A History of Research and a Review of Recent Developments

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The effects of explosive loading
immediately above and below the explosion. The area of demolition fell off
rapidly at lower floors, soon down to 100 square feet. Debris was usually
held by the second and third floor slabs below the explosion, but occasionally
it brought down a succession of floors.
The survey emphasized that the virtual absence of progressive collapse in
well-designed frame buildings meant that floors and walls near the detonation
point were effective as screens. Of equal importance was the ductility and
continuity of the framework, which made the building highly resistant to
explosions. A possible weakness, however, was the lack of ductility and
continuity in bolted or riveted connections. As in many other examples of
structural loading, the crucial factor lay in the strength or weakness of
connections. During the years before the Second World War welded connections
were relatively rare, so there was not a great deal of evidence about the behaviour
of welded frameworks. The authors, however, drew attention to the possibility
of introducing ductility into building structures by clamped connections that
rely on friction to produce the effects of continuity. The resulting structure
could absorb a large amount of energy without collapse.
The following types of damage were noted in single-storey steel-framed
buildings such as hangars, warehouses and workshops: direct damage, primary
collapse and spreading collapse. Direct damage was the cutting of members
by a bomb (before exploding), by fragments or by crater debris; primary collapse
was the collapse of members that depended for their stability on a destroyed
member. Spreading collapse occurred when the forces set up by a primary
collapse were transmitted to adjoining undamaged members, producing
instability in these members. The spreading collapse of a roof could occur
when the cutting of a single member suddenly involved the whole area of a
large structure. The chief risk to roof steelwork arose from the failure of
stanchion to roof-girder connections, which were sheared by violent
displacement or lateral blast, or destroyed by blast uplift.
Dwelling houses, of course, are rarely constructed with a ductile steel
framework, so in assessing the behaviour of houses in explosions, it was
necessary to examine the strength of masonry and brick structures to blast.
Civilian masonry structures are not normally designed to resist explosive
blast. The stone castles and military fortifications of the Middle Ages were
more likely to be threatened by the penetrative action of missiles, and their
wall thicknesses were set empirically by the need to resist damage by local
impact and fragmentation. Civil masonry and brick structures were known
to have relatively little resistance to local explosions, and in earlier times no
attempt was made to predict how they might behave under attack, or what
their residual strength might be. Masonry and stone were not employed much
in the construction of ‘bomb-proof’ shelters once reinforced concrete appeared
on the structural scene. As the Second World War approached, however,
experimental research was initiated to check the behaviour of conventional
building structures when subjected to the general blast from exploding aerial