Blast
Most damage to the urban environment would be due to blast. The rapid rise
in temperature caused by a nuclear explosion creates a mass of gas at
incredibly high pressure. It expands, creating a blast wave which may
travel great distances causing damage. The range of blast damage depends
on factors such as whether the explosion was ground burst or air burst. If
the former it depends to some extent on the ground substrate, and the
immediate surroundings. If air burst it depends upon the altitude.
Blast wave
The front of the blast wave, the shock front, travels rapidly away from
the fireball, behaving like a moving wall of highly compressed air. After
the lapse of 10 seconds, when the fireball of a 1 MT nuclear weapon has
attained its maximum size, about 1 mile across, the shock front is some 3
miles farther ahead. At 50 seconds after the explosion, when the fireball
is no longer visible, the blast wave has traveled about 12 miles (19km).
It is then moving at about 780 mph (1255 kph), which is slightly faster
than the speed of sound at sea level (767 m.p.h or 1,235 km/h).
The
damage resulting from the air blast of the shock wave may be brought about
in two ways. First there is the sudden increase in pressure when the blast
wave arrives. The pressure rises almost instantaneously to a value called
the "peak overpressure" and then gradually falls off, during which a
strong wind blows in the same direction as the front moves, and then
becomes negative, the time taken for the pressure to return to normal
atmospheric being longer at greater distances from the explosion, and for
larger yield explosions. The peak overpressure is greater for larger
explosions. Pressures are measured in units known as Pascals or for the
pressures related to nuclear weapons effects in kilo Pascals. Standard
atmospheric pressure is 101.325 kilopascal (kPa), typical car tyre
pressures are in the range of 170 - 230 kPa. The unit previously used was
the pound per square inch (psi).
The Royal Observer Corps was equipped with Bomb Power Indicators (BPIs)
which recorded peak overpressures.
The distance at which a particular peak over-pressure is produced is
proportional to the cube root of the weapon yield. The peak overpressure
before it strikes anything is known as the incident peak overpressure.
When it meets a surface it is reflected, the pressure wave produced is now
known as the reflected peak overpressure. When this happens the peak
pressure may be increased by a factor of from 2 to 8.
Drag damage is that which is caused by the reduction in pressure
after the peak overpressure has passed, and pressure becomes negative
relative to normal air pressure. Drag damage depends not only on the
pressure but upon the length of time the pressure is applied. This
duration also scales according to a cube root law so that for larger
yields damage distances are greater than would be predicted by scaling
pressure alone. The range of effects are increased for low air-burst
weapons.
Mach effect
When the blast wave strikes the ground, or a surface of different density
to air (e.g. the surface of the sea),it is reflected back, in a similar
way to an echo. This reflected blast wave, like the original (or incident)
wave, is also capable of causing material damage. At a certain region on
the surface, the position of which depends mainly on the height of the
burst and the energy of the explosion, the direct and reflected wave
fronts merge. This merging phenomenon is called the "Mach effect." The
"overpressure," i.e., the pressure in excess of the normal atmospheric
value, at the front of the Mach wave is generally about twice as great as
that at the direct blast wave front.
For an air burst 1Mt weapon at an altitude of 6,500 feet (1981metres), the
Mach effect will begin approximately 4.5 seconds after the explosion, in a
rough circle with a radius of 1.3 miles (2.1 kilometres) from ground zero.
The overpressure on the ground at the blast wave front at this time is
about 20 psi so that the total air pressure is more than double the normal
atmospheric pressure.
Mach front
Initially the height of the Mach front is small, but as the blast wave
front continues to move outward, the height increases steadily. At the
same time, however, the overpressure, like that in the original incident
wave, decreases correspondingly because of the continuous loss of energy
and the ever-increasing area of the advancing front. After about 40
seconds, when the Mach front from a 1Mt weapon is 10 miles from ground
zero, the overpressure will have decreased to roughly 1 pound per square
inch. The distance from ground zero at which the Mach effect commences
varies with the height of burst. In the case of a very high altitude
detonation there may be no detectable Mach effect.
