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THE DESIGN OF BOMB SHELTERS

By

PAUL NEWMAN GILLETT

A THESE

Submitted to the Graduate School of Michigan
State College of Agriculture and Applied

Science in partial fulfillment of the
requirements for the degree of

CW'L ENGZNEER

Department of Civil Engineering
1948

THE DESIGN OF BOMB SHELTERS

by

PAUL NETLAN GILLETT
N

A THESIS
Submitted to the Graduate School of Michigan
State College of Agriculture and Applied
Science in partial fulfilment of the
requirements for the degree of

CIVIL ENGINEER

Department of Civil Engineering

1943

a
1&9092

TA.LE OF CONTENTS

 

 

 

 

Page
CHAPTER I
INTRODUCTION 1
CHAPTER II
TYPES OF SHELTERS 3
A. Domestic One-family Shelters
B. Communal Shelters
1. Within Existing Buildings
2. New Structures Outside Buildings
a. Below Ground
b. Above Ground
CHAPTER III
FUNCTIONAL REQUIRENENTS FOR SHELTERS 13
A. Location
B. Entrances
C. Decontamination
D. Space Requirements
'. ventilation
F. Utilities
CHAPTER IV '
STRUCTURAL REQUIREREKTSIFOR SHELTERS 18

 

A. The Effects of Bombs

1. Types of Bombs

2. Penetration of Bombs

3. The Effect of Explosion on Structures
B.. Structural Design

1. Roofs and Burster Slabs

2. walls and Base Slabs

TABLE OF CONTENTS (CONT'DQ

 

 

Page
CHAPTER V
EXAHPLES OF SHELTER DESIGN 27
A. Family Type Shelters
B. Communal Shelters for Large Groups
BIBLIOGRAPB 29

 

ILLUSTRATIONS SO

 

CHAPTER I
INTRODUCTION

The problem of designing structures to withstand the attack of
artillery projectiles and aerial bombardment has been the subject of
much study since even before the first world war. Research has been
carried out by the governments of all the leading nations and count-
less tests and experiments have been conducted to substantiate or
refute theoretical analyses. The nature of the problem, however, is
such that there still remains much to be learned before bomb-resist-
ing structures can be designed with the same exactitude and certainty
of results as can be anticipated in the design of a bridge or build-
ing frame. Since bomb- or shell-resisting structures have in the
past been exclusively military installations (gun emplacements, fort-
ifications, etc.,) the matter of expense has been secondary and un-
certainties of design could be accounted for by building extremely
heavy, thick structures. With the advent of large scale aerial bom-
bardment of cities, however, with the attendant problem of providing
protection for countless thousands, such haphazard design procedures
could not be applied to air raid shelters because of economic restric-
tions, and more accurate methods of analysis became necessary.

Although there are as yet many unanswered problems in this field,
a great amount of information has been accumulated, both on the
structural effects of bombardment and on the general problem of pro-
viding satisfactory protection for the pepulations of cities.

While the probability of aerial attack on the United States is
small and appears to be diminishing, such an occurance is by no means
impossible. For this reason the problem of air raid shelters should

be of interest to engineers and architects and it is hOped that this

-2-
paper may contribute a small amount of information on this new and

little-understood subject.

-3-

CHAPTER II

TYPES OF SHELTERS

A. Domestic One-Family Shelters. At the beginning of the present war,

 

the one-family shelter was the basic unit of air raid protection in
England. This size of shelter was adepted after a great deal of study
and was based on these considerations:

1. In England, most families live in single houses; therefore
a shelter designed for a single family is reasonable, from
a standpoint of convenience and accessibility.

2. Since absolute protection from a direct hit is economi-
cally impossible, the separation of shelters in small
units prevents mass casualties should one shelter suffer
a direct hit.

3. A single family Shelter is small enough to be transported
readily (if of a portable type) and is simple enough in
construction to be erected by the householder.

4. Functional requirements which complicate the large shelter
are absent in the one-family type. A simple door and
benches to sit on constitute the essential requirements.

Two of the main premises out of which the British A. R. P. scheme
was evolved have been proven by experience to be wholly erroneous. They
are, first, that air raids would be of relatively short duration, say a
few hours at the most; and second, that the destructive efficiency of
aerial bombardment would not be so great but that the protection afforded.
by the relatively frail family-size shelters would be ample.

These conclusions were doubtless arrived at by observing the effects
of bombing in the Spanish Civil war, which we nOW’knOW”W&S on a very

different scale from the present conflict. During that war, excellent

-4-
air raid protection facilities were worked out, especially in the large
cities such as Barcelona and Madrid, and it was quite definitely shown
that under the conditions then obtaining, more persons were killed or
injured from falling debris, fire, panic and splinters than from the
direct explosion of bombs. Thus, shelters giving protection from these
secondary effects were used and found satisfactory, and these facts were
noted in establishing British A. R. P. measures.

