Design Awards

Wimbledon No.1 Court

© AELTC/Simon Bruty

Architect
KSS

Structural Engineer
Thornton Tomasetti Ltd

Steelwork Contractor
Severfield

Main Contractor
Sir Robert McAlpine

Client
The All England Lawn Tennis Club

Constructed over a three-year period, the steelwork element of the re-development of the No.1 Court included the demolition of the existing roof built in 1997, infilling and extension of the level 4 and 5 floorplate steel and new fixed roof, followed by the construction of a moving roof that will provide full rain cover during the Championships.

The main client requirement was that the works to the No.1 Court would still allow the Championships to take place with a minimum of disruption. This resulted in one of the more interesting aspects of the build and necessitated the development of a staged site programme that incorporated into the working methods and temporary works the appearance and character expected of the All England Lawn Tennis Club.

Other objectives for the design were to extend the terraces to maximise the seating within the existing structure and provide new, enlarged and improved debenture/hospitality facilities, whilst optimising grass growing conditions throughout the year both on the No.1 Court itself and the adjacent grass courts. In addition, the movable roof had to be deployed, and the internal environment conditioned, both to eliminate condensation on the playing surface and provide a comfortable spectator environment, all within a 30-minute stoppage of play.

The structure for the east and west sections of the static roof is largely symmetrical. There are long span prismatic trusses spanning 47m between the existing cores, and 80m between new supporting columns installed in the corners. The exception is the NW corner, where the truss is supported on another transfer truss spanning between the cores. The top and bottom chords follow the profile of the cladding surfaces and are large tubular sections. These main elements provide sufficient vertical stiffness to support the weight of the static and moving roofs, good lateral stiffness to resist the lateral forces imposed on the structure from both wind and the moving roof, and create a large open internal space within the truss that facilitates the integration of the MEP equipment.

A secondary structure provides support to the rails that carry the moving roof. The inclination of the prismatic trusses, and the positioning of this secondary structure, enables the load from the moving roof to be located over the bottom chord of the prismatic truss for structural efficiency.
The southern roof utilises two planar trusses spanning across the width of the bowl. The inner truss spans between a planar truss in the corners which transfers the load back to the new corner columns and the existing cores. The rear truss is located such that it is spanning between the cores. This arrangement allows the inner roof structure to cantilever out over the bowl, with the forces being resolved as a push-pull on the two trusses.
The north quadrant of the static roof differs from the south as it carries a large quantity of plant. To achieve the necessary structural strength, the last moving roof truss, that is actually always static, has been incorporated into the static roof structure. This truss is much deeper than the southern roof trusses, and therefore offers a more efficient structural form.

The retractable roof for No. 1 Court is a concertina system using fabric panels tensioned between long span triangular trusses, which in turn span between the east and west sections of the static roof.
The triangular trusses, approximately 6.6m-deep and 1.5m-wide, span in an east-west direction over the court, and are supported on bogeys that run on rails supported by the east and west primary prismatic trusses. Bearings on the bogeys at each end allow thermal expansion and contraction in the plane of the truss and the leading trusses of the two halves have bearings on the east side that are fixed in the east/west direction to provide some stability to the group.

The complex geometry and heavy loading led to the development of jigs that were used in the workshop to achieve the requisite accuracy of fabrication of highly complex connection assemblies, some of which included up to twelve incoming members to a single node.

On site, not only were the existing cores used for permanent support of the fixed roof steelwork, they were also used as temporary support for the four tower cranes used during construction, for which they were temporarily post-tensioned.

The use of structural steelwork, with offsite fabrication and rapid site assembly, allowed the site construction to take place in the windows between Championships. Further, the spans required for the roof trusses and the stiffness needed to control the movements at the critical interfaces between the fixed and moving parts, precluded any form of construction other than steel.

Overall, the structure has been designed to be in keeping with the existing structure, whilst including up-to-date technology. This has allowed the architectural aspects of the existing site to be maintained.

Judges’ comment

Installing a moving roof over No.1 Court involved the adaption of the 1997 building without interrupting the annual tennis Championships. This extraordinarily complex work was carried out over three seasons with minimum public awareness. Large movable steel trusses installed to very exacting tolerances over the existing building provide a roof that can shelter a match from rain within minutes.

London Bridge Station

© Rick Roxburgh

Architect
Grimshaw

Structural Engineers
Arcadis WSP JV

Steelwork Contractors
Cleveland Bridge UK Ltd and Severfield

Main Contractor
Costain

Client
Network Rail

As part of Network Rail’s London Railway Upgrade Plan, London Bridge Station is undergoing a stunning transformation that will deliver a better experience for users and a reduction in delays. It will also ensure greater connection between London’s home counties and increase passenger capacity by two-thirds.

The station transformation includes an enlarged street level concourse underneath the tracks, new entrances and new platforms for more trains, and three of the nine terminating platforms converted to through platforms. The concourse is set to be one of the largest in Europe.

