NIST Technical Note 1870
 

Simulation of a Residential

Wind Driven Basement Fire -

Riverdale Heights, MD

Craig G. Weinschenk 

Kristopher J. Overholt 

Daniel Madrzykowski 

This publication is available free of charge from: 

http://dx.doi.org/10.6028/NIST.TN.1870 

NIST Technical Note 1870
 

Simulation of a Residential

Wind Driven Basement Fire -

Riverdale Heights, MD

Craig G. Weinschenk 

Kristopher J. Overholt 

Daniel Madrzykowski 

Fire Research Division 

Engineering Laboratory 

This publication is available free of charge from: 

http://dx.doi.org/10.6028/NIST.TN.1870 

February 2015 

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 1870

Natl. Inst. Stand. Technol. Tech. Note 1870, 49 pages (February 2015)

CODEN: NTNOEF

This publication is available free of charge from:

http://dx.doi.org/10.6028/NIST.TN.1870
Contents

Contents iii
 

List of Figures v
 

List of Tables vii
 

List of Acronyms viii
 

List of Symbols ix
 

1 Introduction 1
 

2 Fire Incident Summary 2
 

3 Model Description 4
 

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

3.2 Fire . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
 

3.3 Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
 

3.4 Ventilation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
 

3.5 Numerical Mesh . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
 

3.6 Summary of Model Input Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . 18
 

4 Model Results 19
 

4.1 Heat Release Rate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
 

4.2 Pressure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
 

4.3 Velocity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
 

4.4 Temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
 

5 Discussion 32
 

5.1 Simulated Flow Path . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
 

5.2 Assessing the Hazard . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
 

5.3 Tactical Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
 

6 Summary 42
 

7 Acknowledgments 43
 

iii
 
44

48

References

A Dimensioned Drawings

iv
 
List of Figures
 

3.1	 Photograph and computer model rendering of terrain. . . . . . . . . . . . . . . . . 5
 

3.2	 Plan view of basement (top) and first (bottom) floor of structure. . . . . . . . . . . 7
 

3.3	 Photographs showing the front (top) and rear (bottom) exterior of the structure after
 

the incident. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
 

3.4	 Plan view of basement indicating fire location. . . . . . . . . . . . . . . . . . . . . 9
 

3.5	 Prescribed HRR vs. time for the simulation. . . . . . . . . . . . . . . . . . . . . . 12
 

3.6	 Front (top) and rear (bottom) of the structure with 10 cm computational mesh surrounding the structure and 20 cm mesh to the left and right of the structure. . . . . 16
 

3.7	 Overhead front-left side of the structure within the computational domain. . . . . . 17
 

4.1	 Comparison of prescribed and calculated interior HRRs from the FDS simulation. . 21
 

4.2	 Prescribed and total calculated HRRs vs. time from the simulation. . . . . . . . . . 22
 

4.3	 External post-fire image of damage to the basement (top) and FDS model showing
 

external fire prior to the front door of the structure being opened (bottom). . . . . . 23
 

4.4	 Prescribed, calculated domain, and calculated basement HRRs vs. time from the
 

simulation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
 

4.5	 First floor calculated HRR vs. time from the simulation. . . . . . . . . . . . . . . . 25
 

4.6	 Calculated pressure on first floor 1 s before and after the front door opened. . . . . 27
 

4.7	 Calculated velocity on the first floor 1 s before and after the front door opened. . . . 29
 

4.8	 Calculated temperature on the first floor 1 s before the front door opened. . . . . . 30
 

4.9	 Calculated temperature on the first floor 1 s after the front door opened. . . . . . . 31
 

4.10 Calculated temperature on first floor 5 s after the front door opened. . . . . . . . . 31
 

5.1	 Flow path vectors at the front door and top of the stairwell 1 s before the front door
 

was opened. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
 

5.2	 Flow path vectors at the front door and the top of stairwell 1 s after the front door
 

was opened. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
 

5.3	 Flow path vectors at the front door and the top of the stairwell at the estimated time
 

of first floor entry. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
 

5.4	 Flow path vectors at the front door and the top of stairwell 1 s before the front door
 

was closed. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
 

5.5	 Flow path vectors at the front door and the top of stairwell 1 s after the front door
 

was closed. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
 

v
 
5.6	 Interior temperature contours 1 s before the front door was opened (21:13:59). The 

top snapshot shows temperatures on the first floor and basement on the plane that 

runs through the center of the front door. The bottom snapshot shows temperatures 

on the first floor and basement on the plane that runs through the center of the 

interior stairwell doorway. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37 

5.7	 Interior temperature contours 1 s after the front door was opened (21:14:01). The 

top snapshot shows temperatures on the first floor and basement on the plane that 

runs through the center of the front door. The bottom snapshot shows temperatures 

on the first floor and basement on the plane that runs through the center of the 

interior stairwell doorway. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38 

5.8	 Interior temperature contours as firefighters made entry to the structure (21:14:42). 

The top snapshot shows temperatures on the first floor and basement on the plane 

that runs through the center of the front door. The bottom snapshot shows temperatures on the first floor and basement on the plane that runs through the center of 

the interior stairwell doorway. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39 

A.1	 Dimensioned drawing of the basement. . . . . . . . . . . . . . . . . . . . . . . . . 48 

A.2	 Dimensioned drawing of the first floor. . . . . . . . . . . . . . . . . . . . . . . . . 49 

vi
 
List of Tables

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

3.1 Timeline of ventilation changes in the simulation. . . . . . . . . . . . . . . . . . . . 14
 

3.2 Relevant fire model input parameters. . . . . . . . . . . . . . . . . . . . . . . . . . 18
 

4.1 Fire incident and simulation event timeline. . . . . . . . . . . . . . . . . . . . . . . 20
 

5.1 Flow path related LODD/LODI incidents. . . . . . . . . . . . . . . . . . . . . . . . 41
 

vii
 
List of Acronyms
 

EMS Emergency Medical Services 

FDS Fire Dynamics Simulator 

HGL Hot Gas Layer 

HRR Heat Release Rate 

HRRPUA Heat Release Rate per Unit Area 

LODD Line of Duty Death 

NIST National Institute of Standards and Technology 

PPE Personal Protective Equipment 

SCBA Self Contained Breathing Apparatus 

SFPE Society of Fire Protection Engineers 

viii
 
List of Symbols
 

ft foot 

m meter 

min minute 

Pa Pascal 

psi pound per square inch 

s second 

W Watt 

k thermal conductivity 

ρ density 

cp specific heat 

ix 
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 February 24, 2012, within a single-story, single-family residential structure in Riverdale Heights, MD, that resulted in the serious injury of two Price George’s County 

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 which generated ventilation limited (fuel rich) fire conditions in the 

basement area. After the front door was opened, the interior stairwell, aided by strong winds blowing into the basement, acted as a chimney for hot gases in the basement to flow towards regions 

of lower pressure, out through the front door. The temperature of the gases in the interior stairwell 

was estimated to be in excess of 400 ◦C (750 ◦F). Two firefighters were located in the flow path between the interior stairwell door and the front door of the structure, were exposed to these elevated 

temperatures, and suffered serious burn injures. Five additional firefighters were injured during fire 

operations on the first floor. 
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 the Fire Dynamics Simulator (FDS) fire model to provide insight into the fire development 

and thermal conditions of fires that have resulted in line of duty deaths (LODDs) [1–7]. The objective of these studies has been to improve the safety and effectiveness of firefighters. 

