Korea angel bridge
The southwest coast of South Korea is famous for its beautiful islands. Especially in Xin 'an County, which is composed of Mokpo coastline 1004 islands, the tourism value has surged because the Angel Bridge was opened to traffic. Angel Bridge connects Amitai Island and Ape Island and consists of a cable-stayed bridge, a multi-span suspension bridge and a multi-beam bridge. Among them, the cable-stayed bridge is1004m long, and the three-tower suspension bridge is1750m long. This paper will focus on this multi-tower suspension bridge across the main channel.
Design choice
In order to ensure the safety of maritime transportation and resist strong winds, Angel Bridge must be designed and built with a unique shape. According to the bidding documents and the navigation requirements of relevant institutions, two 650m main span layout schemes are determined through three-dimensional full bridge simulation and maritime traffic safety assessment conducted by Korea Ocean University. At the same time, in order to coordinate with the landscape environment, multi-tower suspension bridge is a better scheme, which is different from cable-stayed bridges such as Mokpo Bridge and Zhendao Bridge in the surrounding areas.
Figure 1 Overall Layout of Bridge
main cable
The latest 1960MPa high-strength steel strand is adopted, and the main cable is erected by precast parallel strand method (PPWS), which improves the constructability and economy. Considering the safety and stability of the main cable erection, the sag ratio is set to 1/8. The weight and diameter of the main cable are1900t and 309.4mm respectively.
In the design of main cable, three technologies are improved. Firstly, compared with the existing galvanized steel strand, the zinc+aluminum coating can prolong the fatigue life by 1.4 times and improve the corrosion resistance by 4 times. In addition, this coating can also improve the service life of dehumidification system. Secondly, the rubber wrapping method is introduced into the anti-corrosion system, and the wrapping line is no longer needed. In general, S-shaped winding wires are at risk of corrosion, and there are always durability and maintenance problems in regular spraying. Rubber wrapping method does not need separate metal wires or regular painting, and can ensure the best durability when used in conjunction with dehumidification system. Thirdly, the main part of the saddle at the top of the tower is made of concrete, not cast iron or structural steel commonly used in suspension bridges. Only the groove of the saddle is made of steel. Because the concrete is poured on site, the saddle can be hoisted without large temporary support, and the saddle groove can be easily installed by tower crane.
Fig. 2 Saddle at the top of concrete tower
Average beam
The primary task of main girder design is to ensure the wind stability and economy of main girder. Although reducing the weight of the main girder can ensure the economy, at the same time, the beam that is too light is more susceptible to wind load. Therefore, it is necessary to maintain an appropriate balance between economy and wind stability. In order to design the best section of the main girder, after referring to various literatures, the flat rhombic steel box girder section is selected. Through the wind tunnel test, it is found that the flutter of the main girder can be effectively controlled when the angles between the upper side of the tuyere and the lower flange are 30 and13 respectively. In order to reduce the weight of structural steel, 50 mm epoxy asphalt and supporting diaphragm are used, and the newly developed HSB500 series high-performance steel is used in the main components to improve the fatigue resistance and economy of steel bridges.
Fig. 3 Cross section of stiffening beam
bridge tower
It is no exaggeration to say that in a suspension bridge, the shape of the pylon determines the beauty of the whole bridge. Moreover, in the multi-tower suspension bridge, the stability and bending stiffness of the main tower also play a very important role. In order to improve the structural efficiency and effectively resist the longitudinal force caused by live load, the Angel Bridge concrete tower is designed to be more complex H-shaped. The stronger the main tower, the smaller the deflection of the main girder when the stiffness of the whole structure increases. However, if the displacement of the main tower is excessively limited, the friction balance force between the tower saddle and the main cable may not be maintained. In order to ensure that the tower has enough stiffness without threatening the sliding stability of the saddle, it is necessary to study the stiffness parameters of the tower to determine the best section stiffness characteristics. Considering the sliding friction of the main cable and the deflection of the tower top, the stiffness of the optimized tower bottom section is 6.5 times higher than that of the tower body.
