Design Awards: 2010: Award

Terminal 2, Dublin Airport

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Architect

Pascall+Watson Architects

Structural Engineer And Project Manager

Arup

Steelwork Contractor

Watson Steel Structures Ltd (Severfield-Rowen Plc)

Construction Manager

Mace

Client

Dublin Airport Authority

Terminal 2 was developed to provide a new terminal, pier and road frontage systems to cater for 10-15 million passengers per annum, which would be a simple, efficient and user-friendly experience for passengers and all other end users. This was achieved by providing a flexible, expandable and contemporary facility, which acknowledged current trends and international industry standards for airport and passenger terminal design

The terminal building consists of arrivals, departures, check-in buildings and link bridges comprising of nearly 12,000 tonnes of structural steelwork, together with 55,000sq m of structural metal decking, stair cores and staircases.

In order to meet the challenging build schedule, Dublin Airport Authority took a package approach to various aspects of the expansion project rather than using prime contractors to build the entire system. This allowed the airport more flexibility with the phasing of the packages and accelerated the process. The packages included steel, structure, fit-out and specialist systems, and MEP (mechanical, electrical and public health), which spans communications infrastructure and alarms.

The client’s brief was to provide a sustainable landmark building that could adapt over time to the ever changing requirements of the airport industry. The building was to be light and airy and make the maximum use of natural light, and provide a calm atmosphere for passengers. The project also had to be delivered at the ‘right cost’ in terms of both construction costs and life cycle costs, safely and in a manner that did not affect the airport capacity during construction. Independent consultants, appointed by the Government, have confirmed that the budgeting process and costs of Terminal 2 were in line with best international practice.

The new terminal was designed to utilise appropriate technology, which remains flexible to ensure future ‘proofing’, and to provide enhanced efficiency for both airlines and the operator. Certain elements of the structure are designed to allow for further expansion and also for the required increase in demand.

The building was designed to be highly architectural and the curved shape of the building combined with the extensive use of glass satisfies that requirement. The shape of the roof and the large designed spans clearly pointed to the use of steel as the most practical and economic way of creating the curved shape of the building. Bespoke fabricated box section roof girders were designed and these were fabricated from curved plates and fully welded in the factory. Prior to despatch the roof girders were fitted and bolted together during the fabrication process to ensure the tolerances and fit-up on site were achieved.

One of the main drivers during the design development period was to reduce the amount of work on site and this was achieved by providing large pre-fabricated units, up to 20 tonnes each. These were bolted together at low level to form the main roof girders. Heavy plate girders and plated columns were also used to create large spans.

In order to achieve a very tight site programme, and to avoid disrupting the existing airport operations, all the works were planned on both a day and night shift basis.

The structure presented a considerable challenge in respect of fabrication workmanship. The roof and the sides of the building are curved in both plan and in elevation. The use of 3D modelling and CNC data transfer to the cutting and drilling machines were essential in achieving the accuracy and tolerances in the individual components. The manual assembly of the components to form the complex shapes required considerable experience and skill. Bespoke fabrication jigs were used extensively combined with 3D laser setting out equipment with the data being transferred from the Tekla model directly onto the shop floor equipment.

The internal superstructure frame of the building is fire engineered using intumescent coatings. The roof structure did not require fire protection and was shop painted with a primer with the final coat applied on site during the fit-out process

Throughout the design process there was a high degree of communication between the design team and the steelwork contractor to overcome the more challenging aspects of the design and erection to produce an efficient cost-effective structure that could be erected safely. This was particularly evident when considering the erection sequence and stability issues.

Judges Comment

A large complex infrastructure scheme designed and constructed in a short time in the midst of the day-to-day life of a busy international airport. The intention is to provide an exceptionally userfriendly experience.

The expression of the steel structure is clear, with consistent detailing.

A well executed project which demonstrates close cooperation between all involved, and a fine example of the capabilities of steelwork.

Legacy Roof, London Aquatics Centre

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Architect

Zaha Hadid Architects

Structural Engineer

Arup

Steelwork Contractor

Rowecord Engineering Ltd

Main Contractor

Balfour Beatty Group Ltd

Client

Olympic Delivery Authority

The London Aquatics Centre will mark the gateway to the 2012 Olympic Park. The stunning waveform shape of its complex steel roof sweeps dramatically upwards in a smooth curve from the south end and then down again over the northern cantilever, while the western and eastern tips curve upwards at the edges.

