be written as: 

CH1.8O0.30N0.05 +5.22(0.208 O2 +0.783 N2 +8.34 ×10−3 H2O +3.87 ×10−4 CO2)

→ 6.06(0.129 CO2 +0.156 H2O +0.001 CO +0.035 C +0.679 N2) (3.1) 

10
 
Wood was specified for the exterior basement balcony fuel items, which can be represented by 

the chemical formula, CH1.7O0.74N0.002, with specified product yields of soot (ys = 0.015 kg/kg) 

and carbon monoxide (yCO = 0.004 kg/kg) [19]. A balanced chemical reaction for wood combustion can be written as: 

CH1.7O0.74N0.002 +4.91(0.208 O2 +0.783 N2 +8.34 ×10−3 H2O +3.87 ×10−4 CO2)

→ 5.74(0.168 CO2 +0.155 H2O +6.37 ×10−4 CO +5.57 ×10−3 C +0.670 N2) (3.2) 

The heat of combustion of polyurethane foam and wood were specified as 26,200 kJ/kg and 

16,400 kJ/kg, respectively, based on data provided in the SFPE Handbook [19]. The heat of combustion represents the amount of energy released per unit mass of the fuel. The following text 

defines Eqs. 3.1 and 3.2 for an FDS input file with the two fuels and reaction properties discussed 

above: 

&SPEC ID = 'POLYURETHANE', FORMULA = 'CH1.8O0.30N0.05' / 

&SPEC ID = 'WOOD', FORMULA = 'CH1.7O0.74N0.002' / 

&SPEC ID = 'OXYGEN', LUMPED_COMPONENT_ONLY = .TRUE. / 

&SPEC ID = 'NITROGEN', LUMPED_COMPONENT_ONLY = .TRUE. / 

&SPEC ID = 'WATER VAPOR', LUMPED_COMPONENT_ONLY = .TRUE. / 

&SPEC ID = 'CARBON MONOXIDE', LUMPED_COMPONENT_ONLY = .TRUE. / 

&SPEC ID = 'CARBON DIOXIDE', LUMPED_COMPONENT_ONLY = .TRUE. / 

&SPEC ID = 'SOOT', LUMPED_COMPONENT_ONLY = .TRUE. / 

&SPEC ID = 'AIR', 

SPEC_ID = 'OXYGEN','NITROGEN','WATER VAPOR','CARBON DIOXIDE', 

VOLUME_FRACTION = 0.208057, 0.783214, 0.008342, 0.000387, 

BACKGROUND = .TRUE. / 

&SPEC ID = 'PRODUCTS_POLYURETHANE', 

SPEC_ID = 'NITROGEN','WATER VAPOR','CARBON DIOXIDE','CARBON MONOXIDE','SOOT', 

VOLUME_FRACTION = 0.678838, 0.155754, 0.129475, 0.001139, 0.034794 / 

&REAC ID = 'POLYURETHANE' 

FUEL = 'POLYURETHANE', 

HEAT_OF_COMBUSTION = 26200, 

SPEC_ID_NU = 'POLYURETHANE','AIR','PRODUCTS_POLYURETHANE' 

NU = -1, -5.218633, 6.057861 / 

&SPEC ID = 'PRODUCTS_WOOD', 

SPEC_ID = 'NITROGEN','WATER VAPOR','CARBON DIOXIDE','CARBON MONOXIDE','SOOT', 

VOLUME_FRACTION = 0.670128, 0.155268, 0.168397, 0.000637, 0.005570 / 

&REAC ID = 'WOOD' 

FUEL = 'WOOD', 

HEAT_OF_COMBUSTION = 16400, 

SPEC_ID_NU = 'WOOD','AIR','PRODUCTS_WOOD' 

NU = -1, -4.908330, 5.738119 / 

Note that using the input lines above will invoke the simple chemistry reaction mechanism in 

FDS in which fuel and air react to form only CO2, CO, H2O, soot, and N2. If the inclusion of 

other combustion products is desired, then the user must explicitly define those species and the 

chemical reaction that produces them [7]. Based on the above input lines, FDS uses the default, 

mixing-controlled fast chemistry combustion model. This mechanism states that the rate of fuel 

consumption is proportional to both the local limiting reactant concentration and the local rate of 

mixing, and extinction is based on a critical flame temperature [12]. While FDS provides users 

with the option to use a more complex finite-rate combustion mechanism, there is not a sufficient 

amount of information to justify deviating from the default specifications. 

11
 
Another simplification of the model is that the sources of the gaseous fuel from the polyurethane 

foam and wood had constant areas at fixed locations (five furniture items in the basement interior 

for the polyurethane foam, and the exterior basement balcony and siding for the wood fuel). 

A time-dependent burning rate was specified for each of the fuel packages in terms of a fuel 

mass flux. The fuel mass flux is the amount of fuel vapor per unit area that is released from the 

surface of each fuel package shown in Fig. 3.3. The specified time-dependent fuel mass flux for 

each fuel package corresponds to the HRRs that were described earlier. The HRR from each individual fuel package contributed to the total HRR shown in Fig. 3.4. FDS then determined the 

amount of combustion that occurred throughout the simulation based on the amount of available 

fuel and oxygen at a given location. Each fuel item was modeled as a 3D object with surfaces from 

which a specified amount of fuel flowed into the domain, mixed with air, and burned. 

