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The colossal 1018m long central span of Hong Kong's Stonecutters Bridge is entering the 'most difficult and exciting part of construction'. So says Arup project director Naeem Hussain, who talks to Jessica Rowson about the challenges of designing and building this spectacular engineering feat.

Hong Kong's Stonecutters Bridge is currently experiencing its most exciting period of construction since the project began on site in 2004. Its huge 290m tall single pole towers are complete and the deck sections joining them are about to be winched into place. Once the gap is closed, Stonecutters Bridge will be the second largest spanning cable stayed bridge in the world.

The 1.6km long crossing will be the centrepiece of a new dual three-lane expressway across the Rambler Channel, which will improve access to Hong Kong International Airport, urban areas of West Kowloon and the busy Kwai Chung container port.

Construction of the bridge has always been constrained by the fact that the Rambler channel must remain open to navigation throughout.

The first deck lift took place in December and required some impressive temporary works. Large brackets were fastened onto both sides of the east tower with temporary stay cables. These brackets supported the strand jacks, or hydraulic jacks, which lifted 4000t of deck steelwork up into position.

Subsequent deck sections will be lifted into place directly from a barge using two lifting frames with winches which are secured to the end of the existing steel deck. This method of construction speeds up the lifting operation, which leads to less disruption in the busy shipping channel.

Manoeuvring these deck segments, which each weigh 550t and measure 18m long by 53m wide is no easy matter.

"It's a very controlled operation," says Arup project director Naeem Hussain. "We have to make sure we have the right window for lifting, especially when we enter the typhoon season. The lifting sequence has to run like clockwork, so that there are no surprises."

The main span segments are floated out on a barge, located using GPS (Global Positioning System) to ensure that when the lifting starts, the segment is in the right position. It then takes about 45 minutes to winch the segment into position and it is then secured and welded in place.The welding process takes between eight and ten days, after which the next lifting sequence can begin. All the while ships continue to pass through the channel.

"We are launching the main span which is the most difficult and most exciting part," says Hussain. "A 200m by 200m exclusion zone operates around the barge and we use GPS to make sure it is in the right position. It's a controlled manoeuvre and we don't want the winched section to catch on the last section. There's a lot of shipping going on and we have to not interfere." As the main span segments are winched 70m up into place, they increase the deck out incrementally, cantilevering until they meet in the middle. Three segments have been launched on the east side while launching on the west side is just about to begin. The two halves should meet by November this year.

The main stay cables are erected using a tower crane attached to the side of the bridge tower. One end of the cable is lifted up and fed into the tower anchorage. Then smaller mobile cranes at deck level drag the other end along the deck, where each cable is winched and jacked into a deck anchor point.

Arup worked closely with contractor Maeda-Hitachi-Yokogawa-Hsin Chong joint venture on the construction method, taking high winds into account. Wind speeds can reach up to 117kmph in the typhoon season between May and November.Numerous wind studies were carried out during the design phase and a 1:1500 terrain model test in a wind tunnel was also analysed. Wind measurements came from a 50m tall mast, as well as a review of existing data from the nearby Tsing Ma bridge and the Hong Kong Observatory. All this helped define the turbulent properties of the wind; how it would vary with time, distance and direction.

Extensive testing was also needed for the upper sections of the bridge's two towers. As access for maintenance would be difficult on this part of the structure, Arup proposed fabricating the outer part of the structure from stainless steel. This is corrosion resistant but has not previously been used structurally.

"The bridge features the first use of stainless steel as an engineering material," says Hussain. "We used it as there will be benefits in terms of long term costs in that you don't have to repaint it."

A 1.75m tall prototype was constructed in order to determine the suitable type and grade of stainless steel for the 100m tall tower tips.

The investigation included looking at roll forming plates into a conical shape, fabrication of typical details and the finish that could be achieved on such large scale fabrication.

"We looked at the conical shape, how to get the matt finish and experimented with different (fabrication) techniques," says Hussain. "We wanted to know that what we were proposing could actually be done."

Assuming all goes well, the structure will be opened to traffic in June 2009.

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