Strong transient winds are associated with the passage of the shock and
Mach front. These blast winds are very much stronger than the afterwind
caused by the updraft caused by the rising fireball, which occurs at a
later time. The blast winds may have peak velocities of several hundred
miles an hour near to ground zero; even at more than 6 miles from the
explosion of a 1Mt weapon, the peak velocity will be in excess of 10 miles
per hour. Such strong winds obviously cause significant damage.
A difference in the air pressure acting on separate surfaces of a
structure causes a resultant force on the structure. One of the most
important characteristics is the overpressure, referred to above. The
maximum value pressure at the blast (or shock) front, is called the "peak
overpressure". Other characteristics of the blast wave, such as dynamic
pressure, duration, and time of arrival are also of significance.
As the blast wave travels in the air away from its source, the
overpressure at the front steadily decreases, and the pressure behind the
front falls off in a regular manner. After a short time, when the shock
front has traveled a certain distance from the fireball, the pressure
behind the front drops below that of the surrounding atmosphere and a
so-called "negative phase" of the blast wave forms. At some distance
behind the shock front the overpressure has a negative value. In this
region the air pressure is below that of the original (or ambient)
atmosphere, so that an "underpressure" rather than an overpressure exists.
Pressures
During the negative, rarefaction or suction phase, a partial vacuum occurs
and the air is sucked in. instead of being pushed away from the explosion
as it is when the overpressure is positive. At the end of the negative
phase, which is somewhat longer, in duration, than the positive phase, the
pressure has essentially returned to ambient. The peak values of the
underpressure are relatively small compared with the peak positive
overpressures; typically underpressure is not more than about 4 psi below
the ambient, whereas the positive overpressure is typically much
larger. With increasing distance from the explosion, both peak
values decrease, the positive more rapidly than the negative, and they
approach equality when the peak pressures have decayed to a very low
level.
The destructive effects of the blast wave are related to both the values
of the peak overpressure, and the dynamic pressure. For the majority of
building types, the degree of blast damage depends largely on the drag
force associated with the strong winds accompanying the passage of the
blast wave. The drag force is influenced by mainly by the shape and size
of the structure, but this force also depends on the peak value of the
dynamic pressure and its duration.
The dynamic pressure is proportional to the square of the wind velocity
and to the density of the air behind the shock front. Both of these
quantities may be related to the overpressure under ideal conditions at
the wave front.
Air Pressures and Wind
Velocities
|
Peak
overpressure
in pounds per square inch
(kilopascals)
|
Peak
dynamic pressure
in pounds per square inch
(kilopascals)
|
Maximum
wind velocity
in miles per hour
(kilometres per hour)
|
200
(1,380)
150 (1,030)
100 (689)
72 (496)
50 (345)
30 (207)
20 (138)
10 (69)
5 (35)
2 (14)
|
330
(2,275)
222
(1,531)
123
(848)
74
(510)
41
(283)
17
(117)
8.1
(55.8)
2.2
(15.2)
0.6
(4.1)
0.1
(0.69)
|
2,078
(3,334)
1,777 (2,860)
1,415 (2,277)
1,168 (1,880)
934 (1,503)
669 (1,168)
502 (808)
294 (473)
163 (262)
70 (113)
|
Structural Damage
Most of the material damage caused by a ground burst, or low altitude air
burst weapon is due directly or indirectly to the shock (or blast) wave
which accompanies the explosion. Many structures will suffer some damage
from air blast when the overpressure in the blast wave exceeds atmospheric
pressure by about one-half pound per square inch. The distance to which
this overpressure level will extend depends primarily on the yield
of the weapon, and on the height of the burst.
The degree and type of damage to a structure depends upon the power of a
weapon, on whether it is air or ground burst. and upon the distance of the
structure from the detonation. Additionally it depends upon a number of
other factors which are features of the building under consideration such
as the type and strength of the structure, its size, shape and orientation
with respect to the explosion and upon the number of potential openings,
e.g. doors, windows and wall panels which could fail during the passage of
the blast wave. The damage is the result of displacement which can be
caused by two major forces exerted by the blast. These are the abrupt rise
in pressure as the shock wave hits the building, and passes over it in a
fraction of a second, and the drag force which is exerted by the high wind
throughout the duration of the positive pressure wave and tends to distort
the building or to push it over on to its side.
