Akinada Islands Bridge Project
In order to improve quality of life in islands separated from main land, Hiroshima Prefecture has been proceeded construction of the Akinada Island Bridge Project. This project is to construct bridges between the Akinada islands and main land. By connecting Islands by bridges, easy access between islands and main land becomes possible. It facilitates emergency activity, commute between islands and main land, transportation of fishery and agricultural products, and accelerates development of tourist industry taking advantage of natural beauty of the islands.
The Akinada Bridge, the gate bridge of the Bridge Project was opened to traffic on 18ｔｈ January, 2000. Average daily traffic volume on this bridge is 4,000 vehicles (by May 2000) which exceeds expected number 2,400 vehicles.
The Akinada Bridge(Design)
Akinada Bridge is a suspension bridge whose center span length is 750ｍ, and total length is 1,175ｍ. This bridge is located between a island and main land, i.e. the gate bridge of the Akinada Island Bridge Project. This bridge is a part of the Akinada toll road whose length is about 2.7 km. The construction cost of this toll road is about 50 billion yen, and about 20% of the cost has to be paid back from transportation fee. The construction cost of main bridge is about 35 billion yen.
The construction started on Oct. 1992, and the bridge was opened to traffic on January 18, 2000. 8 years construction of this bridge was completed without any accidents. This bridge is 9ｔｈ longest bridge in Japan, 27 ｔｈ longest bridge in the world.
Neko-seto strait where the Akinada Bridge crosses over is 1 km wide and 90ｍ deep, and maximum tidal current is 5 knot. This strait is good fishery area and busy sea traffic lane. Therefore, in order to minimize effect of construction on sea environment, towers were constructed on shore line instead of in the sea, and as the result, center span length 750ｍ was determined. As for geological condition, main land side is composed of granite layer, and Shimo-kamakari Island side is composed of slate and limestone layer. Other than weak layer found below 3P foundation, geological condition is fairly good. Therefore, 1A and 4A anchorage and 2P tower pier were constructed as spread foundation. On the other hand, 3P tower pier was constructed as piled foundation.
Narrow Decked Bridge
As the Akinada Bridge carries only 2 lanes for automobile and pedestrian road, its deck is narrow for its span length. The ratio of span length and girder width is about 1/40. Because of this narrowness, aerodynamic stability is very important item to be checked in design.
Short and unbalanced side span length.
Side span length of 170ｍ, and 255ｍ are short for center span length of 750ｍ. And side span length differ each other because location of foundations were decided by topological conditions. Steep side span cable due to short side span length between 3P and 4A result in high tensile force in the cable. As the countermeasure, 2 extra strands were placed at the side span to use cable efficiently, and escape from cable slip at the top of 3P tower.
In order to facilitate maintenance work, box girder was employed. The depth of the girder was decided to be 2.5ｍ, which was suitable for inspection work in the girder.
As for cables, dehumidicated air was injected into main cable as anti-corrosion system, and hangers were courted by polyethylene as maintenance free system.
Anti-corrosion system of main cable
Cable was wrapped by wrapping wire which has S shaped cross section, and courted by paint to keep watertight and airtight. Into the airtight cable dehumidicated air was injected to keep the cable in dry condition.
Wind resistant design
3-year field measurement of wind had been conducted to decide design wind speed of 37 m/s at the height of 10ｍ above sea surface, which was calculated as wind speed of return period of 100 year.
As wind resistant design was very important due to slenderness of this bridge, various wind tunnel tests were conducted. As the result, cross section of girder including fairing shape, location of inspection vehicle rails, configuration of handrail etc. were determined to achieve aerodynamic stability in wind speed of 64 m/s.
As for main tower, its cross section was decide to have corner cut at outside edge to minimize wind induced vibration based on wind tunnel test. Additionally, 4 TMD dampers were installed in each tower to suppress vortex induced vibration couldn’t be stabilized sufficiently by changing cross section.
After completion of construction of the bridge, vibration test was conducted to evaluate vibrational characteristics of the bridge. As the result, safety of the bridge was confirmed. As this test included horizontal vibration test which had rarely conducted, important data were recorded.
As design earthquake, earthquake which can takes place in radius of 300ｋｍ whose magnitude are around M=8, and ones which can takes place near the construction site whose magnitude is around M=7, are considered. As the result, design earthquake was decided with return period of 125 years. Maximum responses spectra is 690 gal when structural damping ratio is 0.05.
Girder Cross section
Major Dimensions of the Akinada Bridge
Dimensions of Main Cable
The Akinada Bridge(Erection)
As anchorage is a massive concrete where main cables are anchored, it is necessarily to mitigate cracks due to hydration of cement. Therefore, following countermeasures were taken.
1. Low heat cement was used
2. Each lift of concrete was decided to be less than 75 cm deep, and cold water was circulated in cooling pipes installed in concrete to control temperature of concrete.
3. Anchorage was horizontally divided into 4 sections, and they were separated each other by slots which are 2.5 m wide. After temperature of each section was stabilized, concrete of the slots were placed.
Tower was divided into 14 blocks vertically, and 34 blocks in total including horizontal members. Weight of each block was less than 60 ton because of capacity of crane used for the erection work. The crane used for the erection were climbing crane which was set near the tower. Blocks are connected by welding instead of commonly used bolt connection to make the tower smart looking.
The first rope, pilot rope was erected using helicopter for main span in order not to disturb sea traffic below. Using this rope, catwalk, scaffold for cable erection was erected. After the erection of catwalk, cable strand which was formed in factory was placed into designated position. After all the strands were placed, they were formed into round shape cross section to make it one cable. At the anchorage, the cable were sprayed into strands, and each of them was anchored to anchor-frame installed in the anchorage.
Girder Blocks fabricated in factories were transported to bridge site by barge, and lifted up to designated location vertically using lifting beams placed on main cables. By using self-propelled barge, working time was shortened to minimize disturbance to sea traffics.
The girder where barge cannot proceed below, were erected using so called swing erection. In swing erection, girder blocks lifted up were transported longitudinally using hangers and temporary hangers from lifting beams.
Average length and weight of each girder block were 24ｍ and 170 ton for general section, and 12ｍ and 85 ton for swing erection blocks.