fires have long posed a unique challenge to the fire protection
engineering community. The rack-storage configuration, while being
practical, economical, and efficient, also produces a challenging
scenario with high densities of flammable goods stored at great heights
over a vast floor space.
The general approach taken to protect warehouse storage configurations has been that of suppression, where commodity classification is used to design the parameters of suppression necessary to contain or extinguish fires. In commodity classification, full-scale tests on standardized commodities with appropriate fire suppression systems have established acceptable criteria for the protection of stored goods.or extinguish fires. In commodity classification, full-scale tests on standardized commodities with appropriate fire suppression systems have established acceptable criteria for the protection of stored goods.1,2
Figure 1: Fire Development Over a Group A Plastic Commodity5
Figure 2: Three Stages of Burning of a Group A Plastic Commodity6
implementation and continued development of these standards have
greatly reduced the number of warehouse fires, from more than 4,700 a
year in 1980 to just 1,200 in 2011, the value of direct property damage
has not shown a similar decrease.3 Between 2007 and 2011, storage fires still cost $16 million per month on average.3
storage facilities continue to grow larger and taller, the practicality
of large-scale testing for all possible scenarios has become
increasingly impractical. Some means of determining adequate protection
from smaller-scale test results as well as relating known protection
schemes to new, diverse commodities should be developed.
the dimensional and material complexity of real world storage
commodities is a formidable obstacle. A rigorous approach includes
computational fluid dynamics checked against full scale experiments.
the hopes of systematically reducing the prohibitive costs (actual and
computational) associated with this approach, the industry has already
established significant momentum in this direction, particularly at FM
Global; however, a description of a mixed commodity to use within models
has yet to be ascertained.
study, funded by the SFPE Educational and Scientific Research
Foundation, sought to develop a method to ascertain the flammability
(including burning rate, flame spread rate, etc.) of a mixed warehouse
commodity as a first step towards tackling this problem.
Group A Plastics Test
classification scheme currently used in the U.S. places commodities
into one of seven groups, Classes I–IV for general commodities or Groups
A–C for plastic commodities.1,2 The Group A plastic
commodity represents the greatest "benchmark commodity” fire hazard,
consisting of crystallized polystyrene cups placed within a
compartmentalized, corrugated cardboard box.
more challenging fire hazards exist, such as expanded meat trays,
polyurethane foams, etc., the basis of current commodity classification
approaches for plastics is based around this Group A plastic; therefore,
it was chosen for this study.4
testing, the commodity was insulated on all sides except for the front
face and ignited at the base, in many ways simulating ignition during an
early-stage, rack-storage test. Thermocouples, load cells, cameras, and
heat flux gauges provided data that was used to assess flame spread and
burning rates of the commodity over time. The mixed commodity was found
to progress through three distinct stages of burning, indicated in
Figure 1, due to its unique geometry and material distribution.5,6
ignition of the front face of the commodity, flames spread upward along
the front face of the box with little involvement of interior material.
Therefore, only the properties of corrugated cardboard are necessary to
describe the upward flame spread process, described in detail later.
the front face of the box chars and falls off, it reveals the first
inner layer of segregating cardboard and unexpanded polystyrene cups,
indicated as Stage I in Figure 2. This first layer of cardboard also
pyrolyzes and burns as it is exposed to flames and outside air,
contributing to the burning rate; however, the polystyrene cups inside
do not heat sufficiently to ignite, and only begin to soften and melt.
The resulting heat-release rate in Stage I of burning increases from 0
to a peak of approximately 25 kW over approximately one minute, with
flame heights reaching 1 m (twice the height of the commodity),
contributing to rapid involvement of additional fuel above the ignited
Once the first layer of
cardboard burns out, not enough heat has been absorbed by the
polystyrene cups to ignite them, nor have flames penetrated the second
mixed layer of cardboard and cups; therefore, the heat-release rate and
flame heights decay. With only smoldering combustion remaining, the
commodity transitions to Stage II, where, on average, low heat-release
rates of 10 kW and flame heights of 0.5 m provide a probable opportunity
for extinguishment before ignition of the plastic product.
