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DESEGN OF PREFABRICATED HOUSES

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WCHIGAN STATE COLLEGE
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THESIS

This is to certih] that the

tlwsis entitled
DESIGN OF PREFABRICATED HOUSES

presumed in]

Paras Chandra Bhattacharj i

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of tlw requirements for

7751.5. degree in C'E'

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DESIGN OF PREFABRICATED HOUSES

BY
PARES CHANDRA BHATTACHARJI
M

A THESIS

Submitted to the School of Graduate Studies of Michigan
State College of Agriculture and Applied Science
in partial fulfillment of the requirements
for the degree of

MASTER OF SCIENCE

Department of Civil Engineering

19h9

THESIS

6.!

ACKNOWIEDGEMENT

The author is deeply indebted to Professor C. L.
Allen and Professor C. A. Miller for their guidance and
valuable advice. Appreciation is also expressed to Profes-
sor l. G. Boylan, a member of the Landscape Architecture
and Urban Flaming Department, for .his assistance in aid-
ing as to obtain intonation regarding my field.

1316859

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TABLE OF CONTENTS

Introduction: Evolution of Prefabrication and Its
Application to the Post War Housing Problem ..................

lass Productive Value of Prefabrication ................,.....
Economical Aspect of Prefabrication ..

History of Past and Present Research and Experimentation on
Pnf‘bric‘uon OOOOOOOOOOOOOOOOO0.0.00......OOOOOOOOOOOOOOOOO.

Effect of Functionalist Movement on Prefab Industry . . . . . . . . . .
Designing Factors and Technique of Site Work .................
Application of Engineering Principle in Construction . . . . . . . . .
Building Code and Specifications ................$............

Prefabricationin‘rinber
1. Selection of Wood for House Parts ...................

n. D681“ Pmcipl. 00....00.0.0.0...OOOOOOOOOOOOOOOOOOO

10. Prefabrication in Concrete ...................................

11. Role of Lightweight Concrete in Precast Industry .............

12. Bibliogram OOOOOOOOOOOOOOOOO00.000.000.00...OOOOOOOOOOOOOOOO

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88313131

EVOLUTION OF PREFABRICATION AND ITS APPLICATION TO POST WAR
HOUSING PROBLEMS

According to R. L. Davison, Director of Research, J. B. Pierce
Foundation, "Prefabrication is the assembly, in varing degrees, of parts
of subassemblies into sections to be assembled into a structure, as dis-
tinguished from the assembly of parts during erection of the building:

~Mass production, industrial Specialization and standardization have
made possible the improvement, comforts and conveniences me are getting
today. Prefabrication when applied to residential construction simply
means the manufacture of parts or products in a well-integrated mechanized
plant where they can be more efficiently produced than under the uncontrol-
lable conditions existing at the building site.

The conventional method is a waste of time, labor, and material. WOrk
generally suffers due to climatic conditions. Technicians work with utmost
efficiency and safely because they have the benefit of enough light and the
best mechanical facilities with required sized materials within easy reach.
The raw materials are cut to the lengths required and every small part is
utilized, where as in the site work system this is not at all possible.
Thus, waste is also at a minimum.

One of the main obstructions in the past has been the physical problem
of manufacturing a house in a factory, transporting it in sections, and
erecting it on the chosen site. Now with the event of modern equipment the
conventional houses in most cases are using prefabricated parts. The re-
sult may be that as time goes on it will be difficult to distinguish between

a prefabricated home and one of standard construction.

This industry which promises no radical 'mechanics for living' embraces

2

a rapidly increasing number of factories in which building walls, parti-
tions, ceilings, floors, and roof panels can be prefabricated in completed
units, shipped to the site and erected in only a few hours.

Before the war only a few concerns were regularly producing and
marketing prefabricated homes. Actually war housing and war construction
presented the first real Opportunity to prefabricators to put their systems
to a real test. After the war the wyatt program in l9h6 and the extensive
demand for prefabricated units in 19h7-19h8, have proved "that prefabricated
home industry's success or failure in the post war period will depend upon
the soundness of the method of distribution it likes to follow. During the
'war the industry was in the research and experimental stage-alith the
government as its sole customer. After the war the industry will enter its
distribution phase. If it chooses the right method of distribution, it
'will succeed--if it chooses the wrong path, it will fail. Unfortunately
the industry was not old enough to have solved it before the war".

Only time will prove the rightness or the wrongness in methods of

construction, distribution, and mechanizing of the established houses.

MASS PRODUCTIVE VALUE OF PREFABRICATION

According to the World Economic Conference at Geneva in May 1927,
Rationalization is the method of technique and of
Rationalization organization designed to secure the minimum waste
of either effort or labor or material. It includes
the scientific organization of labor, standardization of both material and
of products, simplification of processes and improvements in the system of
transport and marketing.

Rationalization of the housing industry must clearly begin with the
rationalization of the completed house, involving first the redesign of
its physical units with a view to mass production, convenient transport,
and ready assembly. Its object is to provide suitable homes at reasonable
cost for the entire population and at the same time to compensate all con-
tributing producers at the existing economy.

In the rationalization of housing a complete redesign and a unified
conception of structure is needed. The traditional house is a conglomer-
ation of unrelated materials of all sorts, sizes and shapes, put together
on the site to form a composite whole. Until parts are standardized, mass
production cannot be possible.

The connection between standardization, mass production, between mass
production and rationalization is implied by the words themselves. Accord-

ing to ur. Henry Ford, "the term mass production is
Mass production used to describe the modern method by which great

quantities of a single standardized commodity are
manufactured.....nor is it merely machine production.....Mass production

is the focussing upon a manufacturing project of the principles of power,

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accuracy, economy system, continuity and speed. The interpretation of these
principles, through studies of operation and machine development and their
co-ordination in the consPicuous task of management, gives a normal result
of a productive organization that delivers in quantities a useful commodity
of standard material, workmanship and design at a minimum cost".

Full mass production of houses can be made possible by adopting two
main principles. These two characteristics are uniformity of cross section
at right angles to length, and repetitive features proportioned along the
'length' according to a stated gage.

Again, the parts of a house may be mass produced and mass assembled

If they are designed on a cube module; and, therefore,
Mass Assembled may be prepresented within complete structure by multi-

ples of this cube, because one interchangeable unit of
measure is found only in cubes.

The house parts standardized to meet the requirements of the mass
production method must be so co-ordinated that through selection and group—
ing an unlimited variety of houses can be assembled. Standardization must
therefore be applied to prefabricated house parts, not to the house itself.

Although wood frame construction appears to have no common unit of
measure, the cross sectional dimensions that satisfy the mechanics of struc-
ture, beams strength, post strength and other requirements of different

materials, suggest a simple relationship of a modular
Module character. In reinforced concrete structures, stan-

P13°“ dardization would be directed to the forms. To effect

the highest degree of standardization such forms should be designed on the

cubical modular basis. Whatever the materials or type of structure, its

component parts, if standardized, must be definitely related to a base of

a substantially fixed dimension in all three directions.

ECONOMICAL ASPECT OF PREFABRICATION

When manufactured on a large scale, it eliminates much of the middle
Less eXpensive man's profit and the waste of material and labor which
are involved in the traditional construction.

Numerous tests conducted in wind tunnels, laboratories and in the field
provide ample validity of this fact. New and light materials are being
employed in factory built homes to give increased strength and durability.

