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A GRAND NEW BRIDGE 


The Seohae Grand Bridge, South Korea's longest crossing, features a 470 m center span that is the longest in the country.


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Man-Chung Tang, P.E.
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At 9.4 km, the Seohae Grand Bridge now stands as the longest bridge in South Korea. Rising above the Asan Bay approximately 65 km south of Seoul, the cable-stayed bridge, which took seven years to construct, runs from P'y_ongt'aek on the north side of the bay to Dangjin on the south. The bridge incorporates pistonlike lock-up devices (LUDs) that allow movement in response to temperature changes, creep, and shrinkage but resist such dynamic loads as aerodynamic motions and earthquakes. These devices are of critical importance, because a primary design challenge for the Seohae Grand Bridge was its location in an area of high winds. 

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The crossing consists of several kilometers of concrete box girder spans and the main bridge, which is 990 m long. The main bridge consists of an 870 m long cable-stayed structure and two 60 m long end spans of simply supported composite girders. The cable-stayed portion has three spans-a 470 m center span and two 200 m side spans. The center span provides a 62 m high navigation channel above the bay.

There are six traffic lanes, three in each direction. Together with a 3 m outside shoulder, a 1.2 m inside shoulder, and a 0.8 m median barrier, the roadway width between the inside faces of the outside barriers is 31 m. The centerlines of the cables are placed at a distance of 1.6 m from the inside face of the outside barrier, making the center-to-center distance between the two planes of cables 34 m.

T.Y. Lin International, of San Francisco, performed the detailed design of the main bridge and also provided construction services, including superstructure erection stage analysis, design of special construction equipment, and field assistance.

The 60 m end spans of the bridge (from piers 39 to 40 and piers 41 to 42) are connected to the cable-stayed spans by a hinge at piers 40 and 41. In this way the rotations in both the 60 m span and the cable-stayed spans will be kept independent (see figure 1). However, the hinge connection does not allow relative movement longitudinally.

Large expansion joints placed at the transition between the end spans and the approach spans accommodate the total longitudinal movement of the entire main span. In that location they will facilitate the periodic replacement that is necessary with expansion joints that permit large movements. Placing the joints at the end anchorage zone of the cable-stayed spans would have been less than optimal because that area is, in itself, very complex structurally.

Elastomeric bearings placed under the edge girders at the bridge's two pylons restrain the deck girder in the transverse direction, while lateral bumpers located at the edge of the edge girders further restrain the maximum relative transverse movement between the girder and the pylons. The vertical bearings are sized to provide approximately the same stiffness as the adjacent cables, so the girder behaves more like a floating member. This allowed the design to do without a hard point support, which would have caused a very high bending moment in this area of the girder.

To control the bridge's longitudinal displacements under live loads and other dynamic loading while allowing creep, shrinkage, and movement in response to temperature changes, the girder is hinged at one pylon and is movable at the other. However, to activate the horizontal support of the latter pylon under such dynamic loads as aerodynamic motions and earthquakes, LUDs are installed to connect it to the girder. The LUDs allow slow motions to take place but they lock up, or freeze, under quick motions. Accordingly, the structural system responds differently to static and dynamic loads.

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The LUD is essentially a piston. Normal pistons, for example, those in a steam engine, have one valve at each end so that the piston can be pushed back and forth depending on which opening the steam is coming through. An LUD, by contrast, is a piston with no holes at either end. Instead, there are holes in the piston itself. The cylinder, which is filled with a viscous liquid, can move slowly in response to such forces as creep and temperature expansion because the liquid can flow from one side of the piston to the other through the holes. But when forces act on the bridge quickly, the piston is frozen because the small holes do not allow the liquid to flow quickly enough. By adjusting the viscosity of the liquid and the size of the holes, the LUD can be "tuned" to various degrees of fixity.

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The deck of the Seohae Grand Bridge consists of two longitudinal steel girders spaced 34 m apart, with steel floor beams 4.10 m apart running transversely between these edge girders. The roadway deck consists of precast-concrete panels, the edges of which are all 310 mm deep, and cast-in-place joints. In the vicinity of the pylons, where the axial compression of the girder is high, the slab panels are solid. The panels have the shape of an inverted bathtub and are 260 mm thick in the area closer to midspan, where the axial compression is smaller. The thickness increases to a uniform 310 mm within five cable spaces on either side of the pylons to carry the increased axial compression force. Since the cast-in-place gaps are always placed on top of a steel beam, forming was not required.

