Transport Needs on Both Sides of the Strait
The Akashi Kaikyo Bridge links Kobe's Tarumi ward with Awaji Island across the busy waterway connecting Osaka Bay to the Seto Inland Sea. It forms the northern part of the Kobe–Awaji–Naruto route. Although the Onaruto Bridge at the southern end had opened in June 1985, the northern gap still prevented vehicles from traveling continuously along this route between Honshu and Shikoku. Completion of the bridge provided a direct road connection between Kansai and Shikoku along this alignment.
Shinichi Oe's technical history distinguishes the initial concept from actual investigation: Kobe mayor Chujiro Haraguchi expressed a desire for a bridge in 1953, the city's fiscal 1957 budget included survey funding, and maritime safety authorities were commissioned to make measurements. The Honshu–Shikoku Bridge Authority was established in 1970 as the construction body. Work was suspended and resumed before a groundbreaking ceremony for the Akashi Kaikyo Bridge was held in 1986. The path from aspiration to construction was not one continuous, unchanged plan.
A Suspension System Built Through Divided Responsibilities
The authority coordinated the project, while steel, heavy-industry and civil-engineering companies undertook separate parts. Main-cable construction can be traced to a specific contract: in July 1991, a joint venture of Nippon Steel and Kobe Steel received the authority's cable-erection contract. Its engineers described the wires, strands and cross-strait hauling process in a 1997 technical report, showing how an exceptionally long bridge depended on coordinated materials development, construction equipment and site organization.
The bridge uses a three-span, two-hinged suspension system with a stiffening truss. Deck loads pass through hangers to two main cables, then to the towers and anchorages. The completed structure is 3,911 meters long with a 1,991-meter main span. These dimensions appear in a 2009 bridge-engineering paper and should not simply be copied from the 1997 cable-erection report, which still listed the original 3,910-meter and 1,990-meter design dimensions. The difference relates to earthquake displacement during construction.
Strong Currents and Seabed Foundations
The crossing has a maximum water depth of about 110 meters, tidal currents reaching approximately 4.5 meters per second and heavy shipping traffic. Underwater foundations had to be stable, but the strait could not be treated as a construction site that could be closed at will. Geological investigation used acoustic surveys and drilling to identify seabed strata and faults, and tower foundations were positioned to avoid known faults.
A contemporary 1996 report recorded that the two offshore tower foundations were installed using large caissons. The base of the Honshu-side foundation was approximately 60 meters below sea level and that on the Awaji Island side approximately 57 meters below; the caissons measured 80 and 78 meters in diameter respectively and rested on different geological strata. Forces from the stiffening girders, main cables and towers ultimately passed into these massive seabed structures. The long span was not achieved by the steelwork above water alone.
Carrying Steel Cables Across the Shipping Route
The main cables were erected using prefabricated parallel-wire strands: high-strength galvanized steel wires were assembled into strands, erected across the strait and compacted into cables, each about 1.1 meters in diameter. On November 10, 1993, a large helicopter hauled a pilot rope across the strait in sequence from an anchorage to one tower, the second tower and the opposite anchorage. The pilot rope then drew the hauling system into place, enabling erection of construction catwalks and the main cables.
The main towers, nearly 300 meters high, also had to withstand wind-induced vibration. A paper by Kensaku Hata and colleagues explained that wind-tunnel testing was used to compare cross-sections. The selected form varied the width of chamfered corners with height and incorporated mechanical vibration-control devices; tuned mass dampers inside the towers worked together with construction-stage equipment. These measures addressed interactions among structural shape, vibration and wind, which could not be replaced simply by making the materials stronger.
Surveying and Design Revisions After the Earthquake
When the Southern Hyogo Prefecture Earthquake struck on January 17, 1995, the main cables had been erected and cable compaction was under way, but erection of the stiffening truss girders had not begun. Personnel immediately inspected the structure, resurveyed foundation positions using optical distance measurements and GPS, and investigated the nearby seabed. No serious damage was found in the main structures, but the foundations had moved with the Earth's crust and the distances between them had changed.
Measurements at the time showed that the main span had increased by about 80 centimeters and one side span by about 30 centimeters. Reassessment confirmed that the effects remained within the original design's permissible limits, and the lengths of girder sections still being fabricated were adjusted. The 2009 paper further explained that two midspan sections were each lengthened by 40 centimeters and one end section by 34 centimeters to fit the new geometry. The later commonly cited 1,991-meter main span is therefore a whole-meter dimension resulting from surveying and design revision, rather than a main cable simply stretched by one meter.
Transport Reorganization After Opening
The bridge opened on April 5, 1998, completing the Kobe–Awaji–Naruto Expressway. Kazuya Minoura's 2001 study explicitly recorded this date in its opening and examined changes through a resident questionnaire conducted six months after opening. Assessments of transport access must distinguish between places, users and observation dates; an opening ceremony alone cannot establish that everyone benefited equally.
A freight study in 2000 used questionnaires and interviews with shippers and logistics companies and found that some traffic had shifted from ferries and the Seto Ohashi route to the Akashi Kaikyo Bridge. The area reachable by same-day delivery expanded, while some ferry services ceased or were reduced. Some surveyed shippers reported lower costs from shorter journey times, while others said using the bridge increased transport costs. The engineering breakthrough created new transport choices and redistributed the time, costs and passenger and freight flows of land and sea transport.