After the heavy attacks on England began, the raids were so intense
and of such long durations that many were killed in the small shelters
and the people were forced to spend many hours in the shelters (all
night in frequent instances) so that greater protection and better
accomodations became imperative. For these reasons, the small shelters
gradually were abandoned in favor of larger, safer and more comfortable
structures which were built as a result of these experiences.

The family-type shelter deserves discussion, however, because of
its probable application in the United States. The same factors which
led to its use in England exist here, and the factors which led to its
abandonment there, namely continuous and intense bombardment, are
scarcely likely here.

The family-type shelter in England was designed to meet the follow-
ing requirements:

1. Protection from the blast and fragments of a 500 lb. high
explosive bomb detonated at a distance of 50 feet.

2. Protection from a direct hit of a light incendiary bomb.

3. Sitting accomodations for 6 persons with cubic capacity
of 35 cubic feet per person.

All of the so-called "standard" shelters in this class were
Supposed to fulfill these design requirements. It is believed that

most of them did, and in some designs protection and accomodation well

—5-
above the minimum.were provided.

The most widely used shelter of this class was the Anderson shelter,
made of corrugated iron and distributed to the public by the British
government. Figure 1 is a sketch of this shelter. In most install-
ations of the Anderson shelter it was partly buried in the ground and
part of the excavated earth was placed over the top of the structure
for additional protection. Entry was by means of a hole in the front
plate. There were two wooden benches providing sitting space for six
persons. Some additional protection could be obtained by building up
earth or sandbag revetments around the sides and entrance to the shelter.
The shelter had no floor and there was no means of keeping the interior
dry.

When local conditions such as high ground water or paved surfaces
prevented setting the shelter in the ground, shelters of concrete or
brick were quite common. These were rectangular structures with walls
12 to 18 inches thick and a flat roof of precast concrete about 4 inches
thick. A shelter of this type is shown in Figure 2. The accomodations
were about the same as in the Anderson shelter.

In some cases where shelter materials were difficult to obtain,
family-size shelters were constructed by digging a trench in the ground
about 3 feet wide and 6 feet deep with a roof of boards or corrugated
iron which was covered with a foot or more of earth. These shelters
were so unsatisfactory from a standpoint of safety and comfort that they
were rarely used except in emergencies.

Mention should be made of the family shelters which are built
within dwellings, as these occasionally have been suggested for use
in this country. These shelters are really only refuge rooms, which
are generally equipped with articles needed by the family during an

air raid, blackout curtains, etc. Care should be taken in selecting

-5-
the location of a refuge room, as there is a possibility of being
crushed or trapped by debris if the building should fall, if ready
egress is not provided. In wooden frame houses, it is likely that
any room in the house which is readily accessible and with quick means
of reaching outdoors, is suitable. The value.of the stud walls in
providing protection against splinters is not great, but may be increased
by sandbag revetments if the householder wants to go to this expense
and inconvenience. In a brick house, the 12" exterior walls give a
measure of protection but here there is danger of wall collapse that is
not present in frame buildings. Basements have been suggested for shel-
ters and are excellent from the standpoint of lateral protection from
fragments and blast, but are Open to several objections, the most serious
of which is the danger of being trapped by fire or crushed by debris.
Another is the possibility of flooding from bursting water mains
(numerous instances of this have been reported from London), and the
danger from war gases, which are always heavier than air and tend to
seek lower levels. A sketch of a refuge room is shown in Figure 3.

B. Communal Shelters. There are many instances where one-family

 

type shelters are not suitable. In places where the population density
per unit area is high, as in districts of crowded apartment houses in
cities, downtown office buildings, and factories, the most efficient

type maybe one which houses a large number of persons, from 25 or 50 to
several hundred. The larger the shelter, the less is the per capita cost
of protection, and such shelters can be equipped with facilities for
preparing meals, sleeping and even working. It has generally been the
policy of governments to require that the degree of protection be in-
creased in proportion to the number sheltered in a given shelter unit.
Thus, a shelter for 25 or 50 persons may give protectioanrom all the

effects, including a direct hit, of a one-hundred 1b. bomb, while a

-7-
shelter for 500 may withstand a direct hit of a 500 or 1000 lb. bomb.
Communal shelters may be constructed within existing buildings, or may
be separate external structures.

1. Communal Shelters'Within Existing Buildings.- It is often most
convenient to have the occupants of a building sheltered within that
building. This is especially true in buildings where peeple are working,
as the shelter“within the building makes possible closer supervision
and control and means less working time lost in going to and from.the
Shelter. In.the case of apartment buildings and institutions such as
hospitals, the advantages of a shelter within the building when raids
occur at night are Obvious.

If the building in which the shelter is located is sound structur-
ally and fairly resistant to bombardment, the shelter can be made rela-
tively secure and will offer a high degree of protection. If the
building is old, or of wall bearing masonry construction, it may be very
unsafe and non-fireproof buildings should never be used for shelters.

A building of the skeleton frame type, of steel or reinforced con-
crete is generally very resistant to bombing and ordinarily suffers only
local damage even from a direct hit.

