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Pushing the envelope

Bridges - Bridges are probably the most eye-catching civil engineering projects. Ruby Kitching gives the run down on four major crossings.

Ask the man or woman in the street to describe a civil engineer and the most usual response is, 'someone who builds bridges'.

This is hardly surprising since bridges are among the most awe inspiring forms of publicly accessible civil engineering.

Take a look at these four major crossings currently being built, all with UK input.

Stonecutters Bridge, Hong Kong

Location: 1.6km long crossing between Tsing Yi and Stonecutters Island across the entrance to the Rambler Channel port area.

Bridge type: Cable stay Start date: May 2004 Completion date: 2008 Cost: $346M Client: Hong Kong Highways Department Designer: Ove Arup Checking engineer: Tony Gee and Partners Contractor: Maeda-HitachiYokogawa-Hsin Chong JV.

Stonecutters Bridge will be the next landmark on the Hong Kong skyline, connecting one of its busiest port areas to the mainland. The main feature on the twin three-lane highway will be the 1,018m long cable stay span which will be punctuated by two gleaming, stainless steel capped, 300m tall concrete towers.

The east tower is now 100m high while the west tower has reached 25m. The jump formed towers are rising at a rate of 4m every six days.

An outer layer of stainless steel reinforcement will also be incorporated in the tower concrete to increase its durability and achieve the 120year design life.

Bridge construction involves rst building the concrete towers and back span viaducts, then lifting in the main steel deck sections.

Insitu concrete is used for the backspan deck, to counterbalance the lightweight steel boxes of the main span.

'We're now waiting for the east back span to be completed so we can transfer the falsework onto the west backspan, ' says Arup chief resident engineer Peter Lowther.

Approach viaducts are under construction and the deck units for the main span are being fabricated in China. The east back span will be completed by the end of 2006, and installation of the steel deck will begin in October. Cable installation will begin next year.

Oakland Bay Bridge, US

Location: 3.54km long crossing between Yerba Buena Island and Oakland Bay.

Bridge type: Self Anchored Suspension (SAS) signature span plus approach viaducts, known as the Skyway, Oakland touchdown and transition structure.

Start date: 2006 Completion date: 2013 Cost: $5.4bn Client: California Department of Transportation.

SAS Design checker: Hatch Mott Macdonald SAS contractor: American Bridge/ Fluor joint venture Following the 1989 Loma Prieta earthquake, structural integrity checks on the east span of the Bay Bridge crossing, also known as the Oakland Bay Bridge, led to a decision to replace the entire structure with a new bridge that included earthquake resistant components. The old structure will remain in use while the new one is constructed alongside.

A single tower self anchored suspension (SAS) bridge spanning 395m will be the main feature of the new bridge and will be the longest of its type in the world. The tower will stand 160m tall above mean sea level supporting the deck from one continuous main cable which runs through the deck and up to the top of the main tower.

'Normal suspension bridges have cables which anchor back into the rock. But here we've got 400ft [122m] deep Bay mud, ' says, Hatch Mott Macdonald project manager Ted Brown. 'But if you want a suspension bridge looking thing when there's no rock, this is the only way to do it.' The rest of the Oakland Bay Bridge comprises concrete box girder deck approach spans. Brown explains that 90% of the Skyway viaduct is complete and now incorporates hinge-pipe beams. These are enormous steel pipes placed in stainless steel sleeves which allow the beams to move during earthquakes, 'and the bridge to rock and roll, ' adds Brown.

Service platforms, a cycleway and deck sections on the western end are yet to be installed.

Fabrication of the SAS has begun, and will continue for at least another two years. Pier construction and foundations are complete. The SAS will take six years to complete owing to the extensive temporary works required. These involve building a temporary steel bridge along the line of the SAS deck while the tower is erected.

Permanent deck sections will then be laid on the temporary bridge and connected to the tower via the main cable. Once the suspension cable is fully stressed, the temporary bridge can be dismantled.

Sutong Bridge, China

Location: 8km crossing over the Yangtze River, upstream from Shanghai.

Bridge type: Cable stay Start date: 2003 Completion date: 2008/9 Cost: $785M Client: Jiangsu province Sutong Bridge Construction Commanding Department Concept: Cowi Designer: Second Navigation Engineering Bureau (SNEB) and Maunsell AECOM Contractor: China Harbour Engineering Group, Formwork manufacturer: Doka Contractor's construction manager: Arup This six lane crossing will include the longest cable stay bridge span in the world, beating Stonecutters bridge by just 70m. At 1,088m, the main span will be anked by 300m tall inverted Y-shaped concrete pylons. These are founded on 131, 2.8m diameter, 100m long cast insitu piles. Steel box girder sections, 40m wide and 4m deep, make up the main span. The rest of the crossing uses concrete deck sections.

Formwork maker Doka is using 72 SKE automatic climbers to cast the pylons, which are rising at 4.5m a week and have reached their apex. Installation of steel anchor boxes on each is underway.

Incheon Bridge, South Korea

Location: 12.3km long crossing between New Songdo city and Incheon Airport on Yongjing Island.

Bridge type: Cable stayed central span.

Start date: early 2005 Completion date: late 2009 Cost: $1.28bn Client: KODA Development Corporation ? 30 year concessionaire Designer: Chodai and Seo Yeong Engineering: Design supervisor: Yooshin and Mott Macdonald.

Design and build contractor: Samsung JV Checking engineers: Halcrow/Arup/Dasan JV Project manager: Amec The 12.3km long crossing will carry three trafc lanes in each direction with a hard shoulder on either side. The 1,480m long bridge, which claims to be South Korea's longest spanning cable stay bridge, comprises a steel box girder deck supported by 216 cables and 226m tall concrete pylons.

Approach spans are made from prestressed concrete deck units which are erected using the balanced cantilever method or by incremental launch. Piling for all piers is now complete and the rst three 50m east and west viaduct spans have just been erected. The Korean environment imposes a temperature range from -20 oC to +50 oC and tidal range in excess of 9m.

As a result, engineers have already decided to put concrete pylon construction on hold during the winter. Conventional formwork will be used to cast the pylons at low level, while higher up, self climbing formwork will be used.

Where water is too shallow for barge mounted cranes to erect the superstructure, approach deck sections will be transported on completed bridge sections using gantries then incrementally launched.

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