Click to fit

Prefabrication: the manufacture of component parts of a building and its services prior to site assembly. Of course this is nothing new. Ductwork manufacturers have been prefabricating their products for years, and pre-packaged plantrooms are a common sight on the rooftops of UK plc.

But old habits die hard, and by and large building services are still constructed out on site rather than in the dry and warmth of a factory. Even in 1997, research by the BSRIA revealed that the UK site worker was only effective for 37% of the time1, largely because of poor motivation, appalling site conditions2 and poor project management. Among its many recommendations on ways in which the industry could bootstrap-haul its way to respectful profitability, BSRIA suggested that prefabrication might be part of the answer.

A year later Sir John Egan's report Rethinking Construction arrived at the same conclusion3. "Greater site efficiency will arise from using standard components, precise engineering fit and extensive preassembly," said the report. "Site construction needs to be carried out by a relatively small and dedicated team of multi-skilled operatives who develop their expertise over a series of projects." Coincidentally, the BSRIA had already embarked on follow-up research to test this theory. This is what the project discovered.

Producing the evidence

Previous research by the BSRIA into site productivity had led to the creation of an extensive database of best practice times for over 2500 trade activities, against which observations were made and comparisons drawn. The intention was to do the same for prefabrication and preassembly activities (both on and off-site), comparing them against the best practice times for traditional activities.

Working with some 27 sponsoring organisations, the BSRIA carried out nine case studies which included prefabrication of four-pipe fan coil distribution pipework, boiler plant room pipework and preassembled plant modules and risers. In addition, the BSRIA studied the advanced preassembly techniques used by McDonald's Restaurants.

From these detailed site and factory observations, BSRIA identified significant improvements in overall productivity. In round figures, the improvement was over 2·5 times the productivity found on traditional sites. Prefabrication and preassembly average productivity was also only 15% below what the BSRIA considers best practice, compared with 35% for traditional site activities.

Case study: McDonald's Restaurants

Over the last five years McDonald's Restaurants' use of standardised designs and components has seen the company's construction costs slashed by 50%.

All construction has been moved off-site to a controlled factory environment. This means skilled, cheaper labour producing higher quality construction with a heavy reliance on plug'n' play push-fit connectors for plumbing, lighting and ductwork.

Skilled labour has not been entirely removed from the process. CORGI-registered operatives are still used to connect gas supplies, and qualified electricians still wire the electrical cables to the main distribution board.

Inevitably, McDonald's sheer purchasing power plays a major role. Suppliers tend to work to a low fixed gross profit margin in return for large volumes and payment within 14 days. A downside of this is that manufacturers have limited scope for variations in terms of materials and the level of labour.

Project management has developed to aid the prefabrication process. Every restaurant that McDonald's builds has a reference number that corresponds to the construction drawings, foundation details, structural specification and even the internal finishes and the positioning of cooking equipment. This enables everyone in the supply chain – from the manufacturer of component parts to the specialist preassembly manufacturer, groundwork contractor and erectors who assembled the modules on site – to have access to the information.

Not all is sweetness and light. Groundworks and utility services including drainage have reportedly caused some problems, as they tend to be treated by the groundwork contractors as "out of sight, out of mind", and consequently they are not often set out to the same degree of accuracy as the foundations, laid to match the manufacturing tolerance of the modules: an amazing 3 mm.

Ominously, the UK construction market is relatively small in comparison with the US, and having "gone modular", McDonald's is finding it cheaper and faster to import a product from the USA than to buy it direct from a UK manufacturer.

A McDonald's restaurant can now be ordered and opened in just 10 weeks. And site erection? That can take just two days.

Does prefabrication help partnering?

The BSRIA also attempted to assess the value of prefabrication as an aid to partnering arrangements, and whether or not factory assembly can remove the risk of confrontation.

To do this, the BSRIA's researchers studied the prefabrication of seven, 30 m-tall risers on a fast-track commercial construction project. The decision to "go modular" was taken by the design team in order to overcome severe site constraints imposed by substantial cross-bracing. Although the risers could have been installed in the traditional way, it would have been extremely tortuous with plenty of opportunity for delays and conflicts with other trades.

The main issue was cost. The prefabrication option was priced at around 50% (£200 000) more than the traditionally tendered solution. This was mainly as a result of the additional steelwork required to construct self-supporting steel frames to house ductwork, pipework and cable trays.

To reduce the cost penalty, the project team suggested removing the metal floor decking in the risers, as this could be incorporated in the steel frame. This would also remove the need for temporary platforms.

The 7·5 m-long riser modules were constructed in a factory and then pre-assembled as one 30 m module to ensure that the services were correctly aligned. The modules were then broken down for transportation, trucked to site and craned into position.

The designers had allowed for a 25 mm tolerance between the module and the structure, but some modules exceeded this tight tolerance with the result that some modules fouled the shaft during installation. Nevertheless, instead of the work taking 30 weeks on site, it was reduced to just three weeks in the prefabrication factory and seven weeks on site to install and assemble the risers.

The 20-week saving in construction time had a beneficial effect on other construction activities. It certainly removed opportunity for conflict, as there were fewer site operatives, safer working conditions and less materials handling. Less storage space was also needed for tools and the preparation of materials.

Although the project was a traditional contract rather than a partnering arrangement, the construction team modified the contract arrangements to accommodate prefabrication. The additional expense required by preassembly was performed at cost, with the extra money being pooled from other areas of the project. The client chipped in as well.

Ultimately, the client's building was completed and (more importantly) let on time, thus maximising the return on investment.

Identifying best practice

Despite such positive findings, readers should know that prefabrication is not simply a case of taking an activity off-site.

Good communication between all members of the team, at all levels, is vital to the success of prefabrication and preassembly. Success is also dependent on good site management and programme planning skills.

Project champions are a good way to motivate project teams to adopt ways to drive down costs and eliminate waste. The BSRIA also suggests that project teams should set up value engineering workshops to arrive at cost effective solutions.

Incentives are vital to maintain the interest of all construction team members. The BSRIA research proposed that shared cost savings should accrue from faster erection programmes, the reduction of overall costs and "earlier revenue opportunity" for the client.

Clearly, designers should favour the use of standard components over specials or non-standard sizes of equipment. Plant items should also be rationalised to produce the minimum practical number of types and sizes.

Co-ordination is another major factor. Prefabrication should be considered at the earliest possible stage in a project to achieve economies of space and scale. This may influence the choice of m&e contractor, who may be appointed on the ability to undertake co-ordinated detailed design, or even be capable of prefabricating services elements through good liaison with other design team members and suppliers.

Which raises the issue of contracts. Forms of contract must allow for timely involvement of the contractor and manufacturer. The use of partnering or two-stage negotiated contracts are recommended, although traditional style contracts which include elements of partnering should not be ruled out.