The girder is the simple form of beam bridge.  From approximately 1915 through 1955, the most common type of highway bridge built throughout much of the United States was the steel girder bridge.  Originally wood, girders then were made from iron or steel, or even reinforced concrete.  Following World War II and the expansion of highways, steel and concrete girder bridges became commonplace.  Two basic types of girder bridges may be explained.  First is the I-beam, and the second is the box.  The I-beam consists of two flanges (top and bottom plates) welded to the web (side plate.  The box girder is formed by four plates welded into a box shape, with flanges on top and bottom, and the webs forming the sides.  The other, less-used girders, take the shape of the Greek letter pi or the capital “T.”

     Pictured here is the San Diego-Coronado Bridge in San Diego, California.  This 2.12 mile steel box girder bridge, opened in 1969, connects San Diego with the Coronado isthmus across San Diego Bay.  It has a vertical clearance of approximately 175 to approximately 195 feet, and a height of over 200 feet.  The California Department of Transportation notes it was built at a cost of $47.6 million.  With three span continuous orthotropic box girders, it has the longest continuous box girder in the world and is supported by thirty towers.  "Orthotropic" design means vehicles travel on the bridge structure itself and not on a separate deck.  Its blue color was meant to mesh with the colors of the water and sky.




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