Design Awards: 2010: Commendation

A40 Perryn Road Footbridge

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Architect

Grimshaw

Structural Engineer

Hyder Consulting (UK) Ltd

Steelwork Contractor

S H Structures Ltd

Main Contractor

Carillion Civil Engineering

Client

Transport For London

Perryn Road Footbridge is a high quality steel bridge designed to fit a highly constrained site and to meet challenging accessibility requirements. The bridge layout and architecture required careful consideration to successfully create a crossing that is attractive, safe to use and serves the needs of the local community and the client.

Land availability was severely restricted and compounded by numerous buried services that ran under the footway. Construction of the footbridge had to be undertaken within the constraints of the main contract with regard to working areas, working times and lane closures. The footbridge design allowed construction to satisfy the many conflicting and preexisting constraints and was programmed to minimise disruption.

The architecture provides a clearly understandable link through the “spiralling ribbon” formed by the stairs, ramps and main span parapets. The choice of finishes, architectural lines and structural arrangements successfully integrate the different elements of the crossing. The bridge supports cantilever over the footways with unique asymmetrical spiral staircases winding around and cantilevering from the supports. The ramps are as compact as possible, yet have a maximum gradient of 1:12 and regular landings. Expanded steel mesh screens on the ramps allow good visibility for users and protect the privacy of nearby houses. Both stairs and ramps were provided to cater for all users, and the arrangement allows common entry points for all, and features a bespoke lighting scheme to maximise the security of users.

The bridge supports are major steel fabrications having numerous functions but nonetheless are elegant structural elements. The main span and supports are fully integral, with full moment connections between the truss and supports. To ensure the safety of the crossing the supports were required to withstand the onerous requirements of full collision loading. Dynamic analysis revealed that the supports were important factors in the bridge harmonics. To provide the necessary stiffness and mass, but retain a slender profile, the supports were fabricated in 40mm steel plate and then concrete filled. The only viable foundation locations were extremely restricted areas off the back of footpath. Whilst increasing the main span, these locations avoided costs and risks of delays associated with significant service diversions. To utilise the available foundation areas the bridge supports were required to cantilever off concrete plinths/pilecaps well away from the carriageway. Large mechanical connections, utilising machined shear plates, dowels and high tensile bars, connect the bridge supports to the foundations and transmit significant ultimate limit state loads into the foundations.

The bridge truss comprises curved upper and lower chords that are connected by variously sloping straight bracing members. All the chords and bracing are circular hollow sections, and the sizing and truss geometry was carefully developed such that no stiffening or overlapping of members is required at joint nodes. The chord members are curved to parabolas, the top chords being in vertical planes whilst the bottom chords are in inclined planes to give the bridge deck an elegant vertical profile. Further subtleties to the truss design are that the cross frames are skewed to match the bridge supports; there are elements of symmetry in the truss; and the bottom chord returns in an elegant loop supporting short back spans and the top of the stairs. The structural arrangement of this integral bridge allows a shallow span/depth ratio such that the truss does not rise above the parapets. The bridge deck and guardrails form a continuous ribbon with the link spans, cradled within but distinct from the open tubular steel structure.

The bridge was transported to site, painted and erected as one unit. the bridge supports were erected and concrete filled. During a single night-time road closure, the first permitted on the A40, the lightweight main span was lifted into position and made fully integral with the supports, which had been concrete filled, using bolted connections. Construction of the stairs, ramps and footway works then continued and the bridge was partially opened in June 2009. The footbridge was fully opened in October 2009.

Judges Comment

Steelwork is used imaginatively in this high quality scheme for a very constrained and challenging site in a residential area.

Crossing one of London’s busiest arteries, the skewed deck on cantilevered supports, with spiral staircases and compact stacked ramps, provides access for all types of users.

A well considered and executed urban design produces a robust yet elegant solution.

Energy Recovery Plant, Corus Port Talbot

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Architect

Lazarus & Associates Ltd

Structural Engineer

Siemens Vai

Steelwork Contractor

Rowecord Engineering Ltd

Main Contractor

Rowecord Engineering Ltd

Client

Corus Uk Ltd

The new Energy Recovery Plant will allow the harnessing of the off-gas, rich in carbon monoxide, generated by the ‘Basic Oxygen Steelmaking’ (BOS) process as a source of energy for on-site power plants. To facilitate this new process, a large gas holder along with 3km of associated pipe work and two 70m high stacks, was needed.

