Seto Ohashi Commemorative Hall(Model Descriptions)

1
Land Area on the Shikoku Side
After crossing the strait from Honshu, the Seto Ohashi Bridge separates into road and rail sections at the Ban-no-su Viaduct on the landward side. The road connects to the city of Sakaide at the Sakaide-Kita Interchange, the first interchange on the Shikoku side, and links to National Route 11 at the Sakaide Interchange. It also connects to the Takamatsu Expressway at the Sakaide Junction, making the Seto Ohashi Bridge not only a link between Honshu and Shikoku but also a key hub in the expressway network connecting the Sea of Japan to the Pacific Ocean. The railway line passes through the Ban-no-su Industrial Zone, where it branches into lines heading toward Takamatsu and Tokushima, and toward Matsuyama and Kochi, connecting to the Yosan Line.

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Seto Ohashi Bridges
Construction on the Seto Ohashi Bridge began in October 1978. After nine and a half years and at a cost of approximately 1.12 trillion yen, it was completed on April 10, 1988. The strait section spans approximately 9.4 km, connecting Kojima in Kurashiki City, Okayama Prefecture, to Ban-no-su in Sakaide City, Kagawa Prefecture. It follows the natural topography of the area, passing over five islands and connecting them with a total of six major bridges: three suspension bridges, two cable-stayed bridges, and one truss bridge. Additionally, the Seto Ohashi Bridge is a double-deck bridge with a road on the upper deck and a railway on the lower deck.

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Bridges Which Can Endure Earthquake
The Seto Ohashi Bridge features a flexible structure designed to absorb various types of vibrations. During the design phase, safety was verified from every angle through theoretical research on vibrations, various experiments, and computer simulations of earthquake scenarios. As a result, the bridge was designed to withstand a major earthquake of approximately magnitude 8, even if it were to occur 100 km off the coast of Tosa.

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Wind Tunnel Test
To design bridges that are safe in windy conditions, it is necessary to study the mechanisms by which wind acts on bridge girders. Long bridges are susceptible to wind, making wind resistance a critical consideration. For this reason, during the design of the Seto Ohashi Bridge's long suspension span, wind tunnel tests were conducted repeatedly, exposing a model to winds of various directions and speeds. As a result, the Seto Ohashi Bridge is designed to withstand strong winds of up to 66 meters per second.

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Hitsuishi-jima Bridges and Iwakuro-jima Bridge
The Hitsuishi Island Bridge and Iwakuro Island Bridge are a pair of consecutive cable-stayed bridges known as the "Twin Bridges." They have a total length of 792 meters, a central span of 420 meters, and a height from sea level to the top of the towers ranging from 148 to 161 meters. The clearance from the sea level to the underside of the deck ranges from 32 to 40 meters, and the bridges have a 1% upward gradient toward the Shikoku side. This is because the height from sea level to the underside of the deck of the South and North Bisan Seto Ohashi Bridge on the Shikoku side, which are part of the international shipping route, is higher than that of the Hitsuishi Island Bridge and the Iwakuro Island Bridge.

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Structure of Cable-stayed Bridges
Cable-stayed bridges use cables running diagonally from the towers to support the deck. Unlike suspension bridges, where the deck is suspended by cables, in cable-stayed bridges, the deck and cables form a single unit that supports the bridge.
Their design is structurally complex, and the advent of computers has led to a dramatic increase in the spans of cable-stayed bridges.

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The Minami-Bisan and Kita-Bisan Seto Ohashi Bridges Selected as a Japanese 20th Century Heritage Site
On December 8, 2017, the Japanese National Committee of ICOMOS, the advisory body to UNESCO on World Cultural Heritage, selected the Bisan Seto Ohashi Bridge as one of the "20 Selections of Japan's 20th Century Heritage." The selection covered both the South and North Bisan Seto Ohashi Bridges, describing them as "an outstanding example of suspension bridge technology."

• Key features of the South and North Bisan Seto Ohashi Bridges include:

1)The South and North Bisan Seto Ohashi Bridges are a pair of suspension bridges that share a central anchorage. This anchorage features a world-first design that secures the cables of each bridge.

• During the construction of the South and North Bisan Seto Bridges, the following technology was established.

