A test of their metal

Contractor Llewellyn specified Forge's steel system on the £35m Central Oakridge scheme for client Oakridge Partnership, which had briefed the contractor to select materials with minimal environmental impact. Project architect Amanda Taylor at architect HTA says that Forge's steel panels fitted the bill, being recyclable and having a low embodied energy.

The development will include mixed-tenure apartments and houses as well as community buildings, retail units and a pub. Housing will be built at 43 units per hectare, which easily meets the requirements of PPG3 planning rules, and far exceeds the density of the 143 council maisonettes the new homes are replacing. There are also a range of other innovative concepts, including adaptable designs, and mixed-tenure "pepperpotting".

Forge was the obvious choice for manufacturer as Llewellyn owned the company at the time and wanted to make use of in-house technology. It calculated that by using the lightweight steel frame and pre-decked floor cassettes it could erect the shell and upper floors of a dwelling within a day, thereby dramatically reducing build times.

Although the potential rewards of using steel were great, Llewellyn and the project team knew there were risks. Steel frame's rigidity means that it has much tighter tolerances than traditional construction methods, and is hard to adjust once on site. This meant Llewellyn had to find subcontractors that could build to a high quality as well as a keen price.

Forge also had to come up with construction details for HTA's outline designs, which had been drawn up before the steel panel system had been chosen. For the experiment to come off, the three teams would have to co-operate closely and translate HTA's designs into steel constructions that Llewellyn could build quickly and cost-efficiently. The fact that HTA, Forge and Llewellyn had never built and designed in steel on this scale added a little excitement to the proceedings, as success could by no means be assured.

The specification of other materials was also crucial. Not only would components such as the timber trusses and grp dormer balconies have to interface perfectly with the steel frame, their supply and installation would also have to fit within the tightly scheduled build programme.

And the development is not all steel and glass – there were traditional materials used on the scheme. As well as the timber truss roof, the ground-floor outer skin of the terraces and apartment blocks were built with brick – the preference of the residents, who were consulted throughout the design phase. HTA considered brick slips, which could have been prefabricated, but project architect Taylor says she was unhappy about the amount of waste created when the bricks are cut to form the slips.

During the first phase of construction, having brick on the ground floor and render finishes on the upper floors presented Forge with a tricky design challenge. The depth of the build-up on the ground floor wall was 125 mm greater than that of the first floor wall: 200 mm compared with 75 mm. To align the render with the brick, Forge had to extend the first floor cassette so that the upper floor wall panel could be supported 125 mm beyond the ground-floor panel.

Where brick is not present, such as at the bay windows, the design is simpler. Steel panels 2.5 storeys high can be designed and fabricated to create entire elevations, which can be erected in one go.

The brick-and-render elevation also presented Llewellyn with a sequencing problem in the build programme. To benefit from the speed of steel panel erection, Llewellyn needed to create a weatherproof envelope as quickly as possible so subcontractors could start work on the fit-out. The problem with the build-up of the first-floor wall was that the only waterproof layer was the final skim of render. This meant that the top of the building was not protected against the weather until the render had been applied, which delayed the internal trades. The insulated render build-up could not be installed in wet weather either, leading to more potential hold-ups.

Unlike the render build-up, the build-up between the steel and brick skin could be left exposed to the elements because it featured a Tyvek breathable membrane. However, the brick wall still had to be built before the internal trades could start work because the bricks supported the Rationel prefabricated window units, which form an integral part of the weatherproof envelope.

Richard Crook, site manager at Llewellyn, knew that changes had to be made. "We went down the wrong route originally. We put brickwork on the critical path, which meant we had to wait longer before working on the interior. We realised that brick had to be treated as cladding, not part of the structure," he says.

To rectify the situation the project team made two modifications to the designs prior to the second phase of construction. Firstly Llewellyn created 10 mm thick galvanized steel brackets to transfer the 120 kg weight of the window unit from the brick skin to the steel frame, which meant that windows could be installed before bricks were laid.

More fundamental was the decision to replace the insulated render with blockwork-and-render on the upper floors. Using blocks may look like a retrograde step, but it offered Llewellyn two advantages. It meant that it could use the same weatherproof breather membrane as it used for the ground-floor brick walls, and by adopting this approach the outer skin of the upper floors could be taken off the critical path.

The second benefit from the change in this specification was that walls on the ground and upper floors were now of equal thickness, which meant that first-floor steel panels no longer had to be stepped out over the ground floor panels. As a result, larger steel panels could be used for the elevation and the floor panel extension detail could be eliminated.

Geoff Harris, director at Forge, says that the design details would have been more straightforward if the original designs had been devised with steel in mind. "We were working within the parameters of the original masterplan. It helps if you get the architect on board as early as possible because then it can understand the limitations of the system," he says.

HTA admits the learning curve was steep but now feels it knows what the system is capable of. "We've learned what to do, and not what to do," says HTA's Taylor. "By designing in a certain way we can reduce the number of panels Forge need to produce, which cuts down on costs."

The architect was also able to feed its expertise into the design process. One example was the interface between the timber trusses and the steel frame. The trusses originally had to be aligned with the vertical steel studs, which would carry the loads from the roof. To provide the erectors with more tolerance on site, HTA suggested that Forge reinforce the steel beam to which the trusses were fixed. This means that installers no longer have to accurately position the trusses above the steel studs: the loads could be taken along the strengthened beam, before being transferred down the steel studs.

Having built 55 homes on the first phase, Llewellyn's Crook feels that the project team and subcontractors have now got to grips with steel frame: "We had some hiccups because of the innovation but now we understand the principles, it's straightforward." Crook is so confident about steel construction that he believes that the five-year build programme can be reduced by up to 18 months.

Forge is in the process of designing a steel-frame development near Oakridge. The three-storey building will take prefabrication a step further and have kitchen and bathroom pods. Harris also hopes to do away with brick and blockwork in favour of render and timber cladding.

Crook certainly believes steel frame poses a threat to traditional building materials. "At the moment steel is experimental, but I can see it becoming a regular method of construction within 10 years," he predicts. And if that watering can is anything to go by, those buildings will be weatherproof and rust-free by the time the next building materials revolution comes around.