Central to the scheme was the new 75,000m3 gas holder which has been constructed roughly equidistant between the power plants and the BOS plant. Approximately 2,700 tonnes of steelwork went into this large structure which is 63m high to the apex of its rooftop vent, 54m in diameter and erected from steel plates.

To avoid excessive working at height, and to minimise the use of scaffolding, the entire gas holder was built from the roof down – one ring of steel at a time using hydraulic jacks to lift the whole structure up to allow the next ring to be welded to the one previously installed.

Early works on site involved ground preparation and piling, prior to concrete slab and steel floor plate being installed. This then allowed the main steel erection programme to begin, with the uppermost ring of plate steel installed first. Each steel plate is 2.4m high x 11.3m long and one circumference – with welded connections – took seven days to complete.

Next the roof framework was erected consisting of a series of 600mm deep rafters which fan out to the perimeter from a central point that was temporarily supported on a trestle. The roof structure also includes steel bracing while all connections for this part of the structure were bolted. Once the roof was clad the structure was raised by a series of hydraulic jacks placed around circumference. The jacking process took approximately five hours and raised the entire steel ring 2.4m, which allowed another complete circumference of steel plates to be installed – this was repeated until the gas holder reached its full height.

As each successive circumference of steel plates was inserted the structure’s weight increased and so the number of jacks had to be increased from 24 at the start to 120 units at end. Likewise, the steel plates’ thickness also increased – due to extra loadings – from 10mm for the upper rings to 16mm thick plates at the bottom.

The final steel ring included a 6m wide access opening, which served the dual purpose of allowing the cherry pickers, used inside the structure’s footprint, an exit route and allowed the gas holder’s internal steelwork to be inserted. Internal steelwork was brought to site ‘piece small’ and once inserted was bolted together. This steelwork included the 18m high piston which when assembled weighed 480 tonnes. As gas enters the holder the piston, which rests on a lattice framework at ground level, rises to 1.5m from the roof when full. As the piston rises an attached rubber membrane, fixed to both the piston and the wall of the holder, unfolds and acts as a seal preventing gas leaks.

This is a project with environmental, cost and energy efficiency credentials.

Judges Comment

In a tough industrial environment, this large scheme shows how steelwork contributes to enormous environmental benefits.

Waste gases (previously flared to atmosphere) are now treated and transported via nearly 5 kms of large pipeline on complex steel support structures, to a large welded steel gas-holder for re-use. The challenges of working in an operational steelworks were huge. The project saves energy and Corus reduces atmospheric emissions

Structural steelwork on a large scale is key to the astonishing pay-back time of less than 3 years! This is outstanding.

Cathedral Bridge, Derby

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Architect

Ramboll

Structural Engineer

Ramboll

Steelwork Contractor

Briton Fabricators Ltd

Main Contractor

Dean & Dyball

Client

Derby City Council

Cathedral Bridge is a new pedestrian link and a cycleway which has also been designed to form a meeting place where people can sit and take in views of the surrounding area.

The new swing bridge had to respond to site constraints associated with a lowbanked, fast-flowing river that can rise quickly by up to 1m. The bridge provided a flexible structure low enough to integrate well with the flood-plain environment, swift enough in its mechanics to respond to water fluctuation as needed; its sharp steel profile evokes associations with the tools – scissor, needle – used throughout the local history of textile manufacture.

The basic form is derived from tailors’ shears, in particular the action of the hinged blades as they open and close. The bridge deck evokes one of the blades with the pivot along its length and the force to open and close the blade applied at the opposite end.

It was the client’s wish to re-instate the 18th Century mill race once used by the famous Silk Mill. This was achieved by designing a single bridge that could span both the river and the mill race in one economic structure. The bridge’s kinked back-span links across the mill race while also contributing structural efficiencies to the design, since it counterbalances the main bridge deck which spans across the Derwent. The 20m high needle-like mast is connected to the hollow box steel section bridge deck by three pre-stressed steel cables: these provide support along the deck’s thicker outer edge, keeping the overall structural depth slender. Operation of the swing bridge mechanism is effortlessly controlled by one person standing at the tail end of the bridge on the western bank, using a specially designed consol.

Feasibility analysis took prominence in early design phases as this complex bridge had to be tuned to function under variable loading conditions both in its open and closed positions, as well as in its transitional mode. During transition the bridge rotates about a vertical axis on a central massive cast steel pivot bearing located under the mast. The tail end bearing is supported along a concealed track concentric to the pivot point, and a roller mechanism concealed beneath the elbow of the bridge deck provides continual vertical support. The nose end of the bridge is entirely unsupported during transition. Projected deflection values were calculated to within a hair’s breadth, pre-cambering the structure for efficiency of construction. Robustness was built into the design to account for unexpected behaviour during the building programme, but the first swing of the bridge came in absolutely level.

