Design Awards: 2007: Award

Newport City Footbridge, Newport

Newport City Footbridge, Newport

Architect

Grimshaw

Structural Engineer

Atkins

Steelwork Contractor

Rowecord Engineering Ltd

Main Contractor

Alfred McAlpine

Client

Newport Unlimited and Welsh Assembly Government

Newport Unlimited is an urban regeneration company established by Newport City Council and the Welsh Assembly Government. Its city centre master plan identified the need for improved pedestrian access to the town centre. The Newport City Footbridge plays a critical part in the city’s accessibility strategy, linking the east and west banks of the River Usk and allowing people to travel quickly and safely between the two.

As the bridge was to be the first element of the master plan it was critical for it to be a landmark structure and to be delivered within published cost and time constraints. The bridge now acts as powerful evidence that the regeneration scheme is under way.

The bridge is located between two older road bridges, approximately 1km apart, immediately to the east of the commercial centre of Newport and is surrounded by future development sites. The River Usk flows into the Severn Estuary and has a large tidal range at this point.

The bridge’s dramatic crane structure provides a symbolic link to the site’s earlier use as trading wharves. Placing the main supports on the west bank also reflects the pronounced change in the urban scale and grain from the commercial heart of the city on the man-made west bank to the domestic uses and soft landscape on the east side.

The deliberate concentration of major structures on the west bank has many practical advantages. The vast majority of temporary and permanent works were kept away from the nearby dwellings and river bank wildlife on the east side. Construction work was simplified, with no requirement for any works within the tidal riverbed and avoiding impact on the local river ecology. The existing car park on the west bank also provided an ideal construction site for final assembly of the structural components before installation.

At the competition stage the design allowed plenty of flexibility in the position and geometry of the bridge enabling in to easily be ‘tuned’ to suit final briefing requirements and site conditions.

The primary supporting structure is four masts, standing in pairs, which support the 145 metre long bridge deck from the west bank. The bridge deck loads are transferred to ground level by two 120mm diameter cables which also act as stays for the masts. The forward mast is 80 metres long and has a maximum diameter of two metres. The back mast is 70 metres long, but because of the angles at which the masts are positioned, the back mast is the tallest part of the structure, 67 metres above ground level. The deck is 5 metres wide and 4.1 metres above water at mean high tide level.

Fabrication of the 850 tonne structure began in August 2005 and pre-assembly work started on site in January 2006. Planning for the erection phase was complicated with numerous constraints in relation to site access, environmental impacts, stability issues and exceptional lift weights. Rowecord created a rapid lifting and construction sequence lasting a little over one week to build the bridge, utilising Britain’s largest mobile crane – a 1,200t Gottwald AK 680 in conjunction with a 500Te capacity Liebherr telescopic crane.

Phase 1 of the erection scheme consisted of raising the back mast, placing it on its trunion support, rotating it backwards onto a temporary prop and connecting the rear anchorage cables. The front mast was then lifted and installed at 15 degrees to the vertical. At this stage strand jacks connected to the temporary prop were attached to the front mast to allow release of the main erection crane. Following attachment of the forestay cables the front mast was lowered on the strand jacks to its final attitude at which point the forestays cables became taut and pulled the back mast forward thus releasing the load from the temporary prop and strand jack system.

Phase 2 consisted of the erection of the five deck sections and two pre-cast abutments. The centre section was lifted into position using the Gottwald crane on a revised boom length to achieve clearances over the front mast hairpin and cables. The centre of gravity of the second deck section required moving via the use of kentledge to remain within the erection radius of the main crane. The introduction of out of balance forces that would have occurred on release of this deck section was catered for by tying the bridge sections back to the east bank abutment anchorages. Erection of the third section restored natural equilibrium to the bridge.

Final closure of the bridge was achieved via erection of the first and fifth deck units and pre-cast abutments utilising the 500Te Liebherr on either side of the river.

In addition to the structural steel, the bridge includes nearly three kilometres of stainless steel wire. With a load bearing capacity to carry 2,000 people, the structure also includes 20t of integrated dampers to prevent vertical and lateral oscillation. The masts are constructed from rolled and welded sheet steel and ‘fixed’ inmountings with 450mm long stainless steel pins weighing 500kg each. With a design life of 120 years, the bridge should be standing for many years to come and, with maintenance and cable replacement all factored into the design, there is every confidence that its upkeep will be straightforward.

