Keeping
Disaster
at a
Distance
To safely site potentially explosive
materials, engineers use computer
models to predict where building
debris might fall after a
catastrophic explosion.
By Patricia Moseley Bowles and PA Cox
W
enever ammunition is being stored, there exists the slight
but daunting prospect of accidental explosions.
These internal explosions can occur through impact, thermal
changes, friction, static electric discharge, or the instability of aging
munitions. But whatever the cause of such catastrophic accidents, the
potential for destruction of property, equipment - or more importantly, human life - is sobering.
This potentially explosive situation is being addressed by engineers and analysts in the Engineering Dynamics Department of
Southwest Research Institute (SwRI), who have been developing and
perfecting sophisticated debris prediction tools that can help to minimize the devastation resulting from the accidental explosions of
ammunition stored in hardened aircraft shelters or similar magazines.
Numerous studies have been conducted over the past 20 years
to address the effects of accidental explosions. Many were conducted
by SwRI for the Norwegian Defence Construction Service (NDCS)
and the Klotz Group*, an informal international explosives safety
Tests of concrete walls, such as the sequence shown here, were
integral to the development of OISPRE2, a PC-based computer
code developed at SwRI for predicting the debris scatter and air
blast from accidental explosions in structures.
8
Technolo
*The Klotz Group, formerly known as the Klotz Club, is currently composed of delegates from
Norway, Germany, The Netherlands, Sweden, Switzerland, and Singapore. Other participating
countries have included the United States, the United Kingdom, and France.
Toda • Summer 2000
E1 35662
working group . These efforts
have led to the development of
DISPRE2, an accurate and userfriendly PC-based code that can
predict the likely blast field or
h azardous debris "throw" outside
aircraft shelters or munition sstorage structures. Sw RI and its
contractors hope it w ill become
the ultimate debris prediction tool
for the safe siting of structures
containin g explosives.
With this critically important
p redictive tool, the safe siting of
these shelters in relation to surrounding structures, equipment,
and p ersonnel can be expedited
w ith a minimum of wasted space
and a m aximum degree of safety
assurance.
Software history
Patricia Moseley Bowles served as a principal
analyst in the Engineering Dynamics Department
of SwRl's Mechanical and Materials Engineering
Division. She is a specialist in predicting the
dispersion of fragments and debris following
internal explosions in structures, and has been the
DISPRE program manager since 1990. Bowles
joined the staff of Applied Research Associates in
San Antonio as a senior analyst in July 2000.
A 1981 study for NDCS correlated an approximate engineering
analysis with the exp erimental results of
1:20 and 1:100 scale model tests of the
explosive effects on a third-generation
Norwegian aircraft shelter. The objective
was to determine a method to predict the
blast field characteristics outside the shelter and the m aximum exp ected debris
d istances from fragmentation of such a
shelter following an accidental explosion
of ammunition stored in a chamber
beneath the floor.
In a follow-up phase, the objective
was to use these test results to develop a
m odel for better estimating the p robability of lethality for humans from such an
explosion . The reproducibility and directionality in these tests provided a reliable
database, enabling engineers and others
to predict how and where build ing debris
might fall after such a catastrophic event.
On any given site, this inform ation
vastly facilitates the placement of such
storage magazines in relation to
P.A. Cox is a staff engineer in the Engineering
Dynamics Department. A structural analyst with
special interests in transient response, limit design,
and the finite element method, most of his work has
been in applied or basic research related to weapon
and vehicle dynamics, ship tank design, and the
evaluation and minimization of explosive hazards.
rubble berm surrounding the structure.
surrounding buildings, equipment, and
the personnel who use them.
The initial velocities and angles at which
concrete debris flew from an aircraft shelThis goal was achieved only
throu gh years of testing using h ardened
ter following an internal detonation were
aircraft shelters, w hich are specifically
carefully documented . This project
designed to resist blast and fragment
included 1:15 scale model tests of three
effects. Throu ghou t the 1980s, SwRI p arshelter designs: combined Norwegian /
ticipated in several large-scale (1 :4 to 1:3)
U.S., third-generation U.S., and thirdto full-scale tests of these shelters, w hich
generation German .
were subjected to internal detonations.
In 1992, SwRI began developing a
These programs contributed significode to calculate initial debris p aram eters
cantly to the growing
database of hazardou s
LAUNCH VELOCITY AS A FUNCTION OF SCALED
debris throw and the
COVER DEPTH FOR VARIOUS LOADING DENSITIES
air blast distribution
following an explosion.
