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Channel Challenge

Engineers claim that Abu Dhabi’s unique Sheikh Zayed Bridge is the most difficult ever built thanks to its complex and irregular structural form. Bernadette Redfern reports.

Eighteen years ago the Abu Dhabi Public Works Department first invited tenders for the design and supervision of an 850m crossing of the Maqta channel, which separates Abu Dhabi island from the mainland.

Nine consultants submitted bids. High Point-Rendel (HPR) was eventually appointed but its designs were never approved. "The client wanted something more spectacular," explains chief bridge engineer Joe Barr. "So in 1997 they requested Zaha Hadid present architectural concepts and models," he says.

The result was two very different structures, one a linear frame that appeared to zig-zag over the water, the second an undulating pair of asymmetric arches that resembled sand dunes. By 1999 the client, which is today known as the Abu Dhabi Municipality, had its spectacular structure in the shape of the second design and HPR had to make it work.

Click here for Zaha Hadid's amazing bridge design

"I would say this is the most difficult bridge ever built," says HPR assistant resident engineer Mike Davies. "Nothing about this bridge is regular, all the spans are supported differently, every piece is specifically designed and engineered. It is all one-off stuff," he says.

It took the team two years to produce the detailed design which essentially consists of 11 deck span sections and three major arches with four main piers and two additional sets of supports at the western end.

The entire structure is supported by 16km of 1.5m diameter bored piles, each around 30m long. There are 144 under the central pier alone and the pile cap is 5m deep.

"The early design just wasn’t possible," says Barr. "Part of the compromise to achieve structural stability was the inclusion of cross beams and the central pier was deepened." Following the lengthy and complex design process, the bridge moved into construction but the project did not get any easier.

The AED 635M (£116M) contract was awarded to Archirodon Construction Company in July 2003 as a 44 month contract, but the timescale soon slipped. The completion date moved from the end of 2006 to the end of 2009, with Archirodon taking most of the risk. A new price, understood to be in the region of AED 800M (£147M), was negotiated by Archirodon.

The first delay occurred when it emerged that Archirodon had a different construction sequence in mind to that proposed by the designers. "The irregular shape of the bridge meant that no obvious sequence presented itself, and the one chosen by the contractor was different from the one assumed for design," explains HPR technical director John Dawson. "Since the support structure is continuous, this change in construction sequence – and hence structural system during construction – required a review of the complete design to check that the stress distribution throughout the system remained within the design limits."

Archirodon therefore bought in its own engineering advisor, Buckland & Taylor from Canada. The original sequence created by the designers was to build the arches and then the deck spans. But Archirodon wanted to go from west to east building arch and deck as they went, so had to remodel the structure to check its method was buildable. But late arriving arch steelwork scuppered this plan.

The 60m tall arch sections were designed in steel rather than concrete to simplify construction, but the Thai fabricator was struggling to supply them on time. Archirodon therefore changed the construction sequence so that the main arch could be done later, allowing work to continue elsewhere on the structure. This introduced more redesigns which added further delays.

Another challenge was connecting the steel arches to the concrete piers. "We have a steel jacket around the concrete. It was the only way to handle the high torsion in the concrete as there is bending in both directions," explains Davies. Full penetration butt welds up to 100mm deep were then used to connect the steel segments to each other and to the jacket. The secondary arch segments were lifted into position using strand jacks located on a turntable supported by a portal frame system of towers and beams.

The next arch to be lifted in is the Marina Arch which will be positioned on temporary supports by a 1600t crawler crane. It is hoped this will happen next Spring, dependent on the steel fabricators. Finally the main arch will be lifted in using the same method as the secondary arch and the plan is to complete the structure by the end of 2009.

Sheikh Zayed Bridge from design to reality
John Dawson, HPR technical director explains the challenges of taking the Sheikh Zayed Bridge from design to reality.

Zaha Hadid created the shape of the Sheikh Zayed Bridge to symbolise the desert dune hills. Its splayed Marina Arch opens outwards on the Abu Dhabi side to form a gateway to the island city. However, the design posed considerable engineering challenges. The deck had to form a continuous ribbon across the entire 845m length of the crossing at a relatively high level. The north and south decks needed to be separated, with the space between clearly visible, and the arches had to form a continuous flowing line.

The environmental conditions are extremely hostile. The climate is hot, often humid and the bridge is in a marine environment. The outer mat of reinforcement in the tidal and splash zones is stainless steel. Cover to reinforcement is generous by most standards (up to 180mm), and concrete mixes have a low water/cement ratio and incorporate ground granulated blast furnace slag, together with the selective use of calcium nitrite corrosion inhibitor.

To protect the internal surfaces of the steel arches from corrosion, a de-humidification plant will continuously circulate dry air through the arches and piers. The asymmetric suspension system made it essential to employ a number of crossbeams between the twin decks. To meet the architect’s desire to make the deck to appear as a continuous ribbon above the Maqta channel, half-joints were used at all pier supports, and the crossbeams had to be kept within the depth of the deck. Seismic design condition proved critical and the unique form of the bridge means that adopting a conventional approach is not practical.

To limit horizontal loads on the piers, a fused isolation system was developed between the deck and the substructure. The final system of support comprises pot bearings, hydraulic dampers, rubber springs and rigid fuse-link devices, all of which are uniquely designed. Additionally, the complex system of support and geometry requires an enormous effort in temporary works design to support the structure during construction. Since the piers are all formed by a series of inclined arms, the deck is effectively supported by a series of springs that move during the construction phase.

To help attain this control over shape, the whole bridge including steelwork, formwork, pre-stressing and reinforcement, has been computer modelled in three dimensions to facilitate the production of shop drawings.

In a bridge, the primary structural elements form the visible shape of the structure, unlike buildings where the final shape may be achieved with cladding. In summary, this was not a straightforward task. But creating iconic infrastructure rarely is.

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