BLAST LOAD ASSESSMENT BY SIMPLIFIED AND ADVANCED

METHODSPrepared by

Dr Peter D SmithReader in Protective Structures, Cranfield University, Defence

Academy, Shrivenham, UK

Dr Andrew TyasSenior Lecturer, Department of Civil and Structural, Engineering

University of Sheffield, UK

Data sheet componentsAim of data sheet - to outline methods of assessing the characteristics of a blast wave produced by the detonation of a high explosive charge.Empirical techniques – methods based on the concept of scaled distanceSemi-empirical techniques – methods for use in more complex geometries (e.g. explosions inside buildings)Numerical techniques – Computational fluid dynamics softwareReferences

Empirical techniquesBased on ‘cube root’ or ‘third power’ scalingThe general ‘Sachs’ scaling laws are simplified in ‘Hopkinson-Cranz’ scalingThe non-dimensional Sachs scaled distance is

This can be reduced to the Hopkinson-Cranz scaled distance Z, given by:

R = range (m) and W is charge mass (kg) expressed in terms of an equivalent TNT charge.

3/1

3/10

ERpR =

3/1WRZ =

Charge configurationsSPHERICAL FREE AIR BURST – the charge detonates above the ground and the blast wave propagates with a spherical wave frontAerially-delivered munitions with fuses designed to operate above the ground would be so classified

HEMISPHERICAL SURFACE BURST – the charge detonates in contact with the ground and the blast wave propagates with a hemispherical wave frontTerrorist vehicle bombs or military munitions fused to detonate on impact with the ground would be so classified

Sources of informationThose in YELLOW are commercially available

Those in WHITE are of more restricted availability

1. TM5-1300: Design of Structures to Resist the Effects of Accidental Explosions US Department of Defense, 1991

2. TM5-855-1: Fundamentals of Protective Design for Conventional Weapons. US Dept of the Army Technical Manual, US Army Corps of Engineers, 1987

3. Baker WE, Cox PA, Westine PS, Kulesz JJ, Strehlow RA Explosion hazards and evaluation Elsevier, New York, 1983

4. Smith PD, Hetherington JG Blast and ballistic loading of structuresButterworth-Heinemann, Oxford, 1994

5. Mays GC, Smith PD Blast effects on buildings Thomas Telford, London, 1995 [New edition (ed by D Cormie, GC Mays and PD Smith) is in preparation – publication Spring 2009]

6. ConWep: Conventional weapons effects program. Prepared by DW Hyde, ERDC Vicksburg MS, 1991

7. PSADS: Protective structures automated design system v1.0 US Army Corps of Engineers, 1998

Blast parameters vs. scaled distance

�Scaled Distance, R/W 1/3

Figure 3-7. Shock wave parameters for hemisphericalTNT surface bursts at sea level

0.1 1.0 10 100.001

.01

0.1

1.0

10

100

1000

10000

1.E+5

1.E+6� �P r , psi� �P so , psi

� � �I r , psi-ms/lb 1/3� � �I s , psi-ms/lb 1/3� � �t a , ms/lb 1/3� � �t o , ms/lb 1/3

U, ft/ms

Figure taken from Reference 7

Output from ConWep (Ref 6) for 1000kg TNT hemispherical

charge

Range, meters

Pres

sure

, MPa

Pressure vs. RangeHemispherical Surface Burst

0.5 0.7 1 2 3 4 5 6 7 8 910 20 30 40 50 70 100 200 300 5000.0020.003

0.005

0.01

0.020.03

0.05

0.1

0.20.3

0.5

1

23

5

10

2030

50

100

200300

500

1000

Charge weight 1000 kilograms TNT

Incident Pressure, MPaReflected Pressure, MPa

ConWep compared with experimental data (Ref 8)

-200

0

200

400

600

800

1000

1200

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2

Time (ms)

Ref

lect

ed O

vepr

essu

re (k

Pa)

Experimental data (270mm square target - Optical gauge at mid-point)

ConWep prediction

Semi-empirical techniquesBlast assessment is based on scaled distance in conjunction with an analytical approach (to deal with ‘shock wave addition’).For an explosion occurring inside a building, a complex blast pressure-time loading develops on the walls and other reflecting surfaces.In addition, detonation products also contribute to the loading: the ‘gas pressure’ or ‘quasi-static’ load. The complete pressure-time history (a series of blast wave-fronts + quasi-static loading) at a particular location can be calculatedSoftware routines such as BLASTXW (for multiple-roomed buildings) and SPIDS (for tunnel and ducting complexes) are available as part of Ref 7

BLAST WAVE + QUASI-STATIC LOAD CALCULATION IN CUBIC BUILDING

EXPLOSION AT CENTRE OF BUILDING

TARGET POINT (MONITORING LOCATION)

P vs t history output from BLASTXW (Ref 9)

GAS PRESSURE LOADING

REVERBERATING SHOCKS

Comparison of semi-empirical prediction with experimental data [Ref 4]

Numerical techniquesIn highly complex geometries (eg an urban environment or in a building of complex internal geometry), a fully-analytical approach may be necessary.Eulerian numerical techniques have been developed employing finite volume and finite difference solvers including SHAMRC (Ref 10), ANSYS AUTODYN (Ref 11) and Air3d (Ref 12) among others. In such codes, air is treated as an ideal gas and the detonation is modeled using an appropriate suitable equation of state for the explosive material.

Comparison of experimental data (from Sheffield University) and AUTODYN (Ref 11) for blast inside a vented room

Blast wave propagation in an urban environment: Aird3d (Ref 12) simulation compared with experimental data (from Cranfield University)

Time (msec)

Pres

sure

(kPa

)

0 1 2 3 4 5 6 7 8 9 10 11 12-60

-40

-20

0

20

40

60

80

100

120

Experiment4 levels

EXPERIMENT and Air3d SIMULATION

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