Early in 1991, Sw RI
began another test pro• Shallow Underground
gram for N DCS to furTunnel Test, 20,000 kg
100
ther study the debris
• Fould
2,500,000 kg
effects from aircraft
'C
A Pre Gondola
c:
o
shelters, this time
20,000 kg
u
en 80
adding a protective rock
• Essex I
Q)
10,000 kg
-
Wall Debris Velocity
(Basler & Hofmann)
" Small Concrete
Bunker
The relationship between launch velocity and scaled cover depth (scaled
using the cube root of the charge weight) seems to follow a similar trend
for a wide range of loading densities. The determination of similar rela tionships for debris launch angle and debris mass distribution was not
only considered possible, but became a focused goal for explosives
safety. These data could be used to develop, computerize, and refine the
prediction of debris velocities, angles, and initial mass distribution to facilitate the calculation of debris effects for a variety of structures and to aid
in the subsequent siting of the structures.
• Aircraft Shelters
1,500 kg /m 3
(UET untamped)
0.06 kg/m 3 •
1,500 kg /m 3
(tamped)
10.2
10.1
10
Scaled Cover Depth , Kilograms Per Meter1J3
T prhn o l o p'v Tori
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and throw from such detonations.
The development of a computer
code based on new and existing
test data, as well as first-principle
calculations for such parameters
as launch velocity, angle distribution, and mass distribution,
would eliminate the need to test
each new design or modification.
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The OlSPRE2 model is a self-contained
software package, including numerous
FORTRAN modules and Visual Basic
pre- and post-processors designed to
run in a Windows ™ environment on an
IBM-compatible personal computer.
The calculation models are based on
the physics of structural breakup and
a large database of mostly aircraft
shelter response data.
The model
This predictive model was
developed by SwRI several years
ago with funding from the U.S.
Department of Energy (DOE) and
the U.S. Department of Defense
Explosives Safety Board (DDESB).
The model was called DISPRE,
for" dispersion prediction," and
was approved as a siting tool for
explosives processing and handling facilities in November 1990
by both DOE and DDESB. Various intermediate calculation models and three
computer codes developed by the Naval
Civil Engineering Laboratoryt comprise
DISPRE Version 1.0: SHOCK, short for
shock loading; FRANC, short for frangible panel; and MUDEMIMP, the acronym
for Multiple Debris Missile Impact
Simulation.
The DIS PRE model has proven
effective in reducing required siting
distances for many explosive material
processing structures when used within
OlSPRE2 includes the
prediction of air blast
around a storage
magazine. Air blast, as well
as debris, can cause
serious damage to
structures and personnel in
the vicinity of an explosion
and is a significant factor in
determining the safe
separation of
storage magazines.
its constraints, based on the limits of the
test data used to validate the model.
Version 1.0 can be used to predict building debris throw for charge weights up
to 120 kilograms (260 pounds) in a rectangular structure.
Because of a lack of other accurate
predictive models at the time, the model
was frequently used outside its validated
limits. Under funding from the Klotz
Croup, Sw RI modified the DISPRE model,
rather than simply extrapolating from it to
predict throw. This newest model, known
as DISPRE2, compares the differences
between the internal loads and breakup of
above-ground rectangular structures containing less than 120 kg (260 lb) of TNTequivalent explosives, and arch-shaped
magazines and rectangular above-ground
magazines storing up to 5,000 kg
(11,000 lb) of such exploSives.
Because the safe separation of magazines also depends on external air blast,
DISPRE2 includes the prediction of air
blast around the magazine. The software
is based on an analYSis of existing data
and fundamental calculations and has
been validated to a level of accuracy consistent with the existing test data chiefly aircraft shelter breakup, debris,
and air blast data.
Version 1.0 predicted hazardous
debris density - defined as no more than
one hazardous debris piece per 55.7
square meters - and thus, safe siting distances, reasonably well over a fairly wide
range of loading densities. More refined
versions have expanded
the number of structure
types that can be analyzed and has
improved the accuracy.
Recent efforts have
resulted in two versions
of the software. The
first is specifically for
aircraft shelters. The
second development
version covers seven
types of magazines,
including aircraft shelters. DISPRE2 Version
2.9 contains both versions of the software.