3.3 Materials

Whereas the fires were represented as fuel sources with a constant area (Section 3.2), from a heat 

transfer perspective, it is important to define the material properties of the ceiling, walls, and floors 

(density, thermal conductivity, specific heat, and thickness) to account for heat transfer and energy 

storage. In this study, the material properties of gypsum board [20] were specified on the finished 

ceilings, walls and floors in the structure. Table 3.2 shows the material properties of the gypsum 

board that were used in the simulation. 

3.4 Ventilation

The simulation in this study accounted for changes in ventilation due to a combination of fire 

department operations (opening doors) and fire acting on the structure (breaking windows). The 

ventilation times and ventilation areas represent our best understanding of the incident. The estimated times of ventilation changes for doors and windows are provided in Table 3.1. 

During the time at which the downed firefighters were found, the state of some windows and 

doors in the structure was known, which resulted in the establishment of a flow path in the interior 

stairwell leading into the basement. However, the exact time of operation for other doors was not 

known. Therefore, the following doors were assumed to be open at the start of the simulation: 

1) the front door, 2) the interior door leading from the hallway on the first floor into the stairwell 

towards the basement, 3) the door leading from the garage into the first floor landing, and 4) the 

overhead garage door. The rear basement windows and exterior basement door on the left side of 

the structure were specified to open at certain times based on observations of failure (i.e., window 

breakage) or fire department operations (i.e., forcible entry) from on-scene photographs and postincident reports. 

Air leakage in structures has been shown to be an important consideration when modeling 

enclosure fires [21]. The open front door at the start of the simulation and the vents created during 

the simulation are significantly larger than the total effective area of leakage of a structure of this 

type. Therefore, any leakage in the structure would have a relatively negligible impact on the fluid 

mechanics and combustion within the structure and was not included in this study. 

12
 
Table 3.1: Timeline of ventilation events in the simulation
 

Simulation Time Ventilation Area Ventilation Type Ventilation Location 

(s) (m2) 

0 1.47 Door Exterior Front Entry 

0 10.1 Overhead Door Exterior Front Garage 

0 1.68 Door Interior Stairwell 

0 1.68 Door Interior Garage 

342 1.89 Sliding Glass Door Exterior Rear Basement 

383 1.89 Sliding Glass Door Exterior Rear Basement 

383 1.68 Door Exterior Left Basement 

418 2.52 Window Exterior Rear Basement 

440 2.31 Window Exterior Rear Basement 

450 3.78 Sliding Glass Door Exterior Rear Basement 

3.5 Numerical Mesh

For the simulation, a measure of how well the flow field is resolved can be estimated by using the 

non-dimensional expression D∗/δx. Here, D∗ is the characteristic fire diameter, δx is the nominal 

size of a mesh cell, and Q ̇ is the total heat release rate of the fire: 

� �

2

Q ̇ 5

D∗ = √ (3.3) ρ∞ cp T∞ g 

From the FDS User’s Guide [7], the characteristic fire diameter is related to the characteristic fire 

size via the relation Q∗ = (D∗/D)5/2. Here, D is the physical diameter of the base of the fire 

specified in the simulation. Based on validation work performed for the U.S. Nuclear Regulatory 

Commission, D∗/δx values that ranged between 4 and 16 produced results that were adequate for 

engineering calculations [22]. Following Eq. 3.3 and using a grid cell size of 10 cm, the ratio of the 

characteristic fire diameter to cell size (D∗/δx) was 18 for the initial couch fire (combined HRR 

of 5 MW), 31 for the secondary flashover fire in the basement (combined HRR of 19 MW), and 

32 for the maximum prescribed fire size in the structure (combined HRR of 20 MW). Therefore, 

the grid resolution used in this simulation results in a D∗/δx ratio that exceeds typical engineering 

values. Note that Eq. 3.3 relates to gas-phase phenomena, which is the primary concern of this 

study because the fires were explicitly specified instead of attempting to predict the amount of 

pyrolysis or flame spread in the scenario. 

As discussed in Section 3.1, the structure had exterior dimensions of 8.5 m (28 ft) by 17.7 m 

(58 ft) and a height of 5.9 m (19 ft) above street level. To ensure adequately resolved fluid flow 

in and out of the structure, the computational domain was extended beyond the volume of the 

structure. The dimensions of the computational domain were 11.5 m (38 ft) by 22 m (72 ft) and 

a height of 13.6 m (45 ft). A grid resolution of 10 cm (3.9 in) was used, which resulted in a total 

of approximately 3.4 million computational cells. As a result, all of the obstructions, ventilation 

openings (doors and windows), and fire areas were snapped by FDS to the nearest 10 cm. Figure 3.5 

shows the front and rear sides of the structure rendered in Smokeview with the 10 cm grid cells. 

13
 
Figure 3.5: Front side (top) and rear side (bottom) of structure with a 10 cm computational mesh.
 

14
 
The domain was divided into 16 meshes (each containing between 184,000 and 248,200 grid 

cells) that could be processed in parallel, which reduced the amount of required calculation time to 

approximately 2 days. Figure 3.6 shows the entire structure within the computational domain that 

has been divided into multiple meshes. 

Figure 3.6: Rear-left side of the structure within the computational domain with multiple meshes.
 

