Page 261 - Dynamic Loading and Design of Structures

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life of 60 years, had a 2 per cent risk of one of the flats having a structurally damaging
explosion in 60 years.
The material and human consequences of such incidents are so severe that low risk is not
an adequate reason for ignoring the danger. Secondary consequences such as loss of public or
business confidence can be equally costly.

6.1.1 Philosophy of design

Since Ronan Point collapsed, the stability of all buildings over four storeys in the UK must be
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checked with key elements designed for 35 kN/m static loading in the critical direction, or
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with continuity to limit the area of collapse if a key element fails. The 35 kN/m static load
has no statistical significance as an impulsive load, either from blast or impact.
The limit state philosophy for structural design uses elastic response for service loads,
plastic response for ultimate loads and prevention of overall collapse disproportionate to a
local failure. Many buildings have brittle or non-structural elements such as windows and
suspended ceilings that are extremely vulnerable to blast pressure and produce hazardous
debris, both within and outside a building.
The resistance of the fixings and supports of external cladding on a building as well as of
the panel itself determines the blast or impact resistance and the interaction with the
characteristics of the blast loading function, but there is little blast design guidance available
for cladding fixings.
Cladding fixings are often hard to inspect but some indication of damage can be obtained
from the residual deformations in frames and cladding panels (EPSRC, 1997; Pan and Watson,
1996).
Structures must have safe and serviceable paths for all loads, including extreme loads.
British Standard Codes of Practice since 1972 have recommended that by using nominal
peripheral and horizontal ties buildings would be more robust to resist extreme loads.
Explosions and impact loads may differ in both magnitude and direction from static design
loads and produce local damage such as cratering of concrete elements or local buckling of
steel elements that would reduce the moment or shear capacity locally. Deflections are very
similar for structures under distributed static or dynamic loads but not when the load is
concentrated (Watson and Ang, 1984).

6.1.2 Diagnosis of extreme loads
The damage to structural elements from extreme loads can be back analysed to find the load
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parameters, such as the 35 kN/m equivalent static loading from Ronan Point (1968). From an
analysis of damaged lamp posts at Flixborough (1974), Roberts and Pritchard (1982)
estimated the peak dynamic pressure produced by the explosion. Sadee et al. (1976) estimated
overpressure-distance curves from observations of damage to brickwork and concrete
structures. The case study in Section 6.4 uses the damage to buildings from an explosion to
evaluate the dynamic loads and so assess the cause of damage to other buildings on the site.