The rolling redevelopment programme started in 2012 and has been scheduled in such a way as to ensure the station remains open for business at all times. On 2nd January 2018 the final section of the massive new concourse and five platforms opened to the public, with the remaining redevelopment works to be completed in the spring.
Elegant curves are integral to the station’s design and respond to the track geometry and curvature of the site. Steel is the natural material for the project as it allows the necessary design flexibility. It also offers sustainability benefits as it is recyclable and lightweight.

All 15 platforms have been rebuilt to be covered by a striking undulating canopy of steel and aluminium, fabricated and installed by Severfield. The eye-catching canopy roof is modularised using open sections where each module is approximately 9m deep by 3m wide. There are an astonishing 1,200 prefabricated steel cassettes, with each one a bespoke unit due to the changing rooftop geometry. To save time cassettes were prefabricated offsite and then craned into position, allowing the canopy to be built during short night-time construction hours.

The canopy structure comprises Y-shaped columns supporting a longitudinal spine beam formed from fabricated box sections that have extended webs to create service routes. Platforms and canopies sit outboard of the bridge girders, supported on transverse ‘elephant ear’ frames, and as trains pass over the bridges any deflections cause the tips of the ‘elephant ears’ to move longitudinally. The plates that connect the frames to the bridge girders are designed to balance strength and stiffness to resist the applied loads, while remaining flexible enough to avoid fatigue.

The centrepiece of London Bridge Station is the concourse which is nearly 80m wide. There is also an expansive central space at the heart of the concourse which deals with the level changes across the site. The large span of this space was achieved by using a longitudinal V-column to support a 5m deep Vierendeel truss, and this allowed for glazing between the vertical members to form the rooflights above.

Cleveland Bridge supplied steelwork for the rail bridge decks spanning the new concourse. The work has included fabrication, trial erection at the company’s Darlington facility, painting, delivery and installation.

The concourse bridge decks are made up of three to four spans of simply supported decks for each rail line. Each rail bridge deck comprises six main girders braced together and tied at the ends with trimmer beams, delivered and erected as pairs.

Following installation the beams were mass filled with concrete and fitted with platforms, rail lines and canopies.

The main plate girder lengths (spans) were such that no longitudinal splices were required. After fabrication all components were placed in pairs together for a trial assembly to ensure perfect fit and alignment, de-risking the operation on-site. Upon completion of the trial erection, the deck was separated into component pairs ready for dispatch to London.

The main logistical challenges for the project were the severely restricted site access; a requirement to consider scheduling for follow-on trades, and the essential need to keep the station fully functional. The architect Grimshaw designed the station and complex staging process based on the concept of prefabrication and modular offsite construction. This reduced the pressure on the construction programme and again the use of steel was advantageous. For the installation of the decks and canopy the project was split into six phases.

The possessions for working were ‘Rules of the route’ (very short windows when trains are not running) synchronised with restricted short possessions for delivery vehicle road closures. The entire project took place in a busy city centre location with narrow streets through which to move delivery vehicles, large plant and equipment.

The lifting schemes for all steelwork installations included the innovative use of heavy capacity scissor lifts mounted on the top of Self-Propelled Modular Transporters (SPMTs) to solve access problems.

The aim of Cleveland Bridge’s work was to maximise the level of offsite fabrication and preparation to significantly reduce the on-site programme. As the station was operational throughout the project, health and safety was paramount and the overall project was delivered within budget and ahead of schedule, exceeding the client’s expectations.

Judges’ Comment

The project has produced a major upgrade to the existing station, which remained operational throughout. Collaborative offsite manufacture minimised disruption during the project. The use of steel has allowed the design team to create open concourse spaces beneath the tracks and elegant curves to the canopy structures above. The project is a great example of ‘designing for construction’.

The Leadenhall Building, London

© Paul Carstairs/Arup

Architect
Rogers Stirk Harbour + Partners

Structural Engineer
Ove Arup & Partners Ltd

Steelwork Contractor
Severfield

Main Contractor
Laing O’Rourke

Client
C C Land

The Leadenhall Building is a 224m high steel-framed commercial office tower in the City of London. In order to meet the client’s aspiration for a landmark tower on this sensitive site, the architects proposed a wedge-shaped building. This produced the highest office floors in the City, while minimising the impact on a cherished view of St Paul’s Cathedral.

The use of steel is fundamental to the value of this building. It is visibly integrated into the architecture to an extent that is highly unusual for a skyscraper, creating a powerful tectonic quality which enables people to appreciate and take delight in the way that the building is constructed.

Panoramic lifts were placed on the vertical north elevation so they could serve all the office levels. As a result there is no central core, and stability is provided by the perimeter braced steel ‘megaframe’ placed outside of the building envelope.

This steel design allows the floors to be exceptionally open, with views in every direction and spans of up to 16m, so that there are only up to six internal columns within floorplates of up to 43m x 48m, making them very flexible and attractive to tenants.

At the bottom of the building, floors are cut away and hang from the levels above, creating a vast open space, the ‘galleria’, which connects and relates directly to the surrounding public realm, regenerating the local environment and creating new pedestrian routes.

