Quality housing is in short supply in the world. It is estimated that the
world is short of 200million dwelling units. To overcome this shortage within a
reasonable time is not easy. EvenConsidering that 100 years might be a
reasonable enough time frame for this mammoth task,each dwelling unit will have
to be delivered at the rate of 3.74 seconds during a normal 40-hour workweek.
The non-delivery of adequate quality housing at a stage and time of the world's
evolution when human population is expected to double within the next twenty
years does not bode
well for social and political well being. To address this challenge requires. Unconventional
and innovative methods of housing construction. Prefabricated homes, often referred to as prefab homes, are dwellings
manufactured off-site in advance, usually in standard sections that can be
easily shipped and assembled. Many of the current prefab home designs on the
market have jovial, eclectic elements of postmodernism or the clean, simple lines of futurism. Prefab homes have not been particularly marketable; possible reasons for
this include:
· Homes
are not currently produced cost effectively enough for current demand.
· Homes
are not considered a realistic housing solution by the average consumer.
· The
consumer is either not familiar with the concept, or does not desire it.
· Difficulties
obtaining finance due to stricter guidelines being used by lenders to assess
prefab home loans.
Recently, however, modern architects are experimenting more often
with prefabrication as a means to deliver well-designed and mass-produced modern homes. Modern architecture forgoes referential decoration and
instead features clean lines and open floor plans. Because of
this, many feel modern
architectureis better suited to benefit from prefabrication.
The word "Prefab" is not an industry term like modular
home, manufactured home, panelized home, or site-built
home. The term is an amalgamation of panelized and modular building systems,
and can mean either one. In today's usage the term "Prefab" is more
closely related to the style of home, usually modernist, rather than to a particular
method of home construction.
In the United Kingdom the word "Prefab" is often associated with a specific type
of prefabricated house built in large numbers after the Second
World War as a temporary replacement for housing that
had been destroyed by bombs, particularly in London. Despite the intention that these dwellings would be a strictly temporary
measure, many remained inhabited for years and even decades after the end of
the war. A small number are still in use well into the 21st Century.
A new development of modern prefabricated homes are currently being built
in Milton Keynes, England. Designed by renowned architect Richard Rogers, designer of
the Lloyd's building, the Millennium Dome and the Pompidou
Centrein Paris, these new
'prefab' homes are part of a wider government objective to breathe new building
methods into the housing market within the United Kingdom and show that
'Prefab' is not only still alive, but also well respected.
The prefab home or house requires much less labour as compared to
conventional houses or homes. Most of the companies are selling complete pre
manufactured prefab modular homes or houses called "mobile homes" or
"manufactured homes". Prefab homes are becoming popular in Europe, Canadaand United
States as they are cheap. Local building codes (LBC)
in the US do not apply to prefab homes or houses; instead, these houses are
built according to specialized guidelines called (Federal HUD regulations in
the United States) for manufactured housing. Manufactured homes are not
permitted in some communities and therefore, one should check from their local
city to find about prefab building and construction laws regarding prefab homes
before considering purchase.
INTRODUCTION
Mass housing-a
case for prefabrication:
Prefabrication
owes its birth to the acute housing problems that followed the Second World
War. the devastation of the war rendered millions homeless. Today we are faced
with no less serious a problem. the costs have gone up spirally. Our only
consolation is the availability of cheap labour in India. There is popular
misconception that prefab does not generate employment, uncertainty remains
over the degree of mechanization we can save lot through management economy
alone, using the same materials and labour. The obsolescence of the
conventional irrational methods needs to be assessed. the housing needs to be
taken up at large scale as it needs to be mass produced.