On February 24, 2012, a fire in an uninhabitated residential structure injured seven Prince 

George’s County firefighters. NIST examined the fire dynamics of this incident at the request 

of the Prince George’s Country Fire/Emergency Medical Services (EMS) Department. Computer 

simulations of the fire incident were conducted using the NIST Fire Dynamics Simulator [8] and 

Smokeview [9] software to provide insight into the fire development and thermal conditions that 

likely existed inside the residence during the fire. The specific objectives of the simulations detailed 

in this report are: 

1. To examine the effect of the flow path (including temperature, pressure, and fire conditions) in 

this single-story residential structure with a basement using physics-based calculations. 

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

members of the Prince George’s Fire Department likely experienced during the course of their 

interior operations. 

3. To provide a basis for analyzing tactical operations in terms of fire dynamics. 

This document describes the input and the results of the FDS (version 6.1.1) simulations. The 

simulations were 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 simulations, 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 and first floor of the structure. 

1
 
Section 2

Fire Incident Summary

The account of events for this incident was documented by the Safety Investigation Team of the 

Prince George’s County Fire/EMS Department [10]. At 21:11 hours on February 24, 2012 Prince 

George’s Fire/EMS Department responded to a residential structure fire in Riverdale Heights, 

Maryland with a fire in the basement [10]. For the purpose of this analysis, the details regarding the 

timeline are considered to be approximate values. The Prince George’s County report [10] includes 

narratives of actions of the response personnel. Note that term following the apparatus number (e.g. 

Truck 809 Can) refers to the seating assignment and operational duties of the firefighters on board. 

The following narrative of the incident, focused on the events which led to serious burn injuries to 

two emergency-personnel, was taken from the Prince George’s County report [10]: 

Truck 809 responded from quarters, as the first due truck company, and arrived on 57th 

Avenue right behind Engine 807B. When Truck 809 stopped to let Engine 807B layout, 

Truck 809 Can dismounted the truck and had to walk up the street as it pulled away. Truck 

809 Officer observed a lot of smoke moving extremely fast through the front yard and 

across the street upon exiting the truck. Truck 809 Officer and Truck 809 Forcible Entry 

proceeded directly to the front door . . . of the structure. Both of them entered the structure 

with full PPE, including SCBA, but without the protection of a hose line. Truck 809 Officer 

and Truck 809 Forcible Entry began primary searches on the first floor. At some point 

during the primary search, as conditions worsened, the front door closed, trapping Truck 

809 Officer and Truck 809 Forcible Entry inside the structure. This situation could not 

have occurred if a charged hose line was operating inside the structure at the time the door 

slammed shut. They were the only firefighters operating on the first floor at that time. 

Truck 809 Forcible Entry was unable to self-evacuate, and remained trapped inside. Due 

to rapidly deteriorating conditions, Truck 809 Officer was forced to self-rescue through 

another small window on [the front of the structure]. 

After exiting the structure, Truck 809 Officer [alerted other firefighters] that a firefighter 

was trapped inside. Truck 809 Officer then proceeded to the front door . . . in an attempt 

to search for and rescue Truck 809 Forcible Entry, who was trapped inside. At this point 

multiple firefighters on the exterior assisted Truck 809 Officer in the search, rescue, and 

removal of Truck 809 Forcible Entry. Both Truck 809 Officer and Truck 809 Forcible Entry 

sustained serious injuries and were transported to MedStar/Washington Hospital Center. 

2
 
From a combination of on-scene video and the narrative accounts from the Prince George’s County 

Fire/EMS Department report [10], Table 2.1 shows an overview of the timeline of events. Table 2.1 

represents the best estimate of the events. 

Table 2.1: Abridged Prince George’s Fire/EMS approximate fire event timeline [10]. 

Incident Time Fire Behavior / Fireground Operation 

[hh:mm:ss] 

21:08:26 First 911 call is made. 

Dispatch for a “box” alarm assignment- four engine com21:11:03 panies, two truck companies, one rescue squad, and a commanding officer. 

21:12:55 First unit (Engine 807B) arrived on-scene. 

21:13:44 Volunteer Chief 809A establishes incident command (IC). 

21:13:56 Truck 809 arrived on-scene. 

21:14:00 Front door opened. 

21:14:42 Truck 809 personnel enter structure. 

21:15:47 Basement door opened and water applied to fire. Front door blown closed. 

21:15:51 Lower pane of front egress window broken. 

21:16:01 Top pane of front egress window broken. 

21:16:16 Truck 809 officer bailout through front window. 

21:16:44 Front door opened. 

21:17:15 Truck 809 forcible entry firefighter rescued. 

3
 
Section 3

Model Description

Fire Dynamics Simulator [8] is a computational fluid dynamics (CFD) model 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 one larger volume known as a computational domain. The CFD model 

computes the density, velocity, temperature, pressure, and species concentration of the gas 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 advantages of FDS is that it 

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

expected results, given sufficient input data [12]. A complete description of the FDS model is 

provided in the FDS Technical Reference Guide [13]. 

Smokeview is a software tool designed to visualize simulation results from FDS [9]. Smokeview visualizes smoke and other attributes of the fire simulation using traditional scientific methods such as displaying tracer particle flow, two dimensional (2D) or three dimensional (3D) shaded 

contours of gas flow data such as temperature and flow vectors showing flow direction and magnitude. Smokeview allows the fire and fire movement to be visualized. This is done 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 also visualizes static data at 

particular times using 2D or 3D contours of data such as temperature and flow vectors showing 

flow direction and 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: the geometry of the building and the compartments being modeled, the size and location 

of exterior and interior ventilation openings, and documentation of fire damage to the building. 

The following information was gathered from witnesses, first responders, reports, and recorded 

media such as fire ground radios or videos: information on the timing of the fire development, the 

sequence and approximate timing of ventilation openings to the outside, and weather conditions at 

the time of the fire. 

The analysis of the simulation results is focused on the impact of ventilation. Specifically on the 

interior conditions of the first floor following the establishment of a flow path through door control 

and how wind impacted these conditions. Based on the goals of the analysis and the information 

4
 
collected, several engineering approximations were made. For example, heavy smoke conditions 

were reported at the front of the house at the time of arrival [10]; however, the exact ignition time, 

fire growth rate, and heat release rate (HRR) of the fire were not known. The ignition time and 

growth rate were estimated to reduce the amount of required calculation time while adhering to the 

event timeline (Table 2.1) and documented observations. The HRR of the fire was estimated using 

documentation of the potential fuel load within the structure and relevant literature. 

3.1 Geometry

The structure involved in this fire incident was a single-story, single-family structure with a basement. The structure was platform-framed wood construction with a gable roof. 1 The structure had 

a first floor footprint of 9.14 m (30 ft) by 7.77 m (25.5 ft) with the basement being slightly smaller, 

9.14 m (30 ft) by 7.16 m (23.5 ft), as show in Fig. 3.2. This resulted in a first floor area of 71 m2 

(764 ft2) and a basement floor area of 65 m2 (700 ft2). The interior walls were composed of gypsum board over wood framing [10]. There was a single flight of stairs in the center of the structure 

which connected the first floor to the basement. 