heat is continually absorbed by the polystyrene cups in Stage II, they
eventually absorb sufficient heat to ignite, significantly increasing
the heat-release rate of the overall commodity, with a peak of 40-50 kW
and observed flame heights of 1-1.5 m, shown in Figure 1 and Figure2 as
Stage III. This stage continues as layer after layer of cups is exposed
to air, illustrated in Figure 2. The segregated nature of the commodity
allows burning to progress in a relatively steady manner, involving
cardboard and plastic as earlier-ignited layers burn out.
segregated nature of the commodity, illustrated in Figure 2, aides not
only in a controlled transition between stages for the commodity, but
also in access to fuel, providing a somewhat averaged behavior within
each of the three stages, pointing to a potential means of simplifying
the analysis of the mixed burning of the commodity. In Stage I, for
instance, combustion is likely to be described by the geometry and
properties of cardboard alone, while in Stage III, it is the burning
rate and properties of the plastics, now melted and dripping while
burning, that control the burning rate.
objective of this work was to develop an approach that was appropriate
to measure small-scale fire behavior (at the scale of one or more
commodity packages) up to behavior in large rack-storage tests. This
significant challenge was not accomplished under this short-duration
project; however, some advancement and probable concepts were presented.
B-number, which appears as a boundary condition at the fuel surface in
the classical Emmon’s solution for forced-flow flames over a condensed
fuel surface,7 was suggested as a possible means to present
the burning behavior of a commodity package and serve as a relatively
flammable comparison tool. This dimensionless parameter is a ratio that
compares a summation of the various impetuses (e.g., heat of combustion)
for burning to a summation of the various resistances (e.g., heat of
vaporization) to the process. Originally a purely thermodynamic
quantity, its definition can be extended to encompass effects of
different heat-transfer processes, including radiative transport.8,9
unexpected finding of three stages with distinctive burning behaviors
added complexity to this approach by necessitating averages of the
B-number for each stage of burning (1.8, 1.4, and 1.9 for Stages I, II,
and III are reported5). This in some ways simplified matters,
as Stages I and II only include flaming combustion and later smoldering
of corrugated cardboard, while Stage III is a mixed product of
cardboard and polystyrene combustion.
For Stage III, some possible methods for determining the B-number of mixed materials were presented,5,10 but
more fundamental research needs to continue in order to establish a
firm methodology for utilizing such averaged approximations.
Upward Flame Spread Over Corrugated Cardboard
parameter of significance for suppression applications in warehouses is
the time to sprinkler activation, which largely depends on early-stage
flame spread and heat release rates. Focusing on Stage I of the Group A
plastic commodity tests, flame spread rates were shown to increase with
time to the 3/2 power profile rather than traditional time squared
experimental results, this behavior was hypothesized to be due to the
unique properties of C-flute cardboard, which consists of a corrugated
layer of paperboard glued between two flat sheets. As the outer layer
burns, it delaminates from the corrugated surface and "curls” directly
into the boundary layer, obstructing the flow of hot gasses and
projecting the flame outwards and away from unburnt cardboard, shown in
This behavior is
significant as the projected flames reduce heating rates above the solid
fuel surface, into the preheating region, thereby slowing the
development of flame spread and possibly delaying sprinkler activation
times. This reduction occurs even though progression of the burning
process into the interior of the commodity (including involvement of
plastics) will proceed as usual.
results of these tests have yielded alternative scalings that may be
better applicable to some situations encountered in practice in
warehouse fires.11 Understanding the time-dependent
interaction of both the upward flame spread and in-depth fire growth
processes also may be important for proper predictions of fire behavior
Impacts on Practical Warehouse Design
Ultimately, it will take years for the fruits of this labor to directly impact the design of fire protection systems, but some of the general insights should be useful in everyday designs. First, the ultimate flammability or fire hazard of stored commodities may not be as simple as a percentage classification of plastics and cellulosic materials.1,2 The increasing number of exceptions to standard commodity classification listed in NFPA 13 and FM Data Sheet 8-1 is particularly revealing, in that the list of stored items that do not fall under traditional commodity classification schemes is growing; therefore, current methodologies cannot capture all relevant behavior without full-scale test methods.1,2
3: (left) Front video footage during a representative test. The blue
contour across the width indicates the measured height of the pyrolysis
region. (right) Image taken from the side of a sample during a
representative test. Curling of the front layer of cardboard is visible
in both images, but the extent of three-dimensional effects is more
clearly seen in the side image.5
test methods here are shown to capture some of the complex behavior of
stored commodities that, with future incorporation of suppression system
performance, may be one piece of future system designs. Increasing
progress in numerically simulating warehouse fires may help in this
regard, but a method for simulating the in-depth combustion of mixed
materials must be firmly developed. The ability to extract
nondimensional burning behavior from a single warehouse commodity also
is one approach for developing a useful comparison between actual stored
commodities and standard commodities used in full-scale tests, possibly
limiting the number of large-scale tests in the future.