Features of mass
housing
India faces a
deficit of 30 million housing units presently. The only way to clear this
backlog is mass housing. due to the skyrocketing prices of residential land and
building materials, private residences seem beyond the reach of the common man.
thus mass housing with a higher density and floor area ratio seems to solve the
nation's problem considerably common spaces like stair cases, corridors
etcetera are shared and so are common walls, services etcetera. This reduces
cost on individual owners, sharing of building material per unit and per
cluster, reduces cost equally. Mass housing facilitates economic layout of
services like common sewer lines, man holes, septic tanks, etc. this process of
sharing results to economy. Maintenance cost of common facilities like parks,
garages etc are likewise shared. mass housing
further economises by standardising materials, structural components thus
resulting in efficient management of materials and resources.
Prefabrication
as an alternative strategy
Prefabrication
is an industrialised construction method, whereby mass produced components are
assembled into buildings. the building work is carried out in two stages.
a] manufacture
of components in the factory or at the site casting yards
b] Erection and
grouting at final location at the site.
in the case of
conventional construction the quality of the finished product is mainly
dependent on the skill of the mason, whereas in the case of prefabrication the
components being machine made, the finished product has better consistency in
terms of quality.
Comparison
between prefab and cost in situ construction
in the
competition between the precast and the cast in situ structures, prefabrication
is gaining an ever increasing prominence because it is accompanied by an
improvement in quality, whereas the requirement of materials, the working time
and the cost show a decrease in tendency
wherever a
choice has to be made between the two methods of construction, it is worthwhile
to bear in mind that monolithic method of construction is suitable for
underground and such a lean structure, special architectural constructions and
small, non-repetitive structures, whereas prefabrication is a worthwhile
proposition where the use of large number of standardised members can be made.
Only modern methods of housing manufacture and erection, utilizing
principles such as computer integrated
Manufacturing (CIM), flexible manufacturing systems (FMS), and lean
manufacturing
Coupled with
innovative ideas can hope to erect houses at fast speed and high throughput.
The cam-nut and
cam-screw method of joining large panels was explored for further
Development
Tests revealed that this jointing
Method is adequate
to carry all design loads. Structural testing indicated that structural
Adequacy could
be assured if structural homogeneity could be assured. Finally, analysis was
Undertaken to
determine how best to optimize construction speed within the technological
Constraints
posed by large panel manufacture and erection for mass housing.
The prescribed
cam-nut and cam-screw method furthers the principles and objectives of
Flexible and
lean construction.
There is a tremendous housing shortage in the world that can only be
overcome by Innovative
designs and enlightened production management. This paper presents a method for fast
erection of apartment housing units that have architectural flexibility,
manufacturing flexibility, and
erection flexibility. The paper describes innovative jointing methods for large panel erection
and presents characteristics of an appropriate structural system to correspond to the
mechanical jointing and quick erection needs. Erection speeds using this method
are about ten times
as fast as conventional methods. Details of erection requirements and equipment are
given.
LEAN PRODUCTION
APPLIED TO CONSTRUCTION
Essential
production principles applied to lean construction include production
parameters such
as the following
· avoiding waste,
· standardization
of repetitive work,
· creating a
uniquely custom product,
· reduction of
resource idleness,
· reduction of
average waiting time,
· reduction of
time between delivery of finished products,
· reduction of
variability,
· decrease in time
for processing parts to traverse the system,
· decrease in
costs,
· reducing
inventory, and
· increase in
production rate.
It is frequently observed that above parameters are interrelated. Thus, it
is important to tackle production at a
holistic level where all above parameters are sought to be optimized
The reduction of waiting time and resource idleness can be overcome by
assuring that crews move in
parallel for erection and grouting activities. Decrease in time for processing parts is assured
by using the innovative methods developed herein and by assuring that crews are occupied
through low resource idleness. Collectively, the above assure high production rate, and cost
reduction occurs due to economies in mass construction.