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

floor was located at street level but the basement was below street level (Fig. 3.1). On the rear side 

of the structure, the slope allowed the basement to be a walkout. Therefore, access to the structure 

could be made on the front side to the first floor and on the rear side to the basement. 

Figure 3.1: Photograph taken from front side street level showing the slope down to basement on 

the rear side of the structure (left) and Smokeview rendering of model representation of sloped 

landscape (right). 

The fire, which is described in Section 3.2, started in the basement and spread to the stairs 

1Platform-framed construction means that floor joists are completely covered with a sub-floor. This forms a platform from which walls are built. 

5
 
which connect to the first floor. The door at the top of the stairs was open at the time of the fire. 

Discussion of the fire’s impact on the interior conditions is provided in Section 4. Figure 3.3 shows 

the exterior of the front and rear of the structure after the incident. Note that the majority of the 

damage occurred in the basement of the structure. Fully dimensioned drawings of the interior of 

the basement and first floor are shown in Figs. A.1 and A.2, respectively. 

6
 
1

1

2

2

A A

B B

FRONT

REAR

7.16 m

9.14 m

6404 57th Avenue

Basement Level

Basement Door

Stairs

Basement Window

B

a

s

e

m

e

n

t

W

ni d

o

w

BASEMENT 

Living Area

Bedroom

Bedroom

PRODUCED BY AN AUTODESK EDUCATIONAL PRODUCT

PRODUCED BY AN AUTODESK EDUCATIONAL PRODUCT

P

R

O

D

U

C

E

D

B

Y

A

N

A

U

T

O

D

E

S

K

E

D

U

C

A

T

OI N

AL P

R

O

D

U

C

T

1

1

2

2

A

B

7.77 m

All exterior walls are .15m

de

All interior walls are .11m

de

Ceiling height 2.44m

All doors and windows are

1m from ceiling

9.14 m

REAR

FRONT

6404 57th Avenue

First Floor

Stairs

Front Egress Window Front Door

FIRST FLOOR 

Bedroom

Living Area Bedroom

Kitchen

Figure 3.2: Plan view of basement (top) and first (bottom) floor of structure. Stairs and key windows 

and doors are identified using measurements collected by NIST. 

7 
Figure 3.3: Photographs showing the front (top) and rear (bottom) exterior of the structure after the 

incident. 

8 
3.2 Fire

To estimate the type of fuel, fire size, and structure ventilation, on-scene videos and post-incident 

reports were used. Based on these sources of information, the FDS model includes two source fire 

locations in the basement: one fire in the kitchen area and one in the adjacent bathroom (Fig. 3.4). 

At the time of the fire, the structure was unfurnished so the the dominant fuel was wood: wood 

paneling on walls, exposed ceiling joists, sub-floor, cabinets, and stairs. 

1

1

2

2

A A

B B

FRONT

REAR

7.16 m

9.14 m

6404 57th Avenue

Basement Level

Basement Door

Stairs

Basement Window

B

a

s

e

m

e

n

t

W

ni d

o

w

BASEMENT 

Living Area

Bedroom

Bedroom

Kitchen

Bathroom 

Figure 3.4: Plan view of the basement indicating the location of the two fires in the FDS model. 

Wood fuel can be represented by the chemical formula, CH1.7O0.74N0.002, with specified yields 

of carbon monoxide (yCO = 0.004 kg/kg) and soot (yC = 0.015 kg/kg) [14]. The product yields 

are expressed in terms of the amount of carbon monoxide or soot emitted per unit mass of fuel 

consumed (kg/kg) and can be found in the Society of Fire Protection Engineers (SFPE) Handbook [14]. A balanced chemical reaction for wood combustion can be written as: 

CH1.7O0.74N0.002 +4.91(0.208 O2 +0.783 N2 +0.387E-3 CO2 +0.834E-2 H2O)

→ 5.74(0.636E-3 CO +0.168 CO2 +0.155 H2O +0.670 N2 +0.613E-2 C) (3.1) 

The value for the heat of combustion of wood used in this simulation was 16,400 kJ/kg, based 

on data provided in the SFPE Handbook [14]. The heat of combustion quantifies the amount of 

energy per unit mass of the fuel. The following text defines Eq. 3.1 for an FDS input file with the 

desired fuel and reaction properties discussed above: 

9
 
&SPEC ID='WOOD', FORMULA='CH1.7O0.74N0.002' / 

&REAC ID = 'wood' 

FUEL = 'WOOD', 

HEAT_OF_COMBUSTION = 16400, 

SOOT_YIELD = 0.015, 

CO_YIELD = 0.004/ 

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 for 

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

chemical reaction that produces them [8]. 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. Extinction is based on the critical flame temperature approach [13]. While FDS provides users with the option to use a more complex finite-rate combustion mechanism, there is not 

sufficient evidence in this case to justify deviating from the default specifications. 

To estimate the heat release rate per unit area (HRRPUA) of wood, Babrauskas and Grayson [15] 

conducted experiments in a cone calorimeter2 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 . 

To calculate the total fire size, the appropriate burning area of the basement needed to be 

determined. Based on the floor plan of the structure and post-incident images, two distinct fires 

were considered: 1 source in the basement bathroom and 1 source in the basement kitchen. The 

bathroom fire was estimated to be comprised of wood paneling along the four walls (17.3 m2 

or 186 ft2) and wood subfloors and studs along the ceiling (2.9 m2 or 31 ft2). This resulted in 

20.2 m2 (217 ft2) of wood burning surface area or a 1 MW fire using a 50 kW/m2 HRRPUA. The 

remainder of the wood combustable load in the basement was lumped into the localized kitchen 

fire. The ceiling and wall areas included in the basement kitchen, stairway, and main living area 

totaled approximately 86 m2 (926 ft2). This resulted in a 4.3 MW fire. 

Initial simulations were conducted using a HRR for the two fires totaling 5.3 MW, however, 

the fire behavior did not align with observations made during the fire incident or post-incident 

images. Specifically there was insufficient fuel to drive the basement to an underventilated condition. One reason is that while the structure was unoccupied at the time of the fire [10], there 

were additional fuels that were not accounted for: transient fuels, sparse kitchen cabinetry, and an 

unknown bathroom vanity among other fuels. A second reason is that the 50 kW/m2 HRRPUA 

used to estimate the fire size does not account for an observed increase in burning rate during wind 

driven fires [17, 18]. It is not clear how to quantify the impact of wind on burning rate within a 

structure from first principles, so several iterations of the model with increased fire sizes were run. 

The final estimated fire size was a 1.8 MW fire in the bathroom and a 6.8 MW fire in the kitchen. 

This resulted in a total peak specified fire size of 8.6 MW. 

Since the time at which the fire started is not known relative to the notification of a fire or how 

fast the fire spread, several simplifications were made. The fires started at the beginning of the 

2The cone calorimeter is an experimental apparatus used to gather data about ignition time, mass loss, combustion 

products, and heat release rate among other properties associated with burning small samples of materials [16]. 