15
 
3.6 Summary of Model Input Parameters

Table 3.2 shows a summary of the model input parameters for the simulation that was conducted 

as part of this study. In reality, the quantities associated with model input parameters are not fixed 

values; rather, a model input parameter can be thought of as a point estimate from a distribution 

of possible input parameters with some associated amount of uncertainty. Any change in an input 

parameter (such as the HRR) for a given scenario results in a change in the output quantity (such 

as the hot gas layer (HGL) temperature). 

For example, according to the McCaffrey, Quintiere, and Harkleroad [23] empirical correlation, 

the HGL temperature in a well-ventilated compartment fire is proportional to the HRR raised to the 

two-thirds power. Following this relationship, a 7.5 % increase in the HRR would result in a 5 % 

increase in the HGL temperature [24]. More detailed discussion on the propagation of parameter 

uncertainty in fire models is available in a validation study that was sponsored by the U.S. Nuclear 

Regulatory Commission [24]. 

Table 3.2: Fire model input parameters 

Parameter Description Discussion
 

Simulation Time 9 min – 

Grid Cell Size 10 cm Section 3.5 

Ambient Temperature∗ 20 ◦C (68 ◦F) – 

Reaction: Polyurethane Foam [19] Formula: CH1.8O0.30N0.05 Section 3.2 

Soot Yield: 0.131 kg/kg 

CO Yield: 0.010 kg/kg 

Heat of combustion: 26,200 kJ/kg 

Reaction: Wood [19] Formula: CH1.7O0.74N0.002 Section 3.2 

Soot Yield: 0.015 kg/kg 

CO Yield: 0.004 kg/kg 

Heat of combustion: 16,400 kJ/kg 

Peak HRRs Initial couch fire: 5 MW Section 3.2 

Basement flashover fire: 14 MW 

Basement balcony fire: 1 MW 

Material: Gypsum Board [20] Thermal conductivity: 0.28 W/(m ·K) Section 3.3 

Density: 810 kg/m3 

Specific heat: 1.0 kJ/(kg ·K) 

Thickness: 1.2 cm ∗Initial interior temperature was assumed to be 20 ◦C; actual exterior temperature was 14 ◦C [9]. 

16
 
Section 4

Simulation Results

To examine the results of the simulation, it is important to link the timeline from the fire scene to 

the simulation timeline. Table 4.1 shows the fireground timeline [9] along with the corresponding 

simulation times. 

17
 
Table 4.1: Fire incident and simulation event timeline
 

Incident Time Simulation Fire Behavior / Fireground Operation 

Time 

(hh:mm:ss) (s) 

10:45:00	 Dispatch for a curtain fire due to an electrical short at a 

residential structure. 

10:48:00	 E26 arrives on scene, reports light smoke conditions, and 

makes entry into the front door with a 1 3⁄4 in hoseline. 

10:53:00 0	 FDS simulation begins. 

10:54:00 60	 E26 crew states that the fire is located below the first floor. 

10:56:00	 180 BC9 observes smoke but no fire at the left rear corner 

of the structure. 

10:58:42	 342 Fire self-vents from the rear side of the structure when a glass 

window in the basement fails. Additional glass windows on the 

rear side of the structure fail within the next two minutes. 

10:59:23	 383 BC9 forces open the basement door on the left side of 

the structure, reports heavy fire and smoke, and requests a 

second hoseline. 

11:00:00	 420 BC6 notices a severe change in conditions (heavy black smoke 

from garage). 

11:01:00	 480 Heavy fire and black smoke observed at rear of structure. 

Incident command (IC) attempts to contact E26 several times 

via radio with no reply. 

11:02:00	 540 E32 crew makes entry into the basement with a second hoseline 

and begins suppression operations. FDS simulation ends. 

11:08:00	 Two downed E26 firefighters are found on the first floor. 

11:09:00	 The downed E26 firefighters are carried out of the structure. 

For specific information regarding ventilation areas used in the simulation, refer to Table 3.1. 

18 
4.1 Heat Release Rate

In FDS, a HRR is specified, which results in a specified amount of fuel vapor (or pyrolyzate) 

being released from a fuel surface. FDS then determines the amount of combustion that occurs 

throughout the simulation based on the amount of available fuel and oxygen at a given location. 

As a result, the prescribed HRR that was input into the simulation can be different than the HRR 

calculated by FDS based on the ventilation conditions. 

Figure 4.1 shows the HRR vs. time based on the fires that were described in Section 3.2. In 

this figure, the solid line represents the prescribed HRR that was input into the simulation (based 

on a prescribed mass flux from the fuel items), and the dashed line represents the HRR that was 

calculated by FDS based on the ventilation conditions. The vertical dashed line represents the time 

at which the first basement window on the rear side of the structure failed (342 s). Additional 

windows on the rear side of the basement continued to fail within the next two minutes. Note that 

the rear window failures were manually specified in accordance with the event timeline rather than 

specifying failure temperature criteria. 

0 100 200 300 400 500

Time(s)

0

5

10

15

20

25

30

35

HRR (MW)

Rear Window Failures Begin

Prescribed HRR

Calculated HRR

Figure 4.1: Comparison of prescribed and calculated HRRs from the simulation. The vertical line 

indicates the time at which the rear basement windows began to fail (342 s). 