The 11,000m2 structure spans a column free area 160m long and up to 90m wide. It is supported on bearings on two concrete cores 54m apart near its northern end and on a concrete wall at its southern end. The roof contains about 3,200 tonnes of structural steel, of which 2,000 tonnes are fabricated plate girders with the structural connections totalling around 600 tonnes.

The roof structure comprises a series of long span trusses spanning lengthways over the main pool hall from a transverse truss mounted on the southern retaining wall bearings to another transverse truss spanning between the northern concrete cores. The main trusses lie in a fan arrangement to create the plan shape of the roof. The centre fan trusses cantilever northwards beyond the north transverse truss to form an overhanging canopy over the main public entrance plaza of up to 30m.

The centre fan trusses carry load in truss action, spanning between the north and south transverse trusses which carry the load down to the supporting bearings on the concrete structure below. Due to the roof geometry, arches are formed in the wing areas to the west and east of the central area. Under uniform loading the two opposite inclined arches in the wing areas balance each other, forming a compression hoop around the roof perimeter. A tension force arises from the change in geometry of the compression hoop in plan at the kinks which occur at the wing tips, and this is resisted by a tension tie across the centre and a resulting tension force occurs in the central fan trusses.

Due to the arched shape of the northern transfer truss, lateral thrusts are developed. In the final condition these are resisted by tensions in the plaza level slab. However, as this slab could not be cast until some time after completion of roof erection, it was necessary to install a temporary tie comprising eight high tensile steel bars between the north cores. This was pre-stressed before the roof was lifted off the temporary trestles.

Lateral stability is provided by a system of horizontal and diagonal cross braces in the roof surface between the top chords of the fan trusses. All of the trusses are formed from fabricated H-sections. The plate thicknesses of the sections vary along the length of the trusses to ensure efficient use of material, with plate thicknesses varying between 8mm and 120mm. At site the members were bolted together to produce erectable truss lengths of around 30-40m. The trusses were lifted onto preerected lines of temporary trestles and joined together with bolted splices.

In the permanent condition the roof is designed to be fixed on plan at its northern bearings and free to slide longitudinally at the southern end. However, due to site constraints, it was necessary to construct the roof from south to north, starting with erection of the southern transverse truss which weighed just over 70 tonnes. It was necessary to initially restrain the roof in a longitudinal direction with temporary works at the southern end and then, later in the programme, a controlled transfer of restraint to the northern bearings was carried out whilst simultaneously releasing the southern end.

When 50% of the roof had been erected one of the intermediate lines of trestles had to be removed to allow excavation to start for the deep dive pool. This was achieved by jacking the roof up at the trestle positions to relieve the load from them. The remaining two main lines of trestles were left in position until the main roof structure was complete. On completion of the main erection, the roof was lifted using strand jacks mounted on temporary towers at the south end and allowing it to rotate about the northern bearings. Once the roof was clear the strand jacks were locked off while the trestle heads were dismantled before the roof was lowered to its final position.

All of the bolted connections in the primary structure were designed to be nonslip using tension control bolts. In situations where bolt access was limited by geometric constraints, tension control studs were used. The structure contains about 70,000 bolts.

Due to the highly corrosive environment, rather than leaving faying surfaces unpainted they were coated with zinc silicate paint, slip tests having first been carried out to establish that a suitable slip factor could be achieved. The zinc silicate was used as a primer generally and exposed surfaces were over coated with MIO. In the final condition the steelwork is all concealed by upper and lower surface cladding and so no decorative coat needed to be applied.

A network of 600 linear metres of steel walkways installed throughout the roof space will provide access for regular inspection and maintenance of the structure as well as lighting equipment and other plant.

Temporary stands to the west and east of the structure will provide seating for 14,700 of the full complement of 17,500 spectators for the Olympic mode. These will be removed and recycled on completion of the Olympic and Paralympic Games. The final perimeter façade will then be installed for the legacy mode to provide outstanding community facilities for East London’s future.

Judges Commment

An heroic engineering achievement which has overcome severe programme and constructional problems. A necessarily complex structure delivers the form and shape at the heart of what will become the emblematic and beautiful icon of the London 2012 Olympics.

This is a high profile success for structural steelwork.

The Infinity Footbridge, Stockton-On-Tees

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Structural Engineer

Expedition Engineering

Steelwork Contractor

Cleveland Bridge UK Ltd

Main Contractor

Balfour Beatty Civil Engineering

Client

Stockton-On-Tees Borough Council

The Infinity Footbridge, a unique linkage for pedestrians and cyclists, is formed of two graceful steel arches that flow into one another, spanning 60m from the south bank to the central pier and a further 120m to the north bank of the river, with the 2:1 ratio reflected in the rise of the arches. Such is the length of the north span that it makes the Infinity Footbridge one of the longest footbridges in the UK.