INNOVATION FOR
LEAN PRODUCTION
Continuous
improvement is a hallmark to the way in which lean principles can be applied to any production
process. With improvement, waste in time, material, and money can be reduced,
thereby contributing to efficient lean production. In addition, innovation
contributes the same way as improvement. Indeed, innovation is essential for
lean production , since innovation has the characteristic of streamlining work
flow— either through innovative design or through innovative task planning— it
assists in the enhancement of lean principles. Mass manufacture is known to
assist with lean production. However, mass prefabrication has hitherto had the
drawback of reduced product variety and reduced agility. With
architectural flexibility, the innovative production method developed is
enhanced, thereby contributing to agile production.
INDUSTRIALIZED
PRODUCTION AS CENTRAL THEME
It is desired to
simplify site erection by reducing the number of parts that are required to
fulfill a whole building.
Though buildings
can be erected with columns, beams, slabs, and walls, the erection is
simplified in
our proposal through only the use of large panels.
LEAN
CHARACTERISTICS OF PROPOSED PREFAB SYSTEM
The developed
system using fully embedded cam-nut/cam-screw for joining concrete components
allows immense repetition of work tasks for shell erection that can be undertaken using
a single crew. Consequently, set-up time between activities is minimized and there is
continuity to the process. In addition, after optimal speeds through learning
are reached, those
speeds can be sustained since there is no change of activity for construction
of the mass
housing. Again, management coordination is easier, since fewer subcontractors must be
coordinated; there are fewer workers on site; and fewer number of different activities.
Since the number of type of products required for shell erection is brought
down to unity, there are
fewer inventory categories. In addition, the
proposed system is a unique custom product. The design, too, is conducive to
simplified workflow. Finally, the time to erect and cost to construct are significantly
lower, which are the ultimate objectives in many a production system.
ARCHITECTURAL
CHARACTERISTICS
The basic
general architectural plan proposed consists of a 2,815 square foot floor plan,
Figure 1.
Various other basic general plans can also be proposed. The proposed unit has
one three, one two,
and two one-bedroom units, or modules. This basic layout can be configured to develop
larger building units; thus, there is in-built architectural flexibility. The
modules can be placed in
different locations to meet various needs. These modules are standardized to allow for
efficient manufacture, but still have flexibility in their layout, providing
many possible floor
plans. An axonometric for the basic configuration is given in Figure 2. Other configurations
based on different combinations of the modules of Figures 1 and 2 are provided in
Singh.
A maximum story
height of seven stories has been set, motivated primarily by the
following
factors:
· Building design
and fire codes are less constrictive for buildings of seven stories
or less.
· Site erection is
simplified since tower cranes are not required.
· Construction
costs rise significantly for buildings more than seven stories
MECHANICAL
CHARACTERISTICS OF CAM-NUT AND CAM-SCREW
Erection of the
multi-story building using conventional methods is a time consuming task. Having the
mechanical joint such as cam-nut/cam-screw for the assembly of walls, floors
and columns can
increase the speed and quality of construction compared to that of the conventional
methods.
Figure 1:
General Plan Layout
The cost of erection of multi-story building using the cam-nut/camscrew mechanical joint
is much less than those using conventional methods. This is made possible due to
ease of assembly of the components and correspondingly less use of labor. Before selecting
the cam-nut/cam screw, several mechanical joint types such as bolt and nut, and welded joint
were studied. The cam-nut/cam-screw was chosen due to better mechanical Performance and
faster assembly opportunities than other methods (Singh 1998).
Figures 3 and 4
show the schematic of cam-nut/cam-screw at loose and tightened positions. The
cam-nut is embedded in the panel and the cam-screw is attached to the side of the adjoining
panel. The joint was designed in such a way that as the two panels assemble, the cam-screw is
inserted in the cavity inside the cam-nut; the cam-nut is then tightened
Figure 2:
Axonometric View
using pneumatic
or electrical power tools. The cavity inside the cam-nut is designed in a way that as the
cam-nut is tightened, it pulls the cam-screw inside. Since the panels
have protruding screws, there are limitations to how the panels may be connected to
each other. In addition, as it is expected that the architect will exercise his privilege of
design flexibility, different types of panels, having varied loading conditions
will be produced.
