Preassembly of building services has really taken off. The demand for higher quality, less waste and faster construction means that more pipes and ducts are being made in the factory. How does prefabrication benefit supply chain management?
How quickly can you build a house? Two months? Two weeks? Seven days? How about a matter of hours?

A team of housebuilders in San Diego constructed a four-bedroom, double-garaged bungalow in two hours 53 minutes. That includes digging and pouring the concrete foundations, installing the plumbing, wiring and air conditioning and landscaping the gardens.

Amazed? Well, it took about three months pre-production planning and a cast of 700 to achieve it, as well as typically overwhelming amounts of Californian “can-do”.

Now, of course, the implication for the UK construction industry is that trebling site labour will increase productivity, which is not the message coming out of BSRIA’s latest research (Record). No, the message one should take from San Diego’s sprint builders is the virtue of preplanning every step, and having your supply chains organised so that large elements of the building can be assembled nearby while other trades are preparing the site. In the UK, the same principle applies, if you can preassemble items that usually have to be installed on-site, you will reduce the number of site operatives, up the quality, cut waste and improve on the build time.

The Crown House experience

Crown House set up its manufacturing centre in Wolverhampton five years ago. Since then, the firm has been developing its supply chain management techniques, setting up partnering arrangements with suppliers and liaising with manufacturers to develop products designed specifically for its services modules.

“We have been getting more involved with sub-suppliers, specialist contractors and customers to refine the modules,” said the Centre’s operations manager Gary Connolly. “By involving suppliers very early in the design process we have been able to identify areas where we can add value and save on costs.”

Nowhere is this more evident than the 230 services modules Crown House has produced for the 6 Brindleyplace office development in Birmingham. Working closely with developer Argent and the m&e designers, Crown House has developed what it regards as the “ultimate four-pipe fan coil unit.” The above-ceiling services module not only contains all the pipework and ductwork, but also the fan coils, control valves and modular wiring.

“We made a prototype module and invited the client and engineers to look at it and work out how to improve it,” said Connolly. “We simulated a ceiling space and checked access to filters and control valve handles. By working with the suspended ceiling contractor we could ensure than the ductwork access panels co-ordinated with the tiles of the suspended ceiling.”

“We also measured the fan coil for sound and vibration, and tested the hydraulic circuits by connecting it to a multi-headed manifold.”

So how far away are generic services modules? Connolly reckons that while the Brindleyplace module would be suitable for many other similar projects, the lack of uniformity in consultant’s specifications will preclude total standardisation.

“While consultants favour particular suppliers, we will have to design flexibility into our modules to take different types of fan coil unit,” says Connolly. “But if we can get consulting engineers to modify their in-house specifications to a particular standard, that will help with prefabrication,” he adds.

If we can get consulting engineers to modify their in-house specifications to a particular standard, that will help with prefabrication

Gary Connolly

To solve these issues, Crown House is working closely with Waterloo Air Management to develop a fan coil unit more suited to preassembled modules. It is also working with consulting engineer Colledge Trundle & Hall to develop a pre-assembled modular wiring system.

Crown House has set up a principal supplier arrangement with pipes and fittings manufacturer BSS. This, says Connolly, has given Crown House a very effective procurement cycle.

“It’s got to the stage where BSS maintains our storage requirements,” explains Connolly. “BSS guarantees our equipment needs by contract. It knows exactly how many tees and fittings we need, and supplies them on a just in time basis.”

Crown House is currently involved with Nottingham University on an EPSRC project aimed at disseminating the virtues of preassembly. Along with its property arm Carillion, Crown House is working with Laing Technologies, Ove Arup & Partners and the BSRIA to produce evidence showing the virtues of modular services construction. A CD offering guidance to clients and designers on how to go about procuring prefabricated services is due to be completed in October 2001.

Crown House is also involved in a research project lead by Loughborough University: Cost Model for Preassembly and Standardisation in Construction. The output will be a CD-Rom containing an interactive model for measuring the benefits of preassembly and standardisation.

The KRW experience

M&E contractor Kvaener Rashleigh Weatherfoil (KRW) is trialing new forms of prefabricated horizontal services modules on the latest Slough Estates project at 208 Bath Road. Following the successful application of prefabricated pipework modules on the 268 Bath Road project, KRW and its consulting engineer Robert & Partners were challenged by Slough Estates to shave 10% off the cost of the m&e services, with a view of 20% cost savings on following projects.

  The team’s first port of call was the BCO specification for offices, which gives upper and lower limits for comfort cooling loads. Based on the lower limits in the BCO specification, they decided that the density of fan coils could be reduced to one module per 7·5 m bay module of the perimeter zones, and one fan coil per 80 m2 for the inner zones. In effect, this cut the number of fan coils down from 44 to 32.

What this decision also enabled was a single 1·5 m wide services spine, which would run through the ceiling voids of the building’s two identical wings. The design team concentrated on getting all the main services into this spine, including a busbar-based modular wiring system, and a single dimensioned duct with iris dampers for maintaining the right supply rate to all fan coils (see diagram). While this cost a nominal £3000 extra over a conventional tapered air supply duct, the design team more than clawed that back through prefabrication and good supply chain management.

“The design of the services spine involved everyone in the construction process, from the architects right down to the commissioning engineer,” said Roberts’ Mike Baker. “We even went to the extent of figuring out where the back propping of the slab of the level above should be so we could install the spine while the concrete was still curing.” The design of the services spine was then passed to a local firm Bedford Fabrications for assembly.

As well as the design of preassembled services spine, Bedford Fabrications has been evaluating various different types of prefabricated pipework modules using different materials. Different pipework systems and materials were compared against a conventional 4·8 m four-pipe steel module used on previous Slough Estate projects. The results from the evaluation of two of these systems are described in the box story opposite.

Pre-fabrication: Is it really quicker by tube?

Kvaerner Rashleigh Weatherfoil got to grips with preassembled pipework racks on Slough Estates’ office development at 268 Bath Road. Work studies carried out by the BSRIA during construction showed the above improvements in productivity when compared to sites using traditional installation methods (Source: BSRIA).

Kvaerner Rashleigh Weatherfoil has also been testing the possibility of prefabricated modules using plastics pipes and fittings. George Fischer’s Instaflex system was considered for a trial module. The design team found that while the module was light, its flexibility required secondary bracing to allow it to be transported to site. While it was also faster to make, fusion joints were considered more expensive and requiring greater skill. The conclusion was that plastics pipework does not lend itself to multi-service module arrangements.

Another alternative considered by Kvaerner was the plastic-coated, thin wall steel pipework system made by Mannesman. Jointing is carried out by purpose-made fittings, which are mechanically crimped to the tube. An integral O-ring maintains the water seal. In tests, it was found that a number of specialist tools were needed to bend the pipes and crimp the sockets. While the module was lighter, there were doubts over the modules’ inherent rigidity which would limit the extent of multi-servicing run-ons.

At 208 Bath Road, the designers optimised the density of fan coils from 44 per floor to just 32, and served each wing of the building by a single 1·5 m ceiling-mounted module. This carries chilled water and lthw pipework, a single diameter air supply duct and a busbar-based modular wiring system. All this is prefabricated locally by Bedford Fabrications.

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