The architects wanted the building to express its engineering systems wherever possible. This significant challenge demanded a holistic and creative approach, with the engineers and architects working closely together from the outset. The most striking example of this is in the ‘megaframe’. Alternative bracing arrangements were proposed, studied and then optimised, leading to an arrangement that is both structurally efficient and architecturally coherent. Vertical columns are provided where they are most needed, on the east, west and north faces, and a diagrid structure on the more lightly- loaded south face. Connections are made through a family of separate fabricated node pieces. This ensures that the complex geometrical relationships between members are always resolved within welded joints and the site connections remain simple and standardised.

Within the ‘galleria’, floor beams are exposed, enhancing the character of the space. At level 5 these project beyond the ‘megaframe’ to form a canopy over Leadenhall Street. The levels below are suspended via hangers whose bespoke end connections provide a seamless transition between the rods and the supporting steel beams.

The ‘megaframe’ columns and braces around the ‘galleria’ are unrestrained over a height of 28m. Standard ‘megaframe’ sections are therefore subtly adapted, with tapering webs and additional stiffening plates, to significantly increase their buckling resistance without undermining the node connection principles or aesthetic proportions.

Steelwork is corrosion protected and fire protected where required. In external areas, epoxy intumescent coatings are employed for durability. Cast intumescent caps were placed over the ends of the ‘megaframe’ fasteners to preserve the ‘nuts and bolts’ aesthetic.

The most complex steelwork details were developed in workshops based around Arup’s Tekla BIM model. The model fed directly into the procurement process where it was used to explore the construction methodology and co-ordinate the temporary works. It also fed directly into the steel fabrication models, driving automated shop processes.

80% of the building was constructed offsite, reducing waste and improving quality, safety and programme.

All wet concrete was eliminated above level 5 by replacing conventional composite floor slabs with an innovative precast concrete panel system. The panels have pockets which enable dowels to be installed into the neighbouring units via cast-in couplers to provide diaphragm action. These dowels pass through circular openings in shear tabs pre-welded to the tops of the steel beams, to provide the required shear connection.

The primary steel system within the north core was built as a series of storey-high tables, with the services and concrete floor slabs pre-attached to them, minimising the number of crane lifts required.

The building was predicted to move sideways to the north during construction. An innovative approach was deployed to counter this, known as ‘active alignment’. The structure was initially erected straight and movements regularly monitored. At a later point, adjustments were made to the ‘megaframe’ diagonals which pulled the building back sideways, reversing the gravity sway. This allowed the ‘megaframe’ nodes to be fabricated with a simple orthogonal geometry and improved the overall accuracy of construction.

The Leadenhall Building provides the public with a unique and dramatic new space at ground level, offers tenants some of the most desirable office spaces in the City, and forms a sensitive and elegant addition to London’s skyline.

Judges’ Comment

This project had a committed client, architectural and engineering excellence, fabrication precision and construction ingenuity and innovation. They all combined to make a project whose achievements are even greater than the sum of the parts.

Structural steel is rigorously controlled to generate an architecture that is clear and legible throughout the building. Like most ground-breaking projects there were lessons to be learned, but the client and the team persevered to achieve final success.

This world-class project is an exemplar for large commercial buildings.

London Olympic Roof Conversion

London Olympic Roof Conversion

Architect
Populous

Structural Engineer
BuroHappold Engineering

Steelwork Contractor
William Hare

Main Contractor
Balfour Beatty Major Projects

Client
London Legacy Development Corporation

When the London Olympic Stadium was designed it was with an ethos of ‘embracing the temporary’ in the knowledge that, post- Games, its function would change and, as a result, the structure would need to change too. Such foresight paid dividends when it was announced that the stadium would become the new home of West Ham United football club at the beginning of the 2016/17 season.

One of the main stipulations for the future use of the stadium was that it would retain its running track. To prevent this from adversely affecting the atmosphere at football matches, an automated system of retractable seating was included in the new design, with all four sides of the lower bowl able to move over the running track when in football mode. To meet UEFA rules, the roof needed extending to fully cover the retractable seating.

Work began on the project to transform the venue in late 2013. The new structure included 8km of steel cables weighing 930 tonnes, 112 steel rafters, 2,308 purlins, 422 struts, 9,900 roof panels and 14 light paddles each weighing 43 tonnes, with the whole structure weighing in at around 4,700 tonnes which is nearly six times the weight of its predecessor.

In order to preserve some of the Olympic Stadium’s identity, the iconic triangular lighting tower design that used to stand over the old roof has been inverted and they now appear to hang underneath the new larger roof.

Early works involved the deconstruction of the old roof and the strengthening of the existing structure, foundations, V-columns and the perimeter compression truss.

Strengthening of the existing structure was one of the major challenges. Due to the additional weight of the new roof, it was necessary to replace and/or strengthen the existing V-columns and significant strengthening works were carried out to the existing compression truss.

For the compression truss strengthening work alone the amount of hierarchical complex calculations involved the steelwork contractor developing his own in-house software to process over 10,000 calculations – a task that would have been impossible using traditional methods.