On the other hand machine operations are usually mone-
Better construction precise than hand operations, and one finds an exact-

ness in measurements and fitting in mass production
homes which are absent in traditional buildings,and the result is a stronger
better, and longer lasting house. Glued construction is far superior to
nailing.

The engineers and architects engaged in prefabrication are always try-

ing to draw a plan of maximum comfort, attractiveness
Better design and durability at the least possible cost. Thus, the

house which is reaching the home owner is the result
of research and designs far in excess of what any individual can afford in
a single house.

Greater the saving in time and labor, greater the plant is well-equipped

‘with the production equipments. The house can be constructed within a few
”hours on the site, which is a great handicap over the

Speed traditional orthodox system. ShOpwork can be system-
atized and is faster than field work. Foundation work

can be carried out simultaneously. There is no delay due to adverse weather

conditions.

HISTORY OF PAST AND PRESENT RESEARCH AND EXPERIMENTATION ON
PREFABRICATION

At the beginning of the present century many designers, like inventor
Thomas A. Edison, tried to apply industrial techniques to the construction
of concrete houses. Their ideas were rejected due to several practical
difficulties.

During the depression of 1929 many steel fabrication companies and
manufacturers of insulating board, such as American Rolling Mills, United
States Steel and Republic Steel, Homosok Company, and the Celotex Corpor-
ation established a housing section to push the use of their materials.
However, due to high prices and lack of mass production, they failed to
create interest in the public.

Three endowed agencies which have pioneered with various types of
construction, studied and tested structural materials and methods, and
continually influenced the prefabrication movement are: The Pierce Founda-
tion Housing Research Division organized in 1931, The Purdue Research
Foundation Housing Project opened in 1935, and the Bemis Foundation estab-
lished in 1938.

The John B. Pierce Foundation, Housing Research Division, Raritan,
New Jersey is a noneprofit institution which over a long period of years
has developed an unusual system of prefabrication. Standardized mass
produced parts are designed for various house plans. Framework joists
may be either wood or steel. Wall panels are erected horizontally in
three tiers; the bottom supports the floor load; the top supports the

roof; the middle encloses spaces between windows and is non-structural.

Exterior panels are Shop fabricated box girder units, glued, nailed and

insulated faces on both sides with 3/8" plywood, framed with wood and
formed and fabricated-spotawelded sheet steel. Door and window units are
completely prefabricated. Heating, plumbing and wiring,engineered and
other parts of the design are assembled on the job.

The Housing Research Project of Purdue University first offered five
houses of different materials such as wood, steel and reinforced concrete
for the commercial market. The houses were studied both from the view-
point of the values of the different types of structure involved and of
the cost factor. Special studies were made on.low-cost housing.

Before the prefabrication movement had taken its true shape, Bemis
Industry, Inc.*-maintained its laboratory and experimented with different
types of construction using various materials for wall surfacing and for
structure. They also did research in the use of Special composition like
gypsum blocks, precast gypsum slabs for walls as well as with composition
board and steel panels for houses. A few models also appeared on the
commercial market. Bemis industry first tried to develop modular system
to simplify field construction through the use of standard members.

The war emergency gave prefabrication its first real test in the
field. The prefabrication movement is being provided with a vast reservoir

of actual experience which it lacked in the prewar period. Now it has

 

*Mr. AlbertTar-wen Bemis was the President of Bemis Industry and after
his death a Foundation was established in Massachusetts Institute of
Technology, Cambridge, Messachusetts to carry out his research program.
This Foundation has collected many valuable materials and research data
from a survey of about 100 companies and intends to publish it for general

information.

10
dev60ped to a point where they can no longer be ignored and where they are
bound to influence every aspect of the postawar building picture.

The Farm Security Administration, the Tennessee Valley Authority and
the Public Building Administration all gave prefabrication the chance for
which it long had been'waiting.

The Forest Service, U. S. Department of Agriculture in collaboration
with the technical staff of the Housing and Home Finance Agency tried to
assemble most of the essential scientific information on this subject for
all those interested in planning or constructing wood prefabricated houses.

Farm Security Administration accepted many prefabricated systems for
its rural communities and acted as field laboratory of prefabricated housing.

Tennessee Valley Authority first started their project in a conven-
tional way but later on adopted the design of factory-fabricated houses of

truckable sections.

EFFECT OF FUNCTIONALIST MOVEMENT ON THE PREFABRICATION INDUSTRY:
IDEAS OF SHAPE ENGINEERING

With new scientific inventions the old designing and constructional
idea no longer fits with the present day needs. Only better production
technique with a completely new outlook can face this problem. The func-
tionalist movement of 1925 and the spirit of modern architecture represents
an attempt, but in most cases they failed to channelize their Utopian ideas
into reality.

Buckminister Fuller has designed his famous "Mast House" in 1927.
According to Hr. Fuller's statement, 'The Dymaxion House is still as it
has been for years--a theory only. Despite pragmatic criticism it has

conscientiously been kept so. While theoretical, it
Dymaxion I is immediately improbable by every scientific advance.
’18. A It might even be more broadly stated that the Dymaxion
House has been merely an attitude. An attitude of willingness to think
truthfully. .....Dymaxion House may be conceived of as progressive composites
of the best means of living as determined by universal survey'.

The house has five rooms and weighs 6000 lbs. with all accessories.

In this house the top deck was utilized as a recreational area and covered
with a pneumatic floor system to neutralize any sagging effect. The stru-
ture was tied back to the ground with further guy rOpes. The walls of the
house were to be double plated of casin with a vacuum between. A central
lighting system was designed to diffuse light to all parts of the house by
means of prisms, mirrors and lenses.

Hr. Fuller's second Dymaxion structure is based on the monocoque

principle. In this case Special attention was paid for betterment of living

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conditions and to utilise unused spaces for placing bathrooms and other
service unite. The main idea in this 'llast House' is
Dylaxion II to incorporate all utility services into a compact
form. Dynaadon should be regarded not merely as a
house but as an expression of an entirely different philosophy of living.
llr. Heutra has long been concerned with the possibilities of prefabri-
cation. The Diatalum Dwelling is but one example of his effort in this
field. The possibility of using prefabricated blocks based on diatomaceous
earth appears to have been suggested by Heutra as early as 1923. The basic-
ally interesting thing is that this house is the suspension principle using
a central meet from which walls are suspended in tension. Provision was
also kept for adding successive units if expansion of
Diata‘hna Dwelling the house is required. lasts or posts, made of a pair
Fig. B of channels, are welded into prefabricated hollow foot-
ing blocks of precast vibrated concrete each half weighing about 3/1. of a
ton. Floor Joists in this case, shown to be prefabricated of a pair of
angles and a web plate, are framed to the masts to support the inner edges
of the prefabricated diatom floor slabs. me diatalum panels are made of
infueorial earth chemically compounded and hardened under stea- pressure.
They are light and said to be adequately strong.

According to Mr. Neutra, "The current study although intended to
demonstrate in breadth and depth the problaatics of such post war planning,
concerns itself largely with one of the new elusental materials, its develpp—
nent, combination and application to housing.

'.....Diatomaceous earth and its derivative products may fill a chapter
in the book of substances, and that of human shelter as well.

