Barrett Byrd Associates

London’s Docklands are getting a spectacular new lift bridge designed by the same team which created the Gateshead Millennium Bridge. Jon Masters reports.

Lifting force for the Great Wharf Road Bridge now under construction in London’s Docklands will be provided by a massive 900mm diameter hydraulic ram, said to be the biggest in the UK. This gives some idea of the size of the structure now taking shape on a large pontoon within Canary Wharf, designed by a team renowned for making something spectacular out of the functional: Gifford is lead consultant and structural engineer, Wilkinson Eyre the architect and Bennett Associates the mechanical and electrical consultant, the same three part team that designed the Gateshead Millennium Bridge.

The Great Wharf Road bridge promises to have as much visual impact. The design brief called for a 15m square navigable envelope above water level, so the first decision made was to have an opening bridge. Construction within the dock was not permitted, neither was significant alteration to the existing southern quayside structure. This ruled out a swing bridge or vertical lift structure and has led to a design with a single 67.3m span between bearings.

The bridge’s horizontal and vertical alignment was also fixed from the start and client Canary Wharf Contractors (CWC) wanted a minimum carriageway width of 12m for four lanes of traffic and provision for a footway on each side of the bridge. The result will be a deck weighing in at some 800t, requiring a peak lift force of 650t from its single telescopic hydraulic ram, which will extend to 18m at its full reach.

“The bridge will appear to consist of an arched truss on each side of its deck, although the structure is really a curved beam,” says Gifford technical director Peter Curran.

“The beams themselves are hybrids of structural types. Each splits to form a vierendeel truss with a curved top chord trapezoidal in section and becoming narrower and deeper towards the top of the arch.”

Gifford has designed its lift bridge using a series of finite element models. Relatively simple models were developed to assess global affects of applied loadings – including forces subjected when the bridge lifts – while more detailed modelling confirmed hand calculations of the bridge’s more complex elements, such as the transfer beam at the northern end of the structure.

“The bridge is effectively simply supported when in the closed position, on steel spherical pivot bearings at the southern end and elastomeric bearings below the transfer beam, which is therefore also simply supported transversely when the bridge is down,” says Curran. “As the ram extends and the bridge opens, the transfer beam undergoes complete stress reversal, hogging over its new single support point.”

Wind loadings during the opening phase needed careful attention, says Curran. Wind tunnel tests were carried out for increments of the opening cycle to check for potential aerodynamic instabilities. Secondary load effects also had to be considered.

“The bridge is at a slight skew to the dock, which will cause the deck to move sideways as it lifts, dragging the ram cylinder over and generating horizontal forces. These have been taken into account in the design of the bridge deck and hydraulic ram.”

The Great Wharf Road bridge deck has been formed from four parts fabricated by Hollandia for CWC’s steelwork contractor Victor Buyck and brought over on barges from Krimpen in Holland.

Two end pieces, each around 11m in length, were delivered at the end of August and transferred onto multi-wheeled transporters, which backed them up to the edge of the dock and lowered each 130t piece onto temporary support barges.

Two central sections were united by joining their barges to form a 50m by 24m pontoon in the north dock of West India Quay. After the two halves were welded to a longitudinal deck splice, the pontoon was moved into place between the two end sections. Strand jacks on the end pieces then lifted the 520t central section up for welding and completion of the deck steelwork in September this year.

Designing the Great Wharf Road Bridge has been all about working within tight constraints imposed by the site and operating requirements of the finished structure, while also getting the aesthetics right for such a high profile location, says Curran. The width of the trusses was constrained – the 22m maximum width includes 8m of cantilevered footway as well as kerbs, trusses and carriageway – so the top chord of the truss design has been turned through 90degrees with thicker steel sections to gain the necessary stiffness.

To minimise on weight, the thickness of the steel deck plate has been kept to a slender 12mm, the design having been “optimised” to anticipate volumes of traffic and types of vehicle expected to use the bridge.
Then there are the constraints on alignment. On the south side, the deck ties in with the elevated structure of Great Wharf Road, which was built at around the same time as the adjacent HSBC tower to CWC’s design philosophy of roads separated in grade from pedestrian promenades and walkways.

A concrete approach viaduct, being built by McNicholas, will create the same split level effect on the northern quay. It is being built alongside Billingsgate Fish Market, whose position has necessitated the new bridge’s slight skew across the dock.

The 40m viaduct will house the plant room, below deck level and behind the northern promenade. The design will also allow pedestrian access beneath the viaduct and behind the lift bridge’s concrete abutment and elastomeric bearings.

“To onlookers, Great Wharf Road Bridge will suit the new office developments that dominate its surroundings and it should be something worth seeing in its own right, particularly when it lifts,” says Curran. Glass faced stainless steel canopies – articulated to allow for the deck flexing as it lifts – will ultimately cover the deck walkways, springing from the arched trusses down to walkway level. Wedgewire panels will also clad the structure between the vertical truss chords to provide separation between pedestrians and highway traffic.

BOX– M&E
A German manufactured Hunger Hydraulik two stage telescopic hydraulic ram will lift the Great Wharf Road bridge deck. With a cylinder diameter of 910mm, a top rod diameter of 800mm and maximum stroke of 18.2m, the ram is claimed by Bennett Associates – the design team M&E consultant – to be the UK’s largest hydraulic ram. The ram, which will be installed by its designer Kvaerner Markham Engineering, has a lift capacity of 660t and will be powered by three 132kW hydraulic pumps, arranged to ensure the bridge still lifts if up to two of the pumps fail. The top rod end of the ram will connect to the deck via a spherical ball joint assembly and the cylinder will be trunnion mounted to make the ram act as a pin ended strut during lifting. The cylinder will be disengaged when not working to prevent it being subjected to fatigue stresses.

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