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Cable cat's cradle

Roped access - Installing a 1km tightrope high above South Korea's Han River was something of a balancing act. Jon Young reports.

Seoul in South Korea held its first World High Wire Championships last month using a high wire crossing claimed by the organisers to be the longest ever built.

Consultant Tony Gee & Partners (TGP) were responsible for the design of a high wire that stretched 1,000m across the Han River in central Seoul, supported by 25m high temporary towers on each bank.

The design was complicated by the need to meet a number of unusual serviceability criteria, including the steepness of the sag at each end of the wire and the need to provide clearance for the competitors' balancingpoles ? avoiding the secondary cables either side of the main wire ? as well as more normal concerns such as allowing clearance for shipping.

The result was a net of three spanning cables, two of which followed a pronounced horizontal curvature as well as the vertical sag under gravity, plus numerous transverse stabilising cables.

The towers were designed by a local contractor. TGP associate director, Ian Smith, headed up the cable design, working from the UK.

'The system was a set of three spanning ropes, developed in the early stages of the project. The main cable was 30mm diameter spiral strand; the two secondary cables were 22mm spiral strand. Transverse cables [used to link the main and secondary cables] were 6mm diameter, but were a lower spec cable, ' explains Smith.

The project faced two significant challenges, the first being time.

'It was a very short programme, ' says Smith, 'the programme to us had been short because there had been some uncertainty as to what they needed.' 'We made proposals to the Koreans ? but we needed approval from the contractor [Steel Life] and confirmation that the tightrope walkers were satisfied with the set up and any obstructions on the line, like the transverse wire links, would be OK.

'This was quite a challenge.

We could propose a solution from a structural point of view but needed to know it matched what the walkers wanted.' The second challenge was installing the cables in position over the river.

'We initially hoped to have the cable in place with no more than a one day closure of the river but there was no float in that programme so really we were looking at a two day programme, ' says Smith.

'The cable bundle was fully assembled along the river bank, ' says Smith.

'It was then turned out over the river round steelwork corner pieces we called slide shoes. The river bank working area where the cables were assembled was only 800m long so a second turn was needed to turn the bundle 180infinity back down the bank.' A boat pulled a lead line across the river and the cable bundle was then winched across over a row of 17 barges, dipping into the water between each barge.

'As the lead cable was winched out there was a tendency for things to want to twist. It would have been so difficult to get a twist out that we didn't want to get one in the first place.' 'It would have been disastrous to get a twist when you came to stress it ? that would have been the only point when it would have become apparent as the cables were lifted out of the water.

'The lead cable had a swivel at its connection with the bundle to help prevent twists and a winch was also fitted to the rear of the bundle so that it could be slightly stressed during the pull.

This helped keep it near the surface of the water, preventing snags on the river bed.' The cables were then installed into sheaths on the tower, and six winches, one at each end of the three cables, were fitted. The winches were continuously live because of the very short duration of the work.

'Creep in the cables still went on because of the short time scale, so the winches needed to remain live to take it in, ' explains Smith.

Smith notes that 'the behaviour of the cables under wind loading was a bit peculiar because the secondary cables were not on the same plane.

The up-wind cable rose in the wind while the down wind side dropped so the whole system twisted under loading.

The project had a design life of just five days to serve the three-day festival which ran last month.

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