10
 
simulation and increased to a value of 8.6 MW in 10 s. The 10-s ramp up time was used to prevent 

abrupt changes in the velocity at the fire boundary condition in FDS. Since the actual ignition 

time is unknown, the fire was ramped quickly to reflect the conditions determined from reports 

and post-incident images. The second assumption is that the sources of the gas phase wood fuel 

have constant areas at fixed locations in the basement (1 source for the kitchen, 1 source for the 

bathroom). At 207 s, using a simple decay model, the fire size decreases linearly to 25 % of its 

peak value in 60 s. This is to account for the impact of firefighters putting water on the basement 

fire (Table 2.1). The following lines define the source fires in the FDS input file: 

&SURF ID='burner1', HRRPUA = 2000, COLOR='BURNT ORANGE', RAMP_Q='BURN' / 

&SURF ID='burner2', HRRPUA = 4500, COLOR='BURNT ORANGE', RAMP_Q='BURN' / 

&RAMP ID='BURN', T = 0, F = 0. / 

&RAMP ID='BURN', T = 10, F = 1.0 / 

&RAMP ID='BURN', T = 207, F = 1.0 / 

&RAMP ID='BURN', T = 267, F = 0.25 / 

&RAMP ID='BURN', T = 300, F = 0.25 / 

&OBST XB=0.6,1.2,5,6.5,0.0,0.4, SURF_IDS='burner1','INERT','INERT' / Burner1 

&OBST XB=2.5,4,5.0,6.0,0.0,0.4, SURF_IDS='burner2','INERT','INERT' / Burner2 

Note that the coordinates here are unique for the vents are specific to the input files used for these 

simulations. The key point is the 2-D planar area. 

Figure 3.5 shows the overall prescribed design fire for this scenario, which is the sum of the 

HRRs of the two source fires. In this figure, the text indicates the prescribed steady peak HRR 

and the decay phase associated with firefighter water application. 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. 

11
 
0 50 100 150 200 250 300

Time (s)

0

2

4

6

8

10

HRR (MW)

Steady Burning

Water on Fire

Figure 3.5: Prescribed HRR vs. time for the simulation. The text labels indicate the prescribed 

steady peak HRR and the decay phase associated with firefighter water application. Time zero 

represnts 21:21:20, see Table 4.1. 

12
 
3.3 Materials

While 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 (density, thermal conductivity, 

and specific heat) to account for heat transfer and energy storage in the ceiling, walls, and floor. 

In this study, the material properties of gypsum board [19] were specified on the finished walls 

and ceilings on the main two floors of the structure, and the material properties of wood [20] were 

specified on the unfinished portions of the structure. The following lines define these materials in 

the FDS input file: 

&MATL ID = 'GYPSUM BOARD' 

FYI = 'Wakili - Journal of Fire Sciences 2007' 

CONDUCTIVITY = 0.28 

SPECIFIC_HEAT = 1.0 

DENSITY = 810. / 

&MATL ID = 'WOOD MAT' 

FYI = 'Incropera and DeWitt, Fundamentals of Heat and Mass Transfer' 

CONDUCTIVITY = 0.12 

SPECIFIC_HEAT = 1.38 

DENSITY = 510. / 

3.4 Ventilation

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

department operations (opening doors), fire acting on the structure (fire breaching windows) and 

wind (restricting flow, closing doors). The ventilation times and ventilation areas represent the best 

understanding of the incident. The estimated times of ventilation changes for doors and windows 

are provided in Table 3.1. 

During the time when the firefighters exited the structure or were rescued from the structure, 

the state of some windows and doors in the structure was known. The state of the vents (opened 

versus closed) resulted in the establishment of a flow path in the interior stairwell leading from the 

basement to the first floor. The exact time of operation for other doors/windows was not known. 

Therefore, the two basement windows on the left rear corner of the structure were assumed to be 

open at the start of the simulation. The states of the front door and front window were specified 

at certain times based on observations (i.e. forcible entry, wind effects, forced window breakage) 

from post-incident reports. 

Structure leakage has been shown to be important when modeling enclosure fires [21]. The 

open rear basement windows at the start of the simulation were significantly larger than the typical 

total effective area of leakage of a structure of this type. Therefore, any leakage in the structure 

would have a negligible impact on the fluid mechanics within the structure and is not included in 

this study. 

In addition to the state of openings within the structure, outlined in Table 3.1, an 8.9 m/s 

(20 mph) wind velocity boundary condition was assumed. The flow direction of the wind was 

asasumed from the rear toward the front of the structure, perpendicular to the rear of the structure. 

13
 
Table 3.1: Timeline of ventilation changes in the simulation.
 

Baseline Ventilation Type State of Side of 

Simulation Area Opening Structure 

[s] [m2] 

0 1.40 Window Opened Rear 

0 0.39 Window Opened Left 

100 1.84 Door Opened Front 

207 1.84 Door Closed Front 

207 1.84 Door Opened Rear 

211 0.26 Window Opened Front 

221 0.26 Window Opened Front 

267 1.84 Door Opened Front 

Based on local weather conditions, mean wind velocities ranged between 4.5 m/s (10 mph) and 

8.9 m/s (20 mph) with gusts up to 13.4 m/s (30 mph) with a direction that angled toward the back 

left of the structure [10]. The following lines define the wind boundary condition in the FDS input 

file: 

&SURF ID='wind', VEL=-8.94, RAMP_V='WIND RAMP',COLOR='INVISIBLE' /
 

&VENT MB = 'YMAX', SURF_ID ='wind' /
 

&RAMP ID='WIND RAMP', T=0,F=0.25 /
 

&RAMP ID='WIND RAMP', T=10,F=0.50 /
 

&RAMP ID='WIND RAMP', T=20,F=0.75 /
 

&RAMP ID='WIND RAMP', T=30,F=1.00 /
 

Note that there is a 30 s ramp time to reach peak wind speed. This was inlcuded to prevent an 

abrupt change at the FDS domain boundary. Since the front door does not open until 100 s into the 

simulation there was sufficient time to for the airflow to stabilize. 

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∗ = √ (3.2) ρ∞ cp T∞ g 

14
 
From the FDS User’s Guide [8], 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. Following Eq. 3.2 and using a grid cell size of 10 cm (3.9 in), the 

characteristic fire diameter to cell size (D∗/δx) ratio is 12 for the 1.8 MW bathroom fire and 20 for 

the 6.75 MW basement fire. Based on validation work performed for the U.S. Nuclear Regulatory 

Commission, D∗/δx values ranged between 4 and 16 [22] and produced results that were adequate 

for engineering calculations. The grid resolution used in this model is within or exceeds typical 

engineering D∗/δx values. 

As discussed in Section 3.1, the structure has dimensions of 9.14 m (30 ft) by 7.62 m (25 ft) 

and a total height of 7.31 m (24 ft). To sufficiently resolve the wind boundary condition, the computational domain was extended beyond the volume of the structure. The large domain led to the 

use of two separate grid resolutions, a 10 cm (3.9 in) resolution and a 20 cm (7.8 in) resolution. 