Debris Calculations
tNCEL is now the Naval
Facilities Engineering Service
Center of Port Hueneme,
California.
LETHALITY PREDICTIONS FOR 10,000 kg ACCIDENT IN AIRCRAFT SHELTER
10-1
Zone 1
Zone 5
Hazard Zones
110'
. 1
. 2-4
.... 5
110'
Zone 6
10-4
Reproducibility of breakup patterns and
debris trajectories in 1:100 and 1:20 scale
tests suggested that the prototype shelter
would fragment in a manner similar to the
rupture patterns observed in the tests, with
differences in reinforcement considered. The
measured external blast field was also similar.
The location of concentrated debris after an
explosion appeared to be quite directional in
both scales tested and in full-scale tests
conducted in the U.S. The reproducibility and
directionality of debris throw aided engineers
in the development of lethality probability
curves for personnel in the vicinity of
an accident.
TOP VIEW
Definition of Hazard Zones
10
100
1,000
Distance From Shelter (10,000 kg Charge)
Where do we go from here?
loading situations and better meet the
storage needs of the Klotz Group members. This expansion will require loading
realm differentiation.
SwRI has collaborated with the Ernst
Mach Institut of Freiburg, Germany, to
develop several program plans for the
Klotz Group that include testing and collecting data, as well as performing analyses, to accomplish this goal.
The upper limit of 5,000 kg (11,000 lb)
within a storage magazine corresponds to a
relatively low loading density for such a
structure. At high loading densities - up to
500,000 kg (1.1 million lb) of explosivesthe distance from the explosive charge to
each structural component and the components' thicknesses also become important in
determining failure
modes and subsequent
debris throw distances.
When the charge
References
amount is very high, the
P.K. Moseley, M.G. Whitney, "Prediction of Blast
charge standoff can be
and Debris from an Accidental Explosion Inside a
quite small, resulting in
Norwegian Aircraft Shelter," SwRl Project 02-5881,
close-in shock loading of
February 1981.
the component. For typical magazine wall
F. Riis, "Third-Generation Aircraft Shelter. Debris
Throw and Air Blast Caused by Accidental
thicknesses, this type of
Explosion in the Ammunition Cubicle. Report lli.
loading causes cataModel Tests, Scales 1:20 and 1:100," Fortifikatorisk
strophic failure of the
Notat 149/80, November 1980.
wall or a portion of the
wall into small debris
particles. Because typical stores for magazines
other than aircraft shelters can well exceed
5,000 kg (11,000 lb) of
explosives, the DISPRE2
model should be
expanded to cover high
A. Jenssen, F. Riis, 'Third-Generation Aircraft
Shelter. Debris Throw and Air Blast Caused by
Accidental Explosion in the Ammunition Cubicle.
Report II. Pressure Measurements within Steel
Model, Scale 1:75," Fortifikatorisk Notat 150/80,
November 1980.
P.K. Moseley, M.G. Whitney, "Prediction of Injury
Levels for Humans in the Vicinity of an Accidental
Technolo
In the high-risk world of explosives
storage, out of sight can never be out of
mind. Mindful of the life and death
importance of this mission, SwRI will
continue working to develop predictive
tools that are as fail-safe as humanly
possible .•:.
Comments about this article? Contact Cox
at (210) 522-2315 or pcox@swri.org.
Explosion Inside a Third Generation Norwegian Aircraft
Shelter," SwRl Project 02-6863, September 1982.
K.A. Marchand, P.M. Bowles, "Scaled Tests and Debris
Analysis of Model Aircraft Shelters," SwRl Project 06-4164,
September 1991.
P.M. Bowles, M.e. Whitney, C-J. Oswald, "A White Paper on
Computer Code for Calculating Initial Debris Parameters for
Accidental Detonations in Magazines," SwRl, January 1991.
"Prediction of Building Debris for Quantity-Distance Siting,"
Department of Defense Explosives Safety Board Technical
Paper No. 13, April 1991.
P.M. Bowles, c.J. Oswald, L.M. Vargas, w.E. Baker, "Building
Debris Hazard Prediction Model," February 1991.
P.M. Bowles, C-J. Oswald, M.A. Polcyn, "Earth Covered
Ammunition Magazines Quantity-Distance Model, DISPRE2,"
SwRl Project 07-5394, October 1994.
Todav • Summer 2000