Akashi-Kaiko Bridge
1. Physical Conditions
The Akashi Strait, which connects Osaka and Harimanada, is about 4km wide. The segment spanned by the bridge has a maximum depth of 110meters and a maximum current speed of 4.5m/s. The Strait has been a productive fishing area since ancient times, and is important waterway, used by over 1400 vessels a day. To ensure the safety of marine traffic, an international water-way 1500m wide has been set by the marine traffic safety law.
Several severe conditions were encountered, such as strong currents, deep water in which divers were unable to explore the seabed. It was also necessary to preserve the fishing area and to ensure the safety of marine traffic. Those difficulties were conquered through investigations and technical examinations.
2. Basic Design Conditions
The basic design conditions of the Akashi-Kaiko Bridge are summarized as
follows:
1. The width of the straits is about 4km, and its depth along the proposed bridge route reaches about 110m.
2. The natural conditions at the main pier site: water depth=45m, maximum
tidal current=4.0m/s and maximum wave height=9.4m.
3. The wind condition: basic wind speed for design(defined as 10 minutes speed at 10m above the water level with the return period of 150 years)=46m/s and reference wind speed against flutter=78m/s.
4. The geological condition: the basic rock beneath the straits is granite, on which Kobe Formation (alternating layers of sand stone and mud-stone in the Miocene), Akashi-Formation(semi-consolidated sand and gravel layer in the late Pliocene and the early Pleistocene) and the Alluvium layer are deposited.
5. The design earthquake: the one which occurs off the Pacific coast about 150km away with the magnitude of 8.5or the ones which are expected within radius of 300 km with the return period of 150 years, and
6. The social conditions: a waterway within 1500m in width and for heavy sea traffic is designed amid the straits, and lands on both shores are highly utilized. The bridge is for 6-lane highway with the design speed of 100km/h.
3. Basic Design Conditions
The Akashi-Kaiko Bridge, therefore, has the following characteristics on design and construction. However, its design is strongly influenced by the construction method, because design of a huge structure cannot but be done without consideration of the construction method and procedures.
1. In theory, the minimum clear span length may be slightly longer than 1500m, which is the width of the waterway, but it is better for the main piers to keep off the waterway with some margin to spare, even during construction to ensure the safety of maritime traffic, as is required by law. Accordingly, several comparative designs were developed for various main span lengths around 20000m, and the conclusion was that the span length from 1950m to 2050m yielded the minimum construction cost. The position of two main piers was finally decided from the topographical and geological conditions after securing the width of the waterway and its side margins, which fact leads to the main span length of 1990m(final length of 1991m was due to the Kobe earthquake).
2. The two main piers were decided to be direct foundations having circular plane shape, and “Laying-down caisson method” was selected for their construction, with which HSBA had been familiar since the construction of Seto-Ohashi Bridge in 1980’s. However, its excavation method and underwater concreting method were improved from previous ones due to various technical developments done after then. The caisson had a possibility to overturn from scouring due to the accelerated flow and horse shoe vortexes both of which were generated by the presence of the caisson itself unless some countermeasures were taken, because the Akashi Strait had its sea bottom covered with sand and gravel and its tidal current was rapid. Accordingly, protection against the scouring with filter units and cobble stones laid on them were placed.
3. The side span length was determined to be 960m so that both anchorage might be located near the original shore lines where obtaining working yard by
reclamation was rather easy. The body of two anchorages was designed to be a conventional gravity type. Because the anchorages body is a huge RC structure to which many people can make easy access, its shape is designed so as to lessen an oppressive feeling, and precast panels were used as remaining form which had a superior external appearance and pattern to avoid monotony of the concrete surfaces. These panels precast in a factory can also raise durability of the mass concrete cast in-site. On the other hand, two foundations were quite different by reflecting difference of the geological conditions. Especially, 1A(Kobe side anchorage) foundation became the world’s largest foundation for a bridge with the diameter of 85m and the bottom depth of 61m.
4. By considering the size of the foundation and relatively soft supporting ground, a new seismic design method was developed and used, in which concepts of “ the effective seismic motion” and dynamic damping effect by interaction between foundation and ground” were introduced. Thanks to this new seismic design method and luck, the bridge withstood the Kobe Earthquake in 1995 without receiving any structural damages.
5. Low heat generating type cement, which contains less clinkers and thus decrease hydration heat, were newly developed and widely used. For various parts of the substructure, several kinds of new concrete mix were investigated and used so as to meet required quality such as the strength, the consistency, the maximum size of aggregate, the adiabatic temperature rise and so on. Some examples were desegregadation highly workable concrete for the anchorage body, etc.
6. The main towers is made of steel, and the shaft has cruciform cross section which is insensitive to wind induced oscillation. However, Tuned mass dampers were installed inside the shafts to suppress the oscillation which was anticipated during the tower erection as well as even in the completed stage of the bridge. A tower shaft was divided into 30 tiers and almost of all tiers were composed of 3 blocks. Each block was precisely fabricated in factories and transported to the site, and then hoisted up for the erection with a self-climbing tower crane which had a lifting capacity of 160 tons(1.6MN). High tension with newly developed fluorine-resin paint which had high durability, and in this coating system, zinc-rich paint put directly on the steel surface played an important role in exerting the anti-corrosion performance.
7. The main cable is made of parallel wires which were erected with Prefabricated Strand Method having been used long in Japan because of its superiority on resistance against wind action and number of required workers, etc. during the erection. A cable is composed of 290 strands each of which contains 127 wires each measuring 5.23mm in diameter. High strength(1800MPa) galvanized wire was developed and used for the main cables, which could avoid using double cables per side even when the span of the bridge was very long and sag/span ratio was kept 1/10. This sag/span ratio enabled it to restrict the height of the main towers. In the cable erection new technology such as the spanning pilot rope with a helicopter, eliminating the storm rope system from the catwalk and so on were utilized. Also, corrosion prevention system for the main cables was developed, in which dehumidified air flowed through void inside the main cable and thus removed the moisture. As for the suspender ropes, parallel wires
wrapped with polyethylene tubes with pin connection at both ends were newly used to ease future burden of the maintenance work.