Transverse and longitudinal deck posttensioning addressed the shear lag effect at the center and at the end of the bridge by creating the necessary compressive stresses to bring the anticipated tensile stress within acceptable limits. The multistrand tendons, each comprising four strands 15.2 mm in diameter, were used for the longitudinal prestressing of the deck. The tendons were posttensioned after the closure segments have been completed.

Edge girders, a steel stringer located in the center of the roadway, and the floor beams support the precast panels. The gap between the panels-which were filled with nonshrinking concrete after being placed-is 420 mm on top of the floor beams and 500 mm on top of the stringer. At the edge girder, a thickened cast-in-place slab of nonshrinking concrete extends the panels. Shear studs transfer the shear force between the steel girders and the concrete slab, and a large number of reinforcing bars-extended from the precast panel into the gaps-ensure continuity of the slab, which is designed as a continuous plate over elastic supports.

The edge girders are mostly under compression. In regions where the bottom flange will experience tension, the vertical stiffeners are bolted to the bottom flange to avoid fatigue problems. The web of the edge girders is 2.80 m deep. The bottom flange is 50 mm thick, except at the end segments, where it increases to 60 mm. The width of the bottom flange varies from 860 to 920 mm. The top flange, which is typically 500 by 50 mm, is located only on the inside face of the web to accommodate the cable anchorage. Because the cable anchorage plate is bolted directly to the outside face of the web, the local bending moment caused by the eccentricity of the anchorage from the centerline of the web (which usually exists in such cable connections) was virtually eliminated.

The side spans of the bridge are 200 m long, which is less than half the length of the 470 m long center span. Several cables are grouped together at the ends of the side spans above each anchor pier, where the edge girders deepen to 5 m to accommodate the cable anchorages and to provide room for the large end girder, which supports the steel girders from the end spans.Two planes of 72 cables fan out from the tops of the pylons. Each cable contains 37 to 91 galvanized, wax-coated strands.

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The steel floor beams run perpendicular to the two edge girders. The web is 2.8 m deep at the center, decreases to 2.5 m following the slope of the deck, and then increases again to the same depth as the edge girder. The top flange of the floor beam is 600 mm wide to provide sufficient room for the support of the precast-concrete panels and to allow for a large gap between the panels so that reinforcing bars can be properly spliced.

The floor beams were designed to carry the full structural dead load as a simple beam, without help from the concrete slab. During construction, however, four inverted king posts, one at each of the four front floor beams, were used to apply a predetermined load, thereby bending the floor beams before the gaps between the precast panels were poured and achieving a certain amount of residual compression in the top slab to reduce the possibility of cracking. This approach also allows repair of the top slab without any temporary support.

The concrete end floor beam, which weighs approximately 1,300 Mg, was precast on the ground and then lifted into place by means of a giant floating crane. The weight of the end beam, together with the reaction from the end span, is sufficient to overcome any uplift reaction under service loads. The end beam is structurally connected at each end to the steel edge girders by transverse posttensioning, which is anchored on the outside faces of the edge girder webs. Additional short tendons provide more capacity to transfer the vertical and horizontal shear forces between the edge girders and the deck slab at the end region.

Rising 180 m from the base, the Seohae Grand Bridge's pylons consist of two hollow rectangular columns, with exterior dimensions varying from 15.7 m at the base to 6.6 m at the top of tower in the direction of the roadway (see figure 2). Perpendicular to the direction of travel the column dimensions range from 6 m (below the deck) to 4 m (above the deck). As a rule, the columns are poured in 4 m segments, although smaller pours-down to about 2 m-are used above the upper crossbeam where the stay-cable anchorages are located.

The bridge's two upper crossbeams were selected for aesthetic reasons by the owner. The upper portions of the pylon columns are vertical and spaced at the same distance as the cables so that all cables are vertical in the longitudinal view. Since the vertical curve of the roadway is not exactly symmetrical with the main span, the two pylons have slightly different heights. The difference, however, is barely noticeable.

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Slip forming was used to simplify construction of the pylons. The lower crossbeam was precast on the ground and lifted to its final elevation by strand jacks attached to the pylon legs. The reinforcing bars at the ends of the crossbeam are coupled to the bars embedded in the pylon legs and the gaps are then filled in to monolithically connect the lower crossbeam to the pylon legs. The lower of the two upper crossbeams, formed and cast on top of the lower crossbeam and then lifted into place with strand jacks, was made monolithic in the same way. After the lower crossbeam was complete, supports were constructed on top of it, and formwork was constructed on top of these supports. The upper crossbeam was then cast on this formwork.

The pier table was assembled on the ground and lifted into place by a large-capacity floating crane.