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'.....l[icroeoopically small infusoriad. sea shells piled up into big
layers geologically ages ago, diatom earth is easily tinned by an industry
which is ready for all-round structural and house fabrication of the first
magnitude. The material is combined with a few others, cement, wood fibre,
plastics from agricultural waste products and aluminum; all available on the
Pacific slope, as well as in many other states....."

llr. G. F. Keck's “House of Tomorrow" , shown at the Chicago Century of

Progress, is also based on the mast principle. Radical
House of Tomorrow beams extending out from the central steel mast were

used for holding floors and walls. The ground floor
was designed for hanger, utility rooms and garage.

Has-tin lagier, city architect of Berlin, has designed a special type

of half-egg duped earthquake, shockproof structure
Igloo House for solving housing construction problems in Turkey.
H” c It gives the idea of construction of light as well as
strong structures by distributing stress throughout its surface.

The Forest Product Laboratory did considerable research on the use

of plywood in an aircraft desigi and soon recognised the advantages of the
plywood box unit, assubled with glue to conform to the
Stressed Skin 'et'reseed covering' principle, as a structural unit for
Structure
prefabricated houses. The surfaces reflect a large por-
tion of the rsdient heat that strikes them, and when used in conjunction with
small air spaces are efficient insulators. Most of our modern timber con-
struction follows this principle.
Hr. Gropins is one of the best know German architects. According to

him: “The dwelling house should no longer look like a fort or a monument.

15
Its walls should be of light construction and many windowed, and it should
aim at 'a beautiful life at home at the least possible cost of space,
material and building memes".

Therefore, the beauty of a modern house consists of Opened walls,
light structure expressive of buoyancy, clearly defined simple forms,
harmonised proportions of all building parts, complete satisfaction of
every material and psychic requirement.

On a previously laid concrete foundation was erected a steel frame
employing channels for sills and girts, I-beams for floor framing and

Liars for studding. In his house the curtain walls
Skyscraper were built of 3'I pressed cork sheets covered with

asbestos board and floors of wood planks. The pos-
sibility of prefabrication of tall dwellings is one of the important items
of Gropdns' theory.

it this period many other architects were thinking about changing
the shape of the structure and Corwin Wilson was one of them. He tried
to make trailers as permanent living quarters. The tensile strength of
plywood was utilised for this egg-shaped structure. His experimental
model was actually constructed in the year 1937.

There are other houses like igloos, houses like trailers, houses
made of cormgated steel, bubble houses of want blown into the surface
of inflated ballons , egg-shaped, round, hexagonal, octagonal houses; but
none of them solved even the major part of the problas of prefabrication.
This is because the demand for such a type of housing was uncertain. The
cost was not low or attractive enough to the prospective home ouster to

invest in a new type of structure.

16

DESIGNING FACTORS AND TECHNIQUE OF SITE WORK

Responsibility of the Designer

The main reason why the prefabrication system failed was because the
pattern of prefabrication was lacking in one or more departments. Some
concerns stressed factory fabrication with little thought to design or
erection; others put the importance on new materials with little stress
on factory fabrication. The person who can coordinate all these steps
might be termed as a real prefabrication designer. It requires a flexible
mind because he will act as an amalgam of technician, architect, artist
and organizer. He must keep a thorough knowledge of materials--their
Iqualities, characteristics, structural strength and limitation; of fac-
tory fabrication-—-mill procedure and organization; of site fabrication;
of field erection--organization of crews and procedure of construction;
of cost, transportation and architectural design.
Selection of Building Materials

Since the ideal material, unvarying homogeneous substance requiring
no tolerance for expansion and contraction is not available, the problem
of dimensional variation must be met. In the design and assembly of actual
parts made of existing materials, compromises must be made. Except for
this eXpansion and contraction, and its tendency to warp and twist, wood
is an ideal building material. Its strength and resisting qualities can
be increased by certain chemical treatment. The develoPment of improved
glues, synthetic resins, has resulted in new plywoods that are said to
Ihave enduring structural strength and lasting moisture resistance. One

of’the most important improvements is the plywood panel, impregnated and

l7
bonded with phenol formaldehyde resin, which is much stronger than ordinary
'wood. Plywood resists change of dimension and warping to a great extent,
and is manufactured with a minimum waste of materials and is relatively in-
expensive. wood pulp, long used for paper, is now made into rigid board
for inside use.

Due to heavy war drains on lumber and an increasing scarcity of growing
timber are two important reasons for which substitutes have been deveIOped.
Houses of various materials such as of steel, aluminum, and plastics are
not coming into use. There are many little known but plentiful minerals
suitable for use in building materials, particularly in lightweight con-
crete blocks and in wall boards. They are already capturing the timber
field. There are also new resins and glues by which former unusable or-
ganic matters are combined into building panels of great strength and
durability.

The lightweight metal has several advantages and that is why aluminum
promises to be one of the principal building materials of the future. At
present two types of aluminum houses are in use. One is the so-called
Butler-built of traditional design and the other is the Fuller house which
is circular in shape. Another advantage of new materials is that they are

in 'on-the-job' construction. The main reason is that
Light metals the metals usually require machine-tools for cutting

and shaping, and panels of wood or wood substitutes,
bonded by a resin, require pressure to perfect good union.

Magnesium alloyed with aluminum is much.lighter than aluminum and is
rapidly coming into use in the construction of buildings. Titanium is

another light metal and is the ninth most plentiful chemical element in

18
the world. This metal is exceeded only by iron, aluminum and magnesium
in metals suitable for engineering usage. Its oxide is one of the princi-
pal pigments used in white paints. The other plentiful lightweight aggre-
gate materials suitable in construction that are now available in this
country include haydite*, foamed slag, cinders, pumice, diatomite, perlite
and vermiculite. Vermiculite gives a concrete weighing only from one—
eighth to one-third the blocks or panel of equal size made with sand or
gravel.

Other notable advances in masonry products are air-entrained concrete,
new glass bricks, gypsum slabs for floors, synthetic stones and aerated
ceramics. Gypsum and mixtures of asbestos and cement formed into rigid

boards are increasingly used. It is relatively cheap
Lightweight and comparatively free from dimensional variation.
aggregate
The recent introduction of air conditioning into houses
has directed attention to insulation for different purposes, that of pre-

venting condensation; and the most suitable form of insulation consists

of bright metal surfaces such as aluminum foil.

 

*Haydite is made from the great variety of clays and shale. Foamed slags
are made by treating hot molten blast-furnace slags with water. Pumice is
a siliceous mineral of volcanic origin. Diatomite is composed of deposits
of siliceous shells of microscopic aquatic plants called diatoms. Perlite
is a natural volcanic glass. Vermiculite is a mineral that eXpands up to

30 times its original volume by a simple heat treatment.

l9
APPIIGATIOI OF ENGINEERING PRINCIPLE IN CONSTRUCTION

The application of engineering principles to the design of houses
presents a complete and logical method for determining allowable loads
for walls , floors and roofs. It also makes it practicable to develop
house constructions that have sufficient straigth yet require the least
amount of material and labor. The Bureau of Engineers followed the pro-
cedure of applying loads to specimens which accurately reproduced the most
important parts of a house. For each part the prescribed methods of load-
ing in the laboratory stimulated the actual loads under working conditions.
It is possible by this method of testing to determine the structural proper-
ties of a new construction without waiting for a performance test over a
period of years.