19
 
In Fig. 4.1, the prescribed HRR that was input into the simulation is different than the HRR 

calculated by FDS based on the ventilation conditions. From 0 s to 300 s, the calculated HRR was 

in agreement with the prescribed HRR because there was an adequate amount of oxygen for all 

of the fuel to combust. From 300 s to 342 s (the time at which the first basement window failed), 

the calculated HRR was lower than the prescribed HRR because the fire conditions in the structure 

became ventilation limited. At 315 s, the maximum calculated HRR was approximately 7 MW 

before the fire conditions became ventilation limited and started to decay. At 335 s, the calculated 

HRR decreased to approximately 2 MW. At this time, there was an insufficient amount of oxygen 

in the structure to burn all of the fuel; therefore, localized burning occurred near the ventilation 

openings where ambient air was available. 

At 342 s, the first window on the rear side of the structure failed, and additional windows 

failed within the next two minutes. As additional window failures occurred (see Table 3.1), the 

abrupt change in ventilation caused the calculated HRR to increase to approximately 32 MW at 

385 s before it reached a steady-state value of approximately 20 MW at 465 s. The calculated 

HRR increased above the prescribed HRR because the unburned fuel and smoke that accumulated 

in the structure during ventilation-limited conditions was mixing with ambient air from the failed 

window openings and burning. 

Observations suggest that the peak HRR calculated by FDS might have been overpredicted due 

to limitations in the combustion model in FDS that are related to the reignition of fuel-rich gases. In 

other words, some combustion occurred in the simulation near the front door and second floor that 

was not consistent with on-scene and post-incident photographs because the fuel-rich gases mixed 

with fresh air and reignited. In the actual fire incident, the fuel vapors and smoke that were exiting 

the front of the structure were not at a high enough temperature and did not encounter an ignition 

source that would have resulted in reignition. However, for the high-hazard areas that are the focus 

of this study (the basement and interior stairwell), the pressures, temperatures, and velocities are 

representative of the hazardous flow path conditions that would have occurred in this fire incident. 

Figure 4.2 shows a time series comparison of on-scene photographs and Smokeview snapshots 

of flaming combustion that occurred through the failed windows on the rear side of the structure. 

In this figure, the first photograph is shown at a simulation time of 321 s (approximately 20 s 

before the first rear basement window failed), the second photograph is shown at a simulation time 

of 349 s (approximately 10 s after the first rear basement window failed), the third photograph is 

shown at a simulation time of 440 s (approximately when the third rear basement window failed), 

and the fourth photograph is shown at a simulation time of 450 s (approximately when all of the 

rear basement windows had failed). Figure 4.3 shows a comparison of an on-scene photograph and 

a Smokeview snapshot of smoke flowing out of the front side of the structure at approximately the 

time when the rear windows began to fail. 

20
 
Figure 4.2: Comparison of on-scene photographs and Smokeview snapshots of exterior combustion 

occurring on the rear side of the structure. The on-scene time is indicated on the photograph (left 

side), and the corresponding simulation time is indicated on the Smokeview snapshot (right side). 

For more information on the fire incident and simulation timeline, refer to Table 4.1. Photographs 

courtesy of the San Francisco Fire Department. 

21 
Figure 4.3: Comparison of an on-scene photograph and a Smokeview snapshot of smoke flowing 

out of the doors on the front side of the structure. The on-scene time is indicated on the photograph 

(top), and the corresponding simulation time is indicated on the Smokeview snapshot (bottom). 

For more information on the fire incident and simulation timeline, refer to Table 4.1. Photograph 

courtesy of the San Francisco Fire Department. 

22
 
4.2 Pressure

As the fire grew in the basement, the simulated pressure in the basement increased and caused the 

hot gases to flow from a region of high pressure in the basement upwards to a region of low pressure 

via the interior stairwell. Figure 4.4 shows the calculated pressure conditions in the structure just 

before and after the rear basement window failed (342 s) in the simulation. 

In Fig. 4.4, the pressure profile within the high-hazard areas is shown via two snapshots in time 

to illustrate the change in the interior conditions. The first snapshot is shown at a simulation time of 

340 s, which is 2 s before the first rear basement window failed, and the second snapshot is shown 

at a simulation time of 343 s, which is 1 s after the first rear basement window failed. After the rear 

basement windows failed, the pressure rise in the basement ranged from a 5 Pa (7.3 × 10−4 psi) 

over-pressure at mid-level height to greater than 10 Pa (1.5 ×10−3 psi) at the ceiling. 

23
 
Pressure 

(Pa) 

0 

2 

4 

6 

8 

10 

Pressure 

(Pa) 

0 

2 

4 

6 

8 

10 

Figure 4.4: Simulated pressures on the right interior side of the structure, 2 s before (top) and 1 s 

after (bottom) the first rear basement window failed. The fire originated in the living room in the 

basement. 

24
 
4.3 Velocity

Gases flow from a region of high pressure towards a region of lower pressure. Once the rear basement windows failed, the gases in the basement at an elevated temperature and pressure flowed 

upward into the interior stairwell and exited the structure via the garage door and front door. The 

contours shown in Fig. 4.5 indicate the magnitude of flow velocities within the structure before 

and after the rear basement window failures. 

In Fig. 4.5, the velocity profile within the high-hazard areas is shown via two snapshots in time 

to illustrate the change in the interior conditions. The first snapshot is shown at a simulation time 

of 340 s, which is 2 s before the first rear basement window failed, and the second snapshot is 

shown at a simulation time of 402 s, which is 60 s after the first rear basement window failed. 

