Prefabrication

[edit] Introduction

The term 'prefabrication' describes assemblies that are manufactured under factory conditions and then transported to construction sites for incorporation into building or civil engineering works.

[edit] Post-war residential tower blocks and residential building

The acute shortage of housing following the second world war led to a boom in high-rise apartment blocks, and contractors turned to prefabricated panelised wall and floor system building in an attempt to meet demand. However, the jointing systems used and the failure to hide large joints produced a number of monotonous, ugly developments, often with condensation and other problems. Faults were so numerous that the Housing Defects Act 1984 created a scheme to compensate owners who had unwittingly purchased dwellings with defects. It was estimated that 31,000 owners were eligible under the scheme. See Housing Defects Act 1984 for more information.

In 1968, a gas explosion at Ronan Point, (on Butchers Road, Newham, London) resulted in the collapse of an entire corner of a twenty six floor tower block killing four people and badly injuring seventeen residents. A pensioner striking a match to boil water for tea, inadvertently ignited leaking gas from a joint in her newly-installed cooker. The resulting explosion was enough to blow her through the kitchen door and save her life. However, it lifted the ceiling/floor slab and pushed the wall out resulting in a 'house of cards' collapse (a 'progressive collapse').

The collapse resulted in amendment of the building regulations, introducing a robustness clause so that if one element of a building failed, others would be strong enough to take the increased load, allowing occupants to escape. However, The resulting lack in public confidence brought this type of construction to a rapid halt from which it has never completely recovered.

For more information, see Ronan Point.

However, there are a great many successful factory-produced buildings in Europe and the Far East. Japan has been producing beautifully-detailed houses for three decades providing many options to the customer, who approach purchase in much the same way as ordering a new car from a production line. The Scandinavians also export highly-insulated timber houses with triple glazing and Germany offers carefully-detailed, unashamedly-modern prefabricated houses.

[edit] Current use

[edit] Repetition

The handling, transportation, craneage and erection costs of prefabrication have to be weighed against in-situ alternatives. Economic justification is often dependent on high volume and repetition. This has led to use in applications such as residential blocks, hotels and houses. Architectural success in disguising panel joints has improved public opinion, helping to combat the perception that prefabrication is 'unsightly' or 'cheap'.

[edit] Time

Where there is a short programme for construction, and so time has a higher priority than cost, prefabrication can allow parallel working on different aspects of a project.

For example:

• The Trustees of Ascot Racecourse were only prepared to lose one year of Royal Ascot racing during which the old grandstand had to be demolished and replaced with a new 50,000 sq m equivalent. As a result, the frame and steppings were manufactured while racing took place under the old facilities. The new structure, consisting of composite concrete and steel, was installed in record time.
• Bridges over motorways are often prefabricated to avoid causing traffic congestion.
• Avalanche shelters have a narrow timescale between seasons in which construction can take place so are mostly prefabricated structures.

[edit] Quality

The quality achievable with factory prefabrication is generally higher than can be achieved on site. This particularly applies to welding and hot trades such as pipe work and mechanical services. Back-to-back toilet and basin units mounted on steel framework are commonplace. Even mini plant rooms lend themselves to prefabrication. This can save a lot of complex site work in confined spaces.

[edit] Advantages

The advantages of prefabrication include:

• Programme savings due to the ability to progress work as a parallel operation in a factory and on a construction site.
• Factory tolerances and workmanship is of a higher quality and consistency to that achieved on site.
• There tends to be less waste.
• Independence from adverse weather and winter working.
• An alternative means of production where there may be shortages of local skilled labour.
• Access to cheaper labour markets. For instance two hundred prefabricated timber lodges for short holiday lets in Pembrokeshire were sourced from Eastern Europe.
• Greater programme certainty.
• The factory environment can allow better safety than the construction site.

[edit] Disadvantages

The disadvantages of prefabrication include:

• Road transport maximum widths.
• The need for police escorts.
• Height restrictions under bridges.
• Daytime traffic restrictions in city centres.
• Maximum load capacities of site craneage and temporary gantries.
• Additional cost of temporary bracing for transportation and/or lifting or permanent framing to support prefabricated assemblies.
• Additional cost of pre-assembly in the factory prior to dismantling for transport and delivery.
• The insitu work abutting prefabricated assemblies requires a higher degree of accuracy than is normally associated with on-site building work to avoid interface problems.
• A mistake in the mass production of prefabricated elements ahead of the measurable site work is a serious risk. Reputedly there is a field in which sixty prefabricated concrete staircases are buried as they had been incorrectly manufactured for a tower block in the City of London.
• Sustainability is an issue regarding the transportation of the materials to the construction site.
• Factory production requires predictable and consistent demand, whereas construction tends to require large numbers at the same time, then none.

[edit] Related articles on Designing Buildings

• Advanced construction technology.
• British post-war mass housing.
• Design for deconstruction.
• Design for Manufacture and Assembly (DfMA).
• Fabricated drainage stacks.
• Flying factory.
• In situ.
• Kit house.
• Modern methods of construction.
• Modular buildings.
• Off-site construction.
• Off-site manufacturing.
• Off site materials.
• Off site, on track.
• Platform approach to design for manufacture and assembly.
• Plug and play skyscrapers.
• Precast.
• Precast flat panel system.
• Pre-manufactured value.
• Ronan Point.

[edit] External references

• Building, Cost model: Prefabrication and preassembly. 2002 issue 06 by Davis Langdon & Everest