The action of a bomb upon hitting a building is either to detonate
on impact, causing extreme local damage to the roof and top story, or to
penetrate several floors or to the basement before exploding, depending
on the fuse setting. weighing the probabilities of fuse timing and
damage from penetration and/or explosion, it seems that the third or
fourth floor down from the roof in a five to ten story building is the
safest place for a shelter. Floors at these levels have the important
advantage of being above the level of gases, and the effects of explo-
sions on the ground (blast and fragments) are lessened. It is probable

that such locations would be more accessible to all the occupants.

-8—
Shelters in buildings higher than ten stories might well be placed
at a relatively lower level, say about halfway down from the top.
Since bombs do not dr0p vertically but in a modified parabolic path,
there is a possibility of bombs striking the sides as well as the roof
of a building; the probability of this occurance is greater for tall,
narrow'buildings.

A building of the wall-bearing type is extremely vulnerable to
damage from explosion and should not, therefore, be used for shelter
purposes. Moreover, such structures often have interior framing of
wood which increases the fire hazard. The fact that some wall-bearing
buildings, especially old warehouses, have thick walls and small win-
dows, has given rise to the mistaken idea that such structures would
make good shelters. Some apartment houses of from 4'to 6 stories are
built of wall-bearing exterior walls with wood floor systems and in-
terior columns of structural steel or iron pipe. These structural sys-'
tems are stable only under vertical loads and should not be used as
places of refuge. -

2. Communal Shelters Outside Buildings.- It has been found desir-
able in many cases in EurOpe to provide shelters for large groups in
separate structures outside existing buildings. The occupants of apart-
ment buildings, factories and institutions are often protected in this
way. Shelters outside buildings may be classed as underground and
surface shelters.

Underground shelters were first used on a large scale in Spain
during the Spanish civil war. These were long, deep tunnels lined with
concrete or brick; they ran under the streets at a depth of about 45
feet and were generally laid out in the form.of a square. Cross gal-
leries connected and intersected the square, and there were several

widely separated entrances. Soil conditions in several Spanish cities

-9-
were particularly suited to extensive and economical tunneling, since
ground water was not encountered at ordinary depths, and the nature of
the earth was such as to require very little timbering. These shelters
were considered to be sufficient to take care of all the inhabitants
and passers-by in the vicinity. The protection afforded appeared to be
almost 100% for the types of bombs then in use.

Underground shelters have a number of advantages. First, there
is no problem of splinter protection since the shelter has a protection
of earth cover. Second, the protection afforded is practically a func-
tion of the depth; thus the shelter can be rendered proof against even
a very heavy bomb by being deep enough in the ground. If the shelter
is 30 or more feet underground, the earth surrounding it becomes the
protecting material, and the problem of construction becomes merely the
driving of a tunnel shaft of suitable dimensions and lining it with
concrete or metal tunnel liners. Under certain circumstances, such con-
struction may be much more economical than building a heavy structure
of steel or concrete on the surface. Sometimes underground shelters
are large, rectangular rooms, built of reinforced concrete and con-
nected to the surface by long ramps, stairs or elevators. Underground
shelters may vary greatly in‘size and accomodation, the smallest giving
shelter to a dozen or less, while the largest built in EurOpe house
several hundred, and are equipped with all necessary utilities.

There are a number of disadvantages to underground shelters.
Difficulty of access is one of the main objections, and the cost of
this type of construction is likely to be excessive in some localities.
The netdwork of underground utilities in most of our large cities would
make it difficult to locate an underground structure without extensive
relocations of gas, electricity, water, and sewer conduits. Occasionally

the fear of being trapped underground causes the public to avoid such

-10-
shelters, and makes handling of crowds in the shelter difficult. The
necessity of providing gas-tight seals and adequate ventilation is
apparent, and some utilities, particularly sewage disposal, are compli-
cated by the depth of the shelter. Preventing the seepage of ground
water is another problem.

In discussions of civilian defence in the United States, it has
been suggested on numerous occasions that existing subway tubes could
be utilized as air raid shelters for large groups. Proponents of this
suggestion point out that subway entrances are designed to facilitate
the rapid movements of large groups, that subway tubes are extensively
used in England for shelters, and that since the tunnels are already
in existence, little or no cost would be involved. The official atti-
tude of the government has been to frown upon this suggestion, however,
on the grounds that since the subway tunrels in most American cities are
relatively near the surface, there would be little or no protection from
a direct hit. This is true; there are numerous instances where the roof
of the tunnel is within a few feet of the street surface, and a bomb
hitting here would doubtless go through and cause tremendous destruc-
tion in the confined space below. Protection would be afforded, how-
ever, from the splinters and blast of near misses, and from debris and
machine gun fire.

Some American cities are located in hilly or mountainous terrain.
Here there is a possibility of providing underground communal shelters
at relatively small cost by driving tunnels laterally into the hills
from.the ground surface. Rock Creek Park, in'Washington, D. 0., is in
a narrow valley in the middle of the city, and good shelter for the
apartment house dwellers surrounding the park could be had by driving
horizontal tunnels into the soft sandstone from the valley floor.

