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Keeping it green

An unusual cable stay arch support has helped construction of an elegant new bridge in Malaysia. Pictures and words by Adrian Greeman.

Why read this

Aesthetics are top of the agenda on a river crossing in one of Malaysia's favourite tourist spots

Construction methods were driven by a brief to protect the environment

UK consultants have worked with local designers and contractors to deliver a solution

Locals come from miles around to taste the Chinese curried crabs and king prawns of the Bandar Seri Manjung district, on the coast of western Malaysia. In the future the government hopes many more people will be drawn to the fishing villages and other attractions of this flat seaside region some 200km north of Kuala Lumpur.

But if tourists are to come, infrastructure is needed. An elegant multi-arch road bridge is now nearing completion across the 1.25km wide estuary of the lazy Dinding river. With two shorter bridges over adjacent estuaries, it will help create a 13km road link between Damar Laut and the big Malay peninsula north-south highway. Development is planned to follow.

Consisting of 13 reinforced concrete arches, the bridge is slightly more expensive than a straightforward pier and beam solution, says Ghassan Ziadat, director at consultant Robert Benaim which is working as bridge advisor with KL-based design firm HMS Perunding SDN.

But the client, the bridges division of regional government body Jabatan Kerja Raya Malaysia, 'wanted something that will be a landmark statement - that will be part of the attraction of the area as it grows, ' he explains.

It was stressed the solution should minimise harm to the environment, especially to local 'bakau' or mangrove swamps along the sides of the estuary, and to the river bed regime itself.

Some use of the river bank for site operations has been unavoidable, and contractor group the Konsortium PanzanaLankhorst must re-plant damaged bakau as part of its clean-up. But destruction has been limited due to a special balanced cantilever system devised by Benaim for the arch construction. This allows the clutter and complexity of river bed falsework support to be largely eliminated from operations. Benaim and Perunding worked up some fifteen alternatives for the contractor when it was bidding for the £31.5M turnkey contract in 1997, and this one was selected even though it was not the cheapest.

Ziadat notes that the contract was bid before the Asian economic crisis struck, leading to chronic devaluation of the Malaysian dollar.

To construct the arches of the 930m main crossing, Benaim suggested a climbing formwork system. The hollow, curving box of each arch has been tied back to a sturdy, temporary steel tower as it advances in 5m sections. Formwork rides ahead of the stay-supported section until the arch is completed with a central stitch.

Before this relatively complex work began however, piers were needed. Ground in the estuary is alluvial silts, says Ziadat, 'supposedly overlying decomposed granite. We went down to as much as 60m with the piles.'

Some 5m to 13m of silt lies over progressively denser sands below. In all, 580 spun concrete piles of 800mm diameter were driven in clusters and topped with a 2.5m thick pilecap. A raked formation was used because of ship impact requirements - the river, which is up to 12m deep, is not heavily trafficked but the largest central arch allows a 40m wide ship channel and 18m height clearance. The bridge must be able to resist a 5,000t vessel travelling at 5 knots, or 10km/h.

Konsortium Panzana-Lankhorst had a little trouble working out the schedule for the arch casting system. 'It was thought a cycle of seven days could be established for casting each 5m segment, ' explains Ziadat, 'but in the event it has been more like 10 days.'

The problem lies in the careful balance required between the two sides of the rising arches.

Though the 36m high temporary towers can handle asymmetric loading, care is needed not to over burden them as concrete is skipped in via barge mounted cranes for the casting of alternate arch sections.

'And the formwork must be adjusted for the precise shape of the arch curve, ' adds Koon. Side arches are a 45m span rising to a 90m span for the central arch.

'The towers are assembled on a starting plinth on the pilecap using 12m high modules, ' says Ziadat. 'Then, as the arches are concreted outwards, they are tied back with 36mm diameter Nejibon high tensile bars.' Four bars hold each segment after it is poured, allowing the travelling formwork to be moved upwards for the next pour.

To minimise bending forces between the concrete and the steel stays the arches have a clever design feature - a hinge at the springing point. Temporary concrete infill behind plastic sheeting is used to pack the hinge point until the arches are complete, when their base points are grouted into a monolithic structure.

However, there is also scope to adjust the arch before the final stitch segment is poured at the top. Last of the arches was constructed in spring this year and in June work crews were finishing the last leaf piers which rise above to hold a composite steel beam and concrete road deck.

The whole crossing is expected to open at the end of this year to create a single lane two way highway for the region.

And by then, say the construction crews, they will have eaten enough prawns.

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