BLAST LOAD ASSESSMENT BY SIMPLIFIED AND ADVANCED METHODS Prepared by Dr Peter D Smith Reader in Protective Structures, Cranfield University, Defence Academy, Shrivenham, UK Dr Andrew Tyas Senior Lecturer, Department of Civil and Structural, Engineering University of Sheffield, UK
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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]
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)