People build bridges for crossing rivers. A big river is navigable. A contradiction occurs. There must be a bridge to connect two banks of the river and the bridge must be absent to provide sailing of large vessels. One of the methods for resolving this contraction (in time) is realized through a drawbridge. Such a bridge or one of its spans located over the fairway is divided into two parts. In the daytime, when there are many cars, the bridge halves are down, while at night they are raised so that vessels can easily pass under the bridge.

A drawbridge is an impressive construction. Those who saw the drawing of a bridge at least once, remember how the lifting mechanisms creak under tension, trying to keep suspended huge metal constructions. Enormous energy is spent to draw the bridge. Breakdown of a lifting and holding mechanisms has far-reaching effects.
In the seventies, the magazine “Technics for young people” published an article written by a young inventor. The reasoning of the author was as follows. We need to have a river section free from a bridge. To do this, it is not necessary to lift the bridge. It is enough to rotate it on its axis so that it is positioned along the river. The rotating support must be in the center of mass of the rotatable bridge floor. Then both parts will be balanced and the construction will have the minimal mass. Such a rotatable bridge is much simpler than a drawbridge and much less energy is needed to drive it. Moreover, the river stream can be used to rotate the bridge. If a board is immersed in the river on one side of the bridge, then the flowing water will turn the bridge to the necessary position itself.
Not that they had not built rotatable bridges before. For instance, one of the most beautiful bridges of Europe – Troitsky Bridge in Saint-Petersburg – was initially rotatable. Its first project was developed by the world-famous engineer Eiffel. The bridge revolved on rolls forming two 23-m wide navigable channels.
Later, when they started building large vessels, the bridge was reconstructed and its rotatable span was replaced with a 100-m wide draw one. But the invention of the boy was really an invention, because by that time rotatable bridges had been forgotten out and away. Besides, an original drive for revolving the bridge was proposed.
Anyhow, the publication of the short item in “Technics for young people” awaked interest in many specialists. Such bridges were built in many countries of the world. One of the most impressive projects is the bridge designed by the Kievgiprotrans Institute and built near Nikolayev (Odessa Region), the rotatable span of which is 134 m long.

The inventive principle used for replacing bridge drawing with bridge turning was later described in TRIZ as “Equipotentiality”.