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Matters of Trust

building study

Feilden Clegg Bradley Architects' high-quality housing for Peabody Trust addresses a difficult context in south-west London, and explores the potential of a hybrid form of prefabrication

Peabody Trust was founded in 1862 by American philanthropist George Peabody for the relief of poverty, specifically in London, largely through providing affordable rented housing for those in work. The trust has since grown to be one of London's biggest housing associations with more than 19,500 homes in 26 London boroughs.

Its concern for healthy living was pioneering, ahead of local authorities at the time, though it was more a caring landlord than the more broadly based innovator it has latterly become. For example, shared facilities such as bathrooms remained for some time; its first self-contained flat was not built until around the start of the First World War.

By the latter part of the 20th century, the housing debate had shifted from housing charity toward housing rights. The trust responded by taking a more proactive approach to housing provision and by supporting its tenants in taking their own more proactive approach to their circumstances, particularly to finding jobs. The range of tenants has increased, such as rough sleepers and low-paid young professionals; the range of provision, such as live/work and workplace starter units; and the range of occupancy, such as shared equity and key worker housing. Support for its tenants in the form of advice, training and help in jobfinding has come with the development of Threshold Centres (eg Whitechapel Threshold Centre by Greenhill Jenner Architects, AJ 1.5.03) and the Digital Learning Ring - the makings of a virtual college for vocational education and job-finding. A community arts programme is being developed with the Royal College of Art.

More familiar to AJ readers will be recent projects exploring the potential of off-site construction, notably two schemes using prefabricated modules - Murray Grove by Cartwright Pickard Architects (AJ 25.11.99) and Raines Court by Allford Hall Monaghan Morris (Metalworks, AJ 25.9.03). Peabody is also working with Arup on the potential of volumetric concrete housing. This building study, of Peabody housing in Lillie Road, London SW6, demonstrates hybrid construction. It is predominantly of light-gauge steel panels, but also includes conventional hot-rolled steel framing and prefabricated bathroom pods (see structure box).

Beaufort Court is, of course, firstly housing rather than construction, providing a mix of 65 flats, maisonettes, single-family houses and Rough Sleepers Initiative dwellings (for those in transition between sleeping rough and coping in the open rental market) - (see table). The site that architect Feilden Clegg Bradley Architects (FCBA) had to address, formerly a school, demolished in 1998, is next to an existing site of four 1912 Peabody slab blocks, to the west.

The central focus of the site is a very large new landscaped court with a raised play area in the centre, for existing and new tenants.

This is raised half a floor with a car park hidden in a semi-basement below. FCBA has built a six-storey block of flats and maisonettes along Lillie Road, to the north of the site, matching the height of the 1912 buildings. To the south is a terrace of twostorey houses, kept low by their proximity to an adjoining conservation area. Completing the encircling of the court, to the east, is a block comprising a tenants meeting suite with rough sleepers' accommodation above.

Sited at the level of the play area, this mid-height volume makes an important transition between the tall flats and the low houses. The large size of the courtyard also means that the flats and houses are read, to some extent, separately, like buildings in adjacent streets, rather than the one overshadowing the other. However the form, materials and details do make the scheme all-of-a-piece.

Both Murray Grove and Raines Court have the modular rhythm of stacked boxes with a deck-access structure attached (at the back at Raines Court), though to be fair, not greatly more so than many a site-built block of flats. Here, FCBA has used externally expressed hot-rolled steel framing to vary the unitary scale, opening up larger areas of glazing at the stair towers, as well as supporting cantilevered balconies. (As FCBA's Julian Gitsham points out, the rooms between these structural cores could still have been modules. ) The effect is to allow what is a large block to be more effectively scaled.

Despite the expanses of glass, the palette of materials is dominated by red terracotta, echoing the red brick of the 1912 blocks, built as stack-bonded terracotta block walls for the two-storey houses and for the lower two floors of the six-storey block, which are maisonettes. With their matching mortar, these walls look a bit featureless. But the three storeys of flats above, set back on the court side with terrace or balcony, are clad with a more animated, grooved terracotta-tile rainscreen. The top floor, set back again and grey-rendered, chimes with the pitched roofs of the 1912 blocks. From the flats' balconies looking across the court, tenants see flat sedum roofs on the other new housing. (Balconies facing north onto Lillie Road are very shallow, more gestural than functional. ) For the flats/maisonettes, their entrance cores, with balconied lift entrances, are large enough to allow standing around and chatting. Dwellings are simply planned. The main planning effect of prefabrication has been that, because the form of construction was not decided at the outset, a planning module width of 4.2m was adopted, in case the design team subsequently chose modular construction with its limits on transportable size of units. When panel construction was eventually chosen, they were already committed to the 4.2m module for planning, a dimension that panels could exceed.