Some of these panels may be having protruding screws on all sides, others on some sides only,
and so on. To accommodate
this difficulty in joining panels when their protruding screws may come in the way,
there are two essential erection scenarios (see Figures 5 and 6). The
fundamental need to remedy
this difficulty is to design "pockets" in the concrete that can
receive the screws before
the screws are slipped into nuts.
Figure 3: Joint at Loose Position
Figure 4: Joint at Tightened Position
STRUCTURAL
CHARACTERISTICS
The building
system is an all panel system. Buildings are made up of wall panels and floor panels, or
slabs. The use of one structural element eliminates beams and columns. This greatly simplifies the design and manufacture
process.
The panels
consist of a 6' thick structural section, a 1.5" thick insulation section
and a 2.5" thick facade
section. The panels are joined with mechanical joints along the panel's
vertical and horizontal
edges. The mechanical joints clamp the panels together with sufficient force to allow them to
work together as a monolithic element. A gasket is placed between each panel and between the
panels and the floor slabs to ensure a weather tight seal.
The floor system
consists of solid precast floor slabs, designed as one-way slabs (Figure 7). These slabs
lie on top of the wall panels and are held in place by passing the cam-screw of the lower wall
panel through the floor slab and into the upper wall panel. This clamps the floor slab in
place. Solid slabs were used because they can tolerate the expansion and cracking
problems, whereas precast hollow core slabs would not be able to. The wall
panels provide bearing
for the gravity loads and the cam-screw provides bearing for lateral loads.
Along the sides,
the slabs overlap each other, allowing for a more continuous floor system.
These joints are
beveled to allow an epoxy grout to be placed between each slab to seal the joints and
provide a flatter continuous floor.
ADEQUACY OF
CAM-NUT/CAM-SCREW
The
cam-nut/cam-screw was designed using finite element analysis. ANSYS finite
element software was used. Design constraints for the mechanical joint were i) the size
of the panel, ii) the amount of distribution of reinforcement, iii) the applied
loads on the
joint, and iv) the material of the joint.
The load was
distributed on the three mechanical joints which are located at horizontal
and vertical
sides of the wall or floor-panels to connect one wall or floor-panel to
adjacent wall-panels or
columns. The loads applied on each joint located at the horizontal and vertical sides of panels
at each level were taken from the ETABS output.
The joint can be
made of AISI 1030 steel, a conventional material. The results of finite element
analysis (FEA) revealed that the maximum stress of the cam-nut and cam-screw
for both vertical and horizontal joints was 12,341 psi and 8,865 psi,
respectively. The stress factor of safety of 4.1 and 5.6, respectively, were
achieved, for the yield strength of 50,000 psi. The cam-nut dimensions were
6"f and 4.5"
thk. The cam-screw dimensions were 4" at the end and 3" at the tip,
by 9.4" in length. The same cam-nut as used in the horizontal and vertical
joints can be used for a vertical joint between walls through a slab, since the
loads are identical and cam-nut sizes are practically the same.
The length of the cam-screw is
6" (thickness of the panel) longer than the cam-screw from the horizontal and vertical
joints. The results of FEA revealed that the maximum stress of the cam-nut and cam-screw was
12,341 psi and 14,860 psi, respectively. The minimum stress factor of safety of 3.4 was
achieved, for the yield strength of 50,000 psi. This high factor of safety is adequate for
this critical mechanical structure.
STRUCTURAL
ADEQUACY
In order for the
panels and mechanical joints to be designed, the maximum loads that the panels and
joints would be subjected to have to be determined. The building was analyzed
for both earthquake
and wind loads, with the larger of the two being used, following the Uniform Building Code,
UBC. Computer structural analysis was performed using a special-purpose FEA program for
building analysis, ETABS. Output from ETABS provided the
necessary axial, shear, and moment loads on the panels. These loads were used to design the
panels and the mechanical joints.
