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Bridges - Conference report

Crossing expertise Feng Shui and how it can influence bridge design was just one of the topics discussed at a recent bridge conference in Hong Kong

Inevitably it was Hong Kong's own record-breaking structures that were the stars for the 130 plus delegates from 18 countries to the second two day conference on current and future trends in bridge design, construction and maintenance in April. More than 60 papers covered a dozen or more countries at the event organised by the structures and building board of the ICE at Kowloon's New World Renaissance Hotel.

And it was the Stonecutters Bridge project that probably attracted most attention, not just because it will be the longest cable stayed crossing in the world when it opens in 2008. For the first time ever on a major crossing the Hong Kong Highways Department decided to proceed via a multi-stage design competition without any guarantee that the designers of the winning entry would also be awarded the lucrative contract for detail design and site supervision. In the event the winning Halcrow/Flint & Neill-led team lost out to Arup earlier this year.

In their paper, Michael Hui and Chris Wong of the Highways Department described the aesthetic development of the five shortlisted entries between the first and second stages of the design competition. The winning design features the world's tallest bridge towers at 290m high, a distinctive single leg design with insitu concrete lower sections and steel upper, and a split steel box girder deck with a main span of 1018m.

No major piece of infrastructure in Hong Kong can be finalised without approval from a Feng Shui expert. The judges sought specific advice on the tower design, which some saw as having an undesirable 'joss stick' theme. There was also concern about the large light features on the towers. These, the Feng Shui expert suggested, should be reduced slightly in size. The joss stick analogy would only be valid if there were three towers, he advised. And his final conclusions were particularly reassuring.

Stonecutters would be a 'dragon', with the light features forming its eyes and the curving approach viaducts its tail. Vessels using the container port behind the bridge would be safer because of the dragon's beneficial influence, the Feng Shui expert ruled. The judges themselves described the design as impressive and powerful, elegant and appropriate to its setting, with a magnificent appearance from directly below.

The engineering constraints which influenced the design were described in a later paper presented by Ian Firth of Flint & Neill and Stuart Withycombe of Halcrow. Stonecutters Bridge will span the Rambler Channel, said to be the busiest in the world. Minimising disruption to shipping was an imperative, and the design team's preferred option was to launch the main span deck units from the sidespans rather than lift them up from water level.

Avoiding obstruction to shipping also led to the veto on various temporary damper ideas intended to stabilise the decks during construction. Instead, tuned mass dampers were seen as the most practical alternative.

To reduce wind loadings by 35% the team chose prefabricated locked coil stays over site assembled parallel strand stays, even though contractors usually prefer the latter option.

An equally fascinating tale of the development of a complex design was revealed in no less than four papers on the West Rail viaducts. These form more than 13km of a new route from West Kowloon to Tuen Mun, most of which runs through a dense urban landscape. Tight noise restrictions led designer Arup towards a twin concrete box design with floating slab track and high curving parapets, as described in a paper by Naaeem Hussain of Arup. Trains would be fitted with skirts, creating noise absorbing plenums under the train and the adjacent trackside walkways.

How the principles underlying this conforming design were retained in the alternative design drawn up for the successful contractor by Robert Benaim & Associates was described by Chris Calton of the KowloonCanton Railway Corporation and Nick Southward of Benaim.

The key differences were the narrowing of the box girders so that the webs were located immediately below the slab track bearings - which significantly reduced noise from wheel-rail vibration - and the adoption of an 'integral' design.

In effect the viaduct is made up of a series of back to back portal frames. This layout greatly increases the resistance to typhoon wind forces of such a narrow deck design. Match casting was used to produce the precast concrete units that made up the deck, a process described in a paper by Hugh Boyd of West Rail viaducts joint venture main contractor Maeda-Chun Wo.

Although Hong Kong projects stole much of the limelight, a number of other papers intrigued the delegates. Professor Wojceih Radomski of the Warsaw University of Technology described the astonishing boom in car use and bridge building in Poland as a result of economic and political liberation over the last 10 years.

Contest's Dr Emanuele Codacci-Pisanelli outlined the rehabilitation of an unusual prestressed concrete 'cable stay' bridge in Libya, while High-Point Rendel technical director Michael King presented a paper on the twinning of South Korea's Jindo Grand Bridge.

An unusual composite 'integral' crossing in Japan was described by Professor Hiroshi Hikosaka of Kyushu University.

Designed for improved seismic performance, the three span Imabeppu river bridge features continuous twin steel deck girders rigidly connected to concrete piers, with a non-composite precast prestressed deck over all.

The connection is made rigid by means of cast-in 22mm thick steel plates drilled with 70mm diameter holes which link the crossbeams on either side of the column head.

These 'perfobond' connectors are used without reinforcing bars. Another unusual aspect of this project was the use of balanced cantilever construction, seen as the most practical option in mountainous terrain. Overall, the designers claim a 5% saving in cost and a 25% reduction in construction time over conventional concrete alternatives.

Perhaps the most unusual project presented to the conference was the Genoa Harbour crossing (NCEI August 2000). High-Point Rendel technical director Abdul Farooq described how the various options for relieving severe congestion in the ancient Italian port were considered. These included a 610m span suspension bridge with a deck clearance of 75m and tower height restricted by flight path limitations, and an immersed tube tunnel. Although both had similarly beneficial effects on traffic flows in the port at much the same capital cost, it was the tunnel that got the final vote.

Farooq told the audience of bridge specialists that a bridge was thought to be too dominant for the site and not be in keeping with the architectural heritage of Genoa.

In his opening address, Hong Kong Institution of Engineers vice president Dr CK Lau had spoken of the looming bridge maintenance crisis in mainland China.

He called on the ICE to hold the next 2003 conference on the mainland, a point that ICE deputy secretary Amar Bhogal promised in his closing remarks to press at Great George Street.

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