This 4.2m constraint is most noticeable in the four-bedroom houses, which are long and thin (three storeys were not permitted because of the adjacent conservation area).

The fourth bedroom is placed at the front on the ground floor and rooflighting introduced to bring light to the heart of the plan - the upper-floor landing and lower-floor kitchen. It works, though the kitchen feels a bit corridor-like.

FCBA's long-established focus on sustainability is carried through into this scheme, the panel construction allowing high levels of thermal and acoustic insulation (wall U-value 2.0W/m 2K), backed up by good airtightness and passive stack ventilation (fan-assisted in the six-storey block).

There is the potential too to deconstruct and reuse much of the structure (see environment box). This form of construction is relatively quiet for neighbouring residents.

The team feels that the construction method contributed to the 150,000 manhours passing without a reportable accident.

Another innovative feature of this project was the use of PPC 2000, the project partnering contract; a transparent way of working with shared responsibilities among the professional team and some of key subcontractors who were prepared to adopt it.

As FCBA's Julian Gitsham says: 'There is no benefit in being awkward.' The onus is on everyone to sort out problems, to share risks.

The contractor only gets overheads for extensions of time.

The process was led by client representative Walker Management. Main contractor Llewellyn ROK had the most prior experience of partnering, but there was generally a lot of learning by doing. To start with, there were many meetings with many people present, until trust was established and people not at meetings did not feel left out. Gitsham thinks it would work much better a second time around, preferably with the same people (though so would many a successful project). You cannot make people change overnight, but the client's choice of this contract did set an agenda of expectations.

The project came in close to the agreed maximum price, with the programme overrunning very slightly at 70 weeks.

It is suggested that prefabrication saved some 16 weeks on the contract duration.Next time there could be more standardisation; for example, despite the time taken in prototyping the bathroom, there were 17 variations on the pod between the 65 dwellings.Generally, a hybrid construction approach does provide considerable design flexibility. And such light-gauge panel construction could go up to 9-10 storeys, suggests structural engineer Michael Eatherley.

FCBA inherited this 0.53ha site with planning permission for 40 low-rise units. It has built 65 units, at 365 habitable rooms/ha.

They are at appropriate scales for the surroundings of the site, respond to the complex housing mix required, yet create a scheme with its own coherence. The significant new shared court provides both old and new tenants with a landscaped prospect and an enclave away from the busy road. The construction method is a means to an end; the end is well done.

Structure

Michael Eatherley and Nirupa Perera, Michael Barclay Partnership

A key project aim was to introduce a significant proportion of off-site fabrication. Three forms of construction were investigated: conventional masonry crosswalls with precast concrete floor planks; room-sized volumetric units in steel or timber; and light-gauge steel wall and floor panel construction. Early analysis suggested all were feasible with little difference in cost. However, the two prefabricated types offered other significant benefits. One reason for opting for light-gauge steel panels was that Peabody had already experimented with volumetric construction at Murray Grove (AJ 25.11.99) and Raines Court (Metalworks, AJ 25.9.03).

Another was that it provided a high level of flexibility in architectural form.

More precisely, the constructional system adopted is a hybrid, comprising prefabricated light-gauge steel panels for floors and walls, volumetric units for the bathrooms (high value-added) and hotrolled steel for balconies and other exposed steelwork. Advantages of this approach include:

a fexible and bespoke construction; in particular, the hot-rolled steel elements allow large openings and extensive balconies;

high sound insulation in party walls and floors;

low weight, resulting in economies in transfer structures and foundations;

minimising transportation costs - direct and environmental;

pre-decked floor panels provide safe working platforms;

predominantly dry and clean construction;

tight tolerances with ±2mm achieved in finished panels;

improved construction quality generally;

substantially faster construction;

ability to dismantle and recycle in future.

The focus here is on the six-storey block.

The light-gauge elements are cold-rolled from thin galvanised, steel strip. Because of the thinness of the material, local buckling is an important consideration in design and makes careful detailing and close attention to tolerances particularly important.

Walls and floors are constructed in the factory as welded panels composed of light-gauge steel elements. The bathroom pods are also structured in light-gauge steel and designed to be an integral part of the structure. Generally balloon frame construction is used, meaning that floor panels are framed into the sides of the wall panels.