1)Technology for safely and reliably constructing underwater foundations in deep waters with strong currents using the caisson method
2)Technology for installing the main cable-which, at 1,070 mm in diameter, was the world's largest at the time-within a short period
3)Technology for erecting bridge girders without obstructing international shipping lanes
Together with design methodologies for seismic and wind resistance, these cultivated technologies became the foundation of long-span bridge construction technology in Japan and contributed to numerous subsequent long-span bridge construction projects both domestically and internationally.

• As a combined road-rail bridge,

1)Fatigue design methods that account for the repetitive stress caused by the passage of heavy trains
2)Special track expansion joints designed to allow high-speed trains to run on these long suspension bridges with flexible structures
We undertook the development of technologies that had never been attempted before, resulting in the realization of one of the world's largest combined road-rail suspension bridges.
These long-span bridge construction technologies have contributed to the development of a multi-axis national infrastructure and the creation of new foundations for exchange in Asia and other regions, and can be described as a globally significant "20th Century Heritage in Japan."

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Kita and Minami Bisan Seto Bridge
The South and North Bisan Seto Bridge is a twin-span suspension bridge with a total length of approximately 3.3 km, connecting Ban-no-su in Sakaide City to Yoshima across the Bisan Seto Strait. The South Bisan Seto Bridge is 1,723 meters, and the North Bisan Seto Bridge is 1,611 meters.
Since the Bisan Seto Strait is designated as an international shipping lane, the clearance from the underside of the bridge deck to the sea level is maintained at 65 meters or more, ensuring that even ultra-large tankers of 500,000 tons can pass through with ease.

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Structure of Suspension Bridges
A suspension bridge is a structure in which the deck is suspended from the cables by suspension cables, and the weight of the vehicles and trains traveling on the deck, as well as the weight of the deck itself, is transmitted from the cables to the towers and then to the anchorages. The anchorages are concrete abutments that play a crucial role in supporting the immense forces transmitted by the cables.

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Yoshima Bridge
The Yoshima Bridge is an 877-meter-long truss bridge with a maximum span of 245 meters that connects Yoshima Island to the uninhabited Wasa Island. The bridge curves as it crosses, with the direction of the girders changing at the piers.
Furthermore, the construction required highly skilled navigation, as large crane ships were used in the narrow strait.

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Structure of Truss Bridges
The Yoshima Bridge is primarily constructed using a Warren truss, with K-trusses incorporated into sections subject to greater loads. At the points where the bridge girders rest on the piers, the weight of the girders themselves, as well as that of passing vehicles and trains, is concentrated. Therefore, bearings-which function much like the soles of a person's shoes-are installed to transfer this weight to the piers. These bearings are located primarily on the upper surface and are made of seamless cast iron.

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The Loop Bridge on Iwakurojima Island
Although the original plan called for installing only an elevator for access to the bus stop, in response to strong requests from local residents, Kagawa Prefecture and Sakaide City covered part of the costs, and a loop bridge was constructed.

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Two-level,Two-section Tunnel Construction Method
The Washuzan Tunnel is the world's first double-deck tunnel, featuring two road tunnels and two railway tunnels arranged in two tiers, one above the other. Although the bedrock at Washuzan was brittle, making it a challenging site for tunnel construction, the project brought together the latest technologies to overcome all obstacles.
Excavation began with the lower railway tunnel and proceeded in a complex sequence, step by step, while continuously monitoring the condition of the bedrock.

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Multi-layer Structure of Anchorage
The surface of the Seto Ohashi Bridge's anchorage features a multi-tiered, sloped structure oriented horizontally at a 5-degree upward angle. This design reflects radar waves from ships upward, thereby preventing false echoes. Additionally, radar-absorbing material has been applied to the bridge towers-which cannot be structurally modified-to weaken the reflected radar waves.

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Cross-section of the Seto Ohashi Bridges
The Seto Ohashi Bridge is a two-level combined bridge; the upper level is a four-lane highway, while the lower level consists of a double-track conventional railway line. Its girders possess a flexible strength that accommodates expansion, contraction, and deflection caused by train traffic, temperature changes, and wind. The bridge is also designed to accommodate Shinkansen bullet trains in the future.

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Excavating the Seabed
After blasting the seabed, a large grab dredger was used to excavate the soil down to the base of the foundation. For the hard bedrock, a 125-ton ultra-heavy bucket with a single-dump capacity of 13 cubic meters was used. By leveraging its weight to drive the teeth into the bedrock fractured by the blast, the dredger scooped up the broken rock and sediment. For the sedimentary layers, we used a 25-cubic-meter bucket weighing 60 tons, which is relatively light and has a high excavation volume per pass. Seabed excavation was carried out by selecting the appropriate bucket based on the specific ground conditions.

