protective structures

Explosion Protection—ArchitecturalDesign, Urban Planning andLandscape Planningby

Norbert Gebbeken, Torsten Döge

Reprinted from

International Journal ofProtective Structures

Vol. 1 · No. 1 · 2010

Multi-Science PublishingISSN 2041-4196

Explosion Protection—ArchitecturalDesign, Urban Planning

and Landscape PlanningNorbert Gebbeken, Torsten Döge

Institute of Engineering Mechanics and Structural MechanicsLaboratory for Engineering Informatics

University of the German Armed Forces MunichWerner-Heisenberg-Weg 39, 85577 Neubiberg, Germany

E-mail: [email protected]

Received 28 July 2009, Accepted 3 October 2009

AABBSSTTRRAACCTTThis paper gives first a short overview of some basics of blast wavepropagation and blast wave reflection. The reflected overpressure-timehistory, which is the design load, depends on the shape and geometryof the structures where the blast wave is reflected. It presents someshape studies which show the influence of shaping on the explosiondesign load. These shape studies include horizontal shapes (e.g. crosssection of columns), vertical shapes (e.g. facades) and spatial shapesof buildings. The paper further shows that shrub plantings reduce blastloads. It also discusses the use of flexible materials and constructionsand the influence of the urban situation on the design.

11.. IINNTTRROODDUUCCTTIIOONNIn this paper, different possibilities to protect buildings against blast are discussed taking theurban situation into account. The paper gives first a short overview of some basics of blastwave propagation and blast wave reflection. On this basis, the article shows architecturaloptions which are favorable for building protection. In this context, the shape and thegeometry play a major role. Various structural shapes are investigated and compared with eachother by numerical simulations. According to the motto: “light, yet secure and protective”,some ideas are presented which illustrate that the construction of hardened buildings does notnecessarily mean that the structural elements or buildings must be extremely massive.

The influence of various shapes on explosion loadings was studied by BARAKAT &HETHERINGTON [1, 2] and RICE et al. [3]. An impressive example for the realization ofarchitectural explosion protection measurements is the newly built Oklahoma City FederalBuilding [4]. SMITH & ROSE investigated the influence of street formations on the blast wavepropagation where phenomena like shielding and channelling occur [5]. Valuable advice forblast resistant design is given by the manuals FEMA 426 [6] and FEMA 430 [7].

In addition to [1, 2], this paper presents some new studies on more effective shapes shownin sections 3.2, 3.3 and 3.4. Another new study demonstrates how plantings can reduceexplosion loadings on buildings (section 4), i.e. the reflected overpressure-time history which

International Journal of Protective Structures – Volume 1 · Number 1 · 2010 1

acts on the building is reduced. To the knowledge of the authors, the influence of plantingson blast loadings was not investigated before.

Section 5 briefly shows how flexible materials and constructions can be used to protectbuildings against blast. The influence of the urban situation on the design is discussed insection 6.

22.. EEXXPPLLOOSSIIOONN DDEESSIIGGNN LLOOAADDAt first, a few basics of explosions are presented, which are necessary to understand highdynamic loadings. In this article, we focus on blast as a result of far field detonations.According to MAYRHOFER [8], explosions with a scaled distance

(1)

can be regarded as far field detonations. In eqn (1), a is the distance in the unit m from thecenter of explosion to the point of interest, and m is the mass of explosive in kg of TNT.

2.1. REFLECTED OVERPRESSUREWhen the overpressure of a blast wave is to be measured by an experiment, the pressuregauges are installed at specified distances to the explosion source in order to measure theside-on pressure. The measuring signal (black) shown in Figure 1 (left) stands for a measuredoverpressure-time history which is typical for 100 kg TNT at a distance of 15 m with ahemispherical blast wave propagation. For comparison, the blue curve shows the result of anumerical simulation. The typical pressure-time history is characterized by the pressure risetime, which only takes nanoseconds, by the peak overpressure, by the overpressure phasewhich is in the millisecond range, and by the suction phase (Figure 1 on the right). The peakoverpressure is the difference between the peak pressure p1 and the ambient air pressure p0.The terms overpressure and pressure must not be confused. For the theoretical and thepractical analysis, the rise time, and thus, the “thickness” of the shock front can be neglectedand is idealized here to be zero. The shock front is the boundary between the air in the initialstate and the state in the blast wave.

z am

= >/( )

( /( )).

/

10 5

1 3

m

kg1

2 Explosion Protection—Architectural Design, Urban Planning and Landscape Planning

15−30

0306090

120150180210240270300

Overpressure [kPa]

Time [ms]

Experiment, freeSimulation, freeSimulation, reflected

25 35 45 6555

Pressure p

Peak pressure

Overpressure phase

Pea

k ov

erpr

essu

re

Sho

ck fr

ont

Suction phase

p1

p0

0 ta t

Figure 1. Explosion, left: overpressure-time histories, measured andcalculated; right: idealized pressure-time history and definitions

If the blast wave hits a rigid obstacle at the same distance from the measuring point as inthe free propagation scenario, a pressure gauge measures the overpressure-time historyshown in red in the left of Figure 1. Here, the peak overpressure is approximately 2.8 timeshigher than in the free propagation scenario.

