PREFABRICATION METHODS FOR MASS HOUSING

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.

·         Social stigma that exists because of low quality mass produced designs used in the past.

·         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 architecture is 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 Centre in 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, Canada and 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"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 epoxygrout

ties 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 five

floors 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.