Osaka Bay Bridges: Konohana Bridge

We are leaving Tokyo for Osaka Bay to look at a self-anchored suspension bridge.

The Konohana Bridge is a suspension bridge with its cable anchored into the bridge superstructure rather than into the ground. Self-anchoring is an advantage when the soil is weak or the bridge is in deep water. The disadvantage is that the superstructure must be designed to handle the very large forces that are transferred from the suspension cable.

There are very few self-anchored suspension structures in the world. The State of California is replacing the seismically vulnerable East Crossing of the San Francisco-Oakland Bay Bridge with a new bridge that includes a self-anchored suspension span. South Korea has built one. Another self-anchored suspension bridge is the Konohana Bridge in Osaka, Japan.
Although the Konohana Bridge and the East Bay Crossing are both self-anchored bridges, they are very different. The Bay Bridge has a single tower, unbalanced spans, a wide deck, and two planes of suspension cables. The Konohana Bridge is narrow, it is almost perfectly balanced, it has two towers, but only a single cable. It is like yesterday's Tsurumi Tsubasa Bridge except with a suspension cable.

Osaka Bay Bridges: Konohana Bridge by Mark Yashinsky is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 United States License.

Tokyo Bay Bridges: Tsurumi Tsubasa Bridge (2)

I was on Ohgishima Island (in 1994) looking at underground natural gas tanks the next time I saw the Tsurumi Tsubasa Bridge. The name means 'flying into the future' and it was suggested by the public. The bridge has a 510 m (1675 ft) main span and 255 m (837 ft) side spans. The towers open to carry the superstructure and then close to carry a single vertical plane of cables. This tower shape has become very popular for both single and double planes of cables. This bridge may still have the longest main span for a single plane of cables. However, it strikes me that the superstructure is less stable, especially for wind and seismic loads. That may be why the Tsurumi Tsubasa Bridge has special dampers and an orthotropic deck.

Cable-stayed bridges have filled a gap between arch or cantilever bridges and the longer suspension bridges.  However, they have had problems such as corrosion of the cables, vibration of the cables, etc.  Also, they are difficult to build and usually require an engineer to design their construction.

Tokyo Bay Bridges: Tsurumi Tsubasa Bridge (2) by Mark Yashinsky is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 United States License.

Tokyo Bay Bridges: Tsurumi Tsubasa Bridge (1)

Immediately east of the Yokohama Bridge on the Shuto Expressway is the Tsurumi Tsubasa Bridge. We visited it while it was being built in 1992. It is a three span continuous steel cable-stayed bridge. To speed the construction, a non-moored barge was used to lift the superstructure segments into place for the first time in Japan.

The bridge was carefully designed for wind and seismic loads. Wind testing was conducted with and without an adjacent structure that was planned to be built in the future. The edge of the structure has a special aerodynamic shape to reduce drag. An oil-damping system was used to control vibrations during earthquakes. We'll look at the completed bridge tomorrow.

Tokyo Bay Bridges: Tsurumi Tsubasa Bridge (1) by Mark Yashinsky is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 United States License.