What is the reason for such a pressure rise? One might first think that the incoming andthe outgoing waves overlap, thus doubling the overpressure during the reflection. However,unlike the case of an elastic individual impact, the blast wave forces many subsequentparticles to hit the obstacle. They cannot be freely reflected because more air particles follow.For this reason, a further phenomenon has to be taken into consideration, i.e. the dynamicpressure. Thus, a so-called reflected overpressure develops and acts on the structure. Thereflected overpressure is the design load.

The maximum overpressures of the free and of the reflected blast wave are called peakside-on overpressure pso and peak reflected overpressure pro, respectively. The side-onoverpressure is the overpressure of a blast wave, which can be measured at a surface, whichis parallel to the direction of motion of the blast wave, thus there is no reflection.

2.2. OVERPRESSURE DEPENDING ON DISTANCE AND TIMEAssuming a spherical explosive in open air with the ignition point at the center, then the blastwave propagates spherically through the space. The peak side-on overpressure decreaseswith the distance, as it is illustrated in Figure 2.

In this figure, the overpressure-time histories for the explosion of 10 kg TNT are shown.The blast wave propagates spherically. The overpressure-time histories were calculated fordistances r of 3 m, 5 m, and 10 m by formulas from KINNEY & GRAHAM [9].

The two most important parameters of blast loadings are peak overpressure and maximumimpulse per unit area or peak overpressure and the duration of the overpressure phase,respectively. The maximum impulse Imax = max (I(t)) is defined as the maximum ofthe impulse I(t) = ∫F(t)dt which is the integral of force over time. For the overpressure-time history, the impulse per unit area I(t)/A = ∫ ta+t

ta p(t) — p0 dt is the integral of theoverpressure over time. The arrival time of the shock front is ta.


500

400

300

200

100

0

0 5 10 15

Time [ms]

Ove

rpre

ssur

e [k

Pa]

20 25 30

−100

10 kg TNT, 3 m, Imax/A = 224 kPa ms10 kg TNT, 5 m, Imax/A = 153 kPa ms10 kg TNT, 10 m, Imax/A = 72 kPa msEnvelope (peak side-on overpressure)

Figure 2. Overpressure-time histories: Explosion of 10 kg TNT, sphericalpropagation of blast wave, distances 3 m, 5 m, and 10 m

Often, the overpressure-time history is reduced to the two values peak side-on overpressurepso (or pro respectively) and maximum impulse Imax for simplification and for comparison.The reduction of the entire overpressure-time history to the two values is not alwaysacceptable, because information is lost. If only pso and Imax are given, then the exact shapeof the overpressure-time history is not known. Therefore, a linear function, which isdetermined by pso and Imax , is mostly chosen for the overpressure phase and the suctionphase is often neglected. This might be a good approximation for the overpressure-timehistories shown in Figure 2 when the suction phases can be ignored. But the simplificationof overpressure-time histories to the two values pso and Imax is not valid, when the suctionphase is important, e.g. for light and flexible structures or if a failure opposite to blastdirection is possible. The simplification is also not valid for indoor explosion, fordeflagrations and if multiple reflections occur, because the linear function is not a goodapproximation for overpressure-time histories describing these explosions.

For the example in Figure 2, the peak side-on overpressure decreases from pso = 482 kPa

for r = 3 m to pso = 34 kPa for r = 10 m. The maximum impulse per unit area Imax/A = ∫

ta+tdta

p(t) – p0 dt decreases from Imax/A = 224 kPa ms for r = 3 m to Imax/A = 72 kPa ms for r = 10 m. Thereby, td is the duration of the overpressure phase.

It can be recognized, that providing stand-off distance is a simple but effective protectivemeasure.

This section 2 explained only a few basics of explosions. The whole topic could not becovered on a few pages. For further readings see e.g. KINNEY & GRAHAM [9], MAYS & SMITH

[10], BANGASH & BANGASH [11], BEN-DOR [12], KRAUTHAMMER [13], and TM 5-855-1 [14].

33.. RREEDDUUCCTTIIOONN OOFF TTHHEE EEXXPPLLOOSSIIOONN DDEESSIIGGNN LLOOAADD BBYY SSHHAAPPIINNGGIn order to reduce the explosion design load for a structural member for a given explosionscenario, the materials, the structural shapes, and the structural dimensions have to beadapted. In this section, the influences of the structural shapes are investigated.

Usually, reflections are subdivided into normal reflection, regular reflection and MACH

reflection [9]. The reflection coefficient cr , which is the ratio of peak reflected overpressureto peak side-on overpressure, depends on the peak side-on overpressure and on the incidentangle as it can be seen in Figure 3. For peak side-on overpressures less than ≈ 0.2 MPa, themaximum reflection coefficient is not at normal reflection (cr ≈ 2.0 to 3.3), but in the regionof MACH reflection at an angle of incidence higher than 40 degree (cr > 3, Figure 3). Forhigher peak side-on overpressures, the maximum peak reflected overpressures and reflectioncoefficients, respectively, occur at normal reflections. In order to reduce the design load,normal reflections should be avoided.

3.1. SHAPE STUDIES (HORIZONTAL)A first simple study illustrates the influence of a blast wave on a structural element. Fourdifferent cross sections were investigated:1. circle: circular section,2. square-corner: quadratic section, a corner directed to the explosion center,3. square-edge: quadratic section, an edge directed to the explosion center,4. rectangle-long edge: rectangular section, a long edge directed to the explosion center.

