Andy Pearson presents six case studies of OSM in action, starting at BAA’s vast Heathrow Terminal 5 project, where off-site solutions have been found for diverting rivers, building a roof the size of five football pitches, and providing the services for the main building

At the Heathrow Terminal 5 site, everything from steel reinforcement for the basement concrete to the roof cassettes has been imported from factories
At the Heathrow Terminal 5 site, everything from steel reinforcement for the basement concrete to the roof cassettes has been imported from factories

The airport contractor BAA had a problem. In the past, construction of its airports had cost too much. Far too much. In fact it was Sir John Egan, the company’s former chief executive, who pointed out that if the airport operator wanted to remain a successful business, it could not afford to continue to build airports at the price it had been paying.

So when a vast £4bn terminal was proposed, the brains at BAA had to sit down and seriously consider the construction process that it was going to employ to keep the project on track. To this end, the construction team has introduced such cutting-edge notions as team integration, smart logistics and the use of a single virtual project model to pre-plan construction using sophisticated IT. In such an innovative climate, it was hardly surprising that an equally innovative solution was adopted to combat the problem of the serious lack of space on the site: everything that can be assembled off site is. Off-site solutions have been found for everything from the steel reinforcement for the main terminal building’s concrete foundations to the pre-assembled roof cassettes. The team has even used precast concrete sections to divert the two man-made rivers that run through the site.

So far, it looks like BAA got it just about right when it opted for such an extensive adoption of off-site solutions. The airport operator estimates that using prefabricated components has led to an increase in productivity of between 10 and 15% compared with that of the average building site. It has also managed to side-step the issue of the labour shortages in southern England. With factories located away from Heathrow, the workforce can be drawn from a far wider pool. When finished, Terminal 5 will have the distinction of being the first airport terminal to be constructed from a kit of parts from across the country.

Installing the services

Probat Garga, Amec Services’ project director, is under no illusions about the scale of the task his company faces. It is undertaking the detailed design, procurement, installation and commissioning of all the main terminal’s mechanical and electrical services. “It is one of the largest M&E installations ever undertaken by a UK contractor,” he says, betraying a hint of anxiety.

In addition to the intimidating size of the project, Amec has decided to make and assemble 60% of the services off site. As a result of this innovative approach, the firm is using less than half the site labour that would normally be required. It is also confidently predicting that six months will be cut from the project’s build programme.

To enable the services to be pre-assembled, Amec developed a modular services system. Each module consists of a lightweight rectangular frame into which the pipes, ducts and electrical cable trays are installed.

More than 5000 modules, based on 11 standard types, will be supplied to the main terminal building alone. The dimensions of the modules vary, but all were designed to be transportable by road. And by the time they have been escorted to the site, much of the job will be done.

Raising the roof

The roof of the main terminal building is huge: to use the standard unit of measurement, roughly the size of five football pitches placed side by side. But even this imposing structure is assembled from prefabricated elements. The curved steel beams of the roof structure, for example, may be 156 m long, but they are made up of 30 m sections, transported to site by truck and then simply bolted together. “We designed the steel structure to maximise off-site prefabrication,” says Steve McKechnie, an associate at Arup, the roof’s structural designer.

Just as impressive as the steelwork is the way that the 68,000 m2 roof is being covered using prefabricated roofing cassettes, developed by roofing contractor Hathaway Roofing. This is the first time that such a system has been used, so to ensure the right design was adopted, 12 prototype cassettes, consisting of roof decking and thermal and acoustic insulation, were built and tested for performance.

The modules are being assembled in a purpose-built annex to Hathaway’s factory. Each cassette measures 3 x 6 m, a size selected to allow 10 of them to fit in each lorry load. By using pre-assembled cassettes, the number of roofers needed on site has been reduced. More importantly, it improves health and safety standards by cutting the number of hours the site-based team needs to work at height. It also helps take the risk of delay caused by bad weather out of the construction programme.

More than 3000 cassettes will be used for the roof of the main terminal building, and a further 950 cassettes will cover the terminal’s little brother, the satellite terminal T5B.

Reinforcing the substructure

Both time and space have been extremely tight for the construction of the concrete substructure beneath the new terminal. Any delay in its completion would lead to a delay in the construction of the terminal building, which in turn would jeopardise the project’s opening date – an option BAA were not prepared to consider. To overcome the site’s shortage of storage space, and at the same time increase the speed of construction, the steel reinforcement for the concrete is being manufactured in a purpose-built industrial shed at BAA’s Colnbrook logistics centre near the airport.

Stuart Barr, demand fulfilment manager at concrete contractor Laing O’Rourke, says the reinforcement’s off-site assembly has many advantages: “Factory production improves the quality of assembly, the factory is a safer working environment and production is quicker under factory conditions.”