In order for the
mechanical joints to work with the wall panels, they must be adequately anchored to the
panels. To do this a system of steel plates and headed studs were designed. The cam-nuts and
cam-screws are welded to a plate and long studs attached to the plate. These studs are
embedded in the concrete during the casting of the wall panels.
JOINTING NEEDS
The
cam-nut/cam-screw can be fastened using a pneumatic or electrical power tool.
The
amount of torque
required to rotate the cam-screw about 190 degrees in order to sit into a
tightened
"click position" can be measured experimentally. However,
commercially available
power tools are
available with large sized screw bits to execute the work. A crew of two
workers is
required to move in parallel alongside the panel erection to tighten the nuts.
ERECTION
SEQUENCING
The erection
sequence of the panels is crucial in the construction process. Although there
is great
flexibility in the erection process, there are still critical considerations to
be followed.
The biggest
concern is the location of the pockets in the panels to accommodate protruding screws. The
pockets along the horizontal sides (Figures 5 and 6), are on either side of the cam-nuts; thus,
the panels can only slide in one direction. This is one of the controlling factors in the
erection sequence. Great care must be made in ensuring the proper sequence of panel assembly
is followed.
It is best to
start construction assembly at one of the outside corners. This provides for
the stiffest structure
as the building is erected. No one outside corner is critical, so any outside corner may be
picked. This allows for flexibility in the erection sequence. To accommodate the pocket
system, work out from one corner in both directions, bringing the subsequent panels up
against the installed one. As each panel is placed, the cam-nuts can be
tightened.
The slabs are
installed after the panels are in place. The slabs are placed on top of the
panels and the
cam-screws pass through the slab. Slabs may be placed before all the panels have been
erected. The only factor controlling this is to ensure that all the panels that
are below the slabs
are in place.
The entire
erection sequence will be fastest if following a predetermined sequence. This is not always possible
though. Many circumstances may be unforeseen. If all the panels do not arrive at
the site, or some become damaged during transit, erection does not have to
stop.
Although there
are a few critical panels, erection can start at any outside corner with
whatever panels
arrive first. If during the construction the next needed panel is damaged or not available,
erection may start at a different area. Since all the panels do not have to be erected for the
slabs to be placed, the erection sequence is not entirely dependent on all the panels being
erected first. This affords considerable erection flexibility.
Figure 8 shows
the layout of the structural panels. There are 33 wall panels numbered one through 33 and
13 slabs labeled A through M. After panels 1 through 12 have been erected, for instance,
slabs A, B, and C can be placed. This allows yet more flexibility in the
erection sequence. Slabs
can be erected on the finished panels while any missing panels are being replaced. This
prevents any waiting time and resource idleness in the construction process.
Table 1 shows
four options for erecting the panels; all options start from the corner of
panels 2 and 3,
Figure 8. Options 1 and 2 simply start from one corner and move
counterclockwise to complete the erection sequence, with slight differences
between them.
Options 3 and 4
start at the same corner but stop at panel 7. Option 3 starts up again with panel 31
proceeding through panel 22, then moving to panel 8 to finish the sequence.
Option 4 switches to
panel 22 proceeding through panel 32, then moving to panel 8 to finish the sequence. This
shows how flexible the erection sequence can be.
Figure 8: Panel and Slab Layout
TRANSPORTATION
OF PANELS
The size of the
panels and slabs are limited by the transportation used to deliver them. By limiting the
lengths of the panels and slabs to under 40 feet and the widths to under 10
feet, standard
trailers can be used. Larger size panels and slabs may be used but special
permitting and routing to
the construction site may be needed, adding to the cost of construction. Coordinating
delivery so that the panels arrive as they are erected adds to efficiency, but because of the
flexibility of this system this is not critical.
