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Choosing the cast

Now virtually complete, the duplicate Tsing Yi South provides a vital transport link between Kowloon and a container terminal on Hong Kong's Tsing Yi Island. David Bennett reports

Amass of traffic lanes and spiralling sliproads merge before crossing the Rambler Channel by a new bridge which connects Kwai Chung with Tsing Yi Island in Hong Kong .

The innovative duplicate Tsing Yi South Bridge - two bridges side by side forming separate dual carriageways - and their five approach viaducts, create the new commercial corridor for transporter and container lorries between Kowloon and the new container port being built on Tsing Yi Island.

The bed of the Rambler Channel is pockmarked with boreholes and studded with piles and caisson foundations from the first Tsing Yi bridge and the Rambler Channel Bridge which form part of the eastern link to Chek Lap Kok airport.

It was not from choice that the client, the Government of Hong Kong, commissioned another pair of bridges across this busy waterway, when planning a new container terminal. It was the residents of Kwai Chung Island who persuaded the authorities to build a separate bridge to keep the container lorries off the existing bridge, improve access to Kowloon and hopefully cut down traffic noise reaching their neighbourhood. Only time will tell whether the rising pollution and noise levels will be tolerable.

The conforming design, specification and tender enquiry documents were sent out by consultant Scott Wilson ( Hong Kong) for pricing in August 1995.

The contract comprised the two 650m long main bridges over the Rambler Channel, 2.4km of smaller span approach bridges, an underpass, sea defences, retaining walls, dredging , land reclamation and associated infrastructure road works.

Contractor Gammon Construction invited engineer Robert Benaim & Associates to prepare alternative designs for the two bridges which would be cheaper and faster to build than the precast segmental solution of the conforming design.

'We redesigned the six, 122m main spans and four, 70m side spans for the duplicate bridge, with slimmer concrete box girder sections using external prestressing, ' says Jeremy Cooper, director of Robert Benaim & Associates' Hong Kong office, 'and by casting them insitu using balanced cantilever construction, we were able to show a significant saving on the internally post-tensioned, precast, segmental solution.'

Benaim is better known for replacing insitu box girder bridges with precast segmental bridges, and with 2.4km of box girder deck for the approach spans, precasting seemed to make sense.

However it was impractical to standardise the segments in large enough quantities for precasting, as the approach spans had to maintain their varying cross-sections. 'In the end we also opted to cast the approach spans insitu using ground bearing falsework, ' says Cooper.

The piers of the main bridge spans were made monolithic with the bridge deck to eliminate expensive bearings.

They were designed to be sufficiently flexible to allow movement of the bridge deck without needing expansion joints.

Benaim's alternative design gave Gammon the most competitive solution and keenest tender price. The contractor was awarded the $HK1.338bn ($171.8M) contract by the Territory Development Department in December 1995.

Adopting external prestressing eliminates the risk of unseen tendon corrosion. In the UK in recent years, serious corrosion damage has been found in the tendons of a few bridges constructed with internal prestressing systems.

Detection is very difficult and expensive, and moreover, corroded internal tendons cannot be de-tensioned, removed and repaired and this can seriously undermine the long term integrity of a structure.

The corrosion environment in Hong Kong is not as severe as the UK as no deicing salts are used on the roadways;

nevertheless tendons can still corrode in the marine environment. The external tendons are protected by placing them in sealed, high density polyethylene ducts which are filled with cement grout, after stressing. Before grouting, the ducts are pressure tested to ensure their integrity and water tightness.

Other factors which increase the risk of corrosion of prestressing tendons are substandard concrete which will carbonate faster, lowering the pH around the tendon and anchor; poor waterproofing of the bridge deck; leakage through the joints of precast segmental sections and incomplete grouting of the tendon ducts.

'With no tendons inside the web sections of the box girder, insitu placing of concrete is easier, ' says Cooper, 'and there is less risk of honeycombing. The strength of the web section is improved because we do not have to allow for tendon ducts.'

For all these reasons cast insitu construction was preferred. In addition a polygonal tendon layout using external prestressing, allows for any angular deviations in the tendons to be concentrated at carefully designed saddle locations, eliminating the unintentional angle changes or wobble effects of internal prestressing layouts.

The main advantage of monolithic piers lies in reducing the number of mechanical bearings, and the consequent need for their replacement working over a busy waterway, and cutting out expensive temporary stabilising works of balanced cantilever construction.

The thin, twin wall pier supports make effective encastre connections with the main spans, so that live loading in one span has a limited effect on the adjacent spans. This simplifies the bending moments envelope in the deck structure and simplifies prestressing layout.

'Making the piers work monolithically with the bridge deck substantially increases the stiffness of the foundation, so that ship impact load is shared between the substructure and superstructure, ' says Cooper. The number of piles required for each foundation was reduced from 20 down to eight of 2.5m diameter.

However pier design was made more complex due to the effects of creep and drying shrinkage of the deck structure.

By tying in the three main spans over the Rambler Channel, a distance of 365m, the movement on the end piers is large.

The cross-section of the pier walls was slender enough so that the bending moment caused by the deck movement was minimised, yet strong enough to resist the bending moment created by movement of the pilecap under ship impact.

The 42 month contract was delayed by late completion of a contract for the strategic sewage disposal scheme, otherwise construction progressed without problems. The Duplicate Tsing Yi South Bridge - believed to hold the record for the longest fully external, post-tensioned span - was finished in 1998 with substantial completion of the project in February this year. Currently the last sections of the approach viaducts are being surfaced, while landscaping, noise barriers and lighting works are being completed.

Project team Client: Territory Development Department, Hong Kong Client consulting engineer: Scott Wilson (Hong Kong) Main contractor :Gammon Construction Bridge design: Robert Benaim & Associates Foundation design: Lambeth Associates

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