The prefabricated cages are craned into place straight from the delivery vehicle – which reduces on-site work and eliminates the need for on-site storage. Once the reinforcement has been attached, the shuttering is added and, later the same day, the concrete is poured.

Colin Potts, T5 substructures production leader for Laing O’Rourke, is as evangelical as Barr about the advantages of using prefabricated reinforcement. “Rather than taking a week to build a column base we can do it in six hours on site,” he explains. “We’ve moved from 20% pre-assembled reinforcement to 80%.” His enthusiasm suggests that the figure may rise further.

Diverting the rivers

One of the critical project milestones has been the diversion of the site’s two man-made rivers. The Duke of Northumberland’s River and the Longford River used to flow across the site. Not any more. To free up the ground two channels have been constructed to divert the rivers’ flow around the perimeter of the site before reconnecting them to their original route.

The space around the perimeter was tight: the channels have been threaded through a narrow corridor between a busy dual carriageway and the airport’s security fence. These channels have been constructed with concrete walls rising up from a clay and gravel bed. More than 8.5 km of river wall have been built. To save time on what would have been an arduous process, about 5 km was constructed from precast concrete sections. “We handed over to the earthworks team three weeks ahead of schedule, on time and on budget,” says Phil Wilbraham, T5 landside’s project leader.

Project team

Client BAA
Structural engineer Arup
Services consultant DSSR
Services designer/contractor Amec Services
Service module fabrication Babcock Engineering Services and WS Britland and Co
Roofing contractor and module manufacturer Hathaway Roofing
Substructure contractor Laing O’Rourke
Pre-assembled reinforcement manufacturer Express Laing O’Rourke

Urban housing

 Dabbs Hill, Northolt, west London
Dabbs Hill, Northolt, west London

Location: Dabbs Hill, Northolt, west London
Brief: To build 30% faster than usual without spending a penny more to achieve the result.

To realise their ambitious goal Willmott Dixon’s housing arm decided to exploit the benefits of using OSM components and reorganise its design, procurement and project management processes.

The firm called the project the “accelerated programme initiative” (API).

The project was based on the use of a prefabricated lightweight steel-framed panel system manufactured by Fusion Building Solutions. Fusion uses a computer-controlled manufacturing process to produce a series of lightweight, insulated steel-framed panels that are then assembled to form the house frame.

Brendan Ritchie, Willmott Dixon’s director of innovation, says that because Fusion’s system “is so dimensionally accurate” it allows interior and exterior doorsets to be slotted in, and the windows to arrive on site fully glazed, decorated and ready for installation. It was a framing system they had successfully trialled on a 10-house development in west London.

The framing system is conventional: the frames still have to be faced in a brick or block skin. However, because this external skin is not part of the building’s structure, it is off the critical path of the build programme and so does not hold up completion of the dwelling. The system also comes without plumbing and electrics fully fitted, so tradesmen are still needed. The steel floor joists do however come with ready-punched holes that allow the services to be threaded through.

To put their 30% target to the test, the contractor teamed up with affordable housing provider Catalyst Housing Group to construct 32 houses at Dabbs Hill in Northholt, west London. To help slash time on site Willmott Dixon also decided to use Fusion’s factory-assembled bathroom pods to eliminate a host of site-based tasks. The pre-assembled, pre-plumbed pods have an integrated section of floor that simply slots in neatly with the other Fusion cassettes. To speed up the process even further, Willmott Dixon used prefabricated stair balustrades, plastic plumbing and boilers with integrated hot-water stores.

Having introduced so many prefabricated components, Willmott Dixon then set about changing the way it built its homes. First, the contractor ensured the building’s design had been finalised before work started. This helped eliminate hold-ups during construction caused by design decisions being made, thereby saving both time and money.

With the design already completed, it then became much easier for the contractor to purchase construction materials in advance of the work commencing on site – again eliminating a potential source of delay. At this point, the specialist contractors were made aware of what was expected from them in terms of committing sufficient manpower to meet the programme. The final strand was managing and planning the construction phase in detail, which included working out the amount of time each specialist would be allotted to complete a task.

So did it work? Ritchie says: “Our aspiration using the API was to cut construction time by 30%. In the end we achieved a figure closer to 20% using the Fusion system. When compared with the time it would have taken to build using traditional brick/block construction, the Dabbs Hill houses were built roughly 35% faster. And we stuck to the budget.”

Willmott Dixon is now refining the API on a 93-unit development in Welwyn Garden City, Hertfordshire, for housing association Circle 33. Unlike the Dabbs Hill project, which used a steel framing system, this scheme uses a Pace Timber Systems panelised timber-frame system. The scheme is nearing completion and is currently on target to reduce construction time by 25%.