In spite of the numerous problems and disadvantages of deep

-11-
shelters, they are in wide use today in England. New structures of a
design and location such that they will eventually form part of an
underground transportation system have recently been built. These
shelters have excellent accomodations and facilities for rapid entrance
and egress and satisfactorily meet the problem of continued air raids
of high intensity. They are deep enough so that occupants are almost
undisturbed by even the heaviest raids and it is possible for essential
workers to get enough rest to maintain efficient war production.

Surface shelters for large groups are not as numerous as the under-
ground type, but they possess certain advantages and large shelters of
this type are known to exist in Germany and some other European coun-
tries. Some have been built in Switzerland, and a number were built in
Spain during the war there.

As has been noted, surface shelters have a number of advantages
over underground shelters. 0n the whole, it is probable that construc-
tion is simpler and more easily accomplished by general contractors with
ordinary construction equipment. Surface shelters tend to be massive
affairs, with walls and roofs of concrete Sometimes several feet thick,
but roof spans are short, and procedure is similar to the construction
of buildings. Since there is no protecting layer of earth between a
bomb explosion and the shelter, the roof must be capable of resisting a
direct hit of at least a medium-size bomb and the walls must withstand
the effects of blast and Splinters. Such construction requires ex-
tremely large quantities of material and is very expensive. Cost esti-
mates usually show a decidedly greater cost per capita for surface
shelters than those built underground.

The problems of structural design in surface shelters are quite
difficult, as the roof structure must be designed to withstand the

extreme local stresses resulting from impact and explosion. Experimental

-12-
data and theoretical analyses are so inconclusive or fragmentary that
they are of little help. European practice has been to make the roof
several feet thick and reinforce it heavily. Naturally, some of these
shelters have withstood bombings and some have not.

The lack of clear space on the ground in the vicinity of most
places where communal shelters are required makes the use of airface
shelters rather limited. In most installations abroad, it has been
found that shelters within existing buildings or separate external

shelters underground are more feasible.

-13-

CHAPTER III
FUNCTIONAL REQUIREMENTS FOR SHELTERS

A. Location. Shelters should be so located that there will be
sufficient time for the whole capacity of the shelter to enter in the
time allowed between the warning signal and the beginning of the raid.
The amount of time available will depend on the type of bombing tech-
nique employed and the efficiency of the air raid warning system. Most
civil defence plans assume a warning time of from 5 to 10 minutes, which
appears to be logical in comparison to European experience. This indi-
cates that a shelter should be not more than about two city blocks from
the most remote person sheltered 'by it.

Some time ago a firm of British architects made an extensive study
on locations for shelters in the metropolitan Borough of Finsbury,
London, Englandl. In this study it was first determined where shelters
should not be located by reason of proximity to dangerous areas (oil
storage tanks, gas holders, etc.), or near obvious targets (railroad
yards and docks), or in places which would be endangered by the effects
of bombing (near reservoirs which might be damaged and flood surrounding
areas). Then, numerous maps of the community were made, with popula-
tion densities for each block, for various times of the day, plotted on
the maps. From these it was possible to determine where shelters had
to be located and what their capacity must be to accomodate all the
persons in the community, at any time in the day or night.

In most American cities it has been planned that occupants of

houses or apartments will be sheltered within their residences, and

 

1This report is contained in the book "Planned A. R. P." by Tecton,

Architects. The British Architectural Association, London.

-l4-
workers in commercial or industrial districts will be sheltered within
their places of employment. In Detroit, persons on the street when an
air raid alarm is sounded are instructed to go into the lobby or street
floor of a building if in a downtown district, or up on the porch of a
house if in a residential district. It is unlikely that protection
would be much improved by following these instructions, but the effect
is to get people off the streets during the raid, which is important.

B. Entrances. Entrances to a shelter should be of such a capac-
ity to allow the total number of occupants to enter in the time avail-
able. A single 4 foot wide Opening without turns, steps or other
restrictions to flow will pass about 60 persons per minute. It is
advisable to have several entrances rather than one large one, to
allow for better control of crowds and to provide for escape if one or
more entrances should become blocked by debris. For this reason, the
smallest shelter should have at least two means of egress.

Since entrances usually constitute the weakest part of the shelter
structure, it is frequently desirable in the larger shelters to have
the entrances protected by a wall and roof or by turning past a baffle
wall, even though these devices impede the flow of traffic.

Doors are usually provided for shelters, to control the flow of
traffic and.to decrease the effect of blast and to provide an air
'tight seal in the event of a gas attack. It has been found that doors
made of steel plate i" thick, well braced and fitted with heavy hard-
ware are satisfactory.

