Page 260 - Dynamic Loading and Design of Structures

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Chapter 6

Loading from explosions and impact

Alan J.Watson

6.1 INTRODUCTION

Commonly, blast and impact loads are of subsecond duration and magnitude tens of times
larger than any other loads in the design life of the structure. The maximum positive or
rebound negative peaks of stress or displacement are critical for the structure’s survival and
subsequent vibrations will only be important if the loads are repetitive. For some industrial
structures blast and impact forces are repeated in-service loads and the response must be
checked as a serviceability limit state including cracking, vibration and fatigue.
The design and construction of structures against accidental or deliberate impact or
explosions is now often considered a part of normal design in the ever increasing importance
of safety against industrial and transportation accidents or terrorism. Ronan Point (1968),
Flixborough (1974), Chemobyl (1986), Piper Alpha (1988), Peterborough (1989), Oklahoma
City (1995), and Eschede (1998) all had a profound effect on design philosophy. These
accidents highlight the fact that safety is a multi-disciplinary activity and have shown that
structural design changes would be beneficial without enormously increasing the cost. If solid
abutments had been used instead of columns in the design of the Eschede bridge, or if the rail
lines had been given a greater clearance, the bridge would have been more robust. If
compartmentalized construction and moment frames had been used in the Oklahoma Federal
Building, increasing the total building cost by 2 per cent, the extent of the progressive
collapse which followed the explosion would have been reduced. The public inquiry into the
collapse of Ronan Point (Griffiths et al., 1968), revealed that the gas explosion produced a
2
peak lateral pressure on the walls of about 42 kN/m for a few milliseconds which, aided by
the upward explosive pressure on the slab above, displaced the top of the wall removing all
support from the floor slab of the flat above. Collapse progressed upwards and impact from
the collapsing floor slabs then caused collapse to progress downward. Ronan Point had little
restraint against rotational or translational displacements between floor and wall slabs and the
blast pressure had been enough to fail the joints designed only for modest wind pressures. A
subsequent risk assessment showed that Ronan Point, with 110 flats and a design