These sections are illustrated in Figure 4. All sections have an area of 0.16 m2, whichcould be the cross sections of columns. The dimensions of the rectangle are 0.8 m times 0.2 m.The scenario is the explosion of 10 kg TNT at a distance of 5 m measured from the center ofexplosion to the centers of the cross sections.


The numerical simulations were performed using the hydrocode AUTODYN [17]. At first,the blast wave propagation was modeled by a spherically symmetric one-dimensional(1D) model with just one dimension in radial direction. When the shock front reached adistance of 4.5 m to the explosion center, the calculation was interrupted and remapped onthe 2D models with planar symmetry. This 2D model is not able to simulate the blast wave


10 10 20 30 40 50 60 70 80 90

2

3

4

5

6

7

8

9

10

11

12

1334.47

20.68

13.79

6.89

3.45

2.76

1.38

1.030.69

0.48

0.00690.0034 0.00140.0138

0.34

0.034

0.21

0.07 0.14

2.07

Angle of incidence α [degree]

Ref

lect

ed p

ress

ure

coef

ficie

nt c

r

Peak incidentoverpressure pso [MPa]

Figure 3. Reflection coefficient depending on angle of incidence andpeak side-on overpressure (based on TM 5-1300 [15] and [16])

Peak pressure[kPa]550

460

370

280

190

100

(a) Circle (b) Square-corner

(c) Square-edge (d) Rectangle-long edge

Figure 4. Cross sections, peak pressures

propagation in the third spacial direction, and therefore delivers too high pressures. In thestudies presented, the amplification of maximum overpressures is less than 4%. Thus, thedescribed procedure is adequate to show the influences of the cross sections on the blastwave. The cross sections were modeled as rigid, which is valid according to [18].

The peak (reflected) pressures are illustrated in Figure 4. For the sections square-edgeand rectangle-long edge, there are in each case almost constant peak reflected pressuresacross the exposed edge. Normal reflections occur at the centers of the exposed edges andregular reflections towards the corners. The peak reflected overpressures are about 6%higher for square-edge than for rectangle-long edge because of the shorter distance of theedge to the explosion. For the circular section, the highest peak reflected pressure appearsat the point of the border which is nearest to the explosion and where a normal reflectionoccurs. The peak reflected pressures decrease towards both sides of the center. At thesection square-corner, the blast wave hits the edges at an angle of about 45 degree. A MACH

reflection takes place. In Figure 4, less peak pressures can be observed at the corner nearestto the explosion than along the adjacent edges. This is caused by numerical inaccuracies,because the formation of the MACH stem can not be simulated at the corner due to the cellsize. Less peak pressures appear at the back sides than at the exposed sides of all foursections. There, the blast wave is diffracted instead of reflected. The maximum peakreflected overpressures are listed in Table 1 for the four cross sections.

Figure 5 shows the maximum impulse per unit area Imax /A as defined in Section 2.2 foreach cell of the numerical simulation. The highest maximum impulses per unit area occur atthe section rectangle-long edge because of the high flow resistance. The maximum impulsesper unit area are listed in Table 1 for the four cross sections.

For a structural element, e.g. a column, the overall loading, e.g. a uniformly distributedload, is decisive for design. The uniformly distributed load is the integral of the pressure pacting in a defined direction over the circumference C of the cross section. For example, ifthe cross section is in a plane with Cartesian coordinates x and y, and the blast wave directionis in opposite x direction, then the uniformly distributed load q, which acts also in oppositex direction, is calculated by

(2)

The integration path runs in counterclockwise direction. The load-time history ispresented in Figure 6. The worst cross section is the section rectangle-long edge because ofthe width exposed to the blast load. In this study, the circular cross section has the lowestloads with a maximum uniformly distributed load of 118 kN/m and a maximum impulse perunit length of Imax /L = 90 Ns/m.

As a result it can be recorded that a circular cross section is in most cases the first choicefor a structural element (see also [3]). Apart from the technical reasons relating to the

q t p x y t dyC

( ) ( , , ) .( )

= ∫�


Table 1. Cross sections, maximum peak reflectedoverpressures and maximum impulses per unit area

Cross Section pro [kPa] Imax [kPa ms]circle 447 312square-corner 411 302square-edge 443 328rectangle-long edge 423 397

structural behavior of this shape (e.g. rotationally symmetrical stiffness of the structuralelement) the above analyses show that, on the one hand, a relatively low peak overpressureand a relatively low maximum impulse per unit length are observed with this shape and that,on the other hand, this shape is independent of the direction of the blast wave.

3.2. SHAPE STUDIES (VERTICAL)In this section, the reflection of blast waves at various vertical shapes, e.g. facades, is studied.

The reference scenario is the explosion of 5 kg TNT in a distance of 3 m to an uprightfacade. The aim of this study is to reduce the loads in the highly loaded lower part of theupright facade. This could be achieved by a parabolic shape (Figure 7b) or a cubic shape


Maximum impulseper unit area

[kPa ms]350

280

210

140

70

0

(a) Circle (b) Square-corner

(c) Square-edge (d) Rectangle-long edge

Figure 5. Cross sections, maximum impulses per unit area

Time [ms]

Un

iform

ly d

istr

ibu

ted

loa

d [

kN/m

] sSquare-edge, Imax/L = 109 Ns/m

Square-corner, Imax/L = 117 Ns/m

Circle, Imax/L = 90 Ns/m

Rectangle-long edge, Imax/L = 260 Ns/m

−50

00 1 2 3 4 5 6 7

50

100

150

200

250

300

350

Figure 6. Cross sections, uniformly distributed loads for structural elements

(Figure 7c) compared to the upright shape (Figure 7a). The loss of space in the building maybe a disadvantage of the curved shapes. To avoid this disadvantage, the facades are comparedwith one that bows outwards (Figure 7d). This, of course, is again unfavorable because of theshorter stand-off distance. Can this shape nonetheless be more advantageously than theupright shape?