Panels and pods are sized to fall within the normal vehicle load restrictions for an inner London site. Wall panels are generally one storey high by 8m long with 100mm deep steel studs spaced at 600mm. Studs are doubled up at lower levels where loads are highest and hot-rolled steel is occasionally incorporated for facade and balcony support. Party walls are built as pairs of panels for acoustic separation. Lift shafts are framed in hot-rolled sections, at the request of the lift supplier. Floor panels are around 4.1 x 3m with factory glued and screwed decking. Floor studs are 250 deep, also spaced at 600mm.

Fully braced crosswalls provide stability front-to-back, combined with decked floor panels acting as diaphragms in both directions. Along the block the spine wall is braced at the east end, coupled with braced lift cores. In a few cases special connections are needed to tie the frame to the foundations against wind uplift.

Vertical and lateral ties are provided to all structural components to prevent disproportionate collapse in the event of an accident.Robustness was also explicitly demonstrated by modelling the behaviour of the structure with key elements removed.

The hot-rolled external steelwork moves independently of the internal structures to allow for differential temperature-induced movement. To ensure longevity, sliding connections are created with stainless steel stud bolts fixed through stainless steel plates with slotted holes.

Erection began at one end of the block (for the houses too), completing a third at a time to allow following trades to proceed.Within each third, erection generally proceeded outwards from the cores, with the internal hot-rolled elements fully integrated in the process. Panel-to-panel and panel-to-hotrolled steel connections were made with self-drilling screws.

After an initial learning period, subsequent stages became increasingly straightforward and the project was completed to time and to budget.

Environment

Guy Nevill, Max Fordham

The sustainability strategy adopted for this project aimed for maximum environmental and social benefit with minimal cost to the project. As with many housing schemes, the budget was tight. It was recognised that designing sustainable measures into the fabric of the building would be more likely to survive future rounds of value engineering.

The method of prefabrication chosen enabled high levels of insulation and air-tightness to be achieved, thereby reducing heating loads. The Peabody Trust had made it a requirement that individual gas-fired boilers within flats should not be used due to ongoing problems the trust had experienced with access to properties for annual maintenance.Various heating strategies were considered to minimise energy use, including central plant and solar hot-water heating. However, the budget constraints eventually resulted in our using electric storage heating in the flats and individual gas-fired combination boilers in the houses.

The lightweight form of construction could be prone to overheating during the summer. The facade was carefully considered to reduce this possibility: balconies provide shading to windows below and elsewhere low-transmission glazing is used in potential problem areas. Secure window opening is provided throughout to enable day and night-time ventilation without risk of intruders.

Use of prefabricated bathroom pods encouraged the team to minimise the services installation. Kitchens and bathrooms are arranged back-to-back, with water cylinders located in the adjacent hall cupboard, thus limiting ducts, pipework and drainage, and therefore cost. Factory conditions are better than those on site and the potential for wastage of materials there much reduced, so the lion's share of the services installation was carried out in the factory.

On-site services installation was limited to the main risers and simple connections to and between the pods.

Compact fluorescent light fittings were specified throughout.

Extract ventilation is by passive stack, assisted in the flats with continuously running variable volume roof-mounted fans connected to humidity controlled extract grilles in kitchens and bathrooms.

CREDITS

TENDER DATE Fourth quarter 2001

START ON SITE DATE March 2002

CONTRACT DURATION 72 weeks

GROSS INTERNAL FLOOR AREA 5,323m2

CONTRACT/ PROCUREMENT PPC 2000/negotiated

TOTAL CONSTRUCTION COST £7,494,790

CLIENT Peabody Trust

ARCHITECT Feilden Clegg Bradley Architects: Keith Bradley, Julian Gitsham, Jillian Jones, Helen Roberts, Richard Marks, Zoë Fudge.

CLIENT REPRESENTATIVE/ COST CONSULTANT/ PLANNING SUPERVISOR Walker Management STRUCTURAL ENGINEER Michael Barclay Partnership

M&E ENGINEER Max Fordham

LANDSCAPE ARCHITECT Grant Associates

PANEL SPECIALIST The Forge Company

CONSTRUCTOR Llewellyn ROK

ENERGY CONSULTANT Rickaby Thompson

SUBCONTRACTORS Steelwok Littlehampton Welding; cladding Baris UK; mechanical JS Wright;

electrical Bargad (Electrical)

SUPPLIERS Argeton rainscreen cladding, terracotta blocks James & Taylor; windows Velfac; core glazing Vista Brunswick; insulated render Alsecco; door sets Polynorm Brunzeel; roof, green roof Erisco Bauder

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