The 10 cm resolution encompassed the structure with a total volume 14 m (46 ft) by 12 m (39ft) 

by 8 m (26 ft) that required a total of 1.3 million computational cells. As a result, all of the input 

model geometry lengths, ventilation openings (holes, doors, and windows), and fire areas will snap 

to the nearest 10 cm. The 20 cm grid resolution extended the computational domain 5 m to the left 

and right sides and 4 m behind the 10 cm mesh to provide sufficient computational domain for the 

wind profile to develop around the structure. The mesh is coarser to cut down on computational 

time and because high accuracy in the fluid mechanics of the air flow far from the structure is not 

important in the analysis of this simulation. The 20 cm resolution resulted in an additional 216 000 

computational cells. 

Figure 3.6 shows the front and rear of the structure and surrounding domain rendered in Smokeview with the 10 cm and 20 cm meshes. Note the finer mesh (smaller cells) surrounding the structure and the coarser mesh on each side. The domain was divided into 12 equally sized 10 cm 

meshes that surrounded the structure (each containing 150 000 grid cells) plus 4 equally sized 

20 cm meshes to the left and right of the structure (each containing 32 000 grid cells) and one 

20 cm mesh in the rear of the structure (containing 56 000 grid cells) for a total of 17 meshes. 

Multiple meshes allowed the simulation to be processed in parallel, which reduced the amount 

of required calculation time to approximately 3 days from over 1 week with a uniform 10 cm 

mesh. Figure 3.7 shows the entire structure within the computational domain that has been divided 

into multiple meshes. The boundary between the twelve 10 cm meshes and five 20 cm meshes is 

indicated by the bold lines. 

15
 
Figure 3.6: Front (top) and rear (bottom) of the structure with 10 cm computational mesh surrounding the structure and 20 cm mesh to the left and right of the structure. 

16
 
Figure 3.7: Overhead front-left view of the structure within the computational domain (with 17 

meshes). The entire computational domain was 24 m (79 ft) by 16 m (53 ft) by 8 m (26 ft). 

17
 
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: Relevant fire model input parameters. 

Parameter Description Discussion
 

Simulation Time 5 min – 

Grid Cell Size 10 cm around house 

20 cm for wind Section 3.5 

Ambient Temperature∗ 20 ◦C (68 ◦F) – 

Formula: CH1.7O0.74N0.002 

Reaction: Wood [14] CO Yield: 0.004 kg/kg 

Soot Yield: 0.015 kg/kg Section 3.2 

Heat of Combustion: 16,400 kJ/kg 

Peak HRR Basement Fire: 6.75 MW 

Bathroom Fire: 1.8 MW Section 3.2 

k: 0.12 W/(m ·K)) 

Material: Wood [20] ρ: 510 kg/m3 Section 3.3 

cp: 1.38 kJ/(kg ·K) 

k: 0.28 W/(m ·K) 

Material: Gypsum Board [19] ρ: 810 kg/m3 Section 3.3 

cp: 1.0 kJ/(kg ·K) 

Wind 8.9 m/s (20 mph) Section 3.4 

∗Initial interior temperatures were assumed to be 20 ◦C; exterior temperatures were 10 ◦C [10]. 

18
 
Section 4

Model Results

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

to the simulation times. Table 4.1 shows the fireground timeline [10] along with the corresponding 

simulation times. 

19
 
Table 4.1: Fire incident and simulation event timeline.
 

Incident Time Simulation Fire Behavior / Fireground Operation 

Time 

(hh:mm:ss) (s) 

21:08:26	 First 911 call is made. 

21:11:03	 Dispatch for a“box” alarm assignment- four engine companies 

two truck companies, one rescue squad, and a commanding officer. 

21:21:20 0	 FDS simulation begins. 

21:12:55	 First unit (Engine 807B) arrived on-scene. 

21:13:44	 Volunteer Chief 809A establishes incident command (IC). 

21:13:56	 Truck 809 arrived on-scene. 

21:14:00 100	 Front door opened. 

21:14:42	 Truck 809 enters structure. 

21:15:47	 207 Front door closed over. 

Basement door opened and water applied to fire. 

21:15:51 211	 Lower pane of front egress window broken. 

21:16:01 221	 Top pane of front egress window broken. 

21:16:16	 Truck 809 officer bailout through front window. 

21:16:47 267	 Front door opened. 

21:17:15	 Truck 809 forcible entry firefighter rescued. 

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

20
 
4.1 Heat Release Rate

In FDS, a HRR must be 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 in the structure vs. time based on the fires 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), and the dashed line represents the HRR inside the structure that was 

calculated by FDS based on the ventilation conditions. The vertical lines represent the times at 

which ventilation occurred: front door opening and closing as well as the breaking of a front 

window for emergency escape. See Table 3.1 for a detailed list of the ventilation that occurred 

during the incident. The maximum size of the fire specified in this study was selected such that the 

conditions aligned with observations and fuel loads from the actual fire incident. 

0 40 80 120 160 200 240 280

Time (s)

0

2

4

6

8

10

HRR (MW)

Prescribed HRR

Structure Inteior Calculated HRR

Front Door Open

Front Door Closed

Bottom Bay Window Open

Top Bay Window Open

Front Door Open

Figure 4.1: Comparison of prescribed and calculated interior HRRs from the FDS simulation. The 

vertical lines indicate the times when ventilation occurred. 

21
 
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 approximately 9 s, the calculated 

HRR was in approximate agreement with the prescribed HRR because there was an adequate 

amount of oxygen for all of the fuel to combust. The differences in prescribed versus predicted 

HRR is a result of either fuel accumulation within the structure not combusting because of a lack of 

oxygen or combustion occurring exterior to the structure where fuel and oxygen can mix and react. 

Fig. 4.2 shows the total calculated HRR within the computational domain (includes combustion 

occurring exterior to the structure) compared to the prescribed HRR curve. 

0 40 80 120 160 200 240 280

Time (s)

0

2

4

6

8

10

HRR (MW)

Prescribed HRR

Total Calculated HRR

Front Door Open

Front Door Closed

Bottom Bay Window Open

Top Bay Window Open

Front Door Open

Figure 4.2: Comparison of total prescribed and calculated HRRs within the entire computational 

domain from the FDS simulation. The vertical lines indicate the times at which ventilation occurred. 

A comparison of the differences between the HRR inside the structure (Fig. 4.1) and the total 

HRR (Fig. 4.2) over the first 100 s of the simulation indicated that on average there was the equivalent of an approximate 0.7 MW fire(s) occurring through open external vents (basement windows). 

This aligns with observed post-fire damage and post-incident report [10]. Fig. 4.3 shows a post-fire 

image of the back left corner of the structure as well as a Smokeview rendering of the fire from the 

FDS simulation prior to the front door being opened by firefighters. 

22
 
Figure 4.3: External post-fire image of damage to the basement (top) and FDS model showing 

external fire prior to the front door of the structure being opened (bottom). 

After 100 s of simulation time, 5.5 min after the first 911 call, firefighters open the front door 

to make entry to the first floor of the structure. The change in ventilation to the structure impacted 

the HRR. After the door was opened combustion no longer took place outside of the structure, as 

shown in Fig. 4.4, however the basement HRR was lower than the total HRR within the domain. 