C. Decontamination. Early in the war there was considerable

 

apprehension in regard to the use of gas in aerial bombardment. All
A. R. P. plans, therefore, included gas defence and air raid shelters
were designed to give protection from war gas as well as from other

phases of aerial attack. Up to the present, however, there have been

-15-
no reports of gas being used and the likelihood of its employment is
diminishing. The technical difficulties attendent to the efficient dis-
tribution of war gases, combined with the fact that means of gas war-
fare are available to all combatants, seem to make this kind of attack
unlikely, especially in the United States. Gas attacks by air have
been feared, however, in certain tropical regions occupied by our
forces, since the extremely humid climates would favor effective employ-
ment of mustard gas, Lewisite and other gases which are highly soluble
in'water.

In view of the unlikelihood of gas attacks in this country it does
not seem advisable to provide for gas defence in the construction of
shelters or in the formulating of civil defence plans, except when
such provision would add little or nothing to the cost or difficulty of
the shelter design.

There are two characteristics of war gases which should be borne
in.mind in providing decontamination and anti-gas protection: the
fact that all such gases are heavier than air and therefore collect in
low places (such as shelters in basements, or trencheS), and the fact
that many gases combine with water and form an extremely corrosive
liquid.

Shelters designed for protection against large scale gas attack
are provided with a double set of doors at each entrance, to form an
air look, so that persons entering from.contaminated outside air must
close the outer of the two doors before entering the inner one. Fans
or blowers provide pure air in the gaslock. The shelter itself has gas
tight doors and the ventilation system either has filters to cleanse
gas laden air coming in from outdoors or is of the regenerative type,
in which the shelter is hermetically sealed and oxygen is supplied to

the atmosphere in the shelter.from.pressure cylinders.

-15-

Since persons entering a shelter from gas laden air may bring in
considerable gas on their clothing, some of the more elaborate shelters
have separate decontamination facilities for men and women, with
showers and changes of clothing. A plan of a shelter with gaslock and
decontamination facilities is shown in figure 4.

D. Space Requirements. Various air raid protection codes have

 

set up standards of cubic space per person for various size shelters.
The minimum.for a family-type shelter is 35 cubic feet per person. It
has been.found that the space ordinarily required for seating arrange-
ments, aisles, equipment, etc. will be such that the cubic capacity per
person is sufficient. It is essential, however, to prevent overcrowding
by rigidly policing the entrance to a shelter, and shelters in Europe
usually have the allowed maximum capacity posted, and the shelter is
closed after that capacity is reached. Aside from the obvious reasons
of comfort and accomodation, there is danger of suffocation if only
natural flow of air is relied on for ventilation. Standards of seating
capacity as worked out by architects for theaters and restaurant booths,
and in the design of buses and railroad coaches, have been found valu-
able in establishing seating arrangements and capacities in Shelters.

E. ventilation. The importance of ventilation in shelters

 

depends on the capacity of the shelter, the probability of gas attack,
and the probable duration of a raid. Small shelters accomodating up

to 25 persons probably need no positive ventilation system, and
shelters for larger numbers may be ventilated by a simple fan or blower
arrangement. Shelters formed by tunnels underground may be ventilated
by shafts or stacks relying on convection currents to provide changes
of air. If air in a shelter becomes too damp, trays of calcium
chloride or other deliquescent crystals help in lowering the humidity.

It seems unlikely that the duration of air raids in this country would

-17-
be long enough to justify elaborate ventilation facilities in any but
the largest shelters.

F. Utilities. In the design of shelters accomodating more than
about 25 persons, some sort of provision should be made for water supply,
toilet facilities and electricity, particularly if the shelter is
planned to be in use for relatively long periods.

water supply presents no problem. A large shelter can be piped
from regular water services, with perhaps a small storage tank within
the shelter for use should water mains be destroyed.

Toilet facilities are essential and should be increased with the
capacity of the shelter, in the ratio of about 1 toilet for each sex
for every 60 persons. Shelters at or slightly below ground level may
have sewage disposal to regular sanitary sewers, but shelters under-
ground will probably require pumps tO lift sewage to sewer levels.
Small shelters may be provided with chemical or bucket-type toilets.

Electrical service is necessary fer lighting, for Operation of
ventilating equipment, pumps, radio, and occasionally for heating.
Regular commercial or domestic electric service is generally available
and satisfactory. Electric lines will withstand a great amount of
bombing but for installations where continuance of power is absolutely
imperative, engine-driven generator sets may be used. In this event,
.air for engine Operation should be supplied separately from air for

the shelter and exhaust gases piped outdoors.

-13-

CHAPTER IV

STRUCTURAL REQUIREMENTS FOR SHELTERS

A. The Effects of Bombs.

 

1. Types Of Bombs.- Aircraft bombs may be classified according
to function as high explosive, armor piercing, incendiary, fragmenta-
tion and gas bombs. There are some other types and some combinations
of function but the above are the most important.

