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Spirit of adventure

BUILDING STUDY: Birds Portchmouth Russum's innovative footbridge across a busy road in London's East Ham joins together two halves of an existing girls' school, bringing a much-needed presence to this down-at-heel part of London

Birds Portchmouth Russum is the last practice one would look to for minimalist design, and the bridge which it completed last year in Plashet Road, East Ham, is, indeed, anything but a shrinking presence in the relatively humdrum landscape of this corner of the borough of Newham. As you turn the corner from the Tube station it hits you in the face, a snaking white tube, carried on angular blue-painted steel legs, striding across the busy road. In term time, you are quite likely to glimpse the faces of a group of schoolgirls peering down at you from a glazed bay in the centre of the structure, waving to surprised passengers on the top deck of the buses which pass below.

The Plashet bridge certainly merits an expedition into the further reaches of the East End. It has already picked up a string of awards and is lined up for more. But, as Mike Russum of BPR ruefully explains, the commission came along in the aftermath of a far bigger, now abandoned, project: for an architectural mise en scene for the major improvements to the A13 trunk road running through Newham.

After years of work, the A13 scheme fell foul of PFI-style funding arrangements. Disappointments of this kind have made Russum sceptical of the government's promises to fight for better public architecture, but BPR duly competed in (and won) the recent CABE-sponsored nursery school competition, without much hope that its investment in the process will see a serious financial return. Would the Chichester car park, the building which launched BPR's reputation in the late '80s, survive current methods of procurement? Anybody who doubts the relevance of architecture to such infrastructural projects may, however, find the case of the Plashet bridge worthy of study.

The practical brief was straightforward:

Plashet School for Girls (1,300 pupils, with a preponderance of girls from the Asian community) inhabits two buildings on either side of Plashet Road. One is the former girls' grammar school, completed in 1932 in depressingly institutional municipal NeoGeorgian style; the other a surprising 1960s work (now somewhat altered) by the maverick firm of McMorran & Whitby, which housed the local secondary modern in a striking eight-storey tower, whose generosity of scale, vision and precise detailing puts its clumsy pre-war neighbour to shame.

For many years, the pedestrian crossing on Plashet Road was the vital link between the two sites for pupils and teachers.

The obvious solution was a bridge, although the disposition of the two buildings - indeed, their lack of any obvious relationship, physical or stylistic - made it an awkward proposition. Unabashed, Newham's borough engineers weighed in with proposals in 1998 for a heavyweight straight steel corridor slung across the road.

The local planners were unimpressed and, in 1999, threw out the plans as a potential blot on the borough's landscape. Enter BPR, working with engineer Matthew Wells.

The first version of the bridge, its boldly curved form clad in stainless steel, with extensive areas of glazing, turned out to be too costly at about £750,000. BPR was asked to rethink the designs, with a ceiling of £450,000. The bridge was now much more than a functional connection. Firstly, it reflected a concern for the site, its increasingly sinuous form allowing it to bypass a fine mature tree which the previous proposal would have felled. Secondly, what would have been a grim experience, like marching along an Underground corridor, was turned into a potentially pleasant one.

Locals have reportedly compared the bridge to a wagon train. By cladding the structure in white Teflon fabric (with a life span of 25 years), held taut on a series of Tsection galvanised steel hoops - 'like a corset', Russum comments - the architects have created a light and cheerful interior. The curving plan means that entering the space is something of an adventure. At the centre, a steel-clad section contains two recessed bays, a break-out point, with windows looking out, a place where girls can pause briefly between lessons and swap tales from school. (Russum cites an unlikely inspiration: a Victorian bridge linking buildings at Marlborough School in Wiltshire).

The bridge is not a place to linger, as it is essentially an 'inside/outside' space, naturally ventilated - and the air wafting up from the street below is not especially fresh. (Anyway, Plashet School is a highly disciplined, motivated place where nobody wastes time. ) It could be compared to a cloister, a connector, but also a place to stop and think. It is made for hard wear and low maintenance - which is what it is likely to receive, judging from the battered and rather grubby state of the school buildings. The natural slope in the structure from east to west has been utilised to provide for draining of rainwater through hefty internal ducts - provision has also been made to incorporate IT and other services linking the two buildings.

