The Design and Construction of Geogeum Grand Bridge

Transcription

1 The Design and Construction of Geogeum Grand Bridge Woo Jong Kim, Kyung Sik Cho DM Engineering Co., Ltd, Seoul, Seoul, KOREA Abstract The Geoguem bridge was recently designed to link Geogeum island to Sorok island on the south sea of Korea. This bridge, under construction since 2002, is one part of the national route 77, which route is not only for the industrial traffics but also for the tourism resources. So the aesthetic point of view was considered as an important aspect. The special arrangement of stay cables is introduced, which is motivated from the scene of the shining through of the cloud. The main structure is a cable-stayed bridge with 2 towers and 5 spans. The center span is 480 m long. The superstructure is designed as composite truss girders. The upper deck is designed for 2 lane traffics and lower deck is for pedestrians and bicycle riders. The basic and detailed design of this bridge was selected as the first prizewinner in bridge competition in This paper describes some detailed features of planning, design and construction of this bridge. Keywords: Geogeum, bundle type cable-stayed bridge, composite truss girder, bell shaped caisson 1. Introduction 1.1. Location The Geogeum bridge was issued by Iksan Regional Office of Ministry of Construction and Transportation in This bridge is the route 77 of the national way, which is located in south and west of Korean Peninsula. It is the fixed connection between Sorok Island and Geogeum Island. The 2-lane highway bridge with its total length of 2,028 m is composed of a main bridge of 1,116m-long and the approaching viaduct of 912m-long. The main bridge is planned as a cable-stayed bridge with a main span of 480 m, and it has 5 continuous spans. The approach viaduct of total 912m-long is a continuous girder with regular spans of 120 m. This bridge is under construction by Hyundai Engineering and Construction. It was Turn-key project where both basic and detail design were accomplished for the tender documents. Design was done by DM, CSE in Korea and LAP in Germany. Figure 1. Location of Geogeum bridge

2 1.2. Feasibility Study The bridge should have a space of 210m wide navigation channel and a clearance of 38.5 m above reference level and crosses navigation channel with an angle of The maximum water depth is up to 37m and these regions are widely spread. And the level of sea bed is changed seriously and weathered soils are required to provide for deep foundations. The bridge had to be designed for high wind speeds as in the South of Korea typhoons can occur. The basic wind velocity V10 is 40 m. In addition, high seismic loads had to be considered in this region, with a maximum acceleration of g. In the original basic design, main bridge was also cable-stayed bridge, which has 250m-long main span and orthotropic steel deck box girder. The approach viaduct of total 1740m-long is a continuous girder with regular spans of 90m. Some problems in the basic design were pointed out. 1) Many deep caissons make the construction cost be so high, 2) The real-time simulation said the 250m-long main span was not enough, and so on. Many alternatives were investigated. Last two of the alternatives are 4 and 6 as below. Alternative 4: CSB with truss girder, main span 468 m, 2 pylons and approach bridge with regular spans of 120 m Alternative 6: CSB with truss girder, main span 300 m, 1 pylon and approach bridge with regular spans of 120 m But the Alternative 6 have many deep marine foundations rather than Alternative 4. Figure 2. Alternatives of feasibility study At the feasibility phase various alternatives had been investigated. First of all, we had to choose the cable arrangement type because the cable arrangement has a big effect on the aesthetics of the bridge. Finally, the bundle type with semi-harp arrangement was selected. The stiffness of girders was carefully studied. (Scheme A) Distributed Type with Semi-Harp (Scheme B) Bundle Type with Semi-Harp Figure 3. Cable arrangement of feasibility study

