A History of Research and a Review of Recent Developments

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The effects of explosive loading
(large) or Ground Slam bomb (22 000 lb). However, the first Tallboy (large)
bomb) was dropped by the UK in operations against the Bielefeld Viaduct, and
it fell 50 ft away from the piers. The loosening of the earth caused the destruction
of six piers and five arches of the viaduct, and although the approximate shape
of the crater was 150 ft diameter, 45 ft deep, the loosening of the soil occurred
over a diameter of 240 ft, and to a total depth of 75 ft. These lessons of the
Second World War were perhaps learned again by the forces of the west in the
Gulf War against Iraq, when the US was reported to be turning away from the
precision bombing of bridges with relatively small bombs to the use of very
heavy bombs that could cause destruction from near misses.
Near miss explosions, as we have seen earlier, can cause high blast pressures
on components such as the relatively thin plating of the webs of deep plate
girders. Damage is then mostly by permanent deformation and distortion, and
removal of the integrity of web members. In certain circumstances this can lead
to shear failure and possible lateral buckling. A final commentary on the damage
and repair of bridges during the Second World War was provided by
Christopherson [8.3], who briefly examined ways of pre-strengthening existing
civil bridges against the possibility of bomb damage. He emphasized the
importance of permeability, ductility and redundancy, and the need to eliminate
the possibility of progressive collapse in viaducts. In general, the results of surveys
in the UK and US suggested that when it came to bombing from aircraft, it was
better to shake down the piers than shoot up the superstructure. The susceptibility
of bridge piers to undermining processes is, of course, well known from studies
of civil bridge structural failure, where it has been found that about half of all
bridge failures is due to scouring under piers and abutments.
The explosive damage to bridges from hand-placed contact charges is also
an important aspect of our review. In 1971, for example, during a local war in
Bangladesh, explosive charge damage was recorded on a number of relatively
small reinforced concrete road bridges, and on one steel truss bridge. The
damage was often in the form of holes in the deck, sometimes involving the
partial destruction of multi-beams supporting the deck slabs, and occasionally
this was severe enough to cause the complete collapse of a bridge span. Damage
was also reported to reinforced concrete piers, which collapsed sideways, and
to masonry abutments. The latter suffered extensive cracking and spalling
and often a partial collapse of the bridge seating.
A famous example of bridge deck damage due to hand-placed explosives
was the attack by German frogmen against the Nijmegen road bridge in
September 1944. This has been reported in detail by Hamilton [8.21], who
recounts that early in the morning of 28 September large explosions were
heard. Over 70 feet of the roadway disappeared, following the actions of the
frogmen, who used naval mines weighing 1200 lb, fitted with float chambers.
These were placed in a necklace around the piers before the float chambers
were released and delay fuses activated; however, the mines released their
impact forces upwards, failing to damage the piers, but disrupting the roadway.