High explosive bombs are by far the most important as regards
shelter design. They vary in weight from 100 lbs. to the large 4 ton
"block busters" and about half the weight is the explosive. While
public attention has been focused on the use Of extremely heavy bombs,
it should be remembered that there are many targets which can be more
effectively bombed with say a thousand 100 lb. bombs than with an equal
weight of two or four ton bombs. High explosive bombs characteristi-
cally have thin cases which are sufficiently strong to resist the shock
of impact with ordinary targets (buildings and civilian structures) but
will break up when striking a resistant structure (armored vessel or
fortification). Their chief effects are extreme structural damage to
buildings and death or serious injury to humans within the range of the
bombs' effectiveness. Both of these effects are caused chiefly by the

blast of the explosion. 1: 2. 3

 

1AutOpsy examinations of many blast victims indicate that death is
ordinarily caused by collapse of the lung structure, which results in
internal bleeding, producing suffocation. Many bombing victims have
been found with no external indications Of injury whatever. See
"Research into the Effects of Air Concussion on Animals, with Special

Reference to the Observed Effects of Air Concussion on Soldiers".

-19-

Armor piercing bombs have been used chiefly against protected tar—
gets, such as armored naval vessels, fortification structures and the
like. To make the bomb strong enough to penetrate a resistant material,
the case must be very thick and consequently little space is left for
the explosive charge. In some bombs in this class, the explosive may
constitute only 10% of the total weight. Armor piercing bombs have been
little used in the present war and they are seldom considered in the
design of bomb shelters.

Incendiary bombs range in size from the one-kilogram bomb used by
the Germans to larger bombs weighing 100 lbs. or more. They generally
contain a charge of thermite, magnesium, or other highly combustible
substance which is designed to ignite on impact. They affect the
design of shelters only in the precautions required for fire hazards.

Fragmentation bombs are small, usually weighing about 20 lbs. and
are designed to inflict injury to humans by the fragments which fly out
from the specially-designed case when the charge detonates. These bombs
have not been used extensively in this war; the fragments from the case
of an ordinary high explosive bomb seem to have satisfactorily lethal
qualities.

There have been no authenticated reports of the use of gas bombs

as yet in the present war, although designs for such bombs are known

 

G.'W. Crile, 001., M. R. C., U. S. A. See also "Experimental Study of
Blast Injuries to the Lungs". S. Zuckerman, Ministry of Home Security,

London.

2See "Blast - E. P. A. R. Memorandum:#l". Institute of Civil Engineers,

London.

3See "Blast - Bulletin B-l". Ministry Of Home Security, Research and

Ex eriment De artment London.
P ,

-20-
to exist and there is reason to believe they may be employed under
certain tactical conditions. Gas bombs are merely vessels containing
a poisonous gas under pressure or a liquid saturated with a poisonous
gas which releases the latter when the case breaks Open. Sometimes a
small explosive charge may be contained in the bomb, for the purpose of
better distributing the gas or liquid. The structural effects of these
bombs are slight, and they affect shelter design chiefly in the provi-
sions for ventilation, decontamination, gas tight closures and the like.

2. Penetration of Bombs.- There has been a great deal of research
and study on fiiis subject, but very little concrete information which
would be useful in designing Shelters has been deveIOped. It appears
that penetration depends on the physical characteristics of the bomb
(weight, shape, cross sectional dimensions), its velocity on impact
and the angle of impact, and the density, hardness, and elastic prOp-
erties of the material struck.

Of the numerous formulae1 expressing the penetration of aircraft
bombs in solid materials, the formula which seems most satisfactory
frOm a theoretical standpoint and from ease of application is the Petry

formula, usually expressed in the form:

1+v2
X'KPMéfilo m

where X = penetration in feet

P sectional pressure of bomb in lbs. per square foot
(sectional pressure ='weight of bomb in lbs. divided

by maximum cross sectional area in square feet)

.<
u

impact velocity of bomb in feet per second

' a constant depending on the material

PS
I

 

1See Samuely and Hamann, "Civil Protection", The Architectural Press,

London.

-21-
Values of K for various material, based on numerous tests, are of the
following order:
limestone 5.4 x 10'3

reinforced concrete (3000#) 4.8 "

plain concrete (2000# ~8.0 “
stone masonry 11.7 "
sandy soil 36.7 "
soft soil 73.2 "

The penetration of a bomb in a slab of limited thickness is
increased by the phenomenon of scabbing, which is the breaking off of
a cone-shaped piece on the opposite side and directly beneath the point
of impact. This is important in the design of shelters, as the scab-
bing may produce serious injury, and it frequently aids in the complete
perforation of the slab.

Penetration also depends on the fuse setting of the bomb. If the
fuse detonates on impact, or before maximum penetration is reached,
the penetration will obviously be less than if the bomb had continued
as a projectile. Thus, bombs aimed primarily at surface destruction
(dwellings, factories and surface utilities), will generally be fused
to detonate on impact, while bombs designed to produce large craters,
disrupt underground utilities, undermine bridge abutments, etc. will be
fused to detonate after maximum penetration is reached.