The vector arrows shown in Fig. 4.6 indicate the direction and magnitude of flow velocities 

within the structure before and after the rear basement window failures. Based on Figs. 4.5 and 

4.6, the velocity in the interior stairwell was approximately 4 m/s (9 mph) before the rear basement windows failed (340 s) and approximately 9 m/s (20 mph) after the rear basement windows 

failed (402 s). 

25
 
Velocity


(m/s)
 

8
 

6
 

4
 

2
 

0
 

Velocity


(m/s)
 

8
 

6
 

4
 

2
 

0
 

Velocity


(mph)
20
 

15
 

10
 

5
 

0 

Velocity


(mph)
20
 

15
 

10
 

5
 

0 

Figure 4.5: Simulated velocities on the right interior side of the structure, 2 s before (top) and 60 s 

after (bottom) the first rear basement window failure. The structure and compartment labels are 

shown in Fig. 4.4. 

26
 
Velocity


(m/s)
 

8
 

6
 

4
 

2
 

0
 

Velocity


(m/s)
 

8
 

6
 

4
 

2
 

0
 

Velocity


(mph)
20
 

15
 

10
 

5
 

0 

Velocity


(mph)
20
 

15
 

10
 

5
 

0 

Figure 4.6: Simulated velocities on the right interior side of the structure, 2 s before (top) and 60 s 

after (bottom) the first rear basement window failure. The structure and compartment labels are 

shown in Fig. 4.4. 

27
 
4.4 Temperature

Analysis of the temperatures from the simulation focuses on the high-hazard areas of the structure: 

the basement, and the interior stairwell that connects the basement to the front door and garage 

door on the front side of the structure. The contours shown in Fig. 4.7 indicate the gas temperatures 

within the structure before and after the rear basement window failures. 

In Fig. 4.7, the temperature profile at these high-hazard areas is shown via two snapshots in 

time to illustrate the change in the interior conditions. The first snapshot is shown at a simulation 

time of 340 s, which is 2 s before the first rear basement window failed, and the second snapshot 

is shown at a simulation time of 402 s, which is 60 s after the first rear basement window failed. 

The vector arrows shown in Fig. 4.8 indicate the gas temperatures and direction of flow within 

the structure before and after the rear basement window failures. Based on the results shown in 

Figs. 4.7 and 4.8, 60 s after the rear basement windows began to fail, the temperature in the basement was in excess of 800 ◦C (1500 ◦F), and the temperature in the interior stairwell was in excess 

of 540 ◦C (1000 ◦F). 

28
 
Temperature Temperature

(

◦C) (

◦F) 

800 

1400 

700 

1200 

600 

1000 

500 

800 400 

600 300 

200 400

100 200 

Temperature Temperature

(

◦C) (

◦F) 

800 

1400 

700 

1200 

600 

1000 

500 

800 400 

600 300 

200 400

100 200 

Figure 4.7: Simulated temperatures on the right interior side of the structure, 2 s before (top) and 

60 s after (bottom) the first rear basement window failure. The structure and compartment labels 

are shown in Fig. 4.4. 

29
 
Temperature


(

◦C)
 

800
 

700
 

600
 

500
 

400
 

300
 

200
 

100
 

Temperature


(

◦C)
 

800
 

700
 

600
 

500
 

400
 

300
 

200
 

100
 

Temperature


(

◦F)
 

1400
 

1200
 

1000
 

800
 

600
 

400
 

200
 

Temperature


(

◦F)
 

1400
 

1200
 

1000
 

800
 

600
 

400
 

200
 

Figure 4.8: Simulated temperatures on the right interior side of the structure, 2 s before (top) and 

60 s after (bottom) the first rear basement window failure. The structure and compartment labels 

are shown in Fig. 4.4. 

30
 
Section 5

Discussion of Simulation Results

The results of the simulation are discussed in the following sections. Section 5.1 addresses the 

results of the simulation as they relate to the flow path that was established in the interior stairwell. 

Section 5.2 examines the simulation results as they relate to the hazardous conditions and exposure 

temperatures in the interior stairwell. Section 5.3 discusses tactical considerations and outcomes of 

the fire incident as they relate to the fire dynamics and flow path conditions that have been observed 

in recent experimental research. 

5.1 Simulated Interior Stairwell Flow Path

The NIOSH incident report [9] indicated that firefighters located in the interior stairwell on the 

front side of the structure determined that the fire was located in the basement. From the simulation 

results (Section 4.4), the conditions in the interior stairwell were initially tenable. After the rear 

basement windows failed, the simulation results indicate a high-hazard area in the stairwell near 

the laundry room landing area that exceeded the conditions of a Class III exposure (temperatures 

greater than 260 ◦C or 500 ◦F) [25]. 

The simulation results indicate that a flow path was established between the basement living 

room area and the doors located on the front side of the structure (the front door and the garage 

door) after the rear basement windows failed. The rear basement window failures resulted in a 

rapid change in the conditions within the flow path. After the rear basement windows began to 

fail, the combustion gases at elevated temperature and pressure in the basement flowed upwards 

towards lower pressure regions via the interior stairwell. The two firefighters were located in the 

flow path between the basement and the doors on the front side of the structure. 