The arches are fabricated from weathering steel to form trapezoidal hollow box sections which vary from 1500mm to 400mm deep and 2500mm to 200mm wide. The arches’ box sections bifurcate on plan over the central pier and are supported by four steel arms. The arms in turn land on two 3-tonne solid machined pieces of high grade steel, forming the central nodes. These neatly resolve forces from the arches, horizontal cables and supporting legs beneath, which then sit on concrete piers below the water line. The deck is made from pre-cast concrete units suspended from the arch by hanger cables and post-tensioned along their length by a pair of longitudinal cables running either side of the deck. These cables also act as ties for the arch which resolve the horizontal thrust within the structure. The deck is finished with a stainless steel handrail which incorporates the bridge’s dynamic lighting system.

Through the design process the bridge was reduced to a minimum number of key structural elements. Each element was considered in order to increase its efficiency and robustness, whilst reducing the need for future maintenance and thus the whole life cost of the bridge. Working with 3-dimensional digital models was central to the design of the Infinity Footbridge. The shape of the arches was perfected using form-finding 3D analysis techniques. The structural analysis model was linked through to the geometrical model, allowing simultaneous updates to both structural and visual models.

As the project moved into the construction phase several contractual barriers were overcome to allow construction information to be delivered in a single digital 3D model. This extensive embrace of the digital 3D model led to project-wide efficiencies and removed the risk of interpretation errors. By avoiding cumbersome 2D paper representations of complex geometry in favour of precise digital 3D models, the project exemplifies the leading edge of coordinated steelwork design and fabrication.

The form-found arches were joined over the central support to provide continuous beam action allowing uneven patch loading to be carried without greatly increasing the depth of each arch.

A rowing course in the Tees meant that any support in the river would need to be off centre. This constraint was transformed into an opportunity by using the shorter stiff span on the south to allow the northern arch to span 120m and come down to a section of just 300x700mm.

This geometry introduced curvatures in the box section plates that exceeded limits in the standard codes which resulted in the 1971 Merrison Report (the father of BS 5400) which covers these limits having to be dug out and dusted off! Through a close working relationship with the Category III checker a methodology was agreed on which ensured the bridge could maintain its graceful form and efficient geometry and meet plate slenderness requirements.

The arches were formed from hollow sections, yet it is not practical to inspect the condition of the steel inside the arch. Weathering steel was therefore used to ensure that the internal surfaces would have adequate corrosion protection. Similarly, the corrosion risk from de-icing salts on the deck was taken into account. As an Icon for North Shore and the whole of Stockton-on-Tees, the Infinity Footbridge is expected to outlive its creators. To combat the significant threat of chlorine attack, the deck has been constructed using stainless steel reinforcement.

The phenomenon of pedestrian induced vibrations on footbridges is well known, hence extensive analytical analysis was undertaken to identify the structure’s natural frequencies and expected behaviour under a variety of cases. This led to the inclusion of seven tuned mass dampers which were discreetly hung within the soffit of the deck units. Following extensive onsite testing of the final structure the analytical model proved highly accurate and the dampers, once released, were seen to perform as predicted.

Lifting the larger steel arch provided a special challenge. Sections of the arch were fabricated in nearby Darlington and then carefully welded together on the river bank. In a single lift using the UK’s largest mobile crane, the large north arch was lifted into place. For the many users of the river there was very little disruption – although there was an impressive display of heavy engineering!

Delivery of the Infinity Footbridge provides both a key driver for regeneration and a well loved local landmark.

Judges Comment

An inspirational project which fulfils the client’s brief for a landmark to open up a development area.

The elegant structure clearly describes the forces on it, and its simplicity belies the technical complexities which were handled by good teamwork. Attention to detail is evident throughout.

This is steelwork at its most dramatic.

Audi West London

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Architect

Wilikinson Eyre Architects

Structural Engineer

Expedition Engineering

Steelwork Contractor

Rowen Structures Ltd (Severfield-Rowen Plc)

Main Contractor

ISG Interiorexterior

Client

Volkswagen Group UK Ltd

The high visibility of the site alongside the elevated section of the M4 in west London presented the client with an opportunity to display his product in an innovative and striking manner. An important aspect of the brief to deliver a landmark project was to consider the product marketing.