Here, the scenario is the explosion of 5 kg TNT with a hemispherical propagation of theblast wave. The center of the explosion is on the ground in a distance of 3 m to the uprightfacade. The blast wave propagation was first calculated by a 1D model until the blast wavereached a distance of 1.96 m. Then, the calculated values were remapped on the 2D modelswith axial symmetry. The facades were modeled as rigid structures, which is valid accordingto [18].

As depicted in Figure 8, the parabolic shape as well as the cubic shape have lower peakreflected pressures than the upright shape due to a lower angle of incidence. The cubicshape was described by a cubic function, which allowed to adjust the angle of the shape atthe ground. An angle of 45° was chosen. For the parabolic shape, it was not possible toadjust the angle because of less parameters in the shape function. The angle of incidenceat the ground is about 33.69°. The cubic shape performs better than the parabolic shapewith respect to the peak reflected overpressure as well as to the maximum impulse per unitarea. The shape function could be further optimized, but the optimization is not the focusof this investigation.


3 m

3 m

1 m 3 m 3 m 2 m1 m

2000

Peak pressure [kPa]

1620

1240

860

480

100

= Source of explosion (5 kg TNT)(b) Parabolic shape(a) Upright shape (c) Cubic shape (d) Cubic shape, moved to explosive

Figure 7. Vertical shapes and peak pressures

Upright shapeParabolic shapeCubic shapeCubic shape, moved to explosive

0 0.5 1 1.5

Height [m]

2 2.5 3 0 0.5 1 1.5

Height [m]

2 2.5 30

500

1000

1500

2000

2500

3000

3500

4000

Pea

k re

flect

ed o

verp

ress

ure

[kP

a]

Upright shapeParabolic shapeCubic shapeCubic shape, moved to explosive

Max

imum

impu

lse

per

unit

area

[kP

a m

s]

0

200

400

600

800

1000

1200

Figure 8. Vertical shapes, comparison of peak reflected overpressureand of maximum impulse per unit area

The question was, whether the cubic shape, moved 1m in direction to the explosive, canbe more advantageously than the upright shape. As to the maximum impulse per unit area,the cubic shape has at the ground similar high maximum impulses per unit area as the uprightshape. But with increasing height, these maximum impulses per unit area decrease fasterbecause the air can better flow upwards. The peak reflected overpressures are higher atthe cubic shape in the lower region of the shape. This disadvantage is due to the shorterdistance to the explosion. Except for the higher peak reflected overpressures in the lowerregion, the cubic shape, which is moved 1m to the explosive, performs better than the uprightshape despite the shorter stand-off distance.

3.3. SHAPE STUDIES 3DIn this section, the reflections of blast waves at various spatial (3D) structures are studied.The shapes of these structures are a cube, a step pyramid, a cylinder, and a hyperboloid(Figure 9).

All four structures have a height of 15 m and a volume of 3375 m3. The cube has thedimensions 15 m.15 m.15 m. The step pyramid consists of four steps with a height of 3.75 meach and edge lengths of 21.909 m, 16.432 m, 10.954 m, and 5.477 m. The cylinder has aradius of 8.463 m. The radius of the hyperboloid at the ground is 12.500 m and at the top6.119 m.

For this study, the explosion of 50 kg TNT on the ground with a hemispherical propagationof the blast wave is investigated. The horizontal distance from the center of the explosive to


Figure 9. Spatial shapes and maximum impulses per unit area

the center of the cross section is 22.5 m. The blast wave propagation was first calculated bya 1D model until the blast wave reached a distance of 9.95 m. Then, the calculated valueswere remapped on the 3D models. The structures were modeled as rigid.

Figure 10 shows the peak pressures and maximum impulses per unit area measured acrossthe height at points of the building’s surface which are closest to the explosion. This actuallyis the center line of the exposed surface area. The peak pressures at the cylinder are higherthan at the cube because of the shorter distance to the explosion. Also, the peak pressures atthe lower part of the hyperboloid are higher than at the cube and the cylinder because of theshorter distance to the explosion. But the hyperboloid has lower maximum impulses per unitarea because the air can better flow around. The step pyramid has the highest peak pressuresand the highest maximum impulses per unit area because of the shortest distance to theexplosion. But the upper stories are protected by shadowing effects. The results in Figure 10are for the center line of the exposed surface area. As depicted in Figure 9, the shapescylinder and hyperboloid have lower maximum impulses per unit area away from the centerline than the cube because of the lower flow resistance of the circular cross section.