Floors are subjected to transverse, concentrated and impact loads.
Transverse loads may result from the weight of furniture and persons; con-
centrated loads occur under the furniture; and impact loads are caused by
objects falling on the floor. For roofs some tests were performed by taking
into consideration the action of wind, snow and weight of workmen and tools.

The wall specimens were also tested for resistance to compressive, trans-
verse, concentrated, impact and racking loads simulating the loads to which
walls of a house are subjected. In actual service, compressive loads on a
wall are produced by the weight of the roof, second floor and second story
walls, if any, by furniture, occupants, and by snow and vdnd loads on the
roof. Transverse loads on a wall are produced by the wind; concentrated
and impact leads by furniture or accidental contact with heavy objects; and
racking leads by the action of the wind on the adjoining walls. For non-

 

 

20

load bearing partitions, impact loads may be applied accidentally by fur-
niture or by a person falling against the partition, and concentrated loads
by furniture or by a ladder or other object leaning against the partition.

'lhe deflection and set under each incument of load should be measured,
because the suitability of a construction depends not only on its resis-
tance to deformation when loads are applied but also on its ability to re-
turn to its original sine and shape when loads are rsnoved.

Bach specimen should be tested in accordance with 8.11.8 2 of the
National Burean of Standards. This latter test will also give the require-
ments for the specimen and describes the presentation of the results of the
tests , particularly the load-deformation graphs.

This approach has opened the way for designers to introduce unconven-
tional materials and unusual methods for fabrication through laboratory
tests to determine whether the construction possesses adequate straigth.

BUILDHG cons AND SPECIFICATIONS

The United States Department of Comerce has recorded a voluntary
standard of minimm requiraaents for one-story and two-story prefabricated
homes. It includes specifications for light and ventilation, space excess ,
structm'al requirements for various parts, thermal insulation and conden-
sation control; and requirements for heating, plumbing, and electric wiring.
m Reflrmmts

1. Each room shall be provided with natural light and ventilation by
means of glazed openings. Ratios between the areas of Openings for light
and ventilation shall. not be less than shown in the table below.

22 of Room or Space Glass Area Qpen__a_ble Area
Living room, dining room, bedroom,
laundry, utility room 1/10 1/20
Kitchen 1/8 1/16
Bathroom and water-closet compartment 1/10 1/20
Spaces between ceiling and roof 1/300
Has-sent I 1/50 1/50

2. Every dwelling unit shall contain at least one bedroom, one bath-
room, one living room, and facilities for cooking in separate rooms.

3. Minimum height for each story shall be: basaaent - 7 ft., main
story - 7 ft. 6 in., second story - 7 ft. 6 in.

h. Each habitable room shall have access to each other without passing
through a bedroom or bath.

5. Hanufacturing tolerance shall be such as will assure assenme and
erection in accordance without creating unanticipated stresses .

22
6. All sections, panels, materials and equipment shall be protected

against damage at all times prior to the completion of the dwelling.

7. Erection shall be in accordance with the manufacturer's drawings
and instructions.

8. Before a proposal can be supported for an experimental building
license, it must show a degree of technical efficiency and must conform with
recomended standards for structural stability, thermal insulation, fire-
resistance, resistance to moisture penetration and vermin infection, and
mat have a party wall that will prevent the propagation of sound.

Strength again“

1. Foundation shall conform to the requirements of B.l[.S. 107.
Foundation walls must be large enough to spread the weight over the support-
ing subsoil and deep enough to extend below the frost line. Basement walls
should be waterproofed and drainage tile laid around the basanent footing
in order to prevent the accumulation of soil water along the wall surface.

2. Strength of each element joints shall be detemined either by
structural analysis or by comparison with certified test data for generally
similar elements.

3. The builder should guarantee that the structure is heavy enough
to sustain a load of 1.0 lbs. per sq. ft. on the first floor, 30 lbs. on the
second floor and 20 lbs. on pitched roofs. where annual snowfall exceeds
60 inches, the roof shall be heavy enough to support 30 lbs. per sq. ft.

Tests shall be made in accordance with the procedures of:

(a) The National Bureau of Standards, Washington, D. C.
(b) The Forest Products Laboratory, Madison, Wisconsin.
(e) 1h. American Society for Testing Katerina, Pennsylvania.

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A. Where the house is built without a basement, air vents should be
installed in the foundation walls to prevent dead air pockets beneath the
frame construction.

5. Where plywood is uployed as the anterior surface material and is
exposed to the weather, it should be bonded with waterproof, resinous glue
and should meet the requirements of commercial standards 0.5. 1.5-1.0 of the
lational Bureau of Standards.

m Mutants ‘

Chi-um: All chimneys and flues shall have masonry foundations and
shall extend at least 2 ft. above flat roofs and one foot above the highest
ridge of a pitch roof. Fire clay flue limings shall be built into all
chimneys, the walls of which should be less than 8 inches thick.

Insulation and condensation control: In order to have effective insu-
lation the ceilings and floors of a house, as well as its exterior walls,
must be fully insulated and windows and doors must be weather stripped.
Homes with very large window areas shall be supplied with double pone win-
dows having an - insulation air space between the two glass surfaces.

leans shall be provided to minimise condensation on concealed or
exposed surfaces of the dwelling and within spaces and materials thereof.

Electrig 1531.33: Both the materials and the workmanship employed
in wiring a house should comply with the National Electrical Code. Several
mioipalities have passed ordinances requiring observance of this code,
and fire insurance companies may refuse to insure premises which are not
properly wired-

glumbigg: Materials and installation should be in compliance with the
"Plumbing Manual“ (3.11.3. 66) issued by the National Bureau of Standards.

25

Prefabrication Practice

There are five methods of construction which are the following:

Precut or rag! cut method: The materials are cut to proper sizes,
shapes and numbered in the factory. Local workmen assemble than according
to the plan; and utility lines are installed in the course of erection
exactly as in traditionally built houses.

Pang method: his method saves only a little more time at the site
than the former one. Building materials are assanbled into large walls,
floors, ceiling and roof sections for quickening the process of erection.
It is still necessary to apply some other finish to the interior and on
the anterior portions of the building. Modular panels are designed to
offer various types of houses simply by changing the arrangement of panels.

Sectional method: Instead of sending prefabricated parts in this
case each section would contain one complete portion of the building:
floors, walls, ceiling and roof with the plumbing and lighting equipment,
bath and kitchen fixtures, heating facilities, windows, doors, even the
clothes hooks in the closets and the towel ranks in the bathroom installed
. at the factory.

mlete assanbly method; This type is seldom used due to the diffi-
culty of transporting a complete house from the factory to the site.

Cast Concrete methog: This is quite a new method and may be divided
into two categories. no first is the precasting of the lightweight concrete
into large floors, walls, ceiling and roof panels which are then carried to
the site and erected into place by a movable crane. For heavy construction
elaborate moulds are transported to the site and into those quick drying

concrete is poured so as to form a complete house in one operation.

PREFABRICATION IN TIMBER

I. Selection of Wood for House Parts

For eXposed exterior wall panels plywood made of phenol-resin glues
of equal water resistance and durability are generally used. But the panels
which are not under direct eXposure are made generally with moisture resis-
tant glues such as soybean or casein. Moisture resistant plywood of wall
board grade is suitable for painted interior wall panels. However, in kit-
chens, bathrooms, or in places of unfavorable humidity, resin glued plywood
is recommended.