Keeping the structure as lean as possible reduced the use of materials, all of which were sourced and fabricated within 15 miles of site. Ecological sensitivity was taken into account. The design is futureproofed to accommodate potential rises in water levels. Analysing the structure to achieve the slimmest possible steel deck depth meant it was possible to calculate a generous tolerance for floodwater rise.

Judges Comment

The skewed swing bridge provides a novel and successful solution to linking parts of the city which previously only had road bridges, hostile to cyclists and pedestrians.

The box girder deck is cable-stayed and slender, and the inclined mast is of Y-shape cross section. The fabrication, particularly of the mast, is of very high quality.

Steelwork shows its capabilities yet again.

The Rose Bowl, Leeds Metropolitan University

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Architect

Sheppard Robson

Structural Engineer

Arup

Steelwork Contractor

Fisher Engineering Ltd (Severfield-Rowen Plc)

Main Contractor

Bam Construction Ltd

Client

Leeds Metropolitan University

The Rose Bowl provides Leeds Metropolitan University with a landmark, easily identifiable building.

Central to the project is the Rose Bowl lecture theatre ‘pod’ which sits ‘half in and half out’ of the atrium and contains one 250-seat theatre, two 140-seat and four 60- seat theatres. As well as having an unusual oval bowl-like shape, tapering outward as it gets higher; the structure is clad in distinctive triangulated reflective glass panels fixed to a bespoke aluminium backing frame system. A series of bridges crosses a semi-public atrium linking the ‘pod’ to the outer U-shaped four-storey main floorplates, which house the offices and ancillary teaching spaces.

The building is built over a two to three storey basement car park which extends significantly beyond the footprint of the building. The sloping site introduces a full storey height difference across the building with access at ground floor into the central atrium beneath the ‘pod’, and at first floor directly into the building and on into the main lecture theatre. From ground level up the structure is steel framed to provide maximum flexibility to the floor plate. Cellular beams were used to allow the coordination of mechanical and electrical services into the void. Clear spans of 15 metres are found throughout the building with perimeter columns set back into the space to create a clean façade. The whole of the steel frame rises off the concrete basement columns and transfer structure located within the ground floor slab.

The outer four-storey faculty block and two-storey plant enclosure were erected in phases to facilitate access to working faces. The faculty building steelwork was initially erected up to third floor level and then used as a working platform for the operation of MEWPs to allow the safe erection of the remaining steelwork to the upper levels.

The pod structure itself consists of a series of Y-shaped feature columns connected to a circumferential truss, or diagrid, extending around the envelope of the pod. Temporary stability during pod erection was ensured by tying the circumferential trusses back to the four internal main columns, which were in turn temporarily tied together to form a central braced core.

The ring of Y-columns at the base required a 100 tonne mobile crane, as each of these columns, brought to site as complete pieces, weighed around 5.5t. The first column to be installed was held in position using the crane, and then the adjacent columns were installed using a second crane and tied together with a beam to provide stability. Once the first two columns were installed and stabilised, the entire ring of columns was completed by tying them into adjacent members and the central braced core.

Once the central core and ring of Y columns were in place, the steel diagrid and floors were erected using the temporary core to control the position of the diagrid and provide stability. On completion of the diagrid shell to third floor level, the concrete deck was poured and the pod became self stable. The remaining diagrid and floors were then installed and the temporary bracing removed.

Fabrication of the Y-shape columns and diagrid members required extremely high levels of accuracy to achieve the necessary tolerances. The Y-shaped columns are complex sections fabricated from individual plates of varying thicknesses of between 20 and 55mm. Each “Y” consists of three tapered branches of triangular section which increase from approximately 350mm in depth at their tips to about 450mm at the central node.

All of the individual plates were blasted before fabrication with the completed assemblies blasted again prior to painting. All steelwork then received intumescent fire protection, including the Y columns to ensure that the optimum level of fire resistance was achieved.

Judges Comment

This university development has three teaching blocks, but the interest is in the splendid ovalshaped structure in the centre.

Tapering outwards as it rises, the Rose Bowl’s frame is supported by Y-shaped two-storey triangular columns, which have been designed and fabricated with crisp corners. The overlying storeys are carried by a vertical diagrid, with elegant bolted connections.

The careful detailing and fine fabrication are commendable.