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Cable cat's cradle

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

Seoul, South Korea is preparing to hold its first World High Wire Championships. At the same time the organisers hope to set a new Guinness World Record for the longest-ever high wire crossing.

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

The design is 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' balancing-poles - avoiding the secondary cables either side of the main wire - as well as more normal concerns such as allowing clearance for shipping.

The result is a net of three spanning cables, two of which follow 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 is a set of three spanning ropes, developed in the early stages of the project. The main cable is 30mm diameter spiral strand; the two secondary cables are 22mm spiral strand.

Transverse cables [used to link the main and secondary cables] are 6mm diameter, but are a lower spec cable, ' explains Smith.

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

'It's a very short programme, ' says Smith, 'the programme to us has been short because there has 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 is 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's no float in that programme so really we are looking at a two day programme, ' says Smith.

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

'It is then turned out over the river round steelwork corner pieces we've called slide shoes.

The river bank working area where the cables are assembled is only 800m long so a second turn is needed to turn the bundle 180infinity back down the bank.' A boat will pull a lead line across the river and the cable bundle will then be winched across over a row of 17 barges, dipping into the water between each barge.

'As the lead cable is winched out there is a tendency for things to want to twist. It would be so difficult to get a twist out that we don't want to get one in the first place.' 'It would be disastrous to get a twist when you come to stress it - that would be the only point when it would become apparent as the cables are lifted out of the water.

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

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

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

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

The up-wind cable rises in the wind while the down wind side drops so the whole system twists under loading.

The project has a design life of just five days to serve the threeday festival which runs from 3 to 5 May. Work started on site last week and the erection of the cables is expected to take place in the week before the festival.

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