The ambitious new cantilevered roof now stands as the world’s largest with every seat in the stadium now covered by the new roof.

The 14 new lighting paddles are positioned beneath the new roof. Each lighting paddle houses up to 41 lamps, many of which are the original lamps that shone over the stadium during the London 2012 games. Four 600 tonne capacity cranes operated in tandem to lift the lighting paddles and the other roof members into position.

The tolerance in the fabrication and quality of finish was expected to be very high and the design was made with security in mind. Most of the geometry was complex and specialised jigs were manufactured to fabricate some of the complex tubular nodes. A total station was employed to set out all of the brackets for the lighting paddles which all lean towards the pitch and are all slanted in three opposing planes.

Not least, the oval shape of the stadium and the movement and tolerance requirements only gave the opportunity for single pieces to be replicated twice, which meant that half of the stadium structure was fabricated with unique members.

Following the V-column and compression truss strengthening work, to maintain equilibrium until the oval was fully formed the erectors worked in two teams at opposite ends of the stadium working in a clockwise rotation constructing the back roof first, then the front roof complete with the lighting paddles and walkways.

To ensure the correct distribution of forces through the cable support structure to the compression truss, the front and back roof are completely independent of each other. However, for the installation of the lighting paddles, the front roof had to be temporarily tied to the back roof to ensure that the lighting paddles did not overturn until the full ring stiffness of a complete oval was achieved.

4D programming using BIM modelling was the key to delivering this successful project to a very high profile deadline, which was originally the 2015 Rugby World Cup taking place in September 2015. However, this was brought forward even more to fit in the Sainsbury’s Anniversary Games which took place in July 2015. This meant that all major construction had to be complete by May 2015.

The new structure now has a lifespan of over 60 years and is set to become the new national competition centre for UK Athletics, and in 2017 will host the IAAF World Athletics Championships and IPC World Championships. The stadium has already hosted five games of the 2015 Rugby World Cup and motor racing’s 2015 Race of Champions.

The stadium has also been upgraded to a 54,000 all-seater UEFA category 4 football stadium, which is the highest category of football stadium possible in the world.

Judges’ Comment

The need to modify the roof and seating of the 2012 Olympic athletics stadium to accommodate a multi-purpose sports venue posed formidable challenges. The geometry and behaviour of the original structure were very complex but, with extremely detailed study and fine engineering skill, most of the original elements have been re-incorporated.

The challenges have been met superbly and the project is a triumph for the team and for structural steelwork.

City Centre Bus Station, Stoke-on-Trent

City Centre Bus Station 1

Architect
Grimshaw

Structural Engineer
Arup

Main Contractor
Vinci Construction Ltd

Client
City of Stoke-on-Trent

Stoke Bus Station has a modern and inspirational design that reflects the character and landscape of the surrounding town.

The canopy of the station is an eye-catching and integral part of the design, protecting passengers from the elements, whilst facilitating wayfinding and creating a real sense of arrival and place.

The curved aluminium-clad roof wraps around the perimeter of the site to enclose a glazed pedestrian concourse providing a total of 22 bus stands.

The steel frame resolves what appears as complex geometry in an efficient manner. It is set out as a panoramic section utilising repetitive detailing. Maintaining a 5m clearance to the west, the steel frame expands and contracts as the concourse rises to the north. Details at junctions were designed to allow flexibility of the frame connection enabling ease of erection and simplifying manufacture.

Judges’ comment

This carefully considered scheme will be a catalyst for future urban regeneration. Located on a major roundabout, its striking curved roof form is supported on steel ‘V’ columns, with a palette of materials including glass, aluminium, timber and steel.

This demonstrates how good design can lift the spirits.

Taplow Riverside Footbridge

© Anthony Prevost

Architect
Knight Architects

Structural Engineer
COWI

Steelwork Contractor
S H Structures Ltd

Main Contractor
Land & Water

Client
Berkeley Group

Taplow Riverside Footbridge is the latest crossing over the River Thames. Opened in November 2018 by the Prime Minister, Theresa May, this 40m arch bridge crosses the river at a site with very limited construction access and in an area of natural beauty.

Since Victorian times, Ray Mill Island, located in the River Thames between the town of Maidenhead and the village of Taplow has been a popular attraction for both locals and tourists. However, access to the island has always been restricted by local geography. The redevelopment of the derelict 48-acre Taplow Paper Mill site by Berkeley Homes offered the opportunity to connect these communities and allow the riverside to be once again open to all.

An initial proposal for a truss-type footbridge was criticised by local residents, who felt the design was ‘industrial’ and out of character with the area. This prompted Berkeley Homes to appoint Knight Architects and COWI to develop a slender and elegant design to suit the natural beauty of the site and enhance user experience whilst still being cost-effective.

The steel structure was designed with structural efficiency in mind but allowing a clear architectural identity to be developed comprising three key features; the arch, deck and flat plate hangers.