3 C L 1.3. Final Layout The selected layout is similar to Alternative 4 and is composed of a two pylon cable-stayed bridge with 480 m main span, a 6.0 m deep steel truss and bundled stay cables The 120 m long spans of the approaches results in the least amount of piers and foundations, again in order to open up the view and to reduce foundation cost. The concrete pylon consists of a delta shape bottom part, merging at about EL m to a double leg structure. The cables are supported by cable anchor box, which is connected to the column of the pylon with studs. Three cable anchor boxes are arranged between the legs, these anchor boxes are installed after the completion of the pylon leg. Figure 4. General arrangement of cable-stayed bridge The cable configuration is very unique. It is bundled that the 84 cables are arranged in bundles of 7 cables for aesthetic reasons and for structural reasons. This type of arrangement is the first try in the world. It represents the sunlight beams shining through clouds or through the roof of a rain forest. The composite truss girder of the superstructure fits also very well to this arrangement, since it provides sufficient strength and stiffness to bridge the gap between the cable bundles. The dominating dimensions of the pylon give the bridge a very strong and safe impression. Concrete Deck Steel Deck Concrete Deck Steel Deck Secondary Crossbeam C.T.C=6.000 Main Crossbeam C.T.C=6.000 Secondary Crossbeam C.T.C=6.000 Main Crossbeam C.T.C=6.000 General Part Cable Part Figure 5. Cross section of truss girder The truss girder section is a double composite in order to improve the efficiency of girder stiffness. Lower deck is rigidly linked with the chord of truss girder as well as upper deck is. Also the lower deck is composed of different two decks, concrete and steel deck. The concrete deck is used at the part of pylon and interior pier where the lower deck of truss girder will be received a large compression forces. At the other parts of lower deck, steel deck plate is used to reduce the self weight and resist the tension forces. The composite truss girder has the advantage that the bottom space, 4.0 m wide, can be used for a pedestrian and bicycle path or for emergency vehicles in case the upper roadway is blocked due to a major car accident.

4 2. Design 2.1. Superstructure The deck is composite truss girder like Oresund bridge. This kind of girder has a very large stiffness. The deck has a m wide concrete slab on the top acting as composite section together with the top chord of the truss. The bottom slab with a width of 6.80 m between the trusses is a steel orthotropic deck in the mid span and a 70 cm thick concrete slab at piers. This double composite section saves a considerable amount of structural steel. Especially at the pylon axis, where axial compression force is high due to permanent loads, the concrete section is more economic than a steel section. The horizontal distance between the axis of the trusses is 7.5 m. The diagonals are inclined with 60. The height of the steel structure is 5.94 m. The top and bottom chord has a size of 700 x 700 mm, the diagonals are 600 x 700 mm. The top flange of the top chord is normally 800 mm wide, in the end span and at the support it is widened to limit the maximum thickness of the steel plates to 75 mm. The warren typed trusses are composed of diagonals only (without vertical members) for aesthetical reasons. The outer surface is plane and clear, for that reason all plate thickness variations take place at the inside of the truss chords. Diaphragms are arranged in the same angle as the diagonals and are provided with large openings to allow the pedestrian lanes passing through. They are arranged at end and side spans at the 1/3-points of the span. Additional diaphragms are provided directly above the supports and at the outer anchorage of each cable bundle. Figure 6. Composite warren truss girder The steel structure is completely welded, including the construction joints, so that the inside of the steel truss boxes is corrosion protected. All steel of the superstructure is of Grade SM 520. The concrete top slab is designed in transverse direction for the full cantilever moment. The slab is prestressed in transverse direction with tendons 0.6"-4. The spacing of the tendon is 600 mm at typical areas and 300 mm spacing at the cable anchor position. The tension stresses for the cantilever moment is designed to be smaller than 1.5 N/mm². The provision of HDRB (high damping rubber bearings) at the main bridge and the approach bridge was considered as the best solution. The use of the isolation with HDRB has proven to be a very efficient technique to protect structures from earthquake to distribute loads to more structures than one fix point only to be sufficient stiff for small wind loads to be sufficient flexible for movements caused by creeping, shrinkage and temperature to get the structure back in the starting position. Figure 7. Bearing arrangement of Geogeum bridge