The structure is a seven-storey, 30,000m2 celebration of automotive engineering and technical expertise. The five-storey superstructure contains three floors of showroom and sales areas, with offices, marketing and conference facilities over the top two floors. Below ground, a two-storey basement contains futuristic workshops and diagnostic facilities.

Steel is the client’s structural material of choice from bespoke steel boxes submerged below the basement slab to trapezoidal tapering raking columns at the southern façade. The use of a steel frame in the superstructure has allowed a highly flexible floor plan for future renovations to the showroom and office spaces, while the use of a steel intensive perimeter sheet-piled basement has provided significant cost, aesthetic and construction programme benefits below ground.

The building architecture was deliberately pared down by the architect to allow the main features, the cars themselves, to stand out. The structural frame and simple floor arrangements maximise light and airiness. From the full glazing of the main façade to the translucent Kalwall cladding of the rear display areas, natural light is brought deep onto the floors.

Steel was the clear choice for a versatile and flexible floor plan. Long span steel plate girders have been used to allow large (19m span) column free floor plates while squeezing the floor sandwich to a meagre metre from ceiling to finished floor level. To enable the tight floor construction, early coordination of the services with the structure was essential.

Considering the thickness of the floor construction required, composite steel floor decking was the natural choice. The benefits of simple fire protection, speed and safety of construction as well as providing a very lightweight floor were obvious from the outset.

The exposed internal roof structure is a key feature providing a backdrop of engineered architecture to the cavernous rear display space. The single curve of the roof is formed of curved rolled sections simply and elegantly braced and connected. The economic design extends to the lightweight steel roof deck, chosen to provide the roof structure, architectural finish and acoustic attenuation. The speed of erection of the roof deck enabled the quick closing of the building envelope following the steelwork erection.

While the primary roof structure is based on a single axis of curvature, the southern M4 facing edge of the building introduces a curve on plan producing doubly curved eaves. This geometrically challenging edge, fabricated from steel, was included in the primary steelwork package to ensure a crisp, engineered finish.

The design team developed 3D model information that was used directly by the steelwork contractor to bend and fabricate the roof edge. Key details, such as the roof edge cantilevers, were developed by the team in 3D ahead of the appointment of a contractor and included in the tender drawings with the aim to transfer as much useful knowledge as possible.

In the two-storey basement, the use of steel sheet piling for the perimeter walls provided the best balance of resistance to imposed loadings, speed of construction – 3500m2 of sheet piling was installed in less than two months – final appearance and cost. One of the greatest successes of the perimeter sheet piling is the highly aesthetic finish gained without the need for a secondary facing.

On entering the building the space immediately opens up with a double height façade articulated by primary steel columns. The tapered trapezoidal columns are fabricated from 20mm plate and lead the eye through the space. They give context for the hanging mezzanine, with the angle of the steel tension rods echoing the geometry of the leaning façade.

The design of the vehicle restraint system made use of steel plasticity theory to develop a catenary restraint and allow a visually lightweight high tensile steel rod to be used in place of an elastically designed structural section.

In the workshops, but hidden from view, are 26 submerged steel boxes. Each box comprises over half a tonne of fabricated stiffened steel plate designed to contain and protect the hydraulic mechanisms of the workshop car jacks. The jacks are state-ofthe- art car supports that can elevate the cars 2m above the workshop floor and then retract below the basement slab at night. The boxes protect against hydrostatic and clay heave forces and continue the waterproofing line of the basement. The plate thickness allows for sacrificial corrosion over the lifetime of the building.

Early consideration of the buildability of the main frame allowed the steel erection to take place with minimum temporary bracing or supports. The building frame was conceived as a central spine comprising two stability cores and the building atrium. From this spine the north volume, braced via floor slabs and roof trusses, spans to slender perimeter box columns. To the south, the floor beams provide a tie restraint to the inclined tapering façade columns.

The construction methodology is expressed in the exposed roof structure, where central primary roof beams taper to a simple pin at the connection to the northern and southern structures either side.

Throughout the project steel has consistently emerged as the optimum material choice. In particular its ability to provide an economic solution while expressing engineered design has worked to great effect. The highly visible steel structure with carefully considered detailing, provides a stunning backdrop to the automotive design on display

Judges’ Comment

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Comprehensively well designed and economical in form, the structure and its details reflect a technical ethos. The building showcases the values that Audi project to its public, with great panache.

This is an appropriately stylish and racy building, showing structural steelwork to great effect.