In structural terms, the bridge is quite straightforward. Two main columns, formed of steel sections folded for strength, carry the weight of the steel decking, with pairs of subsidiary columns set close to the buildings providing ancillary support - the bridge is not structurally connected to either building. The original plan was to paint the structural members a bright red. The school wanted green. The choice of blue seems to have been a successful compromise - and it almost matches the blue of the girls' uniforms. By night, the bridge has a warm glow, provided by lighting neatly integrated into the structural system - standard fittings, welded back-to-back and given a connecting stalk, Russum explains.

The eventual cost of the 67m-long bridge was £530,000 - cheaper than the original, rejected proposal and something of a bargain for a structure which has enhanced the image of the school and eased the daily routine of its pupils and staff. (It has, for example, facilitated better disabled access to the 1930s block. ) Mike Russum sees its tough engineering as a reference to the (now lost) manufacturing traditions of the area - West Ham United FC, for example, located a short distance away, was founded as works team of a shipyard.

The Plashet bridge could be described as cheap and cheerful - neither adjective should be seen as derogatory. East Ham needs cheerful buildings and economy is a virtue in the eyes of public-sector clients.

But it is something more than this: the structure has already become something of a symbol not only for the school but equally for the surrounding area. Its exuberance and total rejection of the purely functional reflect BPR's maximalism.

The practice was, of course, born in the office of the late James Stirling, and the spirit of Stirling still infuses its work.

BPR has a view of architecture which is perhaps unfashionable - a vision of its potential to transform lives and landscapes and to create memorable works of art. In its modest way, the Plashet bridge is, indeed, a work of art for the inner city.

Structure

The design of the bridge combines the architectural and structural functions.Universal carriage beams 914mm deep were selected to suit both the span and balustrade requirements, with the bridge deck acting as the bottom flange and providing the torsional restraint required to accommodate the curved profile of the bridge.

Built at different times, the buildings are staggered in height and on plan. A natural transition is achieved through the S-plan form, which rises as a spline curve as it crosses the road.

The sinuous biaxial curvature of the bridge was achieved by firstly minor axis-bending the carriage beams to close tolerance on a 17m radius at Barnshaws. The beams were then precambered using heat - whereby the bottom flange of the beam is heated in sections along its length. Localised expansion due to heat is constrained by the surrounding cold steelwork, inducing stresses. On cooling the area contracts, shortening the bottom flange and inducing camber in the beam. Once the beam profile had been achieved, the plate of the deck box was welded into place, following the profile of the beams.

A viewing gallery slices through the carriage beams at midspan to form an interlude in the crossing at the centre point. The load path is followed through the seating platform by welded stiffeners forming a cranked beam within its depth.The reduction in stiffness attracts load to the supports, leaving a light transition at midspan.

Plate welding is the predominant theme in the steelwork design. All of the steelwork was fabricated in one of the traditional plate welding yards in North Yorkshire, where steel is still crafted and the welders have a feel for the material.Heavy-duty butt welds are part of the sculptural repertory of the design, allowing the steelwork to flow. Despite the number and variety of welded connections on the bridge, from full penetration butt welds of 32mm plate in the piers to 4mm fillet welds of 8mm plate for the bridge deck box, weld distortion was controlled throughout the fabrication process.

Each weld was carefully sequenced and strongbacks were used for the major butt welds.

The bridge is supported on sculptural steel piers which emerge as distorted fins from the ground and clamp the bridge on both sides.

A wedge is driven into the hollowcore tube to clamp the piers to the bridge.

Longitudinally, the bridge is fixed at one end and free to move along its length. Fabricated from welded 32mm steel plate, the piers are suitably flexible in their minor axis to accommodate the longitudinal movement while providing sufficient rigidity to resist the twist generated by the S-bend. Finite element analysis of the piers allowed us to explore the buckling behaviour of the plate and eliminate the need for stiffeners.

Construction

The design was conceived to enable the bulk of the bridge to be prefabricated and erected on site during the six-week school summer holiday.The bridge came down to London by road in three sections. The central span is 23m long, with the S-bend designed to suit the maximum transportable width of 4.5m.

All three sections were lifted into place in a single 24-hour road closure and the splices sitewelded to form a continuous sinuous curve.

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