The arrows shown in Fig. 5.1 indicate the simulated flow direction of gases on the basement 

floor and first floor at mid-level height, 120 s after the first rear basement window failed. In this 

figure, the dashed arrow indicates a continuation of flow from the basement towards the first floor. 

The two arrows located at the door and windows on the basement floor (towards the rear of the 

structure) indicate bi-directional flow at a vent opening. At the basement door and windows, cool 

ambient air flowed towards the fire near the bottom of the openings, and hot gases and smoke 

flowed away from the fire near the top of the openings. 

31
 
Figure 5.1: Top view of the simulated flow path on the first floor and basement floor at mid-level 

height, 120 s after the first rear basement window failure. The arrows indicate the direction of flow. 

The dashed arrow indicates a continuation of flow from the basement towards the first floor. Two 

arrows located at a door or window indicate bi-directional flow at a vent opening. The contours use 

the same scale as Figs. 4.5 and 4.6 with a range of 0 m/s to 9 m/s (0 mph to 20 mph). 

32
 
5.2 Assessing the Hazard

A person is susceptible to second-degree burn injuries if exposed to temperatures greater than 55 ◦C 

(130 ◦F) [26]. Although firefighters wear protective gear, gear only offers a finite amount of protection. The polycarbonate material in the facepiece of a self-contained breathing apparatus begins to 

soften when the material temperature reaches approximately 140 ◦C (284 ◦F) [27]. Structural fire 

fighting coats and pants are tested to withstand temperatures of 260 ◦C (500 ◦F) [28]. Prolonged 

exposure to elevated temperatures can result in a significant amount of heat transferred to the firefighter, putting him or her at risk. Exposure of equipment to temperatures of 260 ◦C (500 ◦F) 

represents a Class III exposure [25]. Firefighters are at increased risk levels when encountering 

Class III exposure conditions for more than 5 minutes [25]. 

Figure 5.2 shows a top view of the temperatures at a height of 0.3 m (1 ft) above the floor on 

the first floor, 2 s before the first rear basement window failed. At this time, the temperature of the 

gases in the interior stairwell were in excess of 200 ◦C (400 ◦F). 

Temperature Temperature

(

◦C) (

◦F) 

100 

200 

300 

400 

500 

600 

700 

800 

200 

400 

600 

800 

1000 

1200 

1400 

Figure 5.2: Top view of simulated temperatures on the first floor, 0.3 m (1 ft) above the floor, 2 s 

before the first rear basement window failure. The square markers indicate the locations of the 

downed firefighters. 

Figure 5.3 shows a top view of the temperatures at a height of 0.3 m (1 ft) above the floor on 

the first floor, 60 s and 120 s after the first rear basement window failed. At a time of 60 s after 

the first rear basement window failed, the temperature of the gases in the interior stairwell were in 

excess of 540 ◦C (1000 ◦F), which exceeds the Class III exposure temperature of 260 ◦C (500 ◦F). 

As more rear basement windows failed and provided ambient air to the growing basement fire, 

the temperatures in the interior stairwell continued to rise. At a time of 120 s after the first rear 

basement window failed, the temperature of the gases in the interior stairwell were in excess of 

700 ◦C (1300 ◦F), and the temperatures near the door leading from the interior stairwell into the 

first floor hallway were in excess of 200 ◦C (400 ◦F) at a height of 0.3 m (1 ft) above the floor. 

33
 
Temperature

(

◦C) 

800 

700 

600 

500 

400 

300 

200 

100 

Temperature

(

◦C) 

800 

700 

600 

500 

400 

300 

200 

100 

Temperature

(

◦F) 

1400 

1200 

1000 

800 

600 

400 

200 

Temperature

(

◦F) 

1400 

1200 

1000 

800 

600 

400 

200 

Figure 5.3: Top view of simulated temperatures on the first floor, 0.3 m (1 ft) above the floor, 60 s 

after (top) and 120 s after (bottom) the first rear basement window failure. 

Based on these results and the simulation results shown in Sections 4.3 and 4.4, after the rear 

basement windows failed, the simulated flow path temperatures in the interior stairwell were in 

excess of 700 ◦C (1300 ◦F) and the flow velocities were approximately 9 m/s (20 mph). The 

conditions in the interior stairwell changed from tenable to high-hazard very rapidly following the 

rear basement window failure. Within the structure, the hot gases and smoke were moving along 

the flow path from the basement towards the doors located on the front side of the structure via the 

interior stairwell. The simulated temperatures are consistent with the post-incident conditions that 

were documented in the interior stairwell. 

34
 
The hot gases in the flow path were exiting the structure from the garage door and front door, 

as shown in Fig. 5.1. From the NIOSH incident report [9], at a time of 10:58:42 (approximately 

when the rear basement windows failed), another firefighting crew attempted to enter the house via 

the garage door into the first floor landing area leading into the basement and reported “untenable 

conditions in the garage door to [the] house that [forced] them to look for an alternate way to 

enter the structure.” Figure 5.4 shows the temperature of gases flowing out of the garage via two 

snapshots in time to illustrate the rapid change in the conditions at the front of the structure. 