Though exterior-type plywoods are more durable than the moisture resis-
tant type, the latter can be used safely even in a dry climate if the mois-
ture condition is favorable, i.e., no leaking through the roofs.

Floor panels should be selected from the denser hardwoods..

It is impossible to select an 'ideal' wood for a specific job. The
'wood which has high bending strength is sometimes found weak in stiffness.
For specific reasons we have to make some compromise and adjustments—-—a
wood that has low decay resistance can be treated with preservatives to fit
it for the job and so on.

II. Design Principle

 

Prefabrication with stressed plywood panels occupies an important posi-
tion in all wood construction, and all modern timber houses are designed on
this principle. 1

Each panel consists of two plywood faces glued to either side of an
inner structural framework to form a box girder. This helps the entire

panel to act as a unit similar to a box girder, and as a result it will

HOUSE PARTS

RECOMMENDED TIMBER

27

QUALITIES NEEDED

 

Structural Members

Roof Boards

Flooring

Sub-Floors

Siding

Shingles

Sash

Girders, Posts,
etc 0

Southern yellow pine,
Douglas fir, eastern hem-
lock, Spruce, white fir

Douglas fir, western
larch, southern yellow
pine, hemlocks.ponderosa
pine, white fir, spruce

Hard maple, oak, beech,
birch, black'walnut

White fir, larch Spruce
Douglas fir, southern
yellow pine, hemlocks,
ponderosa pine

White pines, western red
cedar, red wood

Western red cedar, north-
ern white cedar, red wood

White pines, red wood,
bald cypress

Douglas fir, western larch,
southern yellow pine

High bending strength and
stiffness

Good nail holding power
high stiffness, little
tendency to warp

Living room floor:

minimum warping and shrink-
age, high resistance to
wear.

Kitchen floor:

ability to withstand
'washing and high resis-
tance to wear.

 

 

'workable, good nail hold—
ing power, high in stiff-
ness

NO'warping,'workable,
good painting character-
istics

High decay resistance,

no Splitting during nail-
ing, little tendency to
curl.

No freedom for warping,
“workable, good paint
qualities, screw holding
power 0

High string and good
decay resistance

28

deflect only fractionally as much under a given load as would the framework
and faces when nailed together.
Calculation of Strength and Stiffness of Panel:

If 'b' is the basic width between longitudinal members and 'h' is the
thickness of the plywood cover, the strength and stiffness of the panel can

be determined from the formula:

 

 

 

 

b a 31 h.\\// h for three plies
parallel plies thickness
‘ h
and, b a 36h‘\\//parallel plies thickness for five plies or
more

Any clear width of covering in excess of 'b' between any two longitu-
dinal members should be neglected. If the clear distance is greater than
'2b', the panel should not be considered as having stress covering.

The stiffness can be obtained by calculating the moment of inertia of
the section and from known values of modules of elasticity of the section.

Strength Variation Due to Opening on Stressed Cover Panel:

 

Generally the covering of a stressed cover panel is designed to carry
a portion of the total load imposed on the panel, so the effect of openings
cut through the stressed covers must be considered in computing the strength
and stiffness of the panel. Reduction in strength or stiffness cannot be
assumed from the decrease of moment of inertia because the reduction in
strength is not so great as that of inertia, while stiffness at design load
appears to be little affected by holes in the stressed covers. For a large
size Opening it should be reinforced with additional joist extending the
full span length.

The exterior panels are constructed with 3/3 inch three-ply plywood on

29

the outside and l/h inch three—ply plywood on the inside. The general size

of the panel is h' x 8' x 3“. The frame work consists of vertical members
made of 1" material 2 3/8" wide, Spaced approximately 1'

‘Wall Panel

Construction apart with two and headers to which plywood faces are
glued. The plywood projects beyond the framework of the

panel forming a continuous right angle groove.

The upper face of the floor panel is of 5/8" plywood of five plies
and the lower faces are glued to a structural framework consisting of three
nominal 2 by 6 inch members placed 2 ft. apart with end headers. All parts

of the panel acts as a unit and therefore the panels can
Floor panel
Construction be substituted for the usual 2 by 10 inch joists Spaced
16 inches apart as ordinarily used in house construction.
With the exception of the kitchen and the utility room the upper 5/8" ply-
‘wood is faced with birch 1/8" thick to form the wearing and finished floor
surface.

The flat roof and the Simple gable roof are the most readily prefabri-
cated type. Stressed cover panels are particularly suitable for flat roofs
because such panels are eSpecially box beams, thus providing both coverage
and strength. Since deflection under is the critical factor in employment

of such a panel, stiffness is generally the most impor-
Roof Construction 'tant prOperty for which these panels are designed.

Pitched roofs can be designed for complete prefabrication,
usually the gable ends are prefabricated in sections for quick assembly at

the Site. In roof construction various types of trusses have been adopted

in order to allow greater flexibility in layout of rooms.

PANEL JOINTS - 30

 

 

 

 

 

 

 

 

 

 

 

 

 

‘- Joint: bcfwun panel; um; the help 0; nab and female Stud!

3. a fypical corner 1mm mm cover of One panel winning edge of“? “h"
c. R Corner Joint th mullion

D. A spline Joinf

Jomrsmaoorsmwm

’4 7/ 7 125/? E. a hug-5y” roof.
EEK/x”; / The upper Chord U firmly held in “Her
K“ g '11? ... [a ! Chord of iron with nail-glued 80/16 flak:
;\\y ‘7 UM: : and fruu faskned 19 We. Plde.

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31

Joint Design:

The joint design is very important in prefabricated house construction
because it governs to a great extent the degree to which parts are inter-
changeable and the ease with which the structure can be erected.

The function of these joints is to resist both lateral and lifting
forces exerted by the wind and resist the outward thrust. This joint trans-

fers loads from pitched roofs to the walls without exert-

Between Roofs
and'Walls ing appreciable thrust upon the walls. The gusset joint

F18° E between upper and lower chords is an excellent device for
transfer of load stresses from the rafter to the joint.

The Forest Products Laboratory has designed a joint for flat-roof houses
employing stressed covered roof panels. The amount of thrust action on the
'walls from roof loads depends upon the amount of deflection produced in the
panel by the loads. If panels are designed for adequate stiffness, thrust
action under load Should be reduced to a negligible amount.

A tongued and grooved joint is the most common form of stressed cover
construction. This joint is usually nailed and the male stud is often beveled
to permit easier fitting during site assembly. A mullion joint is a proto-
type of the former one. It serves as a Spacer between panels and is usually

nailed into place between edges of panel covering material.
Panel Joints It also serves sometimes as a load bearing member. Mhllion
F18. AFB type corner joint has the advantage that it permits a flush

exterior joint between panel and mullion that does not require a corner cover

board. Tongue joint is adoptable for corner and partition joining. Spline

joint is used to connect floor and roof panel as a load-transferring member.

32
The joint is proportionately stiffer as its vertical dimension is increased.
Sometimes it is fitted in place without glue.