The slender arches drew inspiration from the nearby Maidenhead Railway Bridge by Brunel, famously employing the ‘flattest brick arches in the world’. The arches are triangular in cross section and lean outwards to produce a dramatic visual effect, opening up the views from the bridge of the river and surrounding landscape. The arch geometry was devised to avoid double curvature to facilitate fabrication from steel plate, and the triangular shape results in a much greater transparency, emphasising the slenderness of the design. The white colour of the arch was chosen to stand out from the natural backdrop, whilst adding to the slender appearance and giving an attractive profile along the river.

The arches support a slender composite steel-concrete deck formed by a steel tray comprising the edge beams and bottom plate, which was filled with in-situ concrete after the bridge was installed. Transverse stiffeners are revealed below and extend outwards to form the hangers. This composite construction results in improved structural behaviour, particularly from the point of view of dynamic response and acoustics, but also facilitated easy construction. The bridge dynamic response is clearly perceptible but within acceptable limits as defined by the Eurocode.

The flat plate hangers, with expressed pinned joints at their ends, provide lateral stability to the arch, enabling greater slenderness than if wire strands had been used. To maintain the desired transparency achieved by using thin plates, high strength steel was used for the shortest hangers which experience out-of-plane moments created by relative longitudinal movement between the deck and the arch.

The parapets continue the theme with closely spaced thin steel uprights welded to the deck supporting the timber handrail, maximising transparency when viewed from upstream or downstream. The dark grey of the deck and parapet contrasts with the white arch to accentuate the visual slenderness of the bridge.

The site for the bridge presented numerous challenges. Access was impeded to the west by a 1.5 tonne capacity timber bridge, to the east by a boggy marshland and the Berkeley Homes construction site and to the north & south by several protected trees on the riverbank. The only viable access route left was the river.

Offsite fabrication enabled a high quality of finish to be achieved and allowed for a trial assembly, ensuring a more efficient on-site build. The structure was transported in three parts by road to an assembly yard a short distance downstream of the bridge site. The bridge was built up on temporary works enabling a high quality of workmanship and dimensional control to be achieved before the entire steel structure was then lifted onto a pontoon, floated upriver and installed using hydraulic jacks in a single well-planned operation.

The ends of the arches have a simple end plate detail, with a knuckle arrangement which allowed rotation during concreting of the deck to reduce bending effects and minimise plate thicknesses. This was then cast into the abutment on completion to act as a fixed end for live load effects, avoiding the use of a bearing at these points for a maintenance-free solution which is particularly crucial as they are below flood level.

The pinned connections of the hangers to the arches employ stainless steel details, separated from the carbon steel by nylon separators.

The slender, sculptural form of the Taplow Riverside Footbridge has already become a popular local landmark. The use of steel was crucial in allowing the team to achieve a high-quality structure which adds to the landscape of this unique location. The final result far exceeded the client’s expectations, providing an iconic addition to the river area.

Judges’ comment

This elegant bridge comprises twin steel arches, triangular in section, with the deck suspended via inclined steel plate hangers. The result is a distinctive, slender structure providing a valuable link between communities and fitting in sensitively with its environment. The steelwork is beautifully detailed, and trial assembly helped ensure trouble-free installation, using barges on the river, despite challenging conditions.

Knostrop Weir Foot and Cycle Bridge, Leeds

© Paul White

Architect
Knight Architects

Structural Engineer
Mott MacDonald

Steelwork Contractor
S H Structures Ltd

Main Contractor
BAM Nuttall

Clients
Leeds City Council and Environment Agency

The Leeds Flood Alleviation Scheme (FAS) is led by Leeds City Council in partnership with the Environment Agency. It will provide the city centre and over 3,000 homes and 500 businesses with protection against flood events from the River Aire, whilst enabling key regeneration opportunities in the South Bank area. Another objective of the scheme is the provision of new routes for walkers and cyclists, both along and across the River Aire. Knostrop Weir Foot and Cycle Bridge serves to reconnect the much-used Trans Pennine Trail, following the removal of a section of island between the River Aire and the Aire and Calder Navigation for flood risk reduction purposes.

As part of the FAS improvements a replacement weir would be constructed on the Knostrop site, and the clients wanted to explore the possible synergy between the new weir and the construction of a bridge across the river. The final design uses the new weir walls as pier foundations for the bridge above, providing significant savings in budget, time and resources.

Leeds City Council recognised the wider value for the design to be of high-quality and identifiable with its place. Despite the apparent complexity of the final design’s appearance, it only requires a single curvature in the fabrication of the steel plate elements. This served to simplify fabrication and enabled the bridge to be delivered within budget and programme. In views along the river the appearance is simple and sympathetic to the natural context. A curved soffit combines with the changing deck width to translate the varying plan width into a rippling deck edge detail, producing a dynamic ‘sinuous’ quality to mirror the noise and movement of the falling water beneath. Another unique feature of the design is that in elevation the piers are only 50mm thick and almost invisible in long views, creating the illusion of a floating deck. When viewed on closer approach the appearance of the piers changes, emerging as dramatic projecting cantilevers springing from the weir below. Lookout points have been positioned above each pier enabling people on the bridge to stop and enjoy views over the weir and along the river.