5 2.2. Pylon The pylon with 171m-rise above the sea level makes the most dominating view of the bridge. The pylon consists of two inclined legs tied together at the elevation of by the cross beam. Above +85 m, at the parts of the cable anchorage, the pylon legs are connected by 3 steel boxes of 15.5m high. In transverse direction the pylon legs and the cross beam are acting in a body. Due to high wind loads in the final stage, the bending moments in the cross beam require a considerable prestressing. All outer edges of the pylon shafts are rounded with R = 1.0 m to reduce the wind loads and for aesthetic reasons. Figure 8. Round shaped concrete pylon 2.3. Cables and Anchorages The cables consist of 15.7 mm mono strands with a tensile strength of 1860 N/mm². Only 3 different types of cables composed of 55, 61 or 75 mono strands will be used. The strands are galvanized, covered with wax, individually sheathed and then placed inside an outer colored HDPE tube without grout. The outer HDPE tube is provided with helical filets at the outside face to improve the aerodynamic behavior of the cables. Stressing of the strands will be done by applying the Iso-tensioning method which incorporates a mono strand jack. The stressing operation will take place at the lower cable anchorage, where the bottom slab provides good access and room. The anchorage zones of the towers are designed as steel composite structures. It was considered as more efficient to anchor the cables in a steel anchorage box rather than in concrete, since the steel plates carry directly tensile stresses from the side span stays to the main span stays. By introducing the steel type anchorage boxes, the post-tensioning in the wall of the shaft is avoided. A further advantage is, that the fabrication of these steel anchorage boxes can be done in the shop where higher quality and accuracy can be achieved than 85 m and more above ground. The complete steel boxes are designed to be lifted by using heavy lift equipment on the tower top. The composite section will be completed by casting of concrete into the gap between the pylon walls and the steel boxes. Figure 9. Anchorages of girder and pylon

6 2.4. Caisson of the Pylon The bell shaped caisson has a depth of 41 m and 37m respectively, the lower part has outer dimensions of 32 m x 38.5 m while the upper part is 19.5 m x 26 m. And this caisson is supported by 30 piles Ø 2.5 m. This solution was selected from a number of alternatives because of its high capacity to resist the large horizontal forces from ship impact, seismic and water flow and waves. Front view Side view Figure 10. The bell shaped caisson supported by piles 2.5. Wind Tunnel Test To check the aerodynamic behavior, the wind tunnel test was performed. Geogeum bridge has a very good aerodynamic. Design wind speed V10 is 40m/sec. Flutter is controlled under 10cm displacement at 100 year period wind, V(100). Figure 11. Section model test and full scale test of bridge