When the front of a shock wave strikes a building it is reflected and the
pressure on the face towards the explosion is momentarily increased by a
factor of two or more. As the main shock front moves over and around the
building, the pressure on that face falls again rapidly to the normal peak
pressure in the shock front (i.e. before reflection occurred) and this
same pressure is exerted side-on to the building. The shock front
then is then diffracted, or bent, round the opposite end until the
whole building is engulfed in the blast wave and the same pressure is
exerted on all four walls and on the roof. Before the blast wave has
completely surrounded the building there will be a considerable difference
in the pressure on the sides facing towards and away from the explosion
and, consequently a resultant force tends to move the building, as a
whole entity, in the same direction as the blast wave. If the building has
relatively few openings it will be subjected to this lateral "diffraction"
loading for the time it takes for the shock front to pass from one end of
the building to the other. The diffraction loading typically lasts for a
few tenths of a second, but may be sufficient to cause considerable
damage. Additionally if there are few openings such as doors and windows
in the building, there may be insufficient time for the internal air
pressure to rise to the same level as the external pressure, and the
building will be subjected to crushing forces for as long as the positive
pressure phase exists, in the case of large weapons this may be
several seconds. In the case of more average buildings equalisation would
occur more rapidly, and it may be that the internal pressure may
rise above external pressure causing the building to explode outwards.
This is because, typically, buildings are designed to withstand
significant external pressure, but not internal pressure. tests have shown
that typical British houses tend to fail in this way.
Wind drag loading
During the positive pressure wave, which may last several seconds, wind
drag forces act on structures, mainly on those that are relatively small
or of open structure, such as communications masts, telephone poles and
girder bridges. This is because they permit rapid equalisation of pressure
around them, and are not vulnerable to an all round external pressure.
Conventionally when calculating the zones affected by blast, circular
rings are used, in the UK they are described as A, B, C and D. The reality
is that this would only be true for a perfectly flat ground. The same is
true of the effects zones for thermal radiation. In practice the shapes of
hills and valleys would create shadows, significantly modifying the ranges
of damage, in ways that would be difficult to predict. Nuclear tests have
largely only been conducted over substantially flat areas, however
Nagasaki is situated in a valley and therefore the damage zones were
elongated. It is expected that slight damage to typical British houses
would occur when the static overpressure shock front was about 0.75
p.s.i.; at 1 - 5 p.s.i. the houses would need repairs to remain habitable
and they would be irreparably damaged at about 6 p.s.i. Slight damage
would include the loss of roofing tiles, broken windows and the like.
Irreparable damage means that houses would need to be demolished.
| Effects
of blast on buildings and humans |
| Peak
overpressure |
Maximum
wind speed |
Effect on
structures |
Effect on
the human body |
1 psi
7 kpa |
38 mph
61 kph |
Window glass shatters |
Light injuries from fragments occur |
2 psi
14 kpa |
70 mph
113 kph |
Moderate damage to houses (windows
and doors blown out and sever damage to roofs |
People injured by flying glass and
debris |
3 psi
21 kpa |
102 mph
164 kph |
Residential structures collapse |
Serious injuries are common,
fatalities may occur |
5 psi
34 kpa |
163 mph
262 kph |
Most buildings collapse |
injuries are universal fatalities
are widespread |
10 psi
69 kpa |
294 mph
473 kph |
Reinforced concrete buildings are
severely damaged or demolished |
Most people are killed |
20 psi
138 kpa |
502 mph
808 kph |
Heavily built concrete buildings are
severely damaged or demolished |
Fatalities approach 100% |
Blast casualties
Effects on the body
Body injuries
|
Category
|
Characteristics
|
Body
part(s) affected
|
Types
of injuries
|
Primary
|
Unique to explosions, results
from the impact of the overpressure wave with the body
|
Gas-filled structures are
most susceptible
- lungs
- gastro-intestinal tract
- middle ear
|
- blast lung (pulmonary
barotrauma)
- tympanic membrane damage
- abdominal hemorrhage
and perforation
- eye rupture
- concussion (traumatic
brain injury without physical signs of head injury
|
Secondary
|
Results from flying debri
|
Any body part may be affected
|
- penetrating ballistic or
blunt injuries
|
Tertiary
|
Results from individuals
being thrown by the blast wind
|
Any body part may be affected |
- fracture and traumatic
amputation
- closed and open brain
injury
|
Quaternary
|
All explosion-related
injuries, illnesses or diseases not due to primary, secondary,
or tertiary mechanisms, includes exacerbation or complications
of existing conditions
|
Any body part may be affected |
- burns (flash, partial,
and full thickness)
- crush injuries
- closed and open brain
injury
- asthma, COPD, or other
breathing problems from dust, smoke or toxic fumes
- angina
- hyperglycemia
- hypertension
|
Blast injuries may be direct or indirect; the former are caused by the
high air pressure and the latter by missiles and by displacement of the
body itself. Fundamentally the blast injuries caused by nuclear bombs are
similar to those caused by conventional weapons, but are significantly
worse. This is due to the fact that the body is sensitive to the duration
of the pressure pulse, and this is significantly greater for nuclear
weapons except in the case of extremely small weapons (less than 1
kiloton). Indirect injuries resulting from nuclear detonations, in
particular those caused by missiles, are similar in nature to those caused
by HE weapons. However because of the longer duration of the pressure
wave, injuries will occur at lower pressures.