Steel was the obvious material of choice to achieve the required aesthetic and minimise the significant construction challenges of working over water. The 70m long bridge was fabricated in S H Structures’ facility, which is situated just 17 miles from the Knostrop site, and treated at a local facility, minimising the environmental impact of the works.

Construction over a river creates special challenges in order not to harm the waterway and its ecology. Minimising the time and extent of temporary works in the river was an essential aspect of the design. The prefabricated superstructure sections and piers were installed over two weeks using a crane. At the abutments special eel bypasses have been incorporated to allow for migration, whilst a dedicated fish bypass is included in the weir.

Given the accuracy required to successfully realise the complex steelwork geometry and installation, it was decided to embrace Building Information Modelling (BIM) from the outset. The Revit model of the bridge enabled every element to be accurately represented and positioned, including every steel plate in the bridge’s curving geometry and all connection elements. This was particularly valuable when designing the highly complex bolted integral pier connection. During fabrication the BIM model was also utilised to allow every component to be spatially positioned and checked. The model was also used to assist in the design of the workshop temporary works as the complete length was fully assembled, allowing the critical interfaces to be set, checked and maintained during the fabrication process.

In a wet environment over a weir, careful detailing, specification and construction are essential to ensure a long-lasting and durable solution for a bridge. The bridge superstructure is predominantly constructed using weathering steel with a four-coat paint system normally only used for difficult access highway structures. A primary concern was for the durability of the bolted connections between assembled elements of the bridge. This required highly protective details and connections that far exceeded what was needed for structural requirements to minimise water ingress.

One of the key features of this elegant structure is the slim piers. To achieve the required aesthetic and structural performance this area required careful consideration. Once the concrete weir walls had been poured and the holding down bolts installed, a detailed as-built survey was carried out. The recess bolt holes in the curved pier base plates were drilled and machined to match the as- built layout of each bolt group. With this work done, each base plate was trial- fitted to check for fit before the piers were finally installed, surveyed and cast in place. This attention to detail is critical to the successful installation of this type of precision detailing.

Judges’ Comment

This team solved an unusual bridge alignment by producing a thoroughly modern intervention in a post-industrial landscape whose unique qualities are derived from the constraints of the flood relief requirements. Using ingenious geometry and thorough attention to detail, the prefabricated sealed modular deck units appear to float on impossibly slender vertical supports. The result is an economic, robust and graceful solution. The overall rippling effect of the bridge is intriguing, yet it is rooted in logic; a seamless integration of architecture and engineering.

T-Pylon


Architect
Bystrup

Structural Engineer
Bystrup

Main Contractor
Balfour Beatty Power Networks

Client
Nationalgrid UK

The T-Pylon is a structure of only few parts that can be erected quickly and requires virtually no maintenance. It is designed to carry 2 x 400kV, but can be modified to alternative specifications, and is the result of a design competition in 2011 to find a 21st Century power pylon design for Nationalgrid UK. The challenge was to find an alternative to conventional lattice towers that minimised the visual impact on the landscape, whilst being cost-effective and functionally superior.

The use of steel for the T-Pylon has allowed for unique geometries. Contrary to conventional lattice tower designs, the arms of the T-Pylon are slightly raised, which gives the pylon a more optimistic and positive appearance. The few parts making up the pylon have been welded together and subsequently painted white. The tower design is shorter and leaner than traditional lattice towers resulting in improved aesthetics and reduced environmental impact. The use of a monopile foundation also minimises the overall cost, installation time and environmental impact of the T-Pylon.

The alternative design using steel has made it possible to obtain the aesthetic and functional goal, which is to minimise the visual impact on the surrounding landscape, while also providing an economic and durable solution.

The steel structure is designed in accordance with Eurocode 3 and fabricated in accordance with BS EN 1090-2 to Execution Class 3. The structural steel specification for the flanges, monopole and transition piece is for S355J2 to BS EN 10025-2 for thicknesses up to and including 50mm, and either S355NL to BS EN 10025-3 or S355ML to BS EN 10025-4 for thicknesses over 50mm. The steel plate also has to be accompanied by a Type 3.1 specific inspection certificate according to BS EN 10204.

A radical innovation is the re-assessment of the conductor/cable arrangement. The prismatic configuration of the cables allows a reduction in the pylon’s height of more than 30%. The footprint of the power lines, as well as the electro-magnetic field (EMF) radiation, is thus reduced.

The most remarkable characteristic of the T-Pylon design is that all conductors are carried by a single attachment point. Traditionally, such a structure would have three separate arms – each carrying an individual conductor.

This unique attachment point was studied closely to ensure its robustness and resistance to fatigue. Complex analysis and physical loading tests were carried out to simulate climatic conditions such as extreme winds and ice loads. Investigations were made into the dynamic performance of the structure under simulated vibrations.

The pylon is made from S355 steel plates that are curved and welded to form cylindrical sections. The shaft is fabricated in either one or two pieces according to the length needed, the requirements for hot dip galvanizing, and transport limitations. The steel plate thickness used for the shaft is optimised according to the design load cases and varies from 22mm at ground level to 14mm at the top.