In timber construction glue is the strongest bonding material. The
quality of a glue joint is determined by tests under shearing loads; if
shear failure occurs near the glue bonds, in such a test, the strength of
the bond is considered to approach that of the wood in shear. Glue must be
applied with pressure, and the maximum permissible gluing pressure is fixed

by the crushing strength of the wood. Lack of adequate
Glue Joints

vs. and well-distributed pressure is sometimes the cause of

Nail Joints

defective joints. In nail gluing size and Spacing of the
nail, proper driving and direction of nailing are important factors; but in
pressure gluing attention should be paid to adequate distribution of pressure
and maintenance of preper temperature. For hardwood joints applicable pres-
sure varies from 150 to 200 lbs. per sq. in. A low pressure between 100 to
150 lbs. per sq. in. is desirable for softwood joints.

The gluing surfaces of framing members should be planned to a tolerance
of at least l/oh" to assure smooth joining surfaces. The factors which govern
allowable tolerance include: (1) the capability of machines to approach design
dimensions under commercial operating conditions, (2) the property of material

being worked, and (3) the extent to which design dimensions
Allowable can be exceeded on the plus or minus Side. Again the
Tolerance ~

amount of tolerance which should be allowed depends entire-

ly upon the nature of the house to be built and also on the physical and
machining properties of the materials. Under the best machining conditions

and'with proper care, a tolerance up to 1.0005" can be obtained.

To retard the transmission of water vapor into the wall or roof,

33

moisture barriers are used. If the barrier is so located that the temper-
ature on its warm side is above the dew-point temperature of the room, then
there will be no condensation on the barrier. The materials which act as
good moisture barriers are asphalt-impregnated kraft
Moisture Barrier paper, duplex sheets, double faced aluminum foil mounted
on paper core or insulating materials mounted on a vapor
resistant paper. Some of the phenol-resin-impregnated papers are very good
as moisture barriers. All the moisture barriers are generally placed with-
in the panels and against the back of the face near the inside of the rooms.
Flexible or blanket type of insulation is generally used for insulation
of floors and walls. Fill or batt type is used where the wall Space is to
be filled completely with insulation. One type of reflective insulation is
made by cementing aluminum foil to a strong back of paper. Then sheet steel

coated with a non-corrosive material is also another

Insulation and

Fire Resistance variety of reflective type. In many cases cork blocks,

corrugated boards, processed paper blankets, honey combed
paper products are used. They also can be used in partition walls to check
sound transmission.
Resin bonded plywood is generally used for inside walls because they
offer greater resistance to fire than plywood glued with vegetable glue.
To secure greater resistance a mineral wool of high density is often used
as an insulating material.
The Forest Products Laboratory test data helps to determine the thermal
conductivity of a given wood at known moisture content. Thermal factor effects
stressed-cover panel thickness; so in stressed cover con-

Thermal Factor
struction, the designer should first determine the size

3h
of the framing members and wall thickness required to support all loads
imposed.

Good Spraying requires highly Skilled workmen capable of gaging the
amount of paint applied by a nice sense of timing together with knowledge
of the equipment and its adjustment. Painting at the factory presents two
great problems, (1) that of transporting and erecting painted units with-

out much damage, and (2) that of providing for drying of
Painting coatings without extra floor space and delay in produc-
Technique

tion. Generally a sealer and a coat of primary paint
should be applied at the factory; For flat panels rotating rollers are
very convenient. To apply sealers and water repellents, a dipping process
is used; but it is unsuitable for t0p coats. The drying of paint coating
can be hastened by controlling the temperature at lbO deg. F.

Better Construction

 

Inspection of raw materials is essential for checking physical defects.
During processing dimensional accuracy should be checked by standard instru-
ments primarily to parts involving critical joints. The
Quality Control jig governs the dimension of the finished parts to a
degree of precision depending somewhat upon its construc-
tion. True alignment of the bolt holes in the top of the jig is essential
for precision assembly of framing panels. InSpection of workmanship is also
necessary for controlling the quality.
Pneumatically Operated jigs permit rapid assembly of simple panel com-
ponents. Where production volume permits, jigs can be set up for mass pro-
duction of parts. In large scale production handling of frameworks and

panels in various stages of construction should be done by conveyor systems.

35

Overhead conveyor systems are suitable for painting and other finish work
on panels. The continuous moving belt or chain conveyor
Production system is very suitable for mass-scale production, but

Control
their use is limited because it requires very slow Speed

inside to give opportunity to the workers for applying glues, nails, insu-

lations, etc .

PREFABRICATION IE CONCRETE

Precast units erected as complete structures had been used at the end
of the last century so the idea of precasting reinforced concrete parts is
an old one---as old as reinforced concrete itself. With the development
of new techniques and due to present day housing demands, this branch of
reinforced concrete structure is again gaining in importance. Now a pre-
case concrete is just as strong and durable as site cast concrete. If we
consider the vast number of concrete structures that have been built in
place or with structural units, we find that the percentage of failure is
very small. This is because the experienced producers of precast parts
have always avoided the mistakes that caused defects under actual. service
conditions. Precast reinforced concrete has, however, proved a great
success in the Soviet Union, Great Britain, Australia and Germany. Although
the production of precast units has steadily increased during recart years
in the United States, its future market will be determined by its quality,
design principle and efficiency of production.

Advantages of Precast Construction

1. Factories are better equipped with tools and machinery, and the
pemanently employed labor becomes more skilled and better organised.

2. The cost of moulding and scaffolding can be minimised by adopting
standard sizes.

3. It results in higher quality concrete, less shrinkage and mre
accurate placing of reinforcment because technical control in the
workshop is better than on the site.

is. The production of precast units in the factory is independent of
climatic conditions.

37

5. It is easier and faster to erect and dismantle the structure.
6. Wastage is minimised and so ultimate cost is low.
M
They may be classified into three groups. The first one replaces
monolithic design by prefabricated units of similar
Subdivision of
Ionolithic shape but in a workshOp and hauled to the job. The
Structure
precast parts are placed in position and joined so as
to reproduce as nearly as possible the monolithic structure from which they
have derived.
1he second one employs standard units such as columns, beams, joists,

slabs, etc., inch are designed as self-contained units

Special Design

of Structures with a special view to their convenient manufacture,
handling and erection and with certain resistances to

specified loads.

The combination of both precast and in-site concrete is desigled in
such a way that the precast units make up a self-supporting skeleton. At
Combination of the same time, they also form the comlete moulds for
Prefabricated
Units with Parts the rueining part of the structure to be cast in-site.
Cast In-site.

In the first two methods it is essential that the connection between
the prefabricated units restore perfectly all the continuity qualities
possessed by the monolithic structure.

Treatment of Concrete:

Selection of fine and coarse aggregate of suitable quality and

thorough mixing of concrete are tun important factors in making good con-

crete. Sufficient time for proper mixing should always be provided. In

38

otherwords, mixers of adequate capacity must be used so workmen do not have
to neglect the time element to meet production schedules. Timing devices
should be installed on mixers so the operators do not have to guess. More-

over, clearing devices should be installed in mixers so
Siigigg and each batch is completely discharged. Otherwise, lumps of

concrete mixed in the early morning may be used in concrete
units and structures several hours later. This retampered concrete accounts
for structural defects later in service.

When concrete of semi-dry consistencies is employed, experienced.manu-
facturers find it expedient to dry mix the concrete; fine and coarse aggre-
gates for at least two minutes to assure uniform dispersion of cement through-
out the batch. This is very important. Then water is added and the concrete
is mixed for two or three minutes. With plastic mixers an abundance of water
is used to assure preper hydration of the cement and dry mixing is not so
important. But concrete of plastic consistencies, too, must be thoroughly
mixed for two or more minutes.