Project team

Client Catalyst Housing Group
Architect BPTW
Steel-frame module manufacturer/bathroom pod manufacturer Fusion Building Solutions
Contractor Willmott Dixon

The office

Dartford, Kent
Dartford, Kent

Location: Dartford, Kent
Brief: To create a turnkey office system combining fast-track construction, cost certainty and customisation using the minimum number of components.

The staff at Laing O’Rourke’s head office at Crossways Business Park in Dartford, Kent, have more cause than most office workers to know what the three letters OSM stand for. After all, they are the guinea pigs working in their employer’s first building constructed using its customised office solution (COS).

The backbone of the COS is the company’s structural frame system. This is built using precast concrete columns combined with post-tensioned concrete floor slabs, which are cast in situ using prefabricated, reusable steel shuttering. The system also incorporates precast concrete edge shuttering with cut-outs for the floors’ tensioning cables. Laing O’Rourke claims that its solution provides an almost infinite variety of building forms from a limited number of precast and post-tensioned concrete components.

However, the real heart of the process – what makes it more than just a one-off – is not the individual components that make up the building, but the supply chain behind the solution. Because this is so well integrated, it could successfully standardise the interfaces between the building’s different components. This makes the design process far simpler and gives clients a wide choice of building size and layout using the same basic components.

At Dartford, each office is based around a 9 x 9 m structural grid with a standard storey height of 3.9 m, to accommodate a choice of servicing solutions. The standardised dimensions also allow the columns, stairs and cladding components to be similarly standardised, thus reducing lead times. Cladding components such as the curtain wall system are also prefabricated off site. By standardising components, early cost and programme certainty are assured.

The system has already proved itself. The standardised approach meant the lead-in time for construction of the four-storey, 7400 m2 building was just four months, compared to an industry average of between six and eight. The site programme was reduced to 40 weeks because of the extensive use of prefabricated components and the post-tensioned concrete solution, which compares favourably with the industry average of 50 to 60 weeks for a building of the same size.

Just to prove the flexibility of the system, the COS team has just completed an office of a similar size and appearance on an adjacent site for developer Land Securities. This building has a central atrium and differs in the type of cladding, services solution and internal finishes used. And it looks like the team are going to be busy for some time to come: over the next few months there are another three projects due to begin on site that use COS including a new headquarters building and a speculative office development.

Project team

Architect Reid Architecture
Structural, environmental and facade engineer Buro Happold
Cost consultant Franklin + Andrews
Main contractor/structural frame contractor Laing O’Rourke
Precast concrete supplier Malling
Unitised curtain wall cladding Schmidlin
Unitised curtain wall frame McMullen Architectural

Suburban housing

Hawkinge, Kent
Hawkinge, Kent

Location: Hawkinge, Kent
Brief: To build an extremely diverse scheme of 28 houses, two bungalows and four apartments for mixed tenure.

Southern Housing Group has brought modular housing to genteel suburbia at Kettle Drive in Hawkinge, Kent. Whereas many projects look at the client’s requirements and turn to off-site options during the planning stage, this scheme had modular construction in mind from the start. The developer had set itself a challenge. “We wanted the homes to look like they were built using conventional methods for the residents and to meet with the local authority’s planning requirements,” explains Dale Meredith, development director at Southern Housing Group.

The timber-framed modules were assembled in the factory of supplier Rollalong. When the original module supplier dropped out Southern, working with innovation specialist Mtech and contractor Hill Partnerships, turned to the Dorset-based firm, which already had the advantage of having a working relationship with the project architect and cost consultant Calford Seaden.

Manufacturing the modules took just four weeks. They were delivered to site finished internally with the kitchen, bathroom, electrics and plumbing in place. Internal decoration was also completed in the factory environment to ensure the quality of the finishes. “Building homes this way benefits from the factory’s quality-control standards,” explains Meredith.

Generally speaking, each house is made up of four modules. The only variation to this came because of the necessity to transport the modules by road without a police escort. This meant the pitched roof had to be manufactured as two independent modules to ensure that the room modules could pass beneath road bridges.

Once the modules had been lifted into place on prepared foundations, they were topped off with a tiled roof and then a brick envelope was added to give the development a 60-year design life.

All 34 homes were on site and erected within six weeks. Meredith says: “This was one of the first and largest schemes of its type and the mix of house types and tenures has enabled us to demonstrate how flexible the system can be. Close partnership working with the contractor and module supplier enabled us to improve on an already reduced build programme.”

Project team

Client Southern Housing Group
Contractor Hill Partnerships
Innovation consultant Mtech Group
Architect/cost consultant Calford Seaden
Module manufacturer Rollalong

The school

Telford, Shropshire
Telford, Shropshire

Location: Telford, Shropshire
Brief: To innovate in creating new learning spaces for schools. The new classrooms must be transportable.