At the top of the shaft a cast node connects the shaft to the two arms. The node is cast in one piece to ensure the optimal load transfer from the arms to the shaft. The result is a highly effective and smooth node that transfers the shape and forces from the arms to the shaft. The node is connected to the arms and shaft by non-visible internal bolts.

Dynamic external wind loads experienced on the pylon arms result in a bending moment at the pylon foundation. However, the cast node must withstand the transfer of internal stress from compression and tension at the node due to the pylon arm distributed load case. The cast node is designed to withstand both the magnitude and the dynamic behaviour of the load case.

At the end of the arms another node connects the insulator configuration to the arm in an aesthetically pleasing way. Again, the node is connected to the arms by non- visible internal bolts.

For the UK market the pylon is hot-dip galvanized and painted light grey. This duplex coating system gives the pylon an expected lifespan of at least 80 years. For other markets the pylon can be produced in stainless steel or weathering steel.

The design of the shaft is similar to the design of towers for wind turbines. Consequently, it was possible for the steelwork contractor to use the experience from wind turbine towers to produce the shaft using automated processes in controlled factory conditions. Maximising the offsite fabrication simplified on-site operations and reduced the number of operatives required for the installation process.

The new designs have significantly reduced maintenance requirements compared to traditional lattice towers. The durable coating system and lack of edges and bolted connections increases the future maintenance intervals and makes re- painting the towers much faster. Also, no anti-climbing devices are needed for the monopole shaft, which would otherwise require frequent replacement.

Judges’ Comment

The T-Pylon represents a generational step change in power transmission hardware. Analytical design from first principles included re-examination of arrangements for insulation and maintenance.

The result is a family of compact pylons which can be deployed in sensitive landscapes, with prefabrication enabling consistent finish, smaller land take and speedy erection. This is a steelwork design classic.

Harlech Castle Footbridge

Harlech Castle Footbridge

Concept Designer
Mott MacDonald

Structural Engineer
David Dexter Associates

Steelwork Contractor
S H Structures Ltd

Main Contractor
RL Davies & Son Ltd

Client
Cadw

Harlech Castle is one of the finest surviving 13th Century castles in Britain – it is a Grade I Listed Building, a Scheduled Ancient Monument and also part of a World Heritage Site. For many years access to the Castle had been via a series of timber steps, with no provision for those with impaired mobility. With the opening of a new visitor centre nearby, the vision was to connect this to the Castle via a new ‘floating’ bridge.

Due to the sensitive nature of the site, the aesthetics have been a particularly important consideration. Various concepts were explored to satisfy the constraints of functionality, alignment, heritage and visual impacts before finally opting for the ‘S’- shaped low profile Vierendeel truss design.

Both horizontal and vertical alignments were constrained by the need to connect straight through the Castle’s gatehouse, whilst maintaining a suitable gradient acceptable to those with impaired mobility.

To minimise the impact of the views of the distant mountains of Snowdonia, the profile of the bridge was reduced by tapering the bottom chords of the trusses and eliminating any diagonal bracing, thus avoiding a potentially more cluttered appearance. The visual lightness of the bridge is significantly improved by the selection of a stainless steel mesh infill to the parapets. The deck is 2m wide in general, however it widens up to 3m above the middle support to provide an area where people can enjoy the views.

To ensure that the bridge was future-proof provision was made for services to be run in a duct under the bridge deck, allowing the creation of a new venue within the castle where events and performances can be hosted.

Throughout the design process there was continuous dialogue between the project team using 3D BIM CAD modelling to explain design proposals and ensure the design developments were acceptable. The first key area for development was the truss and deck configuration. The original proposal had fin plates welded to the back of the CHS Vierendeel truss bracing elements, which then became the tee web in the handrail upright. This arrangement posed some fabrication challenges and raised the possibility of weld distortion in the fin plate attached to the CHS. An alternative solution was adopted whereby SHS bracing was used and the handrail fabricated tee upright orientation was reversed. The face of the SHS Vierendeel bracing then aligned with the flange of the tee to the balustrade which was tapered to give an elegant transition to the handrail, whilst the bracing also gives improved structural capacity particularly at the joint with the CHS chords for a given section width.

To maximise headroom clearances under the bridge, and to give a more efficient structural solution at the supporting columns, the depth of the truss profile was modified and the bottom chord form was achieved from a combination of curved and straight sections of tube.

The bridge’s dynamic performance required careful consideration in the design. The columns needed to be very stiff in the transverse direction so an elliptical section was used which, when partly filled with concrete, achieved the required result. This choice also had the added architectural benefit of the elliptical column being less obtrusive on elevation, whilst approximately matching the profile of the truss chord.

Before work started on site the existing façade was digitally scanned and the 3D survey was incorporated into the design model to ensure the critical dimensional interface between the Castle entrance and the bridge was achieved.

Bridge sections were set up in bespoke jigs to control weld distortion and maintain their geometry during welding.

The bridge is lit with a bespoke integrated LED lighting system that delivers bright white task lighting to the walkway, but has the added benefit of having a number of colour-changing effects that can be accessed for special events.

