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Stepping back prior to construction to the testing phase, another major hurdle for the engineers was to anticipate potential problems caused by the higher wind speed. A difficult challenge for the Akashi Kaikyo Bridge was how to prevent coupled flutter which occured lower than the design wind speed level, something that was not particularly a problem for Mimani-Bisan Seto Bridge and Kita-Bisan Seto Bridge of the 1,000-meter class.
Dr. Masao Miyazaki, currently general manager of Technical Dept. of the Steel Structure & Process Equipment Group at Sumitomo Heavy Industries, Ltd., headed the wind tunnel testing at Sumitomo Heavy Industries Research & Development Center from the start of testing in 1965. Miyazaki recalls, "The only method of verifying a potential design is wind tunnel testing using scale girder models (section model) that accurately simulate the shape, mass, inertia, and frequency ratio of the actual bridges. More than 200 situations were tested with repeating trials and making improvements over and over. Even so, it was very difficult to clear the 78 meters-per-second wind resistance criterion."
For the bridge girders, perforated gratings like those used on Seto Ohashi Bridges were laid along both sides and down the center of the deck. Vertical stabilizers of 2.5 meters in height were also installed directly beneath the road deck, along the center of the span. The critical wind speed was finally achieved when engineers devised the optimum locations of the maintenance roads and the shapes and layout of trusses and other auxiliary members inside the girder, and everyone involved let out a great collective sigh of relief.
But the tests were not over yet. The unprecedented length of the span, stretching 2,000 meters between the main towers, meant that it was almost impossible to know what could happen. Sentiment grew stronger about the need to verify the bridge's properties using a full-scale model of the whole structure. A large wind tunnel facility was built at the Construction Ministry's Public Works Research Institute at Tsukuba that would be big enough to accommodate a 1/100 full-scale model. The tests conducted there revealed new information that was reflected in section models for further rounds of testing. After extensive testing on the full-scale model, the installation of vertical stabilizers only on the central span was deemed sufficient for achieving bridge safety. |