3. The Effect of Explosion on Structures.- The immediate effect
of detonation of a high explosive in air is the production of a
translatory pressure wave of very high velocity and pressure. This
wave is followed by a wave of negative pressure of lesser magnitude
but greater duration. Following this, there may be a succession of

back-and-forth disturbances until equilibrium is attained. The wave of

-22-
pressure1 is highest in the region of the explosion and diminishes
rapidly the further it moves awa . Everything in the irrediete neigh-
borhood of a big bomb therefore will be exposed to a violent pressure
wave of many times atmospheric pressure, whereas, depending on the bomb,
everything 50 feet away may be exposed only to 2 or 3 times atmospheric
pressure.

A structural element (wall panel or floor slab) when subjected
to an explosive blast may (a) be blown away from the explosion by the
primary pressure wave, (b) be blown toward the explosion by the sec-
ondary suction wave, or (c) be destroyed or damaged by vibration
caused by vibrations in the air which approximate in frequency the
natural frequency of the structure.

An explosion occurring underground (resulting from considerable
penetration by a delayed action bomb) has an effect very similar to an
earthquake, in that a shock is transmitted laterally in the earth by a
slight movement of the ground. I

' The resulting destruction to a wall-bearing masonry building:from
blast may be caused by (a) lateral movement or flexural failure of
walls resulting from blast pressure in air, or (b) lateral movement of
foundations, resulting from earth shock. Any relative movement in the
walls of a wall-bearing structure is very likely to cause complete
collapse of the building. Many hundreds of such structures in England
have been completely destroyed.

When a framed building, of steel or concrete, is subjected to the
effects of blast, wall panels and partitions may be demolished but

since they are not load carrying members, the stability of the struc-

 

lFrom."Protective Construction" Civilian Defence pamphlet issued by the

Division of State and Local Cooperation, Office for Emergency Management.

-23-
ture is seldom affected. Even a near underground explosion may not
necessarily damage the building unless the foundations are subject to
severe movement.

While in the employ of the War Department, the writer participated
in the conduct of some experimental bombings of reinforced concrete
buildingsl. These buildings, which were completely demolished by
numerous direct hits, neverless demonstrated considerable resistance
to bombing and usually were damaged only locally by a single bomb.

The most probable type of failure in reinforced concrete buildings was
found to be caused by reversal of bending moments in girders and

beams, cracking the concrete in places where main steel is absent, as
in the t0ps of beams in mid-span. 'Another type of failure in reinforced
concrete structures is the apparent destruction of bond between rein-
forcing and concrete, caused either by impact of a bomb or extreme
stresses set up by blast of an explosion. Examinations of buildings

in the aforementioned tests showed numerous examples of apparent
separation of steel from concrete. One explanation offered for this
phenomenon points out that since the velocities of prepagation of

shock waves are different in steel and concrete, there may be differ-
ential movement between the two materials, resulting in this separation.

B. Structural Desigp.

 

1. Roofs and Burster Slabs.- The design of a roof system to
resist a direct hit of a bomb presents many new problems not encoun-
tered in ordinary structural engineering.

Frequently the roof is protected by what is known as a burster
slab or detonation slab, which is a thick slab of concrete above the

shelter and overhanging it on all sides, usually separated from the

 

1These buildings are illustrated in Figures 5 and 6.

-24-
shelter by several feet of earth. The function of the burster slab
is to stop the bomb and cause it to either break up or detonate, so
that the only load to the roof of the shelter is the force of impact
distributed over a fairly large area. The procedure of design in such
a structural system would be as follows:
1. Assume size of bomb to be protected against and its impact
velocity.
2. Calculate penetration X from Petry formula (page 20), using
the K value for reinforced concrete.
3. Make the thickness of the burster slab double the calculated
penetration(1).
4. Calculate the kinetic energy of the bomb on impact and deter-
mine force of impact from the following:

. KOE. Of bomb
force or lmPa°t "depth of:penetration

 

5. Assume the force of impact to be spread out through the earth
between the burster slab and roof in a "cone of pressure".
The unit live load on the roof is than equal to the force of
impact divided by the area of the base of the cone.

This analysis gives a reasonable treatment for impact stress alone

but dynamic loads occasioned by explosion occur almost at the same time.

 

(1)There seems to be no exact knowledge of how reinforcing in the burster
slab helps to prevent penetration, and designs for reinforcing vary.

An arrangement frequently used consists of mats of %" round bars at

12" on center'eaeh “my, the mats to be spaced vertically about 2 feet
apart. The burster slab must definitely resist perforation of the bomb,
or else it will be worse than useless, as the bomb exploding between the

burster slab and roof would be able to exert a much greater force.

-25-
Explosion forces are known to be large but as yet no quantitative
evaluations have been developed. Structures resisting impact loads
have in a good many cases also withstood the force of explosion. It
may be that the character of the explosion force is such that stresses
set up as a result are not as serious as has been supposed.