The differences in HRR between the total HRR and the basement HRR can be accounted for 

23
 
100 120 140 160 180 200 220

Time (s)

6.0

6.5

7.0

7.5

8.0

8.5

9.0

9.5

10.0

HRR (MW)

Prescribed HRR

Total Calculated HRR

Calculated Basement HRR

Front Door Open

Front Door Closed

Bottom Bay Window Open

Top Bay Window Open

Figure 4.4: Comparison of prescribed, total calculated domain HRR and basement calculated 

HRRs from the simulation. The vertical lines indicate the times when ventilation changed. 

in combustion occurring on the first floor of the structure. Fuel that was combusting outside of the 

structure due to a lack of oxygen combusts on the first floor of the structure as the open front door 

allows flow to occur within the structure and fuel to mix with available oxygen and react (Fig. 4.5). 

24
 
100 120 140 160 180 200 220

Time (s)

0.0

0.2

0.4

0.6

0.8

1.0

HRR (MW)

Calculated First Floor HRR

Front Door Open

Front Door Closed

Bottom Bay Window Open

Top Bay Window Open

Figure 4.5: First floor calculated HRR from the simulation. The vertical lines indicate the times 

when ventilation changed. 

25
 
4.2 Pressure

The growth of the fire combined with a 8.9 m/s (20 mph) wind flowing normal to the rear of the 

structure caused the pressure to rise within the structure. Hot gases which may have built up within 

the structure will flow from high pressure to low pressure. Based on the event timeline, the front 

door was opened 100 s into the simulation, which would create a low pressure relief on the first 

floor. Due to firefighters entering the structure through the front door, the simulated pressure on 

the first floor prior to and just after opening the door is important. Figure 4.6 shows the calculated 

pressure conditions in the structure just before and after the front door was opened (100 s) in the 

simulation. 

In the top snapshot in Fig. 4.6, the pressure profile shows an overpressure of approximately 

200 Pa (2.9 × 10−2 psi), 1 s prior to the door being opened. This pressure may be on the conservative side (over-prediction) because of wind speed fluctuations, wind direction, and small amounts 

of structure leakage. One second after the door was opened (bottom snapshot in Fig. 4.6), the pressure on the first floor drops to between 75 Pa (1.1 × 10−2 psi) and 85 Pa (1.2 × 10−2 psi). Without 

the presence of wind, the pressure in the first floor prior to the door opening was approximately 

18 Pa (2.6 × 10−3 psi) and after the door was opened the pressure drops to between 5 Pa (7.3 ×

10−4 psi) and 10 Pa (1.4 × 10−3 psi). 

26
 
Pressure 

(Pa) 

0 

20 

40 

60 

80 

100 

120 

140 

160 

180 

200 

Pressure 

(Pa) 

0 

20 

40 

60 

80 

100 

120 

140 

160 

180 

200 

Figure 4.6: Calculated pressure contours on the first floor of the structure 1 s prior to (top) and 1 s 

after (bottom) firefighter entry through the front door at 0.91 m (3 ft) above the floor. 

27
 
4.3 Velocity

Gases flow from a region of high pressure towards a region of lower pressure. Once the front door 

opened, the gases in the basement at an elevated temperature and pressure flowed upward into the 

interior stairwell and exited the structure via the front door. The velocity contours shown in Fig. 4.7 

indicate the magnitude of flow velocities within the structure before and after the front door was 

opened. 

In Fig. 4.7, the velocity profile within the high-hazard area (first floor) is shown via two snapshots in time to illustrate the change in the interior conditions 0.91 m (3 ft) above the floor. The 

first snapshot is shown at a simulation time of 99 s, which is 1 s before the front door was opened, 

and the second snapshot is shown at a simulation time of 101 s, which is 1 s after the front door 

was opened. The velocity contours shown indicate that prior to the front door being opened, there 

was minimal flow on the first floor. However, after the front door was opened, a flow path was established forcing the combustion products from the basement fire to flow to the first floor through 

the interior stairwell. The simulation calculated the flow exiting the structure to be approximately 

11 m/s (25 mph). 

28
 
Velocity Velocity

(m/s) (mph)35 

14 30 

12 25 

10 

20 

8 

15 6 

10 4 

2 5

0 0 

Velocity Velocity

(m/s) (mph) 

Figure 4.7: Calculated velocity contours on the first floor of the structure 1 s prior to (top) and 1 s 

after (bottom) firefighter entry through front door 0.91 m (3 ft) above the floor. Arrows indicate 

direction. 

0 

2 

4 

6 

8 

10 

12 

14 

0 

5 

10 

15 

20 

25 

30 

35 

29
 
4.4 Temperature

Analysis of the temperatures from the simulation focused on the high-hazard area of the structure: 

the first floor area between the front door and the top of the interior stairwell. The temperature contours shown in Fig. 4.8 indicate the gas temperatures within the structure 1 s before the front door 

was opened 0.91 m (3 ft) above the floor. The overlaid arrows represent the gas flow: the length of 

the lines represents the magnitude of the speed and the direction of the arrow represents the flow 

direction. Prior to the door opening, the open living area and kitchen on the first floor were approximately 60 ◦C (140 ◦F), while the peak temperatures were concentrated near the interior stairwell, 

105 ◦C (220 ◦F). Note that there was very minimal flow throughout the first floor. Figures 4.9 and 

4.10 show gas temperature contours at 0.91 m (3 ft) above the floor, 1 s and 5 s after the front door 

was opened. 

Temperature

(

◦C) 

400 

350 

300 

250 

200 

150 

100 

50 

Temperature

(

◦F) 800 

700 

600 

500 

400 

300 

200 

100 

Figure 4.8: Calculated temperature contours on the first floor of the structure 1 s prior to firefighter 

entry through front door 0.91 m (3 ft) above the floor. 

After the front door is opened, hot fire gases from the basement begin to flow through the 

interior stairwell into the first floor. Figure 4.9, 1 s after the front door was opened, shows that the 

temperature contours at the top of the stairwell have increased to approximately 200 ◦C (392 ◦F) 

with a hot spot that approached 425 ◦C (800 ◦F). Figure 4.10, 5 s after the front door was opened, 

shows more significant increases in temperature. The area of at the top of the stairs is between 

375 ◦C (700 ◦F) and 425 ◦C (800 ◦F). The gas temperatures 0.91 m (3 ft) above the floor at the front 

door have also increased to approximately 230 ◦C (450 ◦F) and show increased flow velocities. 

30
 
Temperature Temperature

(

◦C) (

◦F) 800 

400 

700 

350 

600 300 

500 250 

200 400

150 300 

100 200 

50 100 

Figure 4.9: Calculated temperature contours on the first floor of the structure 1 s after firefighter 

entry through front door 0.91 m (3 ft) above the floor. 

Temperature Temperature

(

◦C) (

◦F) 800 

400 

700 

350 

600 300 

500 250 

200 400

150 300 

100 200 

50 100 

Figure 4.10: Calculated temperature contours on first floor of the structure 5 s after firefighter entry 

through front door 0.91 m (3 ft) above the floor. 

31
 
Section 5

Discussion

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 

through the front door. Section 5.2 examines the model results as they relate to the hazardous conditions and exposure temperatures on the first floor. 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 anaylses, modeling, and full and bench-scale experimental research. 

