A new bridge is being built between Yeosu and Dolsan Island in South Korea to provide extra capacity during the 2012 Expo. Kyoung-Jae Lee reports
Construction of the Second Dolsan Bridge in Korea is currently being carried out by Daelim, the contractor responsible for construction of the existing Dolsan Bridge. The first bridge was notable for being the first cable-stayed bridge in Korea and the new project also embraces innovation, incorporating a stiffening edge girder to reduce the self weight of the deck and improve its aerodynamic stability.
The new link is intended to reduce congestion and reduce the distance of the crossing between the city of Yeosu and Dolsan Island. It will also act as an important infrastructure link during the Expo which will be held in Yeosu in 2012.
The bridge is being built by contractor Daelim for client Iksan Regional Construction & Management Administration, and has been designed by Daelim Consortium - a team of Daelim Industrial Company and Yooshin Engineering Corporation. Contract cost is US$60 million, which includes a 460m-long tunnel at one end, and a 280m-long approach bridge.
The main bridge has a total length of 464m, consisting of a 230m-long main span and 117m-long side spans. It will carry four lanes of traffic on a concrete deck which has an edge girder of reinforced concrete consisting of two edge beams about 1.5m deep and 24.2m wide and 25cm thickness of deck slab between each edge beam. These slabs are transversely post-tensioned at 4.5m intervals along the bridge.
This is the first time that a cable-stayed bridge with a concrete edge girder has been built in Korea, and it was necessary to exercise tight geometric control of camber during construction, as well as carrying out thorough checks to minimise cracking in the stiffening girders. Naturally the analysis of the structure required consideration of the bridge's aerodynamic stability during its balanced cantilever construction.
Caution must also be exercised in the choice of construction method and selection of form traveller. The main criterion governing the design and construction of the bridge was the control of tensile stress in the edge girder. For this reason the designers chose to adopt a three-span, continuous floating girder system with extra intermediate pier support, built using an underslung form traveller and with a dual-plane stay cable system.
It is worth noting that the deck system of this bridge is 'floating', meaning that it is directly suspended from the stay cables with no supports, bearings or cross-beams below the stiffening girders of the towers. Use of this structural form eliminates the cost of constructing cross-beams on the towers and the maintenance cost of the bearings. In addition to this, the bending moments caused in the towers by the deck are reduced compared to that of supporting-type systems. The pier table - the permanent deck structure that is supported by the first two stays - was cast at the bottom level of the tower and raised to its final level by jacks with a capacity of 120t. When lifting the pier table, the assumed weight was 540t. An impact factor of 1.15 was also considered to account for momentum change and horizontal deviation of the load being lifted. The pier table itself consists of a pair of edge beams and cross-beams identical to those of the typical deck cross-section. It also has a cross-beam at the midpoint, which is twice as wide as a typical cross-beam.
After setting the pier table, the distribution of cable stress and the variation of the forces in each stay had to be considered in order to maintain successful geometry control. Therefore operations were carried out to equalise the forces and stresses in each of the four stay cables. Intermediate pier supports were also installed at each side, in order to prevent large displacements - and associated cracking - from traffic loading and excessive bending moments. These intermediate pier supports also increase the stiffness of the side span and enable the stay cables on the side span to behave as back-stays.
Casting the concrete edge girder effectively while reducing the negative moment caused by the reaction of the form traveller were the most important considerations in deciding what type of form traveller should be used. An underslung form traveller supported by permanent stays was chosen; this self-launching system is designed for in situ casting of the segment using the free cantilever method. During casting of the segment, the traveller is supported at the leading edge by permanent stays which are threaded through the stressing block anchored at the longitudinal truss, and by hanger bars anchored to the previously-cast segment at the rear through the hanger frame. With a different type of form traveller, deck tensile stresses would have been very close to the stress limits during the launching and segment casting, which would have resulted in cracking. Using an underslung form traveller supported by permanent stays, however, means that the tensile stress in the deck falls within the service limits at all times. Also the time taken to cast each segment was reduced, due to the fact that it was possible to cast the edge and the slab at the same time.
Choice of a dual-plane stay cable system is intended to increase the torsional resistance and aerodynamic stability of the structure. The external pipe of each cable also has helical lines around its surface to improve the aerodynamic behaviour of the cables themselves. There are 52 cables in each cable plane, ranging in size from 23 to 49 strands, each of which are 15.7mm in diameter. The cables were supplied by VSL and installed with a monostrand jack, using its Iso-tension process in which each strand is stressed individually. For corrosion protection, the strands are galvanised, covered with wax, individually sheathed, and then placed inside a plastic pipe without grout. They are stressed from the upper end in the tower, where the concentration of anchorages makes the process more efficient. At the lower end, the cable is anchored below the edge girder.
The Second Dolsan Bridge is a sea crossing, hence the tower foundations had to be constructed below sea level. Caisson and cast-in-place concrete pile foundations were used for PY1 and PY2 respectively, taking into account the results of detailed investigation into geographical and geological site conditions, construction costs, programme and so on.
For the foundation of PY1, two small caissons were chosen. The biggest benefit of this decision was that only the 1,300 ton floating crane was needed to install the caisson foundations, which was cheaper and did not interfere with shipping at the site.
It is common to use caissons for tower foundations because of their large bearing capacity. But cast-in-place concrete piles were chosen for PY2 because of the difficulty of waterproofing such a shallow foundation and the fact that the shallow depth of just 3.9m restricted access by the floating crane. After reclamation work had been carried out, 15 drilled shafts of 2.4m in diameter and up to 55m deep were installed.
Construction of the tower legs was carried out using automatic climbing formwork, in lifts varying from 3m to 4m height. Each segment took about seven days to complete. To control the deflection under permanent loads, a precamber - maximum value of 100mm - was applied during construction, in order that the final position and shape of the towers would be as designed.
Typical H-shaped towers were chosen for the main supports, rising to a height of 90m. The upper cross-beam is located about 60m above the sea level. Initially the intention as to build the cross-beam and tower together, when the climbing form reached a height