focus should not be restricted to suppression systems alone because a
closer look at individual commodities may be worth considering. For
instance, if new packaging could be developed that significantly delays
in-depth combustion while still allowing flames to quickly spread
upward, triggering sprinkler activation, the large heat-release rates of
Stage III may be prevented and the size of necessary extinguishment
systems reduced. Similarly, different types of cardboard may be designed
that speed or slow upward flame spread.
essence, by looking at the constituent pieces of a warehouse fire, it
may be possible to not only design a suppression system for a fire
hazard, but also to modify the fire hazard to match a suppression system
in the future. These approaches would require strict control of stored
commodities; there are many occupancies where this is possible.
Full-scale testing also would be necessary to finally validate these
concepts; however, with further modeling and understanding, there is
room for revolution in the ways storage occupancies are protected.
This work was supported in part by the SFPE Scientific and Educational Research Foundation and AON Fire Protection Engineering, Inc. Group A plastic commodities were donated by Tyco International, Ltd. The work of Prof. Forman A. Williams, Jonathan Perricone, P.E., Prof. Ali S. Rangwala, Dr. Kristopher Overholt, Todd Hetrick, and others throughout the duration of this project are gratefully acknowledged. Experiments were performed at both the University of California, San Diego, and Worcester Polytechnic Institute.
Michael Gollner is with the University of Maryland, College Park.
- NFPA 13, Standard for the Installation of Sprinkler Systems, National Fire Protection Association, Quincy, MA, 2010.
- Property Loss Prevention Data Sheets 8-1, Commodity Classification, FM Global, Norwood, MA, 2004.
- Karter, M., "Fire Loss in the United States During 2012,” National Fire Protection Association, Quincy, MA, 2013.
- Palenske, G., "NFPA 13 Sprinkler System Design Density Curves – Where Did They Come From?," Fire Protection Engineering, Second Quarter, 2012.
- Gollner, M., Overholt, K., Williams, F., Rangwala, A. and Perricone, J., "Warehouse commodity classification from fundamental principles. Part I: commodity and burning rates,” Fire Safety Journal, Volume 46, Issue 6, August 2011, pp 305-316.
- Gollner, M., "A Fundamental Approach to Storage Commodity Classification,” Master’s Thesis, University of California, San Diego, 2010.
- Emmons, H. "The Film Combustion of Liquid Fuel,” ZAMM – Journal of Applied Mathematics and Mechanics, 36 (1956), pp. 60-71.
- Torero, J., Vietoris, T., Legros, G. and Joulain, P. "Estimation of a Total Mass Transfer Number from the Standoff Distance of a Spreading Flame,” Combustion Science and Technology. 174 (11) (2002) 187-203.
- Jiang, F., Qi, H., de Ris, J. and Khan, M. "Radiation Enhanced B-Number,” Combustion and Flame, Volume 160, Issue 8, 2013, pp. 1510 -1518.
- Overholt, K., Gollner, M., Williams, F., Rangwala, A. and Perricone, J., "Warehouse Commodity Classification From Fundamental Principles. Part II: Flame Height Prediction.” Fire Safety Journal, Volume 46, Issue 6, 2011, pp. 317-329.
- Gollner, M., Williams, F., and Rangwala, A. "Upward Flame Spread Over Corrugated Cardboard.” Combustion and Flame, 158. 7 (2011):1401-1412.