The bridge is finished with a timber deck and handrail for which FSC certified Ekki hardwood was selected, this requires no preservative treatment and little maintenance. The deck boards feature anti-slip inserts and seamlessly follow the curves of the bridge.

The biggest challenge to the installation team was the limited footprint of the site and the restricted access through Harlech. These challenges were overcome with the careful selection of the multi-wheel steer mobile crane and rear wheel steer transport trailers.

Steel erection required meticulous planning and attention to detail to ensure a smooth and safe installation process, however the unique historic nature of the site put even more responsibility onto the erection team. Following offsite matching of the deck units the fit-up on site was perfect and the three main spans were installed without any significant problems. With the bridge sections in place, the careful co-ordination of the fitting of the timber deck, parapets, lighting and services allowed the bridge to be completed in good time ready for its opening for the year’s summer visitors.

The new footbridge has been very well received and welcomed as an attractive addition to the historic site, whilst dramatically enhancing the visitors’ experience.

Judges’ Comment

In a very sensitive setting this elegant bridge provides level access to the historic castle, whilst minimising its visual impact. The detailing and fabrication of the curved deck are exemplary. The erection was effected with a high degree of precision despite the limited site and extremely difficult access.

The modern shapes of the bridge create a beautiful counterpoint to the ancient castle it serves.

Derby Arena

Derby Arena 1

Architect
FaulknerBrowns

Structural Engineer
Arup

Steelwork Contractor
Billington Structures Ltd

Main Contractor
Bowmer & Kirkland

Client
Derby City Council

Derby’s multi-sports Arena is a key legacy project that draws on sporting enthusiasm following the 2012 Olympics. The first new-build velodrome constructed in England since the London Olympics, the iconic 14,500m2 building features a 250m Siberian timber Olympic-sized velodrome track.

In addition to the cycling facilities, the Arena allows provision for a large number of community sport and fitness activities, including a large fitness suite and aerobic studios. The main Arena space accommodates 12 badminton courts or three volleyball courts. The Arena has been designed for a range of sporting and non-sporting events and can hold up to 5,000 spectators.

Structural steelwork was used for the majority of the key elements of the project due to its strength allied to its relative lightness, aesthetic appeal and speed of erection. Steel was the ideal material for the large spans of up to 85m that were part of the design concept.

The innovative facility raises the track to allow greater flexibility of the infield for other uses, including court sports activity, events, exhibitions and concerts. The level access for day-to-day use and event logistics, rather than usual ramps and tunnels, makes this particularly attractive from an operational perspective.

The Arena building is diamond-shaped with chamfered corners. Its main entrance is on the western corner and the oval cycling track sits east-west across opposite diagonals of the building at the first floor level.

The geometry created between the curving roof profile and the lifting of the building front and back has been deliberate to create a consistent height to the upper façade. This consistency allows a horizontal strip cladding to be used akin to the boarding of the velodrome track. Whilst the height of the strips is consistent, the vertical jointing is random.

The cladding system is a metal long strip aluminium ‘shingle’ system. The ‘shingle’ system is a ‘soft metal’ rainscreen adopting the building curvature which tapers to create window ‘eye-lids’ and integrated louvres. As a ‘soft metal’ system there will be a subtle and effective distortion and rippling appearance to the sheets which create a shimmering surface.

Three effective ‘eye-lids’ feature on the outside of the Arena building, which provided the challenge of accommodating twisted glazing with the frame. Due to the shape, this proved to be a complex element in the overall production, one which required optimum coordination between the steelwork contractor, glazier and architect to ensure a smooth and accurate execution.

Corrosion protection was achieved through combining offsite applied corrosion primer protection and onsite finishing coats, including intumescent paint as necessary.

Durable cladding systems to minimise materials consumption and waste generation were used to maintain a low environmental impact. Site materials were re-used in situ or sourced from a local supplier to minimise road transport to and from the site as far as possible. Sustainable urban drainage was used throughout the project to limit the impact of the new surface water drainage. Lined gravel filled trenches were used to provide conveyance and storage, and to keep excavations to a minimum within the landfill material. These were combined with large diameter storage pipes and a hydrobrake/surface water pump to control offsite surface water flows.

In terms of energy and carbon reduction, the strategy focussed primarily on the building fabric and achieving a well-insulated and airtight construction. In addition, the high efficiency central heating and hot water plant is supplemented with a combined heat and power (CHP) unit. It has achieved a BREEAM rating of ‘Very Good’.

By working together the design team was able to bring an innovative and futuristic design to life with an ambitious steel structure. With the selection of structural steel as the main construction material the building will stand the test of time, remaining aesthetically pleasing for generations to come.

The project was completed within budget and handed over earlier than the planned construction completion date.

Judges’ comment

A very well-executed project for a new velodrome that challenges the normal configuration by lifting the track to free-up the ground floor for a multi- use sports facility. The highly efficient steel-framed structure, with its 85m spans, exposes the steelwork where appropriate.

The building’s success owes much to careful integration of the architecture and engineering.