POCKET GROUTING
All pockets must
be grouted after installation. For the general plan, there are 113 pockets per floor and 188
cam-nuts per floor. The pockets are grouted by pressure injecting grout through small access
holes. The grout gun injects the grout through the hole and the hole is plugged with a plastic
slug. The grouting can be done as the panels are installed. The grout would be non-shrinking
and pumped or pressure injected. Using a pneumatic, shoulder carried grout pump, a crew of
four can grout approximately five floors per day
SLAB JOINT
GROUTING
The slabs
overlap one another along their long axis. This joint is beveled, has exposed
reinforcing, and
is filled with epoxy-grout. The exposed reinforcing along with the epoxygroutties the slabs
together, providing a stiff diaphragm. The epoxy grouting also smoothes the transition
between the slabs. This system eliminates the need for cast in place concrete over the entire
slab area. Using a grout pump, a crew four can finish approximately fivefloors per
day
EVALUATION OF
TECHNOLOGY
Various
techniques for evaluating construction and building technologies using manufacturing
principles have been provided in literature through mathematical precepts. An evaluation of
the proposed system can be made after a complete process simulation is
executed.
TIME AND COST
ESTIMATES FOR LARGE PANEL ERECTION
Erection costs
are dramatically reduced since construction durations are reduced by nearly ten times.
Prefabrication is currently more economic than conventional construction for residential
apartment complexes in many world locations, such as Finland and Singapore, for example. This
means that with industrial conversion and mass consumption, the economies of
industrialized building can be well exploited.
Four panels can
be erected per hour with crane and four-man crew. Therefore, it would
take 1.5 days to
erect the 46 panels for the entire floor. (These results are based on estimates of prefab
construction from site data, expanded application of learning curves, and Means Manual.) In
contrast, conventional construction can take from 10 to 20 days to finish a
floor on site, which
is 670% to 1330% longer than in the proposed prefab construction.
A 7-story
building will take approximately 11 days to finish; traditional construction
would take 105
days. Extending this arithmetic further, the panel system can build nine full buildings in the
amount of time it takes traditional construction to build one building.
Working 250 days
a year, one crew can erect 22 such buildings having 616 housing units; traditional
construction would have constructed only 2 buildings or 56 housing units. With 100 crews,
61,600 housing units can be built in one year.
Therefore, the
construction time is approximately ten times as fast as conventional construction.
Conventional construction can take from10 to 20 days to cast a complete floor.
Correspondingly,
erection costs are dramatically reduced since construction durations are reduced by an
average of ten times.
With the flexibility
of the system, costs can be further reduced. In traditional construction each sequence of
construction, such as formwork or pouring concrete, stops or dramatically slows the work
of others. Walls for each story must wait until the entire slab of that story
is complete. With
the panel system, walls of the next story can be erected before all the slabs
of current story
are complete. This means that there is less conflict in scheduling the work
crew.
With this system
in particular, the pocket grouting crew does not have to wait for all the panels to be in
place for one floor to start grouting. Once a few panels are in place, they can begin grouting.
Pocket grouting
requires a shoulder carried grouting gun. The pockets to be filled are
7"x7"x4"
or 0.111CF. There are 113 pockets per floor with this typical building, giving
12.45 CF of
grouting per floor. Taking the assistance of Means , we estimate that one
grouting labor,
one equipment operator and one helper can grout approximately 70 CF per day. Thus, they
can do five floors per day. Floor grouting for the longitudinal construction joints of slabs
can be done at similar speeds.
Both nut
tightening and pocket grouting are faster work activities per panel in contrast
to panel erection.
Nut tightening involves only the mechanical tightening of the nut, which can be done by a
two-man crew.
CONCLUSIONS
The following
major conclusions are derived:
·
The cam-nut/cam-screw mechanical jointing method is
mechanically and
·
Structurally feasible.
·
Architectural flexibility, i.e., product variety,
is ensured.
·
The cam-nut/cam-screw joint can increase the speed
and quality of construction.
·
Production agility is increased.
· The success of this system will depend on the
production of large panels under Factory conditions, produced at high
speed using lean construction, flexible Manufacturing, and improved
production management principles.