Generally the way the body responds to blast is similar to small
structures, because of the relative small size the pressure wave passes
rapidly, however the body is subjected to sever compression forces. The
forces decrease as the wave passes, at the same time the drag forces are
considerable and cause a significant displacement risk.
Direct injuries (Primary)
Five parameters of the blast wave can affect the extent of the direct
injuries to the body:Effects of blast on humans and buildings
- the ambient pressure,
- the effective peak overpressure,
- the rate of pressure rise (or "rise time") at the blast wave front,
- the character and "shape" of the pressure pulse,
- the duration of the positive phase of the blast wave and the
associated wind.
The sudden compression of the chest and thorax caused by blast cause rapid
oscillations of pressure in gas/air containing organs, primarily the
lungs. These effects, combined with the transmission of the shock wave
through the body produce damage mainly where tissues of different
densities are in contact such as in joints, and junctions of tissues with
air containing organs. The chief consequences are haemorrhage and
occasional rupture of abdominal and thoracic walls. The lungs are
particularly prone to haemorrhage and oedema. If the injury is severe air
enters the veins of the lungs and from there the heart and arteries. Death
can then follow from air embolisms in the blood vessels of the heart or
brain. Damage to the lungs can also result in haemorrhage and oedema
resulting in suffocation. Blood clots may also form which may travel
through the circulation and cause damage to critical organs. Bodily
activity after blast damage to the heart and lungs is extremely hazardous
and death can result quickly where recovery might otherwise have been
expected.
The biologically effective peak overpressure depends on the orientation of
the individual to the blast wave. If the subject is against a reflective
surface, e.g., a wall, the effective overpressure for direct blast injury
is equal to the maximum reflected overpressure, which may be a from two to
about eight times the incident peak overpressure. In the open at a
substantial distance from a reflecting surface, the effective overpressure
is the sum of the peak incident overpressure and the associated peak
dynamic pressure if the subject is perpendicular to the direction of
travel of the blast wave and to the peak overpressure alone if the subject
is parallel to this direction. For a given incident overpressure, the
blast injury is expected to be greatest it the individual is close to a
wall and least if he is at a distance from a reflecting surface and is
oriented with his body parallel to the direction in which the blast wave
is moving. Soft surfaces such as curtains have the effect of absorbing
energy, but the amount of that absorption is difficult to predict as there
are a considerable number of variables.
The body, like many other structures, responds to the difference between
the external and internal pressures. As a consequence, the injury caused
by a certain peak overpressure depends on the rate of increase of the
pressure at the blast wave front. For wave fronts with a sufficiently slow
pressure rise, the increase in internal pressure due to compression of the
body and air ?ow into the lungs keeps pace (to some extent) with the
external pressure. Consequently, quite high incident overpressures are
tolerable if the rise time is sufficiently long. In contrast, if the rise
time is short, the damaging effect of a given overpressure is greater. The
increase in internal pressure of the body takes a definite time and the
response is then to the maximum possible pressure differential. A sharply
rising pressure wave will be more damaging than if the same peak
overpressure is attained more slowly. If the blast pressure increases at
first slowly and then quite rapidly; the injury potential of a given peak
overpressure is decreased.