In order to assess the structures, Figure 11 depicts the loads on the entire structures. Theforce-time history results from the integral of the component of the overpressure in loading


CubeCylinderStep pyramidHyperboloid

0 3 6 9

Height [m]

12 15 0 3 6 9

Height [m]

12 150

50

100

150

200

250

300P

eak

refle

cted

ove

rpre

ssur

e [k

Pa]

[kP

a m

s]

Max

imum

impu

lse

per

unit

area

0

100

200

300

400

500

600

700

800

900CubeCylinderStep pyramidHyperboloid

Figure 10. Spatial shapes, comparison of peak reflected overpressureand of maximum impulse per unit area

0 10 20 30

Time [ms]

40 50

For

ce [k

N]

−3000

−2000

−1000

0

1000

2000

3000

4000

5000Cube, Imax = 44785 N s

Cylinder, Imax = 34593 N s

Step pyramid, Imax = 33538 N s

Hyperboloid, Imax = 26892 N s

Figure 11. Spatial shapes, loadings on structures

direction over the surface of the structure. The load for the hyperboloid is the lowest in thisstudy, with respect to maximum force as well as to the maximum impulse.

Conclusion: In all examples given above the peak overpressures and maximum impulsesare mainly dependent on the distance to the explosion, the blast wave’s angle of incidence,and the flow resistance of the structural shape. The shape of a structural element or buildingcan decisively reduce the design load.

3.4. NON-CONVEX SHAPESThe reduction of explosion loadings is even more effective than in the preceding sectionwhen non-convex corners are smoothed out as it is shown by the following example of anL-shaped building.

The investigated L-shaped building is 20 m high. The wings of the building have a lengthof 26 m and a width of 10 m. The explosion of 100 kg TNT with hemispherical blast wavepropagation is located on the ground at the coordinates x = –2 m, y = –2 m and z = 0 m(Figure 12d)).

In order to reduce the blast loadings like in the preceding sections, the re-entrant cornerof the L-shaped building was smoothed out (Figure 12 b)). The shape of the smoothing outis described by

(3)

with

in the range 0 m ≤ x ≤ 16 m, 0 m ≤ y ≤ 16 m and 0 m ≤ z ≤ 20 m. Eqn (3) was derivedfrom the formula for a so called superellipse and describes a quarter circle at the ground(z = 0 m, dashed line in Figure 12d)). At the top of the building (z = 20 m), the originalcorner is kept.

Often, it is mentioned that convex shapes are better than non-convex shapes in blastresistant design. This statement is too general. If appropriate shapes are chosen, then non-convex shapes can involve reduced blast loads in comparison to convex shapes.

For example, the smallest convex envelope of the L-shaped building in Figure 12a) wouldbe the extension of the building to the dotted line in Figure 12d) which results in the buildingshown in Figure 12c). The disadvantage of this shape is clearly the shorter distance to thesource of explosion.

The blast wave propagation was first calculated by a 1D model until the blast wavereached a distance of 13.98 m. Then, the calculated values were remapped on the 3D models.The buildings were modeled as rigid.

n zr z

( )

log

log( )

=

1

2

16m 2

= −( ) +

r z z( ) 16 2 1

201m

m

x yn z n

16 16m m

+

( ) (zz )

=1


The L-shaped building (Figure 13 a) and Figure 14a)) has the highest explosion loadingsin the corner because the reflected shock fronts of both wings meet at this corner and causea high peak reflected overpressure. Furthermore, the flow of the air is interfered and thisleads to a high maximum impulse per unit area.

The highest peak reflected overpressure for the L-shaped building (Figure13a)) is 192 kPa.

The highest maximum impulse per unit area is 1160 kPa ms (Figure 14a)).By smoothing out the re-entrant corner as described, the peak reflected overpressure and

the maximum impulse per unit area can be reduced down to 139 kPa and 752 kPa ms,respectively (Figure 13b) and Figure 14b)). This corresponds to a reduction by 28% and 35%.


Figure 13. L-shaped building, comparison of peak (reflected) pressure

Figure 12. L-shaped building, 3D views and plan view

For the convex envelope, the highest peak reflected overpressure is 222 kPa and thehighest maximum impulse per unit area is 1033 kPa ms (Figure 13c) and Figure 14c)). Thehighest peak reflected overpressure is even higher than at the L-shaped building because ofthe shorter distance to the explosion.

Conclusion: The statement that convex shapes should be preferred in blast resistantdesign is too general. Correctly designed non-convex shapes can perform better in blastresistant design than convex shapes.

44.. LLAANNDDSSCCAAPPEE MMOODDEELLLLIINNGG:: IINNFFLLUUEENNCCEE OOFF PPLLAANNTTSS OONN TTHHEE BBLLAASSTT LLOOAADDSSHedges in urban areas have special meanings. They are valuable habitats for insects andbirds, clean the air and enrich the air with oxygen, serve as screens and as delimitation andstructuring elements. Hedges play also a role in noise reduction [19, 20, 21].

In this section, it is investigated whether hedges (shrub plantings) can be used to reducethe explosion loadings. An answer to this question is not easy, because realistic calculationsof the blast wave propagation in planting areas is very complex.

When the blast wave hits the leaves, twigs and branches of plants, it is partly reflected andpartly diffracted. The flow of the air behind the shock front is interfered by the plants, whichleads also to a reduction of the pressures in the blast waves. Thereby, energy of the blastwave is transferred to kinetic and potential energy of the plants. An entire simulation of aplanting area with modeling of all twigs and leaves and with consideration of the fluidstructure interaction is too complex and expensive for current computers. Therefore, asimplified numerical study was performed which shows the influence of a hedge on a blastwave. In this study, the hedge was modeled by small randomly distributed rigid obstacles.