A dense high-strength concrete of low water content can also be pro-
duced by vibration. Vibration can be produced either internally or exter-
nally in the concrete by vibrators attached to the framework. However, in
all cases the desirable degree of vibration must be determined by experience
or tests. The advantages claimed for vibrated concrete are mainly due to
the fact that much drier concretes can be used. By vibration greater density,
increased strength, impermeability and durability, and'better bond'between
concrete and steel can be obtained.

For plastic concrete pressure moluding is generally suitable. Excess

'water is removed by the application of high pressure amounting to one ton

39

per sq. in. Very strong moulds are, therefore, required and consequently
this method is only applied to a restricted range of products of simple
form and moderate size. The advantages of pressure treatment are similar
to those obtained by vibration.

For treatment of plastic concrete, the combination of vibration and
pressure gives still greater compactness. This result is obtained in a
shorter time than if vibration or pressure alone was used. In the combined
method a low pressure is sufficient because the vibration facilities com-
press by rendering the particles of the mixture mobile. Similarly better
and quicker results can also be obtained by the combination of heat treat-
ment with vibration, or with pressure moulding or with both.

Importance of Curing

Proper curing of concrete with water, steam or a combination of both
cannot, under any circumstances, be neglected in the production of sound,
dense and high—strength concrete. No matter how much care has been exer-
cised in the selection of aggregates, time of mining with pr0per amount of
cement, it is of major importance to cure concrete to avoid structural and
other defects and to assure a long life expectancy. Adequate curing pre-
vents the evaporation of'mixing water in concrete of semi-dry consistency
which is needed to prOperly hydrate the cement. Proper curing also pre-
vents the rapid evaporation of the free water in plastic concrete mixtures
thus minimizing volume change, including shrinkage, cracks and other objec-
tionable defects. Careful curing of precast concrete floor and roof gives
maximum density, impermeability and strength.

Advantages of Curing

The advantage of steam curing over water spray is the high temperature

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SOCKET FOUNDATION

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which accelerates hardening of concrete. In southern climates steam curing
is not so necessary because water spray is just sufficient provided the units
are kept constantly wet .

Infra-red curing is also used sometimes in the precast industry. One
advantage derived from accelerated curing is earlier freeing of moulds so
that in mass production, the mould can be restricted to the money expendable
on the mould requirements for two to four hours instead of from twelve to
twenty-four hours.

Precast concrete being factory-produced can be more easily controlled
than other forms of concrete ,and therefore, full advantage can be taken of
improvsaents in strength and other qualities resulting from careful grading,
batching and mixing. The controlling fact in the strength of any mixture
is the ratio of water to cuent, and this can be far more easily maintained
under factory conditions than it can be done outside on the site. Again,
the whole econodcs of precast concrete depends upon a proper choice of
aggregates with regard to type and sise, and the suitable proportioning of
these together with the cement and water in the mixture. By making the
right choice and maintaining the relationship, the time of curing may be
determined.

Joint Dong;

The value of joint between two precast units is measured by the degree
to which the continuity qualities possessed by the corresponding monolithic
structure are provided by the joint in question. Beam joints may be sub-
divided into three classes-«rigid metal joints, welded joints and joints
by concrete case in-site.

Rigid metal joints consist of steel chanels or angles fixed to the ends

JOINT DESIGN

w’ ‘ "‘

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or 41'

If

'._

w“ "

o—.._...-h_

7/...

._~.-—._..._

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Fig. A

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of a beam. During the cast of a beam the reinforcements are held at the

ends by templates to ensure that they will register with the holes in the
steel section. The space between the prefabricated units at the joints are
filled with concrete any time after the connections have been made. Fig. l

The welded joints connect corresponding bars which during manufacture
have been left exposed. This type of joint requires only a small space be-
tween the units vmich is later filled in with high-grade cement mortar.

A joint by concrete cast in-site should have roughed surfaces on those
parts that come in contact with the in-site concrete. There should be no
reinforcement projecting beyond the over-all length of the unit. To improve
the bond between concrete splices cast-in-site and precast units, a few
vertical concrete nibs or bosses are sometimes cast on the ends of the pre-
fabricated units.

3121.!!! Between Columns; Columns need some temporary location during
the time of joint construction but it is difficult sometimes to make accur-
ate connection between colums. This is due to (l) difficulty of keeping
the upper column in proper position until the poured concrete has hardened
and (2) difficulty in getting mu contact at the top surface of the joint.
Sometimes metal joints have been used for connecting columns of large cross-
sections. However, these joints are very expensive and require a large
amount of steel. Another kind of connection between columns is illustrated
in Fig. A. The main beam ,which is in channel form, is placed with the sides
of the channel on brackets forming the head of the lower column. The only
reinforcement projecting into the connection is the main bars from the lower
column. After the upper column has hem placed, the socket is filled with

concrete.

Considerations in Floors and Beams Desig
For the purpose of decreasing weight a rectangular cross-section of

beam may be improved by reducing the depth at the ends of the beam and by
providing openings near the neutral axis at mid-span.

The tee-section is economical in resistance to bending and may be
improved by varying the width of the flange reaching its maximum near
mid-span, in accordance with the diagram of the bending moments. In a
similar manner the web thiclmess may be varied in accordance with the dia-
grma of the shear forces.

The I-section with stiffeners may be improved for the purpose of pro-
viding uniform resistance by thickening the top flange and narrowing the
web towards the mid-span. The sise of the bottom flange is generally de-
fined by the amount cf main steel .bedded and could be decreased also
towards the supports, if some of the reinforcement bars were bent upwards
to assist the shear resistance of the concrete web. Prefabricated I-shaped
concrete beams are sometimes used for erection of complete precast houses.
lhetherit isto beusedas abesmcr as apartof awall, everyunithas
the some cross-section: 9" deep, with a web 1" thick, and flanges 6" tide.

Trussed beams are applied with advantage to longer spans where a large
percentage of bending resistance is taken by the structure itself.

While deciding a type for a given job, one must consider first the
amount of monolithic action required in the finished floor. Then attention
should be paid to decrease the weight of the unit.

All joints, beams, girders and other floor units should show some ml'k
plainly indicating the top of the unit and the sise of the main reinforce-
ment. This mark should also indicate the length, size, type of reinforcing

LS
and the carrying capacity of the unit.