Temperature Temperature

(

◦C) (

◦F) 

100 

200 

300 

400 

500 

600 

700 

800 

200 

400 

600 

800 

1000 

1200 

1400 

Temperature Temperature

(

◦C) (

◦F) 

100 

200 

300 

400 

500 

600 

700 

800 

200 

400 

600 

800 

1000 

1200 

1400 

Figure 5.4: Simulated temperatures of gases flowing out of the garage, 2 s before (top) and 10 s 

after (bottom) the first rear basement window failure. 

35
 
In Fig. 5.4, the first snapshot is shown at a simulation time of 340 s, which is 2 s before the 

first rear basement window failed, and the second snapshot is shown at a simulation time of 352 s, 

which is 10 s after the first rear basement window failed. Before the rear basement window failed, 

the temperature of the gases exiting the garage door were less than 150 ◦C (300 ◦F). After the rear 

basement window failed, the temperature of the gases exiting the garage door were in excess of 

430 ◦C (800 ◦F). 

Ongoing research and experimental work is being conducted by NIST to gain a better understanding of convective heat transfer to firefighter personal protective equipment, including firefighting gear, helmets, self-contained breathing apparatuses, etc. The goal of this ongoing research 

is to parameterize various flow path conditions (elevated temperatures and velocities) and determine their effect on the rate of heat transfer and amount of energy storage in firefighting safety 

equipment. 

36
 
5.3 Tactical Considerations

In this fire incident, the failure of the basement windows on the rear side of the structure coincided 

with a rapid change in the thermal conditions in the interior stairwell. The rear basement window failures resulted in the establishment of a flow path within the interior stairwell with highly 

hazardous conditions. These conditions would be equivalent to or greater than a Class III exposure [25]. 

The interior stairwell acted as a chimney for hot gases in the basement to flow towards regions 

of lower pressure and vent openings located on the front side of the structure. Firefighters should 

avoid placing themselves within a flow path where elevated temperatures and flow velocities can 

present hazardous conditions and increase the rate of heat transfer to firefighting gear via convection. A 360◦ scene size-up by arriving firefighters can help determine the location of the fire and 

identify potential flow paths within a structure. Door control can also be used to avoid creating 

inlet and outlet vents that could result in the establishment of a flow path. 

The timing of the interior attack in the basement occurred after delays in the forcible entry 

operations on the exterior basement door [9]. Fire suppression efforts should be coordinated with 

interior operations and ventilation procedures to reduce thermal hazards related to flow paths within 

a structure. Ongoing research by NIST, Underwriters Laboratories (UL), and others has demonstrated that applying water from the exterior into the fire area of a structure (prior to the start of 

interior operations) can significantly improve the safety of firefighters by reducing the thermal hazard from the fire and reducing the potential for developing high velocity hot gas flows within the 

structure [13, 29]. 

There have been many previous fire incidents [9, 30–43] in which changes in the flow paths 

are thought to have had an adverse impact on firefighter and occupant safety. Table 5.1 lists the 

NIOSH investigation reports from the past 15 years in which it could be determined that a flow 

path played a role in the related incident. This table lists the NIOSH report number, the outcome, 

and a brief description of the flow path details. 

Based on a review of these incidents, it is clear that fires with rapidly developing or changing 

flow paths are a significant hazard to the fire service. The development of (or changes to) a flow 

path could be caused by the failure of a component of the structure, such as a door, window, or 

portion of a ceiling, wall or floor. Environmental conditions such as wind can generate hazardous 

thermal conditions within a flow path. Uncoordinated ventilation procedures can also be the cause 

of increased thermal hazards within a flow path. 

37
 
Table 5.1: Flow path related LODD/LODI incidents
 

NIOSH Report No. No. of LODDs/LODIs Flow Path Details 

99-F01 [30] 3 LODDs From apartment into hallway on 10th 

floor of high-rise apartment building 

99-F21 [31] 2 LODDs Basement to 1st floor 

2 LODIs 

F2000-04 [32] 3 LODDs 1st floor to 2nd floor 

3 civilian deaths 

F2000-16 [33] 1 LODD 2nd floor hallway through 

1 LODI 2nd floor apartment 

1 civilian death 

F2000-23 [34] 1 LODD From ground level to 1st floor then to 

2 LODIs 2nd floor, flow exited through ceiling 

F2000-43 [35] 1 serious LODI 1st floor to 2nd floor 

2 other LODIs 

F2004-02 [36] 1 LODD 1st floor to basement 

F2005-02 [37] 1 LODD Rear to front of the building 

4 LODIs 

F2005-04 [38] 1 LODD Basement to 1st floor 

9 LODIs 

F2007-09 [39] 1 LODD 3 story training burn - flow through 

2 LODIs all levels 

F2007-35 [40] 4 LODIs 1st floor to 2nd floor 

F2009-11 [41] 2 LODDs Rear to front of the building 

F2011-13 [9] 2 LODDs Lower level up stairs and through 

entry door and garage 

F2011-31 [42] 1 LODD Fire extended from lower level apartment 

F2012-28 [43] 1 LODD Attic fire extended into closed 

1 LODI porch and then into 2nd floor 

38
 
Section 6

Summary

Fire Dynamics Simulator was used to provide insight into the fire dynamics of a fire that occurred 

within a multi-level, single-family residential structure in San Francisco, CA, that resulted in the 

death of two firefighters. The fuel, fire size, and fire growth rate that were used in the FDS simulation were estimated by taking into account all of the available information including the NIOSH 

report [9], post-incident pictures, and relevant literature. This resulted in a maximum specified 

source fire of approximately 20 MW in the basement and rear balcony of the structure. Based on 

the limited ventilation conditions in the basement, the FDS simulation results indicated a HRR of 

approximately 2 MW as a result of the limited supply of oxygen. After the rear basement windows failed, the HRR increased to approximately 32 MW before it reached a steady-state value of 

approximately 20 MW. 