7 3. Analysis 3.1. Used Program for Analysis In the design procedure of the Geogeum bridge, program 3D-series was applied as main analysis program. 3D is a specialized bridge analysis program with the spatial frame element. It has some experiences to analyze several cable-stayed bridges, such as Seohae bridge and Samchunpo bridge in Korea. A composite bridge has almost all structural components of bridge, it shows not the behaviors of concrete but also those of steel bridge. So, in order to establish a engineer-oriented analysis program, we have to grasp clearly the whole behaviors of the composite bridge. Some fundamental functions of analysis program will be shown by taking the analysis procedure of a composite cable-stayed bridge. The Composite Beam Element The girder section in the composite cable-stayed bridges varies with the construction sequence. Initially it is a non-composite section with steel girder only, but after completion of curing the concrete slab, it becomes a composite section. To represent systematically these complicated variations of the sectional property and the sectional stresses, two independent elements with rigid link are generally accepted. Concrete Steel Rigid Figure 12. Schematic model of composite beam element But two element model have some problems, such as, the results of that model depend on the number of elements, the rigidity of connect element, and it shows some large local value at node. In conclusion, it strongly depends on the modeling conditions. In 3D-series, to yield more reliable results, the composite beam element is introduced, which make a composite stiffness matrix satisfying the compatibility condition between the steel girder and the concrete slab. And it is based upon the assumption of continuously connect along the element length not only node. Creep Verified Element with Prestressed tendon Generally, the slab of the composite cable-stayed bridge is in a high compressive stress induced by cable force. Then it is very important to simulate the creep and shrinkage. Especially creep behavior must be clarified thoroughly. But creep mechanism of the concrete is influenced by so many parameters that it is one of the most complex numerical problems in time dependent analysis of concrete structures. In the numerical modeling of creep, it is the important factor to develop the procedure to efficient store the stress history of the specimen. This makes it desirable to formulate a creep model which stores the stress history in a compact form. P.S Tendon Element can exactly simulate the behavior of the P.S beam which has a parabolic shaped P.S steel embedded. The concept of P.S Tendon Element is that the self-equilibrated loads extracted from the profile of P.S tendon acts on the member. It is considered under any construction to estimate precisely the prestressing losses induced by creep and shrinkage Construction Commands The construction process of the composite cable-stayed bridge is very complicated. As the result of every single step has effect on the final result, the exact simulation of construction is very important. 3D-series has surplus construction commands enough to simulate any construction stage of bridge. The construction commands adopted in this program are followings. ERECT, REMOVE GIRDER, SLAB, CHANGE SECTION SUPCON, SUPPORT, CHANGE SUPPORT, REMOVE SUPPORT CONNECT, CHANGE CONNECT, REMOVE CONNECT STRESS CABLE, CHANGE CABLE, REMOVE CABLE STRESS TENDON, CHANGE TENDON, REMOVE TENDON SET TRAVELLER, MOVE TRAVELLER, REMOVE TRAVELLER SET DAY, LOAD, CASTING CURVE, MAXMIN

8 Y Z Y Z X X SCALE OF OBJECTS SCALE OF OBJECTS SYSTEM RESULT ISOMETRIC VIEW SUPPORT ELEMEMT CONNECT ELEMENT MAXIMUM( 2179 ) MIMIMUM( 2018 ) ABS MAX( 2018 ) Y Z Y Z X X SCALE OF OBJECTS SYSTEM RESULT ISOMETRIC VIEW SUPPORT ELEMEMT CONNECT ELEMENT MAXIMUM( 2179 ) MIMIMUM( 2018 ) ABS MAX( 2018 ) Y Z Y Z X X SCALE OF OBJECTS SYSTEM RESULT ISOMETRIC VIEW SUPPORT ELEMEMT CONNECT ELEMENT MAXIMUM( 2178 ) MIMIMUM( 2054 ) ABS MAX( 2178 ) Modeling Basically beam element with 7-DOF, which include the warping deformation, and the equivalent modulus cable element of Ernst was used. Anchorage of Girder Support of Pylon Anchorage of Pylon Bearing of Pier Bearing of Pylon Figure 13. Modeling of bridge and details 3.3. Some Results of the Analysis (1) Stiffening girder analysis To get the initial cable jacking forces and the final state under dead load, several times of backward and forward analysis were performed. The following diagrams show the changes of the axial force at the lower chord. 3 D P L O T step01 3 D P L O T step07 3 D P L O T step04 하현재축력도 SUPPORT ELEMEMT CONNECT ELEMENT 하현재축력도 SUPPORT ELEMEMT CONNECT ELEMENT 하현재축력도 SUPPORT ELEMEMT CONNECT ELEMENT SYSTEM RESULT ISOMETRIC VIEW MAXIMUM( 2009 ) MIMIMUM( 2020 ) ABS MAX( 2020 ) SCALE OF OBJECTS SYSTEM RESULT ISOMETRIC VIEW MAXIMUM( 2009 ) MIMIMUM( 2018 ) ABS MAX( 2018 ) SCALE OF OBJECTS SYSTEM RESULT ISOMETRIC VIEW MAXIMUM( 2009 ) MIMIMUM( 2018 ) ABS MAX( 2009 ) D P L O T step10 3 D P L O T step11 3 D P L O T step14 하현재축력도 하현재축력도 하현재축력도 Figure 14. Axial force of lower chord to get the initial cable jacking forces (2) Longitudinal reinforcement of slab The Working Stress Design was applied to the check of the stiffened girder. As the girder of the cable-stayed bridge undergoes considerable big compression, the 2nd effect should be considered. Amount of the rebar was determined by the distribution of the axial force. On the support, i.e. on the cross beam of pylon and hold-down pier, the rebar was heavier than other parts.