Anchor block
The horizontal force at the anchorage point of multi-tower suspension bridge is determined by one of the main spans (650m), not the whole span. In the design, the horizontal force of the anchorage point is reduced to 74200kN. This is also the lowest horizontal force among the existing suspension bridges in Korea. Therefore, this reduction in horizontal force can also minimize the volume of the anchorage, which usually accounts for a large part of the construction cost. The anchoring position is in the soft clay layer more than 0/0 meter from the seabed/kloc-0, and the bedrock is 26 meters below the sea level. If caisson foundation is used in this case, there is a risk of settlement and deviation, and the solution may be very difficult and expensive. After many studies, the composite foundation treatment scheme combining drilling with caisson is adopted, and the performance of foundation structure is improved by compaction pile (SCP) and deep cement mixing (DCM).
Architectural challenge
Build an overpass
Catwalk is a temporary construction access road which is erected under the main cable and parallel to the main cable line shape during the construction of suspension bridge. It is a scaffold for constructors and a construction platform for the main cable system and even the whole superstructure of suspension bridge. Construction personnel finish important work on it, such as cable traction, cable strand adjustment, molding and padding, cable tightening, cable clamp and sling installation, box girder hoisting, main cable winding, protective painting, etc.
The catwalk system of this bridge is supported by six steel wires, each with a diameter of 31.5mm, forming a working area, with a cross section of 4.2m wide, a height of 6.75m and a total length of 2065m. These catwalk ropes are separated at the middle tower. The frame supporting the traction system is arranged on the catwalk with a spacing of 42.5 meters, and the spacing of the main cable forming machine (cable tightener) is also 42.5 meters. The catwalk is located below the main cable, and there is a gap of 1.3m between the main cable and the catwalk.
Installation of tower saddle
In suspension bridge, the installation of cable saddle is one of the noteworthy processes, because the cable saddle is usually the largest and heaviest component in the installation process of cable system. As the hybrid saddle is adopted, the base at the bottom of the saddle is made of concrete structure and the upper part is made of steel structure, and the weight of the components hoisted by the tower crane is only 3 tons, so there is no need to mobilize any temporary equipment during the operation, which has great advantages in practicality and economy. At the same time, the good compression resistance of concrete material has become another advantage of this hybrid saddle. In the construction stage, the saddles on the towers on both sides must be pre-deviated to the central tower to maintain the initial balance of the cable system. For convenience, a skateboard is installed under the saddle.
Main cable erection
Total weight of main cable 1847 tons, diameter of 5.3 mm, tensile strength 1960 MPa. Steel wire is produced by Kiswire Company of Korea. These high-strength steel wires are woven into parallel strands in the factory according to the precast parallel strand method (PPWS), and anchor heads are installed at both ends to form strands. The manufactured steel strand is wound on the drum and transported to the construction site by barge. Pull the cable strand end from one end of the bridge to the other end along the catwalk with traction equipment, and fix it after positioning and strand adjustment procedures. The main cable consists of 2 1 strand, and each strand contains 127 steel wires. The total installation time of the main cable is 40 days.
Deck installation
According to the design, the bridge deck is divided into 85 segments, and two side spans and two main spans need to be hoisted at the same time. One * * * needs six portal frames, two main spans and one side span. However, only four sets of lifting equipment are available. Because there is not enough budget to pay for the other two gantry cranes, it is necessary to adjust this process and use suitable and limited equipment.
Fig. 4 Installation of the bridge deck behind the main span.
In the revised construction technology, the bridge deck members of the main span are hoisted first, and then the gantry is moved from the main span to the adjacent side span. During the movement of the gantry, the bridge deck closest to the tower body and the bridge deck members at both ends are installed by floating cranes.
The key problem in this construction scheme is that the cable force of the bridge tower is unbalanced, which will lead to the slip of the cable saddle and the bending moment at the bottom of the tower. In addition, the geometric shape of the main cable changes greatly, resulting in secondary stress in the main cable. Therefore, for each installation step and each simulation stage, structural effects including bridge tower stress, saddle slip and secondary stress in cables should be checked.
As a landmark building, Angel Bridge has overcome the complex marine environment, saved costs and become the pride of local people. Choosing a multi-tower suspension bridge scheme that meets these strict conditions is expected to provide reference for similar projects. Considering that multi-tower suspension bridge is considered to be a better bridge type among the sea-crossing bridges, the completion of Angel Bridge has promoted the development of the same type of bridge technology and made this type dominate the world bridge market.
This article was published/Bridge JournalNo. 1 No.99.
Author/Sun (Korea), etc.
Source/Structural Engineering International 2020