5.1 Simulated Flow Path

The incident report from Prince George’s County Fire/EMS [10] indicated that two firefighters 

from Truck 809 were seriosuly injured after making entry to the first floor. The interior conditions 

forced one firefighter to bail out through a first floor window on the front side of the structure and 

the other firefighter to be rescued through the front door. From Section 4.4, the conditions on the 

first floor were initially tenable. After the front door was opened, however, the simulation results 

indicated that a high hazard area existed in the first floor that exceeded the conditions that would 

put a firefighter at risk for burn injuries (temperatures greater than 260 ◦C or 500 ◦F) [25]. 

The simulation results indicate that a flow path was established between the open vents (windows) on the rear side of the basement and front side of the structure (through the front door of the 

first floor) after the front door was opened. Strong winds (8.9 m/s (20 mph)) pushed air into the 

rear side of the basement through window failures and combined with buoyant combustion gases 

to pressurize the structure. Opening the front door provided a pressure relief; this resulted in a 

rapid change in the conditions within the structure and the establishment of a flow path. After the 

front door was opened, the combustion gases at elevated temperature and pressure in the basement 

flowed towards lower pressure regions outside of the structure on the leeward side (front of structure) via the interior stairwell. The two injured firefighters were located in the flow path on the first 

floor between the top of the interior stairwell and the front door. 

To examine the impact of opening the front door on the establishment of a flow path, flow 

conditions were examined along the centerline of the interior stairwell doorway and through the 

centerline of front door. Figure 5.1 shows the flow vectors at the first floor stairwell doorway 

1 s before the front door was opened. Note that the length and direction of the arrows indicate 

the magnitude and direction of the flow, respectively. The color of the arrows correspond to the 

32
 
values from the included temperature scales. Therefore, a short blue arrow represents slow moving, 

cool gas while a long red arrow represents fast moving, high temperature gas. Prior to the door 

opening, there was minimal flow through the first floor stairwell door and the temperature peak 

was approximately 125 ◦C (257 ◦F) as shown in Figure 5.1. One second after the door opened, 

Figure 5.2, there was noticeable flow through both the stairwell doorway and through the front 

door. Simulated temperatures at the stairwell increased to approximately 200 ◦C (392 ◦F) and 

temperatures through the doorway were approximately 100 ◦C (212 ◦F). These values are also 

shown in planar contours 1 m above the floor in Figs. 4.7 and 4.9 for velocity and temperature, 

respectively. 

Temperature Temperature

(

◦C) (

◦F) 800 

400 

700 

350 

600 300 

500 250 

200 400

150 300 

100 200 

50 100 

Figure 5.1: Flow path vectors at front door and top of stairwell 1 s before the front door was opened. 

Temperature Temperature

(

◦C) (

◦F) 800 

400 

700 

350 

600 300 

500 250 

200
 400

150
 300 

100 200 

50 100 

Figure 5.2: Flow path vectors at front door and top of stairwell 1 s after the front door was opened.
 

33
 
At the estimated time of firefighter entry to the first floor (142 s into the simulation at 21:14:42; 

see Table 4.1), the simulated flow magnitudes through the first floor (stairwell and front doors) 

remained similar to the conditions just after opening the front door. Velocities through both the 

doorway at the top of the stairs and the front door were approximately 13 m/s (29 mph), which was 

faster than the imposed wind speed (9 m/s or 20 mph) because of the addition of the buoyancy of 

the fire plume. The model temperatures increased through the stairwell doorway to between 400 ◦C 

and 425 ◦C (750 ◦F to 800 ◦F) and through the front door to approximately 375 ◦C (700 ◦F). From 

the approximate timeline (Table 4.1), the front door was forced closed due to airflow pushing 

against it, 207 s into the simulation. Figures 5.4 and 5.5 show the conditions 1 s prior to and after 

the front door closed. The figures show that while the closed door stopped significant flow through 

the stairwell door and onto the first floor, the temperature hazard for the firefighters (temperatures 

at or above 400 ◦C or 750 ◦F) remained. 

Temperature

(

◦C) 

400 

350 

300 

250 

200 

150 

100 

50 

Temperature

(

◦F) 800 

700 

600 

500 

400 

300 

200 

100 

Figure 5.3: Flow path vectors at the front door and the top of the stairwell 42 s after the front door 

was opened, at the estimated time of first floor entry. 

34
 
Temperature

(

◦C) 

400 

350 

300 

250 

200 

150 

100 

50 

Temperature

(

◦F) 800 

700 

600 

500 

400 

300 

200 

100 

Figure 5.4: Flow path vectors at the front door and the top of stairwell 1 s before the front door 

was closed. 

Temperature

(

◦C) 

400 

350 

300 

250 

200 

150 

100 

50 

Temperature

(

◦F) 800 

700 

600 

500 

400 

300 

200 

100 

Figure 5.5: Flow path vectors at the front door and the top of stairwell 1 s after the front door was 

closed. 

35
 
5.2 Assessing the Hazard

A person is susceptible to second-degree burn injuries if skin temperatures exceed 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 up to 260 ◦C (500 ◦F) or 10 kW/m2 [25] for less than 

2.5 min. Exposure to elevated temperatures can result in a significant amount of heat transferred to 

the firefighter, putting him or her at risk. 

Figures 5.6 to 5.8 show two simulated interior views of temperature contours at particular 

snapshots in time. The top image is a planar view through the middle of the front door (from the 

front to the back of the house), which shows the conditions in the living area on the first floor 

and the conditions in the basement just past the bottom of the stairs. The bottom image shows 

temperature contours through the middle of the interior stairwell which connected the basement 

to the first floor of the structure (looking from the rear of the structure towards the front). Prior 

to the front door being opened, (Fig. 5.6 shows that temperatures in excess of 200 ◦C (400 ◦F) 

were confined to the basement of the structure: specifically the rear of the structure and under 

the stairwell. One second after the door was opened, hazard in the structure increased with the 

most drastic change occurring at the top of the stairwell as peak simulated temperatures exceeded 

400 ◦C (750 ◦F). At the approximate time firefighters made entry to the first floor, Fig. 5.8, the 

conditions within the flowpath (interior stairwell through front door) had become more severe with 

simulated temperatures in excess of 400 ◦C (750 ◦F), for which a fully protected firefighter would 

be susceptible to burn injuries. These high-hazard conditions within the flowpath remained until 

the HRR was reduced by the simulated action of applying water on the fire. 

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

door was opened, the simulated flow path temperatures in the interior stairwell were in excess of 

400 ◦C (750 ◦F) and the flow velocities were in excess of 9 m/s (20 mph). The conditions in the 

interior stairwell changed from tenable to high-hazard very rapidly following the opening of the 

front door. Within the structure, the hot gases and smoke were moving along the flow path from 

the basement towards the door 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. 

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
 
Temperature

(

◦C) 

400 

350 

300 

250 

200 

150 

100 

50 

Temperature

(

◦F) 800 

700 

600 

500 

400 

300 

200 

100 

Figure 5.6: Interior temperature contours 1 s before the front door was opened (21:13:59). The top 

snapshot shows temperatures on the first floor and basement on the plane that runs through the 

center of the front door. The bottom snapshot shows temperatures on the first floor and basement 

on the plane that runs through the center of the interior stairwell doorway. 