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Building Foundation
The construction of the Seto Ohashi Bridge was carried out under harsh natural conditions, including a deep and complex seabed topography and strong tidal currents, while ensuring the safety of passing ships. In particular, to construct the foundations underwater and ensure rapid and reliable completion, the project adopted a major strategy of scaling up the size of each construction phase and utilizing prefabrication-transporting components manufactured in advance at factories to minimize on-site work at sea. The caisson installation method employed took advantage of the ocean's unique property that allows even heavy objects to be transported easily by utilizing buoyancy.

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Types of Under-water Blasting
To install the foundations of a suspension bridge on the seabed, it is necessary to remove sediment and soft rock from the seabed and excavate down to the solid bedrock located 10 to 50 meters below the sea surface. To achieve this, the first preparatory step involved breaking up the weak bedrock using explosives. Four different detonation methods were adopted for each foundation, selected based on factors such as tidal currents, depth, topography, geology, and the surrounding environment.
1)Detonating Cord Method
A method in which explosives are detonated by igniting a detonating cord extended to the sea surface. This method was used in areas with calm currents and shallow water depths.
2)Electric Detonation Method
A method in which electricity is passed through a cable to detonate the explosives, allowing for continuous blasting in a dozen or so stages using small amounts of explosives. Since this method reduces vibrations caused by explosions, it was adopted at sites located near oil refineries.
3)Ultrasonic Wireless Detonation Method
A method that uses ultrasonic waves to detonate explosives without wires. Since there was no risk of wires being swept away, this method was used in deep waters with strong currents.
4)Electromagnetic Induction Blasting Method
This method involves passing an electric current through a loop antenna on the seabed to create a magnetic field, which induces a current in a receiver coil on the explosives to ignite and detonate them. It was adopted in areas where the water was deep and the tidal currents were slow, making it impossible to use ships.

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Building Foundation Under-Water Blasting - Setting of Caisson on the Seabed
Seabed Blasting
Using a drilling rig, holes are drilled into the deep seabed bedrock, filled with explosives, and detonated to create cracks in the bedrock.
Seabed Excavation
Using a large grab dredger, the rock and sediment broken up by blasting are scooped up and removed precisely down to the bedrock where the foundation will be placed.
Seabed Leveling
An excavator mounted on the opening work platform levels the seabed surface to ensure the foundation can be installed horizontally.
Caisson Installation
Steel caissons are fabricated at a shipyard, floated out to sea, and sunk onto the leveled seabed.
Coarse Aggregate Filling
To prepare the interior of the installed caissons for concrete curing, crushed rock is first placed inside.
Mortar Injection
Mortar is injected into the caissons filled with coarse aggregate. This completes the underwater foundation.
Tower
The structure is completed by assembling the tower on top.
Anchorage
The anchorage is completed by installing the anchor frame on top.

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Checking the Seabed
Since underwater cameras and other equipment were hindered by floating plankton and other debris, preventing the acquisition of accurate data on the bedrock, engineers had to dive down themselves to sketch the condition of the finished bedrock. The seafloor was pitch black; even with powerful lights, visibility was limited to about 1 meter, so they had to work with their foreheads pressed against the bedrock. Because the time for diving at a depth of 50 meters was limited, they had to repeat the dives numerous times.

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Checking the Seabed
Since working on the seafloor at a depth of 50 meters is impossible using conventional diving methods, the operation was carried out using the diving support vessel "Seatopia." A bell-shaped cylindrical capsule was used to lower four technicians to the seafloor while regulating the air pressure. After the work was completed, the air pressure was gradually reduced in the decompression chamber aboard the "Seatopia" to prevent decompression sickness.

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Placing of Caisson on the Seabed
The installation of the 7A caisson was truly the most critical phase of the project. To accurately position this colossal caisson while battling the tidal currents, a minute-by-minute schedule was drawn up to complete all work within a four-day window during which stable weather was expected to persist. Using winches, large crane ships, and surveying equipment, the caisson was carefully lowered while verifying its exact position, and it was successfully installed within the specified tolerance range at its designated location (±50cm).

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Caisson Structure
When viewed from above, a 7A caisson resembles the character "田" (ta). While the interior of each section is open at the bottom, the lines forming the "田" are constructed as double walls with solid bottoms, allowing the caisson to float on water. Seawater is gradually introduced into these double walls to reduce buoyancy, causing the caisson to sink.