The government wants school architecture to inspire and stimulate young minds. In Telford, Shropshire, two classroom blocks have been produced for two schools with the hope of doing just that using modular construction.

Under the government’s “Classrooms of the Future” initiative, local education authorities are encouraged to put forward innovative building projects to win government grants to help fund their construction. Telford and Wrekin council set out to demonstrate that standard factory-made volumetric modules could be used to create a teaching environment fit for the 21st century. The modules are

also intended to be removable and transportable, should the needs of the education authority change significantly, without leaving the landscape scarred when moved.

The classrooms’ design, by green innovation organisation Integer and its architectural arm Cole Thompson Associates, set out to make the modules look attractive by breaking the “boxy” mould often associated with modular buildings. To this end they added a monopitch roof above a row of clerestory windows and ran a full-height window wall along one side.

Part of the architect’s brief was to incorporate sustainable technologies in the scheme to help improve the environmental performance of the buildings. Green features include a conservatory, a wind turbine and a living sedum roof, fitted with photovoltaic cells to generate electricity from the sun.

To help the new classrooms blend in with the existing buildings, the architect has clad the buildings differently on each site. The rainscreen cladding at Wrockwardine Wood Junior School is constructed from cedar boarding, while at the Lord Silkin School steel mesh has been used, which will eventually be covered in climbing plants.

Contractor Yorkon prefabricated the steel-framed volumetric modules at its York factory. By fitting out the modules in the factory, Yorkon halved the site construction programme. In addition, assembling the units in a controlled factory environment helped ensure consistent build quality and minimised the risk of accidents on site.

Project team

Client Telford and Wrekin council
Architect Integer/Cole Thompson Associates
Services engineer BWP
Module manufacturer/contractor Yorkon

Infill housing

Gypsy Hill, Norwich
Gypsy Hill, Norwich

Location: Gypsy Hill, Norwich
Brief: To make the cost of factory technology competitive, even for small schemes.

How do you embrace the government’s drive to utilise modern methods of construction without exceeding the Housing Corporation’s constrained budgets?

Folio, a partnership of seven organisations, may have the answer. The partnership has just completed the UK’s first two-storey housing scheme built using timber-framed modular construction at Gypsy Hill in Norwich. The partnership claims the cost for the completed scheme of 10 semi-detached houses was comparable to traditional build and well within Housing Corporation parameters.

The project team had a unique opportunity to compare and contrast conventional construction methods and off-site manufacture as they were constructing traditional-build units on the same site. “We started the Folio units last but we finished them six weeks ahead of the traditional-build units,” says Martin Aust, development director with Flagship. In fact the scheme has been so successful that 15 sites are planned over the next two years, and the next 28-unit scheme in Thetford is already on site.

Registered social landlord Flagship Housing Group initiated the partnership. The group had looked at the more commonplace modern methods of construction such as standard timber frame and steel-framed modules, all of which were too expensive. Timber-framed modules, however, appeared to offer Flagship a more cost-effective solution.

The housing group gathered together module manufacturer Omar, contractor Lovell Partnerships and a team of consultants to develop a series of module types for two-, three- and four-bedroom houses, two- and three-bedroom conventional and disabled bungalows and one- and two-bedroom apartments. Folio has committed to build more than 100 timber-framed modular units a year, which gives the manufacturer a guaranteed demand for its product and long production runs.

The key to the system is that it is simple and uses familiar materials and construction techniques. Modules are constructed in a similar fashion to a standard timber frame from timber wall studs with insulation packed between that is held in place by a breather membrane.

Internally, the units are fully fitted with all the bathroom and kitchen units in place. Services are installed in the walls before fitting the plasterboard; the unit is faced in plywood for stability; the windows are added; and finally the completed module is wrapped in plastic for protection.

The system is incredibly quick and simple to install on site. At Gypsy Hill the first semi-detached house, assembled from one ground-floor and one first-floor module, was completed in less than three hours. By the end of the day the roof trusses were in place and the roof was ready for tiling. Once the roof was complete, fully loading the structure, the brick external walls were constructed. Eight weeks after work began, the house was ready for its first occupant. Aust says: “It all went incredibly well in terms of quality and the cost was comparable to traditional construction.”

Simon Medler, regional manager of contractor Lovell East Anglia, says the big advantage of the system is that it is cost-effective even for a small four-unit scheme. “We could build all four units in a total of eight weeks,” he says. Folio is producing a design manual to allow other housing associations to benefit from the scheme.

Project team

Client Flagship Housing Group
Architect Ingleton Wood
Employer’s agent Oxbury & Company
Building Regulations issues Norwich City Council
Module manufacturer Omar Homes
Contractor Lovell Partnerships

Offsite supplement Oct 2004