37
 
Temperature

(

◦C) 

400 

350 

300 

250 

200 

150 

100 

50 

Temperature

(

◦F) 800 

700 

600 

500 

400 

300 

200 

100 

Figure 5.7: Interior temperature contours 1 s after the front door was opened (21:14:01). The top 

snapshot shows temperatures on the first floor and basement on the plane that runs through the 

center of the front door. The bottom snapshot shows temperatures on the first floor and basement 

on the plane that runs through the center of the interior stairwell doorway. 

38
 
Temperature

(

◦C) 

400 

350 

300 

250 

200 

150 

100 

50 

Temperature

(

◦F) 800 

700 

600 

500 

400 

300 

200 

100 

Figure 5.8: Interior temperature contours as firefighters made entry to the structure (21:14:42). The 

top snapshot shows temperatures on the first floor and basement on the plane that runs through the 

center of the front door. The bottom snapshot shows temperatures on the first floor and basement 

on the plane that runs through the center of the interior stairwell doorway. 

39
 
5.3 Tactical Considerations

In this fire incident, the initial failure of the basement windows with strong incident winds on 

the rear side of the structure coincided with a rapid change in the thermal conditions in the interior 

stairwell and first floor after the front door to the structure was opened. The open front door resulted 

in the establishment of a flow path within the interior stairwell from the high pressure side of the 

structure to the low presure side with highly hazardous conditions. 

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 the firefighter 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. 

First suppression efforts should take place at the same level as the fire (i.e. fight basement 

fires from the basement level) with the wind at the back of firefighters. 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 [17, 18]. 

There have been many previous fire incidents [28–42] 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 

ventilation may lead to flow paths that are a significant hazard to the fire service during a response. 

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. 

40
 
Table 5.1: Flow path related LODD/LODI incidents.
 

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

99-F01 [28] 3 LODDs From apartment into hallway on 10th 

floor of high-rise apartment building 

99-F21 [29] 2 LODDs Basement to 1st floor 

2 LODIs 

F2000-04 [30] 3 LODDs 1st floor to 2nd floor 

3 civilian deaths 

F2000-16 [31] 1 LODD 2nd floor hallway through 

1 LODI 2nd floor apartment 

1 civilian death 

F2000-23 [32] 1 LODD From ground level to 1st floor then to 

2 LODIs 2nd floor, flow exited through ceiling 

F2000-43 [33] 1 serious LODI 1st floor to 2nd floor 

2 other LODIs 

F2004-02 [34] 1 LODD 1st floor to basement 

F2005-02 [35] 1 LODD Rear to front of the building 

4 LODIs 

F2005-04 [36] 1 LODD Basement to 1st floor 

9 LODIs 

F2007-09 [37] 1 LODD 3 story training burn - flow through 

2 LODIs all levels 

F2007-35 [38] 4 LODIs 1st floor to 2nd floor 

F2009-11 [39] 2 LODDs Rear to front of the building 

F2011-13 [40] 2 LODDs Lower level up stairs and through 

entry door and garage 

F2011-31 [41] 1 LODD Fire extended from lower level apartment 

F2012-28 [42] 1 LODD Attic fire extended into closed 

1 LODI porch and then into 2nd floor 

41
 
Section 6

Summary

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

within a single-story, single-family residential structure in Riverdale Heights, MD, that resulted in 

the serious injury 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 post incident report from Prince George’s Country Fire/EMS [10], post-incident pictures, and 

relevant literature. This resulted in a maximum specified source fire of approximately 8.5 MW 

in the basement. Based on the limited ventilation conditions in the basement, the FDS simulation 

results indicated that before the front door was opened, there was combustion occurring outside the 

structure at the rear basement windows which had failed prior to fire department arrival. Once the 

front door was opened, there was sufficient oxygen for combustion to occur within the structure, 

with intermittent flaming combustion on the first floor. 

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 through the open front door of the structure. 

After the front door was opened, a flow path was established between the basement and the front 

side of the structure (the front door). The opening of the front door resulted in a rapid change in 

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

9 m/s (20 mph) and the temperature of the gases was estimated to be in excess of 400 ◦C (750 ◦F), 

which puts fully protected firefighters at risk for burn injuries. 

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

front side of the structure. One firefighter was able to break a window to escape the hazardous 

conditions and after a call for assistance, the second firefighter was removed from the structure 

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

medical center where they were treated for their injuries. Five other firefighters were injured onscene and transported to the hospital for treatment. 

42
 
Section 7

Acknowledgments

The authors would like to thank Chief Marc Bashoor of the Prince George’s County Fire/EMS 

Department for their assistance with this study and their dedication to improving firefighter safety. 

The authors would like to thank members of the Safety Investigation Team for compiling the report: Deputy Chief Scott Hoglander, Battalion Chief Sayshan Conver-White, Chief Richard Bowers, Deputy Chief William Goldfeder, Assistant Chief Michael Nelson, Volunteer Safety Division 

Chief Stefan Ganset, Paramedic Captain Roland Berg, Lieutenant Philip Bird, Lieutenant Daniel 

Schrader, Investigator Aaron Tyler, Fire Protection Engineer Lee McCarthy, President Charles 

Walker, and Administrative Aide Xiomara Lozano-Chevez. 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. 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. 

43
 
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47
 
Appendix A

Dimensioned Drawings

1

1

2

2

A A

B B

A

C

7.16

4.88

5.84

6.25

2.82

1.71 1.71

4.00

5.51

1.27

1.47

4.52

4.57

5.73

6.92

9.14

2.20

5.33

5.82

.97 wide stairs with 

.2 rise & .2 run

.76

4.46

6.65

.76 X .51 window 

(flush with ceiling)

.76 X .51 window 

(flush with ceiling)

.76 X .51 window 

(flush with ceiling)

.76 X .51 window 

(flush with ceiling) .97 X 1.45 window

(.56 from ceiling)

1.22 door

.71 

door

.71 

door

.71 

door

.91 door

.91 

door

.91 

door

.91

.97 X 1.45 window

(.56 from ceiling)

6404 57th Avenue

Basement Level

*All units are in meters

*A and C side exterior walls are .15 wide

*B and D side exterior walls are .25m 

wide

*CH is 2.44

*All doors are 2.03 tall

Figure A.1: Dimensioned drawing of the basement. The measurements are accurate to within 15 cm 

(6 in). 

48
 
1

1

2

2

A A

B B

C

A

1.89

4.14

4.58

5.87

.50

3.91

4.48

2.20

8.00

9.14

1.45

4.89

7.57

9.14

5.89

5.49

7.62

3.54

1.96

.77

3.81

1.32 1.32

5.33

6.15

.76 X 1.14 

window

.76 X 1.14 

window

.76 X 1.14 

window .76 X 1.14 

window

.76 X 1.14 

window

.76 X 1.14 

window

.76 X 1.14 

window

.41 X .66 

window

2.41

1.22 door

.91 door

.91 door

.91 door

.71 door

.71 door

.71 door

.71 door

1.19 X 1.14

window

.48 X 1.14 window

(.38 X 1.04 inside frame)

.46 

door

6404 57th Avenue

First Floor

*Units are in meters

*All exterior walls are .15 wide

*All interior walls are .11 wide

*CH is 2.44

*All doors and windows are .41 from 

ceiling

STAIRS

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

(6 in). 

49