The body is able to survive surprisingly high pressures, up to 30 p.s.i.
(206 kilopascals), although British houses are unlikely to survive above
one fifth of that. The percentage of people who would survive such a high
pressure is very small.
An individual inside a building but not too close to a wall would be
subject to multiple reflections of the blast wave from the ceiling, floor,
and walls as well as to the incident wave entering the structure. Since
the re?ected waves would reach them at different times, the result would
be a step loading, although the rise time for each step might be quite
short. In such cases, where the initial blast pressure is tolerable and
the subsequent pressure increase is not too great or occurs in stages, or
slowly, a certain peak overpressure is much less hazardous than if it were
applied in a single sharp pulse. The reason for the decreased blast injury
potential in these situations is that the early stage of the pressure
pulse produces an increase in the internal body pressure, which reduces
the pressure differential associated with the later portion of the pulse.
A higher peak overpressure is then required to cause a certain degree of
blast injury.
A location against a wall is the most hazardous position because the
effective peak overpressure, which is the maximum reflected overpressure,
is high and is applied rapidly in a single step. A location a few feet
from a wall can be expected to decrease the direct blast injury, although
the hazard arising from displacement of the body will, in all probability,
be increased. Oscillating pressures often exist inside structures due to
reverberating reflections from the inside walls.
The duration of the positive phase of the blast wave is a significant
factor for direct blast injuries. Up to a point, the increase in the
duration in- creases the probability of injury for a given effective peak
overpressure. Beyond this point, which may be of the order of several tens
to a few hundred milliseconds, depending on the body size, it is only the
magnitude of the overpressure that is important. The duration of the
positive phase, for a given peak overpressure, varies with the yield and
the height of burst of the weapon, but for most conditions, especially for
powers in excess of about 10 kilotons, the duration of the positive phase
of the blast wave is so long, approaching a second or more, that the
effective peak overpressure is the main factor for determining the
potential for direct injury from a fast-rising pressure pulse.
Death as a result of direct injury is likely to occur for pressures in the
range of less than 50 psi to 100 psi with positive phase durations of the
order of a second, for nuclear explosions. Ruptured eardrums may
occur with peak overpressures of as low as 5 psi.Body injuries
Casualties who survive for 24 to 48 hours in the absence of treatment,
complications, and other injury usually recover and show little remaining
lung hemorrhage after 7 to 10 days. In very severe injuries under
treatment, recurring lung haemorrhage may occur as long as 5 to 10 days
after injury. Experience from Hiroshima and Nagasaki is that those with
severe blast injuries did not survive, but that those with less severe
injuries recovered even without medical intervention
Brain injury due to air blast overpressure alone is improbable, indirect
injury is far more likely, as a result of head trauma caused by missiles,
debris, or displacement of the body.
Indirect blast injuries (Secondary, Tertiary & Quaternary)
Indirect blast injuries are associated with:
- the impact of missiles, either penetrating or non-penetrating
(secondary effects),
- the physical displacement of the body as a whole (tertiary effects),
- exacerbation of existing conditions (quaternary effects)
The wounding potential of blast debris depends upon a number of factors;
these include the impact velocity, the angle at which impact occurs, and
the size, shape, density, mass, and nature of the moving objects. These
are, of course, related to the total kinetic energy of the missile.
Wounding potential is also affected by the part of the body involved in
the missile impact. The associated risk is of a variety of injury types
and severities, ranging from simple contusions and lacerations, at one
extreme, through more serious penetrations and fractures, and critical
damage to vital organs, at the other extreme.
The hazard from displacement depends mainly upon the time and distance
over which acceleration and deceleration of the body occur. Injury is more
likely to result during the latter phase when the body strikes a solid
object, e.g., a wall or the ground. The velocity which has been attained
before impact is then significant. This is determined by the physical
parameters of the blast wave, as well as by the orientation of the
body with respect to the direction of motion of the wave. The severity of
the damage depends on the magnitude of the impact velocity, the area
of the body that makes contact, and the nature of the surface or object
struck. Likely injuries are contusions and fractures, and damage to vital
organs, including rupture.
