Piers and towers of a major cable-stayed bridge in South Korea are currently under construction. Armin Patsch explains the development of the design.
Construction of the main towers is under way on the Second Geo-Geum Grand Bridge which will be part of the fixed connection of the Geo-Geum Island to the South Korean mainland. The two lane highway bridge with its total length of 2,028m is the second stage of this fixed-link project and will connect Sorok Island to Geo-Geum Island passing Dae Hwa Island. Erection of the superstructure and cables is intended to start in 2008 with completion of the whole link currently scheduled for 2010.
This high level crossing is composed of a 912m-long approach viaduct and a 1,116m long main bridge which will carry the road over a 210m-wide shipping channel. The main bridge is a cable-stayed bridge with a main span of 480m, 198m-long side spans and 119m-long end spans, while the approach viaduct is a continuous girder with regular spans of 120m.
The main design considerations of the Second Geo-Geum Grand Bridge were how to combine its functional requirements as a highway bridge, with innovation in design and visual harmony with its surroundings. The stay cables are arranged in a single plane along the centre line of the bridge deck; their semi-fan arrangement with its bundled configuration give it a striking appearance. On completion, the bridge will be the largest of its kind in Korea.
The navigation channel has a width of 210m, a clearance of 38.5m above reference level and intersects the bridge alignment at an angle of 35o. Collision loads of 50MN at the towers and 15.8MN at the anchor piers were considered at the design stage, to resist any impacts from aberrant vessels.
Deep foundations were required to cope with the water depths of up to 35m and weathered soils. Additionally, the bridge had to be designed for high wind speeds as it is located in typhoon region. The basic wind velocity is a 10 minute mean wind speed of 40m/s at 10m above sea level. Finally, high seismic loads had to be considered, with a maximum ground acceleration of 0.385g.
At the feasibility phase various alternatives were investigated and a shortlist of seven options was made; six of these were cable-stayed bridges with spans ranging from 300m to 468m, one or two towers, and a range of truss girder and box girder decks. The seventh was a suspension bridge with main span of 450m and two towers.
The final choice was a two-tower cable-stayed bridge with 480m main span, a 6m-deep steel truss deck, bundled stay cables and approach viaducts with 6m-deep truss girders, typically 120m spans.
The horizontal alignment is straight at the main bridge and the approach bridge is curved with a radius of 1300m; for vertical alignment, the main bridge is curved on a radius of 16,667m over a length of 1200m and inclined at 1.8% over the remainder.
Towers are designed in concrete, consisting of a delta shape at the bottom, which divides at 85m height into a double leg structure. The cable anchor boxes are located between the two legs, providing unobstructed access inside the legs.
One of the most unusual things about this bridge is the cable configuration; 84 cables are arranged in bundles of seven cables for aesthetic and structural reasons. It results in almost-uniform loads in all the cables of one bundle, so loss of cables and cable replacement is no problem. It also means that if damping measures are required, they can be concentrated on the bundle rather than having to be applied to each cable individually.
This type of arrangement was developed specifically for this project and it is believed to be the only bridge in the world where it is used. From a visual point of view, it can be regarded as sunlight beams shining through clouds or through the roof of a rain forest. The use of a truss for the superstructure also fits this arrangement very well, since it provides sufficient strength and stiffness to bridge the gap between the cable bundles.
Another benefit of the truss girder is the possibility that the bottom slab can be used for a future 4m-wide cycle path, or for emergency vehicles if the main road is blocked by a traffic accident.
The 120m long spans of the approach viaduct were chosen with the aim of minimising the number of piers and foundations, both to reduce costs and in order to open up the view.
The steel composite superstructure consists of a truss of 1,116m length for the main bridge and a total length of 912m for the approach bridge. The main bridge is a symmetrical cable-stayed structure with 480m-long main span, 198m-long side spans and 120m-long end spans. The deck has a 15.3m-wide concrete slab on top, which acts compositely with the top chord of the truss. The bottom slab with a width of 6.8m between the trusses is a steel orthotropic deck at mid-span and a 700mm-thick concrete slab at the supports. The concrete bottom slab is only provided where negative moments are experienced, at the supports at the hold-down piers and at the tower axis; use of this concrete bottom flange considerably reduces the amount of structural steel required. Especially at the tower axis, where axial compression force is high due to permanent loads, the concrete section is more economic than a steel section. The 700mm thickness was selected for structural reasons to match the full height of the lower chord and cross-beams.
The horizontal distance between the axis of the trusses is 7.5m and the diagonals are inclined with 60' with an axis-raster of 6m. The height of the steel structure is 5.9m. The top and bottom chord has a size of 700mm by 700mm, while the diagonals are 600mm by 700mm. The top flange of the top chord is normally 800mm wide, but it is widened at the end span and at the support to limit the maximum thickness of the steel plates to 75mm.
The trusses consist of diagonals only, without vertical members, for clarity and aesthetic considerations. The outer surface is plane and clear, for that reason all plate thickness variations are designed to take place inside the truss chords. Diaphragms are arranged at the same angle as the diagonals and have large openings to allow the walkways to pass through. They are arranged at end and side spans at the third-points of the span. Additional diaphragms are provided directly above the supports and at the outer anchorage of each cable bundle.
The grade SM520 steel structure is completely welded, including the construction joints, so that the inside of the truss box is protected from corrosion. The concrete top slab is designed for the full cantilever moment in the transverse direction and is prestressed transversely with four, 15mm VSL tendons at 600mm spacings over typical areas and 300mm spacings at the section of the slab where the b
