A History of Research and a Review of Recent Developments

Introduction xxv
indoor shelters and factory walls to withstand the effects of blast energy from
closely exploding bombs, and to give protection against bomb fragments,
crater debris, and other objects projected violently by the explosion.
Much full-scale test work was undertaken during the wars of the twentieth
century by most of the major participating nations. The need to calculate the
charges required to demolish structures, particularly reinforced and unreinforced
concrete structures, became important, and most countries produced simple
formulae linking the dimensions of concrete protective structures with the
weight of the explosive charge and the internal volume of air space. In steel
and wrought-iron building frames the possibility of progressive collapse was
investigated, and the need for alternative load paths became an important
consideration. Problems due to weakness in the connections of framed structures
were evident, and designers were reminded that the more nearly that connections
could be made to approach the continuity and ductility of the main members
they join, the less the damage that would result from a given bomb explosion.
The framework needed to be capable of resisting collapse if one main member
was suddenly removed, and in no circumstances should it be liable to progressive
failure.
The modelling of structures as simple mass-spring systems and the
comparison of the natural periods of oscillation of such systems with the
duration of the pressure pulse of an explosion were important in the
determination of the level of structural response before the main energy
absorption activity began. The advent of computer-based software in the midtwentieth
century has enabled quite sophisticated modelling to be undertaken
in an effort fully to understand the physics of blast-structure interaction. More
complex, multi-degree of freedom systems can be investigated quickly by the
designer, and the influence on behaviour of structural damping assessed. This
has led to a great increase in the computer-based analysis of structures subjected
to shock loading. It has also become much easier to investigate theoretically
the diffusion of shock waves in structural materials, and to use this to predict
material behaviour resulting from explosive shock characteristics. The spalling
and scattering of steel and concrete, and localized fragmentation at the point
of application of the impulsive loading, can be investigated.
It was clear from the earliest time that structural damage would be
considerably increased if the explosion of a charge took place within the material
of the structure, so the penetration of attacking missiles into structures followed
by subsequent explosion became an important aspect of design. It is not possible
to discuss structural loading fully without discussing penetration, whether it
is penetration of concrete by free-falling bombs or of the armour plating of
military vehicles, naval vessels or military aircraft by armour-piercing shells,
bombs, rockets or missiles. The penetration of high-velocity objects into soils,
stone, metals and concrete has historically been a subject for military engineers,
and much of the work in this field originated in the military establishments of
Europe in earlier centuries.