If the roof of a shelter is not protected by a burster slab, the
problem of structural design is much more difficult. It is next to
impossible to prevent extreme local destruction at the point of impact
and the magnitude of the loads involved is such that ordinary design
methods within the elastic limit do not give reasonable results.
Flexural resistance appears to be of secondary importance, as the slab
will either fail or resist the bomb by local shear resistance before
the structure begins to bend. Current practice in the design of such
structures seems to be to make the slab very thick, use a large amount
of shear reinforcing and provide a steel plate on the soffit of the
slab, well anchored in with welded anchors, to prevent the under sur-
face of the slab from scabbing away under impact.

2. Walls and Base Slabs.- If a structure is built above ground,
the walls should be designed to resist penetration of fragments and the
large explosion pressures of a nearby bomb. Usually walls thick
enough to support a heavy bomb-resisting roof will be ample. If the
'walls are below ground but not so deep as to be below the probable
penetration of a bomb, they must be thicker, of the order of several
feet, to resist the tamped explosion of a delayed action bomb. Walls
in a deep shelter need be designed only for lateral earth pressure
plus a moderate shock wave from a distant explosion. Base slabs or
footings can be designed by ordinary structural practice, and the live
load of the impact of the bomb may be largely neglected in preportioning

footing sizes. If the shelter is not protected by a burster slab, the

-23-
base slab shOuld be designed for the possibility of a bomb penetrating
the earth near the shelter and turning in its path and exploding under

the shelter. This phenomenon has been reported on numerous occasions

in England.

-27-

CHAPTER V

EX.EPL33 OF SHELTER DESITN

A. Family Type Shelters. Before the outbreak of the present war,

 

the Office of the Chief of Engineers, U. S. Army,'was directed to
prepare designs for family-type air raid shelters for civilian use.
These structures were to utilize various materials and were to have
the same amount of protection as their British prototypes, namely,
protection against all the effects of a 500 lb. bomb exploding at a
distance of 50 feet, debris from falling buildings, and a direct hit
of a light incendiary bomb. A number of shelters were designed and
built and testedxvith bombs. The two which most successfully resisted
bombing are shown in Figures 7 and 8. The shelter in Figure 7 is made
of Armco-filo gage sheets and in a commercial size, 6'3" diameter. By
caulking the joints it can be made watertight, and a removable plate
in one end provides a second means of egress. The shelter may be
partly or completely buried, and the entrance or exit may be extended
by adding lengths of pipe to the Openings, The shelter in Figure 8 is
of reinforced concrete, is designed to be buried and access is through
a gas tight door at the bottom of a concrete stair. An escape exit is
provided at the Opposite end. Both of these shelters were subjected
to severe bombings and the resulting damage indicated the shelters
'were much safer than required in the design criteria.

B. Communal Shelters for Large Groups. The Office of the Chief

 

of Engineers also prepared designs for large shelters and two are shown
in Figures 9 and 10. The underground shelter shown in Figure 9 has a
capacity of 100 persons and access is down a long ramp and through a
gas lock. The burster slab just below the surface is intended to step

bombs and overhangs the shelter on all sides by a sufficient amount to

-28-
obviate the danger of an underground explosion near the shelter.

The shelter shown in Figure 10 is a two story structure, based
on British designs, and has a capacity of 200 persons. The roof slab
is 5 feet thick and the walls and base slab are preportionately
massive. The per capita cost of this shelter was about 15% greater
than the structure in Figure 9. Note the baffle walls in front of

each entrance to protect the steel doors from.fragments and blast.

-29-

BIBLIOGRAPHY

 

EStructural Defense, Handbook No. 5 - Ministry of Home Security, London

iBomb Resisting Shelters, Handbook No. 5A - Ministry of Home Security,
London

.Air Raid Precautions in Factories and Business Premises, Handbook No. 6 -
Ministry of Home Security, London

Domestic Surface Shelters, Memorandum No. 14 - Ministry of Home Security,
London

The Design of Bombproof Shelters, D. Anderson - The Institute of Civil
Engineers, London

Experimental'Work on Air Raid Precautions, R. E. Stradling - Institure
of Civil Engineers, London

The Resistance to Collapse of Structures under Air Attack - Journal of

the Institute of Civil Engineers, London, October, 1940

Civil Defense, C. W. Glover - Chapman and Hall, London

 

Civil Protection, Samuely & Hamann - The Architectural Press, London

 

The Design and Construction of Air Raid Shelters, Donovan H. Lee -

 

Concrete Publications Ltd., London
1The Design, Cost, Construction and Relative Safety of Trench, Surface
IBombproof and other Air Raid Shelters, 0. N. Arup - Concrete Publications

Ltd., London

 

 

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IH7313 BINDIXG SPECIFICATIONS

Weight of paper: 20 lb. for original and 16 lb. for carbon copies.
Binding: The thesis should be bound in half black leather and
black canvas. The page, after binding, shall measure 8% x 11
inches. The bound volume shall measure 8% x 11 inches.

The thesis title should be stamped in gold on the outside of

the front cover. rhe prOper arrangement of its imprint is
below:

THE DESIGN OF 3038 SHELTEFS

Thesis for degree of C. E.

michigan State College

PAUL NEWIAN GILLETT

1945

Tbaa DCIGNIS SIS
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