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Placing of Caissons on the Sea Bottom
The position of the caisson during installation changes constantly due to factors such as tidal currents. Therefore, we conducted surveys at two locations using state-of-the-art equipment and immediately analyzed the results using a computer. Any deviation from the planned position was immediately displayed on the screen, and the site supervisor used this information to issue operating instructions to the crane vessel.

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Mortar Pouring Process
If mortar injection is interrupted, the mortar hardens, creating joints that become weak points in the concrete. To prevent this, the mortar plant ship "Seiki" was stationed at sea to ensure a continuous supply of mortar, but the raw materials-cement, sand, and water-had to be transported from land to the ship repeatedly. Consequently, strong winds and thick fog sometimes prevented the timely delivery of materials, causing anxiety among those involved.

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Strength Test of Under-water Concrete
The strength of underwater concrete varies significantly depending on the materials used and the quality of construction. For this reason, samples were taken from the concrete inside the completed caissons and subjected to pressure testing to determine their strength. These tests were conducted on all caissons, confirming the safety of the foundation for the long bridge.

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Types of Anchorages
The role of an anchorage is to secure the cables and anchor the suspension bridge to the ground. Inside the anchorage is an anchor frame that secures the cables and connects the anchorage to them. There are several types of anchorages, depending on the conditions of the installation site.
The South Bisan Seto Bridge 7A
A massive anchorage on the South Bisan Seto Bridge. It withstands the pulling force of 90,000 tons of cable and weighs 1 million tons.
The South and North Bisan Seto Bridge 4A
A shared anchorage that simultaneously supports the pulling forces of 80,000 tons of cable from the North Bisan Seto Bridge and 90,000 tons from the South Bisan Seto Bridge. Because the forces from both sides balance each other to some extent, the required weight is less than if the load were pulled from just one side.
Shimotsui Seto Ohashi Bridge 1A
To preserve the scenery of Mt. Washuzan, a tunnel was excavated through the mountain without cutting into it, and the anchor frame was embedded within it.

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Welding Test Specimen
During the construction of the Seto Ohashi Bridge, the factory fabrication phase required the assembly of massive steel structures, which necessitated welding of large components-a task without precedent-and technically challenging welds. For this reason, in order to verify welding conditions and work methods before welding the actual components, thorough welding tests were conducted using test pieces made of the same material and dimensions as the actual components. This test piece is from the tower section of the tower link that suspends the bridge girders of the Shimotsui Seto Ohashi Bridge.

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Cutting and Grinding End Surfaces
To ensure the tower stood perfectly vertical, the contact surfaces of each tower column block were machined to be straight and parallel. Because this work required extreme precision, skilled workers painstakingly repeated the process of shaving off 0.05 mm at a time during the night, when temperatures were stable. As a result, the 194-meter tower of the South Bisan Seto Ohashi Bridge was completed with an accuracy within 19 mm, compared to the permitted error of 3 cm for tower tilt.

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Erection of Main Towers
A special 130-ton crane known as a crawler crane was used to erect the tower. As the tower blocks were stacked, the crawler crane moved upward along the tower, allowing blocks to be placed at increasingly higher levels.

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Metal Touch Test
A feeler gauge is a tool used to measure gaps; it is made of thin metal plates. On the Seto Ohashi Bridge, a 0.04 mm feeler gauge was used to perform a "metal-to-metal contact" inspection, in which the gauge was inserted between the blocks each time a new layer of tower blocks was stacked. This metal-to-metal contact ensures that the force exerted by the cables is smoothly transmitted to the foundation.

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Erection of Cables
The cable was installed by pulling it out during the day and adjusting its tension at night when temperatures were stable. This process was repeated until the cable was fully installed. During the cable installation work, numerous lights were used for nighttime adjustments, making the structure look like a giant tree of light floating in the Bisan Seto Strait. The lighting for this work was carefully designed in terms of brightness and direction to avoid interfering with passing ships.

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Corrosion-inhibiting Measures for the Bridges
The components for the towers and bridge girders were manufactured at factories across the country and transported to the site by barge. With the exception of the sections to be joined on-site, all of these components were fully painted. This is because salt from sea spray adhering to the components during transport or erection can weaken the paint finish; therefore, painting is performed at factories where work management is easier. On the other hand, the sections to be joined on-site were washed with water prior to installation, and measures such as measuring the amount of salt adhering to the surface were taken before painting.

