MOVERS AND SHAKERS

Movable bridges are vulnerable even to small earthquakes as the mechanisms that allow the bridge to move as well as the shims, guides, and locks that close the bridge are easily damaged. The article discusses the requirements for the design of new movable bridges that must have at least 50% of the inertial force even when the bridge is open less than 10% of the time. The key to retrofitting movable bridges is that the retrofit allow the bridge to move during the earthquake and thus preventing it from collapsing as a result of those motions. Examples are provided to show how this principle was used on different movable bridge retrofits.