Pastor of Safety in Desig

‘nie detemination of factor of safety in precast construction is quite
different from that of monolithic reinforced concrete structures. The allow-
able load on a unit is usually determined by dividing the breaking load by
load factor. The American Concrete Institute reconnaids for the test of
such an individual unit as a simple bone, that it chin sustain without
complete failure a load of at least 2.25 times the design load based on
allowable stresses in bending and shearing as given in its regulation. lith
regard to allowable deflection, the member shall be considered to have passed
the test if the maximum deflection at the end of twenty-four hours does not
exceed a value equivalent to the square of the span divided by 12,000 times
the total thickness of the member.
Production TeMe

All production plants follow more or less the same type of casting
operations. For straight line production method each casting yard must have
at least a few steel trucks on which the steel casting tables should run.
Before casting each table is cleaned with mechanically operated steel brushes
or by an air hose or liquid soap. Outside forms are then set and locked to
the table. All surfaces to be exposed to the concrete are then treated with
a parting agent-us mixture of liquid soap thinned with gasoline. Bucks are
then bolted to the side form for windows and doors and inserts are placed
for roof slab openings. Since each table is used for casting a particular
type, the door and window bucks must be cleaned and placed at locations
handy to the respective tables on which they are fitted. Reinforceaent is
then set in place and concrete is poured in the form by a mechanically

1.6
operated conveyor belt which receives its load from a hopper suspended
directly over the belt. The conveyor belt is rigidly framed and runs over-
head on tracks. With this belt being movable and the table carrying the
form running on a track at right angles to the belt, every part of the form
can be brought under the hopper-gate mouth. When the form has been filled,
the concrete is vibrated and the table is pulled by a small lift truck to
the finishing yard. The truck has a special arm that engages the bottom
angle of the table. When the table has been brought for towed finishing,
the arm is dropped and the truck returns to the next vibrated table. The
table passes through the curing room (for approxinately four hours) to the
steaming room. When panels have steamed for approximately six hours, they
are moved out to a rotator located underneath the craneway. Panel sections
are then rotated to a vertical position and made ready for delivery.

New Erection Technifle

Recently a very important improvement has been made in the method of
tilting the walls into place. With the help of new power equipment, mass
production and also reduction of reinforcing steel in walls can be made
possible. This idea, to use a lifting device of structural steel sections
fastened to the wall, is to take the stress of erection. After the wall
is tilted into position, the lifting rig is removed and is used to tilt
another wall in the same way. The steel is only used to take the tempers-
ature stress. Another method suggested was that of casting the entire wall
on a pallet which will take the erection stress and then tilting the panel
up. This would enable the walls to be tilted up in twenty-four hours to
thirty-six hours, after casting. The pallet would swing free of the wall
after the wall has reached the vertical position.

h?
The concrete floor of the building can be used for casting the walls

in a horisontal position. The edge of the form can be adjusted according
to the wall dimensions.

his plan offers wide architectural flexibility and lends itself to
many economics and short cuts. The need for presently scarce mason labor
is greatly minimised by this method and also carpenter's work is reduced
to interior partitioning.

R013 01' LIGHTWEIGIT CONCRETE IN PRECAST INDUSTRY

The most important development in concrete construction in recent
years has been the production of lightweight units. Lightweight concrete
is of two types, cellular and gas concrete, and those made with a light-
weight aggregate. In the former one hydrogen gas bubbles are generated
in a mixture containing line or cement by the incorporation of finely
divided aluminum or zinc powder. The cellular structure produced this
way is retained after the cement has set and a lightweight product obtained
thereby. Lightweight aggregates are either obtained from natural sources
or are specially processed. The natural lightweight aggregates are nearly
all volcanic in origin, pumice being the most widely used of this group.
Processed aggregates include foam slag, expanded slate, etc.

Pumice stones have sp. gr. 1.12 and can be used for lightweight con-
crete construction having a crushing strength of 1500 p.s.i. at 28 days.
The weight per cu. ft. of concrete is nearly 75 pounds. Time required for
various operations concerned with the manufacture of slabs runs as follows:
Pouring concrete - 12 minutes, vibrating - 7 minutes, finishing - 17 minutes,
curing in steam tunnel - 6 hours, stripping from forms - 10 minutes and
cleaning forms - 15 minutes.

In hot climatic regions pumice concrete was found to be very good.

The material has inherent thermal insulation qualities, freedom from con-
densation, high degree of sound absorption, and can be made earthquake
resistant and thoroughly fire proof. Beautification can also be made either
interiorly or enteriorly to give each unit a ' different look' .

In wet localities, a prefabricated lightweight concrete house can be
built that should prove entirely satisfactory if the roof is pitched to

1.9
drain and provided the exterior concrete walls are waterproofed. Viwl
resins, pigments and solvents can be used as paints for this purpose.
T'nree coats of paints applied in suitable colors would yield a waterproof
job pleasing to the eye.

Pumice concrete and lightweight precast concrete houses presents
a challenge and opportunity to both architects and engineers.

BIBLIOGRAPHY

 

l. Prefabricated Home Manufacturer's Institute, Washington D. 0., "Modern
' Homes by Modern Method".

2. Graff, R. K., Matern, R. A. and William, H. L., "The Prefabricated Houses".
3. Portland Cement Association, "Tilt-Up Construction".
b/he Carr, A. L., "A Practical Guide to Prefabricated Houses".

5. B.M.S.* -62 (N.B.S.*), "Structural Properties of a Precast Joist Concnete
Floor Construction", sponsored by Portland Cement Association.

6. Portland Cement Association, "How to Design and Build Precast Joist
Concrete Floor".

7. Billing, Kurt, "Structural Precast Reinforced Concrete".

8. B.M.S. - 26 (N.B.S.), "Structural Properties of Nelson Precast Concrete
Foundation'Wall Construction", sponsored by the Nelson Cement Stone
COe, Inc.

9. Bruce, Alfred and Bank, Harold Sand, "A History of Prefabrication",
(The John B. Pierce Foundation, Housing Research Division).

10. B.M.S.-10h (N.B.S.), ”Structural Properties of Prefabricated Plywood
Lightweight Construction for'Walls, Partitions, Floors, and Roofs,
Sponsored by Douglas Fir Plywood Association.

L11. Sheppard, Richard, "Prefabrication in Building".
12. B.M.S.-18 (N.B.S.), "Structural PrOperties of 'Prefab' Construction for
Wells, Partitions and Floors", Sponsored by the Harinschfeger

Corporation.

13. United States Department of Commerce, "Prefabricated Homes", (Commercial
Standard CS 125-h7).

1h. Bemis, Albert Farwell, "The Evolving House", Vol. #3.
15. B.M.S.-h6 (N.B.S.), "Structural PrOperties of 'Scot-Bilt' Prefabricated
Sheet-Steel Construction for Walls, Floors, and Roofs, Sponsored by

Gloves4Wernicke Co.

16. National Homes Corporation, Lafayette, Indiana, "Loading Tests of Pre-
fabricated'Wall Panel Sections".

17. B.M.S. (N.B.S.), "Methods of Determining Structural Properties of Low
Cost House Construction".

18.

19.

20.

21.

9.
10.

ll.

51

B.M.S.-112 (N.B.S.), "Properties of Some Lightweight Aggregate Concrete
'With and Without Air-entraining Admixture"

Townes, C. A., Chief, Department of Regional Studies, T.V.A., "Design
of Prefabrication: Some Personal Observation".

Hansen, Howard J., "Modern Timber Design".

McKenuee, O. W., "Prefabs on Parade".

Periodicals

 

Journal of American Concrete Institute, l9b6-h7.

Concrete, l9h6-h8.

Engineering News Records, 19h7-h8.

Civil Engineering, 19h5-h7.

Engineering, l9h7-h8.

American Builders and Building Age, l9hh-h6.

Architectural Records, l9h2-h7.

Architectural Forum, who-1.7.

American Lumberman and Building Products Merchandiser, 19h8.
Science News Letters, 19h7.

Pulp and Paper of Canada, l9b7.

*B.M.S.--Building, Materials and Structures.
N.B.S.-National Bureau of Standards.

 

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