The scenario is the explosion of 5 kg TNT in a distance of 4 m to a wall (Figure 15). Athree-dimensional EULER mesh with the dimensions 4 m · 2 m · 2 m (half symmetry) and with200 .100 .100 elements was generated. Thus, the element size is 20 mm. A hedge is assumedwith 1 m in the height and 1 m in the width. The distance between the hedge and the explosionis 2 m.

SPÄH et al. [21] investigated hedges width densities of 8.7 kg/m3 for Common Dogwood(Cornus sanguinea) up to 28.2 kg/m3 for Fly Honeysuckle (Lonicera xylosteum). That means,that for estimated densities for wood and leaves of about 400 to 700 kg/m3 approximately(8.7 kg/m3)/(700 kg/m3) ≈ 1% up to (28.2 kg/m3)/(400 kg/m3) ≈ 7% of the volume ofa hedge consists of organic material, the other part is air. In the following study, volumefractions of 1% and of 5% are investigated.


Figure 14. L-shaped building, comparison of maximum impulse per unit area


The hedge is modeled in AUTODYN by a simplified approach. Some of the elements in theregion of the hedge, 1% and 5% respectively, are set unused. An unused cell is like a rigidobstacle in the numerical simulation. The distribution of the unused cells is randomly. Thesmall obstacles are fixed at there location, they can not move like the twigs and leaves inreality. The pressure waves are reflected at the small obstacles and the air flow is impeded.At the sides, except the wall, and at the top of the EULER mesh are flow-out boundaryconditions. The explosion was first calculated with an one-dimensional spherical-symmetricmodel with a propagation of the shock front to a distance of 1.95 m to the center of theexplosion. Then, the calculated values were copied to the three-dimensional model.

The peak reflected pressures were reduced in both models with hedges in comparison tothe model without hedge (Figure 16). The highest loaded point on the wall in the modelwithout hedge is the point at the ground with the shortest distance to the explosion. Table 2shows the peak reflected overpressure pro and the maximum impulse Imax for this point.

The peak reflected overpressure could be reduced by 9% by the 1%-hedge and by 39% bythe 5%-hedge. The maximum impulse was reduced by6% and 25% respectively.

These simplified models with randomly distributed rigid cells show that plantings can beused to reduce explosion loadings on buildings. The reduction of peak reflected overpressureand maximum impulse were beyond our first expectations. Of course, this is more a theoretical

2 m

5 kg TNT

1 m 1 m

1 mHedge

Figure 15. Hedge, sketch of model (elevation)

Figure 16. AUTODYN model and peak pressure, left: 0% unused (no hedge),center: 1% unused, right: 5% unused

than a realistic model. More research should be done on this topic in the future. The modelof the plantings should be improved, e.g. the model of the plants should be modeled morerealistically by LAGRANGE elements. Of course, the numerical effort is then much higher,because much more elements are necessary because the dimensions of the leaves are in therange of parts of millimeters. Nonetheless, we presented our study and hope to initiate moreresearchers on this topic.

55.. SSEELLEECCTTIIOONN OOFF MMAATTEERRIIAALLSS AANNDD SSTTRRUUCCTTUURREESSIn this section further ideas are briefly presented, illustrating that the construction ofhardened buildings does not necessarily mean that structural elements and buildings must beextremely massive.

Most structural materials such as steel, concrete or glass have practically no impact on thereflection coefficient and thus on the peak reflected overpressure (also refer to GEBBEKEN &DÖGE [18, 22]). However, the peak reflected overpressure can be reduced by extremely lightand resilient materials (e.g. soft foams).

When using protective cladding it is important to use energy-absorbing materials (e.g.porous materials) in order to reduce the loading on bearing structures. Due to the plasticdeformation of the protective material the blast wave energy is partly transformed intointernal energy (i.e. heat).

It is also described in [18] that the thickness of structural members (from a thickness ofapproximately 0.01 mm and smaller) and their support have practically no influence on thepeak reflected overpressure. But, the reflected overpressure-time history after the first peakcan be reduced by resilient structures such as glazing or membrane facades (refer to [23]).The reflected overpressure-time history in front of a membrane facade is reduced comparedto a rigid facade, since the membrane absorbs energy and part of the energy is transferred tothe air behind the membrane [22].

Another positive effect of such membrane facades is, that the forces are transferred tothe load-bearing structure with a delay in time. When the blast wave impinges on amembrane structure which is externally attached to a building, the membrane can firstbillow like a sail before the forces are fully transferred to the load-bearing structure. Dueto the time delay, the peak force on the load-bearing structure is lower in comparison to arigid facade (Figure 17).

If now the membrane support frame is viscoelastically attached to the load-bearingstructure there are manifold possibilities to adjust the interaction between the blast protectionfacade and the building. This allows to absorb a significant part of the blast wave energy.Additionally, the maximum force which is transferred to the building is reduced compared toa rigid facade.

Based on these considerations additional concepts can be developed. For instance, onecould conceive hightec textiles which are transparent and partly air-permeable [23]. Suchfacade elements could be of interest from a design and economic point of view.


Table 2. Results

Model pro [kPa] Imax [kPams]no hedge 627 5371%-hedge 570 5035%-hedge 381 404

For more details on this topic, see e.g. TEICH & GEBBEKEN [24].Conclusion: The peak reflected overpressure can be reduced by the use of light and soft

materials. Additionally, the materials used shall be energy-absorbing allowing to reduce theloading on bearing structures. Resilient structures can positively influence both the reflectedoverpressure-time history of the blast wave and the forces transferred to the load-bearingstructure versus time.