Like the pylon crossbeams, the pier table, consisting of the steel frame and the cast-in-place deck slab, was fabricated on the ground. After the lower crossbeam was completed and while the upper portion of the tower was under construction, a large barge crane lifted the pier table panel to its final position. The large-capacity crane saved time and made erecting the pier table much simpler.

Hammerhead-shaped anchor piers support the cable-stay bridge end beam, and the 60 m end span girders are supported by hinge bearings on the end floor beam. Two 37-strand tie-downs hold the end beam to the pier to ensure stability of the cable-stayed bridge during construction. They also act to provide stability in the unlikely event that the 60 m side span is separated from the rest of the bridge as a result of an accident or is removed for a retrofit or replacement. During service loads, the anchor piers are always under compression.

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Dual-plane stay cables support the bridge, fanning from the top of the pylons and anchored to the steel edge girders. There are 72 cables in each cable plane, ranging in size from 37 to 91 strands, each 15 mm in diameter, a system supplied by Freyssinet, of Saint-Rémy-de-Provence, France. The strands are galvanized, covered with wax, individually sheathed, and then placed inside a plastic pipe without grout. The external pipe has a rib spirally wound around the outside face to improve the aerodynamic behavior of the cables.

To stress each strand individually, Freyssinet's Iso-tension method-which incorporates a monostrand jack-was used. A load cell mounted to the first stressed strand indicates the appropriate tensioning force in each subsequent strand. The stay cables, which are designed to be replaceable in traffic, are stressed from the upper end inside the pylons, where the cable concentration makes the process more efficient. The nonjacking ends of the cable anchorages are conveniently located above the deck. This eliminates the need for special platforms for regular cable inspection. At the lower end, the cable is anchored into a steel plate, which is attached to the girder web by high-strength, friction-type bolts to avoid fatigue problems caused by welding.

The lower of two upper crossbeams was cast on a lower crossbeam and lifted into place by strand jacks.

After the steel frame of the superstructure's end segments was assembled at the site, the 1,700 Mg end concrete floor beam was formed and poured, posttensioned, and then lifted to its final position on top of the end piers by means of the same barge crane used to lift the pier table segments. The steel frame of each panel consists of three 34 m long floor beams connected to 12.3 m long edge girders. Four custom lifters, one at the end of each cantilever, lifted the steel frame of each panel from a barge to its final position. After the edge girders were aligned and bolted to the cantilever, the same lifter raised the precast panels, one at a time, and placed them on top of the steel frame. The gaps between the panels were filled with nonshrinking concrete.

The erection of the steel girders in the two halves of the bridge proceeded simultaneously, and cable forces were adjusted as new segments are erected to ensure that the deck tension remains close to zero. To allow easy placement of the final, 12.3 m long closure segment, half the girder was moved backward about 30 cm by hydraulic jacks. The last steel frame panel was then lifted and the splice bolted together before the girder was jacked back and the final splice bolted.

With the last steel frame in place, the space between the two middle floor beams was not sufficient to allow the lifting of the deck panels, so the last floor beam was dismantled to provide the required space. This floor beam was reinstalled after lifting of the precast panels, which were temporarily stored on the finished portion of the deck. After completing the erection of the girder, a detailed survey of the bridge was made and the cable forces were adjusted according to predetermined values.

Construction of the pylons began in mid-1998 after completion of the cofferdams and the solid-footing foundation. The bridge superstructure was opened to traffic in November 2000.

Project Credits

  • Owner's representative: Chan-Ming Park, Korean Highway Corporation, Seoul, South Korea
  • Contractor's representatives: Tae-Sup Yoon, Seo-Kyung Cho, and Jong-Gyon Paik, Daelim Industries, Seoul, South Korea
  • Design: Man-Chung Tang, Gloria Hwang, and Dennis Jang, T.Y. Lin International, San Francisco; Dennis Lee, Leedco Engineering, Los Angeles
  • Design review: M.R. Huh and J. Son, Byucksan Engineering, Seoul, South Korea
  • Aerodynamics: Robert Scanlan and Nick Jones, Johns Hopkins University; Jon Raggert, West Wind Laboratory, Carmel, California
  • Construction engineering: Man-Chung Tang, Dennis Jang, J.R. Tao, and Mark Chan, T.Y. Lin International
  • Construction site support: Kook-Joon Ahn, T.Y. Lin International; Erich Aigner, Leedco Engineering
  • Construction inspection: Daewoo Engineering and COWI Consul, Copenhagen, Denmark

Source: CIVIL ENGINEERING  MAGAZINE, February 2001