Degrees of Reading Power (DRP)

     Bridges are built to allow a continuous flow of highway and railway traffic across water lying in their paths.  But engineers cannot forget that river traffic, too, is essential to our economy.  The role of         1        is important.  To keep these vessels moving freely, bridges are built high enough, when possible, to let them pass underneath.  Sometimes, however, channels must accommodate very tall ships.  It may be uneconomical to build a tall enough bridge.  The         2       would be too high.  To save money, engineers build movable bridges.

     In the swing bridge, the middle part pivots or swings open.  When the bridge is closed, this section joins the two ends of the bridge, blocking tall vessels.  But this section          3       .  When swung open, it is perpendicular to the ends of the bridge, creating two free channels for river traffic.  With swing bridges, channel width is limited by the bridge’s piers.  The largest swing bridge provides only a 75-meter channel.  Such channels are sometimes too          4       .  In such cases, a bascule bridge may be built.

     Bascule bridges are drawbridges with two arms that swing upward.  They provide an opening as wide as the span.  They are also versatile.  These bridges are not limited to being fully opened or fully closed.  They can be         5         in many ways.  They can be fixed at different angles to accommodate different vessels.

     In vertical lift bridges, the center remains horizontal.  Towers at both ends allow the center to be lifted like an elevator.  One interesting variation of this kind of bridge was built during World War II.  A lift bridge was desired, but there were wartime shortages of the steel and machinery needed for the towers.  It was hard to find enough         6       .  An ingenious engineer designed the bridge so that it did not have to be raised above traffic.  Instead it was         7        .  It could be submerged seven meters below the surface of the river.  Ships sailed over it.