A History of Research and a Review of Recent Developments
A History of Research and a Review of Recent Developments
A History of Research and a Review of Recent Developments
Create successful ePaper yourself
Turn your PDF publications into a flip-book with our unique Google optimized e-Paper software.
Loads on underground structures 85<br />
have surface structures above them, <strong>and</strong> in certain circumstances the presence<br />
<strong>of</strong> such structures can affect the peak incident overpressure reaching the<br />
underground structure for a large air burst.<br />
It is clear that the loading on underground structures is influenced by the<br />
shape <strong>and</strong> flexibility <strong>of</strong> the structure. For example, because <strong>of</strong> the support<br />
that can be mobilized from well-compacted soil, it is possible to use thinwalled<br />
metal construction for underground structures or for the linings <strong>of</strong><br />
temporary trench shelters or earthworks. Here, the structure is formed from<br />
thin sheet elements that in themselves have low structural strength, but which<br />
can support large overpressures if properly embedded in soil. Corrugated steel<br />
culvert material used in civil engineering is a good example. The limitation to<br />
load-carrying ability is <strong>of</strong>ten the inward buckling <strong>of</strong> the walls <strong>of</strong> the structure,<br />
or the tensile rupturing <strong>of</strong> sheeting due to large membrane forces, rather than<br />
collapse by the formation <strong>of</strong> ultimate moments in structural members.<br />
For heavier, thick-walled construction, typical <strong>of</strong> reinforced concrete, there<br />
are broad, approximate quasi-static loading conditions that can be used for<br />
initial design calculations without recourse to complex analysis or personal<br />
computers. For example, in a shallow-buried rectangular structure, with a<br />
cover depth less than one-half the span <strong>of</strong> the ro<strong>of</strong> slab, it can be assumed<br />
roughly that the ro<strong>of</strong> <strong>and</strong> floors are loaded with the ground surface peak<br />
overpressure, p 0. In a dry soil the effect <strong>of</strong> attenuation might limit the side<br />
pressure on the walls to 0.5 p 0, but in saturated conditions with a high water<br />
table this would increase to p 0. The loading conditions on ro<strong>of</strong> elements would<br />
be the full overpressure, p 0, plus the dead load <strong>of</strong> the ro<strong>of</strong> structure, the soil<br />
cover, <strong>and</strong> any debris that might fall on the surface. For side wall elements the<br />
static earth <strong>and</strong> water pressure would need to be added to the blast overpressure.<br />
Examples <strong>of</strong> reinforced concrete underground shelters to withst<strong>and</strong> unclear<br />
explosions, designed on this rough quasi-static basis, were given in the UK<br />
Home Office publication, Domestic Nuclear Shelters [4.15]. It is interesting<br />
to note that in the UK domestic nuclear shelter design <strong>of</strong> the 1960s <strong>and</strong> 1970s<br />
the loading overpressures were limited to three atmospheres (315 kp a or 45<br />
psi), because at pressures above this the effects <strong>of</strong> Initial Nuclear Radiation<br />
would probably kill the occupants.<br />
Perhaps we should pause for a moment <strong>and</strong> consider radiation doses, since<br />
they are a fundamental part <strong>of</strong> nuclear explosions. They are measured in various<br />
units such as roentgen, gen, rad or rem, depending on the precise kind <strong>of</strong> radiation.<br />
We will only consider the roentgen here. Acute radiation sickness is produced<br />
by a brief exposure <strong>of</strong> the whole body to 50 roentgens; for exposures between<br />
50 <strong>and</strong> 200 there could be weakness <strong>and</strong> fatigue in addition to sickness, but<br />
only about one person in twenty would need medical attention. Between 200<br />
<strong>and</strong> 450 roentgens there would be moderately severe illness, <strong>and</strong> perhaps one<br />
quarter <strong>of</strong> the people so exposed could die. Exposures <strong>of</strong> over 600 roentgens<br />
would lead to death in less than 14 days in almost every case. Exposures in<br />
excess <strong>of</strong> several thous<strong>and</strong>s causes severe brain damage <strong>and</strong> death within hours.