9 Figure 15. Axial force of slab due to DL and the amount of the rebar (3) Substructure The pylon is very slender, so the additional forces due to the secondary effect should be clarified. To do this, the buckling analysis and P- analysis were performed. These analyses said that there were moment magnification of 2.1% in the longitudinal and 5.9% in the transverse direction. Using the moment magnification factors which were valuated by P- analysis, the stability of the column was checked with P-M diagram. Maximum axial force Maximum transverse moment Maximum longitudinal moment Figure 16. P-M diagram of pylon leg (4) Detail design Finite element analysis is accomplished at the complicate region of stress behavior, anchorage part of girder and pylon. Figure 17 shows the result of FE analysis. Anchorage of girder Anchorage of pylon Figure 17. Finite element analysis results of anchorages

10 4. Construction The project has started at December, 2002 and is scheduled for December The total construction period is planned as 1800 days. The fabrication of steel casing pile and installation of concrete pile were all completed. Also the fabrication and erection of steel caisson of main bridge were finished by The inner concrete of caisson will be placed in these days and then the pylon legs will be erected to the level of cross beam in this year. 4.1 Caisson of Pylon This foundation type also has advantages regarding the construction method as the water depth is up to 35 m deep and the solid foundation level of the soft rock is 13 m below ground. First, all the piles Ø 2.5 m will be constructed, then the caisson consisting of a steel casing is floated in by using a heavy barge crane with 3,000 tons lifting capacity. The hollow steel casing walls will be filled with concrete and the caisson subsequently is lowered onto the piles. Finally the bottom caisson which connects the piles with the caisson and the top cap of the caisson are filled with concrete. The bell shaped steel caisson is installed on sea bed by 3,000tons F/C. For positioning and guiding of the caisson handling, 4 piles are used as guide piles. After positioning, concrete is cast into the water to seal at bottom of the caisson. To reduce the inertia force under the earthquake, inner part of the caisson remains empty. Finally, the cap beam can be established. Driving the guide piles Positioning the Caisson Puring the concrete into the water Forming the footing in dry state Figure 18. Construction sequence of caisson in design phase But actually the construction method was changed by certain circumstances. Especially the 3,000 tons lifting crane was replaced as the 35,000 tons submerged barge and 1,500 tons crane because of the cost and efficiency of equipment. 35,000 tons semi-submersible barge 1,500 tons floating crane to positioning of caisson Figure 19. Actual caisson erection method

11 4.2. Superstructure According to main span construction, the already composite segments of 72m-long are installed at the temporary false work on the jack-up barges. The weight of these segments is up to 2,400 tons. So, 3,000tons F/C will be used. The cables are installed as long as the segments are supported by the barges. The end span segments with lengths of approx. 160 m will be lifted-in and completed similar as the approach bridge. Erection of main span with jack-up barge Erection of end span with floating crane Figure 20. Erection method of superstructure Figure 21. Erection sequence of superstructure

12 5. Conclusions The Geogeum bridge was selected as a winner at a design and built competition, which was issued by Iksan Regional Construction Management Office in The construction was begun by the contractor Hyundai Engineering and Construction in the end of the This bridge is the first bundle type cable-stayed bridge in the world. It will show us a splendid and panoramic view. Figure 22. The image of bridge 6. References Armin PATCH (2003), Design of the Second Geo Geum Grand Bridge, IABSE Conference on Mokpo. Hyundai Engineering and Construction Consortium (2002), Design report of the Geogeum bridge (Korean). Woo Jong Kim and Kyung Sik Cho (2003), Design of the Cable-stayed bridge linking Geogeum island to Sorok island in Korea, 7th Japan-Korea Workshop.