66.. IINNFFLLUUEENNCCEE OOFF TTHHEE UURRBBAANN SSIITTUUAATTIIOONN OONN TTHHEE DDEESSIIGGNNFor numerous aspects of sensitive building design the urban situation plays an importantrole. When planning the design of a building the following aspects shall be considered anddiscussed with the customer or the authorities as required:• Is the occurrence of extraordinary risks such as explosions to be expected in the built-

up or new urban development area? Not only the possibility of a terrorist attack but alsothe danger of normal accidents has to be taken into account. One might remember forexample the tanker accident of Ryongchon in North Corea in 2004 or the accident inthe fireworks factory of Enschede (Netherlands) in 2000.

• How dense is the population in that area? How many buildings with which dimensionsare there and what are the distances between them? Which building systems have beenused for the construction of these buildings?

• Which access paths exist to this area? Are all roads publicly accessible and to whichvehicle types are they open? Is it possible to restrict and/or control the access to thearea in accordance with the intended use and the related category of persons (barriersor checks of individuals)?

The list is not exhaustive but it includes important aspects to be considered when examiningthe urban situation. If it has to be assumed that an explosion can happen in the area underconsideration two aspects are of special interest for the construction of hardened buildings:1. Distance of the buildings to the origin of explosion: According to the motto “distance

protects” care should be taken that carriers of explosives cannot be found in closevicinity to the buildings.

2. Consideration of (multiple) reflections: Blast waves are reflected at building surfacesseveral times and can thus intensify the blast, put a second strain on the building underconsideration and finally unintentionally damage or destroy other unhardenedbuildings whose surfaces are put under load for the first time.

The blast wave propagation in a built-up area with multiple reflections is illustrated inFigure 18 by means of a graphical representation of the velocity evolution from a numerical


Preliminary protection volume

Protectionmembrane

Non blastresistantglazingor walls

FD

FD

FZ

FZ

pso

pso

0

40

80

120

For

ce F

D [k

N]

0 30 60 90 120Time t [ms]

Stiff facadeResilient facade

Figure 17. Membrane facades, reduction of blast loadings (schematic)


Figure 18. Simulation of blast wave propagation in a built-up area [25]

simulation. The sequence shows a detonation in close vicinity to a main building. Inparticular the last five pictures show that the blast wave which is reflected at the mainbuilding for the first time, propagates to the crescent-shaped building from which it isagain reflected and hits the main building again. This illustrates the influence of thesurrounding buildings on the blast wave.

Some considerations on safety engineering are shown in the series of pictures in Figure 19[26]. The planned transparent building located along a street shall be protected against ablast wave generated by an exploding bomb placed in a truck. The safety engineerrecommends in a first approach that a protecting wall should be erected in front of thebuilding to provide appropriate protection (Figure 19 a)). The architect does not agree thatthe building would be hidden behind a wall and prefers another solution. It can bedemonstrated by numerical simulation that appropriate protection can also be provided bya banquette (landscape modelling, Figure 19 b)). To achieve this, the street would need tobe on a lower level at the place where a truck could be parked. The blast wave is reflectedat the banquette resulting in a reduction of the peak overpressure and impulse which are thedecisive design parameters (Figure 20 c)).

The protection wall (Figure 20 b)) is a more effective protection measurement in comparisonto the banquette. In this example, the banquette already reduces the peak reflected overpressureby up to 24% and the maximum impulse per unit area by up to 17% (Figure 21).

To keep the front exposed to the load relatively narrow the safety engineer recommends thatthe building should not be too long (Figure 19 c)). Additionally, reinforcing structures might beincorporated in the front area, while such structures are not required for the sides and the back.

Furthermore a circular shape would be recommendable (refer to the conclusions drawnfrom Figure 9).


H = 5 m H = 3 m

B = 3 m

B = 15 m

D = 5 m

T = 15 m

D = 15 m

(a) Protection wall

(c) Shape of building(d) Design of facade and structural elements

(b) Banquette

D = 15 m

D = 15 m

Figure 19. Safety engineering [26], comparison of measures


Pressure [kPa]

300

240

180

0

120

60

Building

Building Building15

m

15 m 10 m 5 m5 m7 m 3 m

3 m

5 m

(b) Building with protection wall(a) Unprotected building (c) Building with banquette

= Source of explosion (100 kg TNT)

Figure 20. Blast wave protection, numerical simulations, pressure plots

0

100

200

300

400

500

600

700

800

900

1000

Max

imum

impu

lse

per

unita

rea

[kP

a m

s]

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0 3 6 9 12 15Height [m]

0 3 6 9 12 15Height [m]

Pea

k re

flect

ed o

verp

ress

ure

[kP

a] (a) Unprotected building

(b) With protection wall

(c) With banquette

Unprotected building

With protection wallWith banquette

Figure 21. Comparison of peak reflected overpressure and of maximumimpulse per unit area

Finally, a curtain wall could be attached to the facade partially absorbing the blast fromthe truck or rucksack bomb before the blast hits the bearing elements (Figure 19 d)).Furthermore it is recommended to choose a circular design for the columns as alreadydemonstrated in the cross section study.

