Examining Forces on Bridges

 At any given time, two main forces act upon a bridge: compression and tension.  It is the job of engineers to design bridges capable of withstanding these forces without buckling or snapping. Buckling occurs when compressive forces overcome an object's ability to handle compression, and snapping occurs when the tensile forces overcome an object's ability to handle tension.

 In a BEAM bridge, the top undergoes compression while the bottom undergoes tension.  In an ARCH bridge, compression is pushed down the ‘legs’ and the edges take the tension.  In a SUSPENSION bridge, the ropes take the tension and the posts.

 BEAM BRIDGE- a simple bridge with vertical posts with a horizontal beam. They are usually between 200 and 600 feet.  ARCH BRIDGE- a curved structure with vertical abutments supporting each end. Very strong, they stretch from feet.  SUSPENSION BRIDGE- cables are strung between major posts. These go from feet.

 Cut a strip of cardboard approximately 4 inches long and 1 inch wide.  Draw lines along the length.  Push your pencil in the centre of the cardboard strip. What happens to the black lines?  Where are the compression forces acting? Where are the tensile forces acting?

 Create an arch bridge with your piece of cardboard WITHOUT books for support.  Push in the centre of the bridge like before. What happens?  Now place books as “abutments” on either side of the bridge. Try again. What happens?  What type of force do the abutments put on the arch, compression or tension?

 Create a suspension bridge like the one pictured below.  What type of force is being applied on the string, tension or compression?

 A river that is 300 feet (91 m) wide  A ravine that is 1,000 feet (305 m) across.  A body of water that is 10,000 feet (3,048 m) across.  A small patch of swampy land.  A waterway in which tall ships must pass through.