Asked questions in bridges

　Our research target is design and construction of new bridges and maintenance of existing bridges as a part of our civilization and their maintenance for the sustainable development of our global society. So our research will cover field of the bridge engineering but not cultural history of bridges

　In Wikipedia on Internet explanation on "Bridge" is started as "A bridge is a structure built to span a gorge, valley, road, railroad track, river, body of water, or any other physical obstacle, for the purpose of providing passage over the obstacle. Designs of bridges will vary depending on the function of the bridge and the nature of the terrain where the bridge is to be constructed.". bridge is simply a tool to pass over any obstacles. This purpose results different understanding, that bridge can be understood as a door to glorious future. Besides it is also constructed to show a boundary, such as sacred are of shrine and others. And it can be favourable landmark of the area and the city, such as the Golden-Gate Bridge, San Fransisco, U.S.A..

　The bridge in its history was born with the action to place a stepping stone in the old days in the place to want to cross to and then to hand a tree on it. However, at present, the feature of the bridge in the stepping stone is left and not a few examples will be found at gardens Arch bridges made of stone, a suspension bridge made of vine and the covered bridges, wooden structures of which there is a roof to protect against severe weather, have been built. Their histories are very long
. However, it is appropriate that moder bridges were built after the Industrial Revolution by high quality steel.

A word "the cable-supported bridge " may not accustom but it means to so-called the suspension bridge and the cable-stayed bridge.
It is the bridge which is hung with the cable but it means what doesn't support external force mainly with the deck while the bridge structure behave under the dynamics of the cable. The extra dozed bridge, constructions of which are increased recently like the Odawara blue weigh bridge, resembles a cable-stayed bridge in the whole configuration, but the mechanism to support external force efficiently with the deck distinguishes from the cable-stayed bridge. The stress-ribbon bridge is the one of the cable-supported bridge, but due to tightly set cables in the deck it becomes possible to do passage about. It doesn't feel to be hung but its dynamic behavior is just the vibration of the cable.

　The vine bridge of Iya which is accompanied by the legend of surviving soldier at the Heike in the 1100s is very famous.
The first modern suspension bridge is the 70 ft suspension bridge at Jacob's Creek in Pennsylvania, U.S.A. by James Finley (1756-1828) in 1801. In Europe, the Menai Straits Bridge ( 1826 ) in Wales, U.K. by Thomas Telford is known as early suspension bridge. Main cables of these bridges are of chain. The more modern one will be the Brooklyn Bridge ( opened on May 24, 1883) by the Roebling family (John Augustus, Washington and Emily Warren Roebling) who used parallel wire cables for the main cables which are essential for the present suspension bridge.

　It is often said to that the history at the modern suspension bridge is the history of the battle with the strong wind. The Menai Straits Bridge suffered damage with the strong wind from the early stages and it had to repeat repairing of the structure and a lot of suspension bridges experienced actually many vibration damages by the wind. The extreme example of the collapse is the Tacoma Narrows Bridge ( the Nov.1940 collapse ). A detailed movie, and the photograph were taken during its construction and collapse. They appear even now as the shocking case image on the mass media. But after this great lesson of the accident detailed investigation was done and it becomes a foundation of the present wind-resistance design technology.

Since the accident at the Tacoma bridge, the importance of the wind-resistance design was recognized by the design of the long span suspension bridge. In the wind-resistance design, a stability to the vibration by the unstable phenomenon, safety of the wind loading ,built in the structural-design, are verified. In any case, because it is a wind-induced phenomenon, i.e. aerodynamic-structural dynamic problem, the experiment by the wind tunnel is the main technique of the design procedures. It constructed a lot of long-span bridges by the Honshu-Shikoku-bridge project and the mechanism of the experiment precision homogenization was introduced and the wind-resistance design standard was made.

　橋のどこに不具合があるかを調べ不具合を修理すること，あるいは，最近では不具合が起きそうなので前もって手当をしていく方法が維持管理の基本です．不具合が知るために，橋の状況を知ることをモニタリング，英語ではhealth monitoringといいます．橋の状態を知って，それから，さらに不具合を知ることはモニタリングの鍵ですが，かなり難しい技術です．

　橋の振動をコントロールすることを制振といい，振動の対象が地震だったら制震を使います．免震あるいは免振は，振動の伝達経路をたち振動を抑えることです．簡単には，基礎と構造を切り離せばいいわけですが，具体的には支沓やゴム材など支持力を持ちながら何らかの方法でスリップさせればよく，振動を抑えるために減衰器も併用される例もあります