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Erection of Girders
The installation of the suspension bridge's girders began near the towers. Large girder sections, which had been prefabricated in a factory, were lifted 90 meters above the sea by a large crane ship and installed in a single operation. This required advanced technical skills to join six components simultaneously, and as many as 3,000 bolts were used to secure the blocks. Using these large blocks as scaffolding, the components were assembled one after another, extending the bridge girders to the left and right.

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Tower Links
In the tower section, the bridge deck is suspended by tower links. The tower links are pinned at the top to the tower and at the bottom to the bridge deck, and they move like a clock pendulum in response to the movement of the bridge deck.

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Blow Holes
Although the fabrication of the bridge girders involved a significant amount of welding work, any flaws in the process could result in small internal voids known as blowholes, which can lead to cracks or damage over time. For this reason, we conducted repeated experiments in advance to develop the optimal welding method before beginning production. Furthermore, after welding, we performed thorough quality control by checking blowholes larger than 1 mm using a newly developed automatic ultrasonic flaw detector.

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Air Spinning Method
The air-spinning method involves installing the cable by laying individual wires directly. A spinning wheel moves back and forth along a catwalk, drawing out four individual wires and bunding them into strands on the spot. This allows for a reduced number of strands, enabling the anchor frames used to secure the cable to be narrower. Since the anchorage for the Shimotsui-Seto Bridge is a tunnel-type structure built on Mt. Washuzan, the AS method was selected to minimize the tunnel cross-section as much as possible.

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Railway Expansion Joint
The most distinctive feature of the Seto Ohashi Bridge is that trains travel across it at high speeds. Suspension bridges are inherently prone to deformation; when a train passes, the bridge girders deflect, and a phenomenon known as "corner buckling" occurs at the ends. The railway buffer girder was developed to mitigate this phenomenon and ensure smooth train operation. It is designed to absorb deflections of up to 1.5 meters and corner buckling of up to 0.5 degrees.

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Hanger Rope Pulling Device
Screw the rod of the retraction mechanism into the bottom of the socket attached to the end of the hanger rope. Use a hydraulic jack to retract all four hanger ropes simultaneously and secure them in the grooves of the brackets on the side of the bridge girder.

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Luminous Buoy
This is a large buoy installed in construction zones. It was specially developed to withstand strong currents and waves.
Since the Bisan Seto Strait is a key maritime traffic hub with 1,000 ships passing through daily, this buoy was installed to designate areas where vessel entry is prohibited.
The actual buoy is on display in the exhibition plaza east of Marine Dome. Please take a moment to see its actual size.

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124"Eyes"Watching the Seto Ohashi Bridges
The Seto Ohashi Bridge is monitored 24 hours a day, 365 days a year. Anemometers, seismometers, accelerometers, and other instruments are installed on the Seto Ohashi Bridge, and data from 124 monitoring points is collected in real time, 24 hours a day. If the measured values exceed acceptable limits, safety-first measures are taken, such as displaying speed limits or traffic restrictions on road information boards. The valuable data collected is organized and stored, and will be utilized for technological development in the construction of future long-span bridges.

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Inspection Vehicle for Hitsuishijima Truss Viaduct
The bridge girders are equipped with rails that allow inspection and maintenance vehicles carrying personnel to move along them. There are two types of vehicles: those that operate on the inner surface of the girder and those that operate on the outer surface, both of which are used for maintenance tasks such as inspections and painting.

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"Sistes Bridges"of the Seto Ohashi Bridges Golden-gate Bridge
On April 5, 1988, the Seto Ohashi Bridge and the Golden Gate Bridge entered into a sister-bridge partnership. This initiative-the world's first sister-bridge partnership between two major bridges-aimed to further deepen mutual exchange and friendly relations between Japan and the United States by using these two iconic bridges as a bridge not only in bridge-building technology but also in all aspects of transportation, tourism, the economy, and culture.

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"Sister Bridges"of the Seto Ohashi Bridges Faith-Sultan-Mehmet Bridge
The Seto Ohashi Bridge and the Fatih Sultan Mehmet Bridge established a sister-bridge partnership on July 3, 1988. Completed around the same time as the Seto Ohashi Bridge, the Fatih Sultan Mehmet Bridge plays a similar vital role, connecting regions previously separated by a strait and serving as a major transportation artery that has contributed significantly to regional development.