Judges’ Comment

s:

Drawing inspiration from the city’s historic wharves, this crane-like structure supports a 145 metre span footbridge/cycleway across the River Usk, with a mast rising 67 metres.

Fabrication and erection were a demanding task for the team.

The result headlines steel in a big way, and provides a magnificent, iconic landmark in the heart of the regeneration area.

Sheppy Crossing, Isle of Sheppy

Sheppy Crossing, Isle of Sheppy

Architect

Yee Associates

Lead Designer

Capita Symonds

Viaduct Structural Engineer

Cass Hayward LLP

Steelwork Contractor

Fairfield-Mabey Ltd

Main Contractors

Carillion

Client

Highways Agency and Sheppy Route Ltd

The completion of the new ‘Sheppey Crossing’ in 2006 has provided the first fixed link from mainland Kent to the Isle of Sheppey, a high level viaduct. The viaduct was constructed as the primary feature of the Highways Agency DBFO Contract. The A249 dual carriageway now enables free flow of traffic. The bridge, which uses 10,000 tonnes of fabricated steel plate girders and 60,000 tonnes of structural concrete, is 1,270m long with 19 spans, the longest being 92.5m over the central navigation Channel of the Swale. The spans grow in length gradually from the abutments towards the main central span and the bridge depth increases proportionately to a maximum of nearly 4m at mid crossing. This unusual arrangement produces a most elegant elevation, which is enhanced by the sweeping curve of the highway rising to a crest of 30m above the estuary.

The bridge spans over the Saxon shoreway and due to the flat terrain there is a visual impact envelope of up to 30km. The flora and fauna in the area directly affected by the build were relocated to other parts of the marsh site.

The decision to launch the vast majority of the structure (15 spans in three phases using 14 separate launches) heavily influenced the design and also reduced the work load done by large capacity mobile cranes during the construction phase. The design programme mirrored the construction programme closely so that the release of deliverables allowed timely procurement and fabrication of the steelwork therefore avoiding congestion both in the works and at site.

The four main girders at 5.5m centres continuously varied in depth from approximately 1.5m at the abutments to 3.5m over the central navigation channel. 940mm deep cross girders at 3.5m longitudinal centres were fabricated in long lengths and then cut to the required length to suit Fairfield-Mabey’s automated processes. Plan bracing to the central seven spans located between the two inner main girders ensured temporary stability during the launch and acceptable aerodynamic behaviour of the completed deck. The plan bracing was located at an aesthetically pleasing 300mm above the bottom flanges. Material grades used were S355J2+N (6 to 61mm) supplemented with S355 K2 (62 to 77mm) and NL (78 to 98mm) as necessary. Over 185,000 High Strength Friction Grip bolts were used in the structural connections, totalling approximately 100 tonnes in weight.

By using modern fabrication facilities and forging a team of the designers, main contractor and steelwork contractor, the high level bridge was fabricated and erected within programme and budget. Over 11,000 tonnes of permanent and temporary steelwork went into the construction of this landmark structure that was efficiently erected between December 2004 and December 2005, bringing substantial benefit to the local community.

Judges’ Comment

s:

The new road bridge replaces the previous road/rail lifting bridge, and provides much-needed improved access to the Isle of Sheppey.

For ship passage, the structure is some 30 metres high, but the team have achieved a solution which minimises intrusion into the flat landscape. The main plate girders carry cross-beams within their depth, and they taper continuously from abutments to mid-span, both in plan and elevation. The structure was launched from the north end, probably the longest of such operations. Having solved the primary challenges, the team have then refined the design to its successful conclusion.

Clyde Arc Bridge, Glasgow

Clyde Arc Bridge, Glasgow

Architect

Gillespies Architects

Structural Engineer

Halcrow

Steelwork Contractor

Watson Steel Structures Ltd

Main Contractor

Edmund Nuttall

Client

Glasgow City Council

This new landmark bridge, known as the ‘Clyde Arc Bridge’, crosses the River Clyde in Glasgow and significantly improves access to Pacific Quay Area and is a major catalyst in the redevelopment of this once derelict site.