The fire originated in the basement, and the interior stairwell acted as a chimney for hot gases 

in the basement to flow towards regions of lower pressure and vent openings located on the front 

side of the structure. After the rear basement windows failed, a flow path was established between 

the basement living room area and the doors located on the front side of the structure (the front 

door and the garage door). The rear basement window failures resulted in a rapid change in the 

conditions within the flow path. In the interior stairwell, the flow velocities were approximately 

9 m/s (20 mph) and the temperature of the gases was estimated to be in excess of 700 ◦C (1300 ◦F), 

which exceeds the Class III exposure temperature of 260 ◦C (500 ◦F). 

Two firefighters were located in the flow path between the basement and the doors on the front 

side of the structure. After a call for assistance, both firefighters were removed from the structure 

and immediate medical treatment was provided. The two firefighters were transported to the local 

medical center where the lieutenant was pronounced dead and the fire fighter/paramedic died two 

days later. 

39
 
Acknowledgments

The authors would like to thank Chief of Department Joanne Hayes-White, Battalion Chief Jose 

L. Velo, and Assistant Chief David L. Franklin of the San Francisco Fire Department for their 

assistance with this study and their dedication to improving firefighter safety. The authors would 

also like to thank John Culbertson, Director of the Montana Fire Services Training School, for 

his assistance in researching and compiling the flow path related LODD/LODI incidents shown 

in Table 5.1. Thanks to the NIOSH Fire Fighter Fatality Investigation and Prevention Program for 

their cooperation. Finally, the authors would like to thank Joseph Willi of the University of Illinois 

Urbana-Champaign for his assistance in creating dimensioned drawings of the structure. 

40
 
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44
 
Appendix A

Dimensioned Drawings

45
 
1

1

22

A A

B B

C

Roof Deck B

ALCONY

Legend

Symbol Meaning

door

.20 thick wall

window

CH is 2.44

overhang (1.22 from ceiling)

opening for stairs

.76 1.07

1.22

2.92

3.76

5.03

6.15

5.61

7.32

8.53

6.38

7.47

2.20

4.98

6.93

7.94

8.43

10.99

10.10

12.48

17.01

17.73

18.55

133 Berkeley Second Floor

*All measurements are in meters*CH is 2.74 (unless specified)*All doors are .81 X 2.06*All walls are .11 thick (unless specified)*All windows are .38 from ceiling*Floor is .36 thick*Roof deck walls are 1.52 tall

(shaded portion is 1.22 tall)

*Balcony walls are 1.22 tall

A

1.09

X 

1.75

2.24 X 

2.06

.91 X .91

1.02 X 1.14

2.24 X 2.06

2.64 X 2.36

.41 X .84

.10

.22

.83

1.03

.43

.43

9.37

.61 X .61 Figure A.1: Dimensioned drawing of the second floor. The measurements are accurate to within 15 cm (6 in).
 

46
 
1

1

22

A A

B B

A C

.76 1.78

2.67

3.05

4.55

5.16

6.38

8.53

Legend

Symbol Meaning

door

.20 thick wall

window

stair landing (elevated floor)

1.37

3.71

4.86

6.08

6.93

8.27

9.02

11.34

10.35

12.37

13.45

17.73

9.17

11.51

17.46

7.34

3.45

.25 .81

.08

.23

133 Berkeley First Floor

.61 X 1.45

2.24 X 1.45

1.63 X 1.14

1.14X 1.63

4.88 X 2.13 

.22 .30

Up to Second Floor

.08 .11

10 stairs.18 rise .21 run

5 stairs.19 rise .20 run

.53

*All measurements are in meters*CH is 2.44*All doors are .81 X 2.06 (unless specified)*All walls are .11 thick (unless specified)*All windows are .38 from ceiling*Floor is .36 thick

10 stairs

.18 rise

.22 run

.41

 X 

.71

7.24

Figure A.2: Dimensioned drawing of the first floor. The measurements are accurate to within 15 cm (6 in).
 

47
 
1

1

22

A A

B B

A C

2.68

3.71

6.08

6.85

8.48

9.40

17.22

18.30

.97

4.04

2.36

6.38

7.44

8.53

1.22

 4 stairs.19 rise .22 run 

 .11 

from wall

 10 stairs.2 rise and .19 run

1.78 X 2.06 1.14 X 2.06

133 Berkeley Basement

*All measurements are in meters*CH is 2.44*All doors are .81 X 2.06*All walls are .11 thick (unless specified)*All windows are .38 from ceiling

(except stair landing window)

*C side windows are .11 apart*Balcony walls are .91 tall

.46 X .71 

1.22 above landing 

BALCONY

Up to First Floor

.08

3.01

4.09

7.58

Legend

Symbol Meaning

door

.20 thick wall

window

stair landing (elevated floor)

.15

8.14

Figure A.3: Dimensioned drawing of the basement floor. The measurements are accurate to within 15 cm (6 in).
 

48