This simplified example shows that there is a variety of possibilities to avoid a bunker-like building design without neglecting an appropriate protection against explosive loads!

77.. CCOONNCCLLUUSSIIOONNIn this paper, some numerical investigations have been presented which show how differentarchitectural measures can effectively reduce the loadings from explosions. The main points,which have to be considered, are• stand-off distance,• use of circular shapes, especially for columns,• avoidance of normal reflections,• correctly designed non-convex shapes can be better than convex shapes,• plantings reduce blast loads,• landscape modelling,• use of energy absorbing materials and constructions,• taking into account the urban situation in the planning process.

Numerical studies, presented in this paper, have shown that these measures can decisivelyreduce the blast design load.

RREEFFEERREENNCCEESS1. M. Barakat and J.G. Hetherington. New architectural forms to reduce the effects of blast waves and

fragments on structures. Structures Under Shock and Impact V, pages 53–62, 1998.

2. M.A. Barakat and J.G. Hetherington. Architectural approach to reducing blast effects on structures.Proceedings of the Institution of Civil Engineers: Structures and Buildings, pages 333–343, 1999.

3. D. Rice, J. Baum, D. Tennant, and R. Löhner. Effect of Reinforced Concrete Column Geometry on BlastLoading and Response. In 11th International Symposium on Interaction of the Effects of Munitions withStructures, Mannheim, 2003.

4. B. Fortner. Symbol of Strength. Civil Engineering, 74(10):36–45, 2004.

5. P.D. Smith and T.A. Rose. Blast wave propagation in city streets – an overview. Progress in StructuralEngineering and Materials, 8(l):16–28, 2006.

6. Federal Emergency Management Agency. FEMA 426: Reference Manual to Mitigate Potential TerroristAttacks Against Buildings. FEMA, 2003.

7. Federal Emergency Management Agency. FEMA 430: Site and Urban Design for Security – GuidanceAgainst Potential Terrorist Attacks. FEMA, 2007.

8. C. Mayrhofer. Methoden der dynamischen Grenztragfähigkeitsberechnung. In K. Thoma, N. Gebbeken,and H. Thünemann, editors, Workshop “Ban-Protect” Bauliche Strukturen unter Belastung durch Impaktund Sprengwirkung, pages 169–184, Freiburg, 2004. Fraunhofer-Institut für Kurzzeitdynamik, Ernst-Mach-Institut.

9. G.F. Kinney and K.J. Graham. Explosive Shocks in Air. Springer, Berlin, 1985.

10. G.C. Mays and P.D. Smith. Blast effects on buildings: Design of buildings to optimize resistance to blastloading. Thomas Telford, 1995.

11. M.Y.H. Bangash and T. Bangash. Explosion-Resistant Buildings: Design, Analysis, and Case Studies.Springer, 2006.

12. G. Ben-Dor. Shock wave reflection phenomena. Springer, 2007.

13. T. Krauthammer. Modern Protective Structures. CRC Press, 2008.


14. Defense Special Weapons Agency and the Joint Departments of the Army, Air Force, and Navy. TM 5–855–1,Design and Analysis of Hardened Structures to Conventional Weapons Effects, 1997.

15. US Army Corps of Engineers. TM 5–1300, Structures to Resist the Effects of Accidental Explosions. USDepartment of the Army, 1990.

16. Department of Defense. Structures to Resist the Effects of Accidental Explosions, UFC 3–340–02, 2008.

17. Century Dynamics Inc. Autodyn - Theory Manual. Century Dynamics Inc., 2005.

18. N. Gebbeken and T. Döge. Der Reflexionsfaktor bei der senkrechten Reflexion von Luftstoßwellen an starrenund an nachgiebigen Materialien. Bauingenieur, 81(11):496–503, 2006.

19. D. Aylor. Noise reduction by vegetation and ground. The Journal of the Acoustical Society of America,51:197–205, 1972.

20. G. Watts, L. Chinn, and N. Godfrey. The effects of vegetation on the perception of traffic noise. AppliedAcoustics, 56(l):39–56, 1999.

21. M. Späh, L. Weber, and P. Leistner. Schallschutzpflanzen - Optimierung der Abschirmwirkung von Heckenund Gehölzen. Technical report, Zwischenbericht Fraunhofer-Institut für Bauphysik, 2009.

22. N. Gebbeken and T. Döge. Vom Explosionsszenario zur Bemessungslast. Der Prüfingenieur, 29:42–52,Oktober 2006.

23. N. Gebbeken. Grundsätzliche Ideen und Konzepte zur Blast-Sicherheit von Glas-Membran-Fassaden. InWorkshop “Vitrum-Protect”, Freiburg, Germany, 2006.

24. M. Teich and N. Gebbeken. Infrastructure Protection with Flexible and Soft Structures. In 13th InternationalSymposium on the Interaction of the Effects of Munitions with Structures, Br hl, 2009.

25. N. Gebbeken, S. Greulich, and T. Döge. Blast resistant urban planning and design. In N. Gebbeken, M.Keuser, M. Klaus, I. Mangerig, and K. Thoma, editors, 2. Workshop “BAU-PROTECT”, pages 409–421.Berichte des Konstruktiven Ingenieurbaus 06/4, University of the German Armed Forces Munich, 2006.

26. N. Gebbeken et al. Blast Resistant Shaping: A knowledge based strategy to reduce the effects of blast waveson structures! Presentation in Dubai, 2005.


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