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Pre-fab strikes back

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A renewed interest in prefabrication, partly encouraged by the intractable economic problems of conventional construction, is using steel as its basis. Martin Pawley reports

Above left, from top: McDonald's module being located into position; the restaurant was completed in just one day from the delivery of modules - a world record; plan of a typical McDonald's restaurant. Above right, top to bottom: sequence of construction at Murray Grove - module and roof, access works and external cladding, lift and stair

Nearly 60 years ago, at the end of the Second World War, the uk's housing stock had been so reduced by bombing and neglect that a million new houses were needed. Because speed of construction outweighed all other considerations, the innate conservatism of the housing market was overwhelmed. As a result, between the end of the war in 1945 and the sterling crisis of 1947, a window of opportunity opened up for innovative housing - 170,000 examples of 20 different types of pre-fabricated house, of a total of 1400 designs submitted, were erected on new estates all over the country.

Many of these houses were assembled from components made in factories that had just ceased producing military vehicles or aircraft, and demonstrated considerable ingenuity in their transfer of new technology to the problem of housing families fast.

But although considered a panacea for construction delays, pre-fabrication failed to provide the whole answer to the post-war crisis. In time, like those other wartime words 'austerity' and 'rationing', 'pre-fabrication' became unfashionable. Later still it became associated with the system- built housing and tower blocks of the 1960s, an episode in the history of housebuilding that is still wholly discredited in the public mind.

Today only those of very mature years still connect the word with the years when the Emergency Factory Made programme brought forth the 'pre- fabs', that are still considered to be among the most advanced industrialised dwellings ever produced in this country.

Nowadays pre-fabrication is generally defined as the manufacture of the components for a building prior to its assembly on site, a definition that includes modular building, although the word still has a distinct meaning. If a three-dimensional module is assembled from manufactured components, as is the case with steel framing, it is said to be pre-fabricated. A good example of the distinction is offered by Foster's 1986 Hong Kong and Shanghai Bank, which included 139 separate service modules made in Japan. These modules were assembled from pre-fabricated framing components before they were shipped to Hong Kong and installed in the building.

If the most impressive demonstration of pre-fabrication to date required the urgency of a war to bring it about, current interest in the resurrection of the technique on a large scale is being driven by growing concern at the intractable economics of conventional construction.

Among its main recommendations, 'Rethinking Construction' - the 1998 Construction Task Force document better known as the Egan Report - called for annual reductions of 10 per cent in construction cost and construction time. These targets, which the task force declared had been achieved by several major construction clients in recent years, point conclusively towards a general increase in off-site fabrication of buildings and a reduction in the amount of site work. In short, more pre-fabrication and modularisation and the maximum use of dry construction.

Typical of the new generation of advanced pre-fabricated buildings emerging from this new thinking is the prototype cost-rent steel-frame apartment building designed by Cartwright Pickard for the Peabody Trust that is now on site in Murray Grove, Hackney, East London. Comprising 30 apartments on a corner site divided by a circular stair and lift tower, the building consists of 90 steel-framed and fully equipped modules which rest on structural steel legs on top of one another, side by side to a height of six storeys.

These 8 x 3m accommodation modules have been manufactured by Yorkon of Huntington. Dry-lined on the inside and injection insulated, they are framed in a combination of hot-formed angles and cold-formed channels and clad in flat galvanised steel sheet. Once stacked in position, they receive a bolt-on tubular steel facade that will support their balcony- access walkways and require no fireproofing. Diagonal stainless-steel cross bracing between the steel columns of this facade will ensure that this whole assembly will act as a beam if a column is struck by a motor vehicle.

Between the two wings of the building is a circular lift and stair tower in steel. This too is pre-fabricated. The central lift shaft is a single 16m steel structure encircled by modular steel stairs and landings. The lift shaft supports a three-part circular steel roof.

Thanks to its high level of pre-fabrication, Murray Grove will only be on site for six months instead of a year and, although at £2.2 million it will cost 20 per cent more than a conventional structure, according to architect James Pickard there are plans to follow it with a 250-apartment version of the same design that will take only two and a half, rather than four, years from go-ahead to completion.

This time the building will take more advantage of scale and experience and be 10 to 15 per cent cheaper than any equivalent conventional structure. Pickard stresses that these speed and cost savings will not be at the expense of quality. At Murray Grove, in addition to its much higher tolerances and lower defects count, the low-e glass and high-quality terracotta rainscreen cladding used are much higher-quality materials than would normally be employed in social housing.

Elsewhere in the uk light-steel framing is helping to achieve the goal of continuous performance improvement identified in the Egan Report. Today in its factories in South Wales, British Steel pre-fabricates galvanised strip-steel-framed panels, up to 3 x 3m, as planar elements for dry-lined modular box structures for a wide variety of building types.

These panels are strong, lightweight, durable and resistant to long-term structural movement. Assembled into modular units, they have been used in student halls of residence, hotel extensions, office buildings, fast- food restaurants and apartment buildings. At the University of Wales a student hall of residence was used as the prototype for the tmt modular building system developed by Ove Arup & Partners. Using this system it is possible to stack pre-fabricated modular units up to 12 storeys high without a supporting structural frame.

Forte Posthouse was the first hotel chain to adopt light steel framing rather than conventional construction. The 60-bedroom extension to the Posthouse hotel at Dublin Airport was the first to be completed using a new system of steel framing perfected by Volumetric.

Perhaps the best-known light-steel framing achievement is the Britspace/McDonald's 'Drive Thru' at Shopping City, Runcorn. There, a world record was established by erecting a complete restaurant on a prepared site in only 24 hours. The switch from conventional to steel-framed modular construction at first reduced the typical contract time for a McDonald's restaurant from 16 to eight weeks. This has now been reduced further to four weeks, with the period from module delivery to completion brought down to eight days. Over 300 of the chain's restaurants have now been completed, incorporating 1800 Yorkon or Britspace light-steel-framed modules.

As with modular building in general, light-steel-framed modular construction is often criticised for its bland and artless appearance. After silently absorbing this objection for years, the Modular Steel Industry Group (which comprises the Steel Construction Institute, British Steel and senior executives from major companies) now welcomes the intervention of architects.

As David Phillips, marketing development manager for British Steel puts it: 'We have systems that work to everyone's benefit. Now we want architects to come on board to raise the standard of design. We can't create better buildings on our own.'

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