The 96m span single arch rib and the hangers are a prominent yet complementary feature in the surrounding environment. The single arch rib straddling the deck is the first such structure in the UK to be tied to the deck. The diamond shaped arch section is also unique.

A condition that arose from the planning consent process was that the bridge should incorporate provision for a light rapid transit tram system in addition to the four lanes of traffic. This included increasing the surfacing on the bridge to 230mm to accommodate standard tram rails and the reconfiguration of the bridge articulation by moving the expansion joints from the abutments to the piers. These changes had fundamental implications for the design of the structure in that increasing the dead and live loading and reducing the relative stiffness of the deck significantly increased the load carried by the hangers and arch. However these changes were readily accommodated without changing the original design concept.

Minimising the environmental impact was a key factor in the development of the conceptual and detailed design. This included the use of tubular piles and pre-cast concrete set above river bed level to minimise disturbing riverbed sediments. Maximising the use of off-site prefabrication in terms of steel and pre-cast concrete also minimised the duration of on-site works and significantly reduced the possibility of concrete spillage.

The solution offered in response to the client’s brief is a bowstring arch with a single arch rib straddling the deck and inclined hangers connected to outriggers on the edge of the deck which creates interest for users of the bridge in terms of driving through the arch and hanger. The single arch also clearly expresses the structural principles of the bridge and is not confused by a second arch rib or bracing members.

The diamond shaped arch rib is a unique feature of the bridge and significantly enhances the slenderness of the arch while also providing significant visual interest from the constantly changing reflection of sunlight and from the projection of the architectural lighting.

The structural relevance of the diamond shape is also demonstrated by the fact that in cross section the hangers intersect the arch rib at angles approaching 90 degrees. The arch springings are also a unique architectural feature of the structure with the transition from the steel diamond shape to a square concrete section which tapers out and when intersecting with the circular piers results in the interesting scallop features. Other key architectural features include the two platforms that provide a vantage point for viewing the river and surrounding environment and the architectural lighting.

The arch is 130 metres long around the curve and was fabricated in sections each weighing approximately 50 tonnes. The fabrication was difficult due to the doubly curved nature of the side plates. A 3-dimensional model was used to produce the fabrication jig that was set up accurately on the shop floor using XYZ coordinates. The individual plates that had been accurately precut were fitted into the jig and the components tack welded together. The arch sections were then lifted out of the assembly jig with specially designed carrying and turning frames that allowed the sections to be rotated so that the longitudinal seam welds could be placed in a down hand position.

The construction of the abutments, piles, composite deck and steelwork were totally integrated resulting in efficiencies and cost savings. For example a large floating crane was required for the installation of the steel piles and the steel deck girders and pre-cast units were sized around the lifting capacity of this crane.

Four temporary trestles were installed in the river to support the deck during construction. Every lift was planned in detail and, in order to save hook time on the expensive floating crane, a hinged detail was developed so that all the splice plates could be fixed to the beams on the shore with a small crane and, once in position, the splice plates could be manually swung into place by the erectors. The steelwork erectors also placed the large full depth pre-cast concrete deck units using the same crane.

The nine arch pieces were delivered, assembled and welded on the deck into three large sections each weighing 150 tonnes. Two 500 tonne capacity cranes were then used to tandem lift the three sections onto the temporary trestles. The support trestles were then de-jacked and removed and the hangers installed. Finally the deck weight was transferred to the hangers and arch and the temporary supports in the river removed. The whole of the superstructure was completed between November 2005 and June 2006.

This impressive bridge meets all the client’s aspirations and was completed to programme within budget. It is an excellent example of the benefits that early contractor involvement, associated with an integrated design team, can bring to a project.

Judges’ Comment

The new bridge provides strategic access to the Pacific Quay area, and is already a strong catalyst for redevelopment of this once derelict part of the city.

The single arch has a diamond-shaped profile, giving a slender appearance enhanced by reflections of light from the sky and the river surface, strikingly augmented at night by architectural illumination. The design of the arch, the hangers and deck, is satisfyingly and effectively resolvedwith clear expression of their functions.

This landmark structure is thoroughly professional, meets the aspirations of the client and is a major addition to the skyline of Glasgow.