Crossing the River

Crossing the River. . In this age of technological innovation, the act of crossing a river may be taken for granted. And yet, there is always a magic to bridges: How is the river parted to lay the foundation? What stops the bridge from washing downstream? How are so many cars and trucks supported? , Let every man praise the bridge that carries him over. -- Anonymous , Redefining the arch , The design of the bridge, selected from among seven competing entries, reflects the arch tradition of other Potomac River crossings without actually using the classic arch. True Arches create large horizontal forces at the foundations -- a problem in the Potomac River where soils are particularly poor. An innovative system of V-shaped piers is used to transfer the load vertically through soft soil to firmer soils 200 feet below. , a balancing act , When tall ships approach, staff in the operator's House engage motors and machinery to open the drawbridge. The movable span -- called a bascule -- is a device that is counterbalanced so that it moves on a pivot, much like a seesaw. The bridge is so finely balanced that only small motors are needed to move the 34 million pounds of bridge, possibly the largest movable span in the world. , the construction process , This bridge, which replaced a deteriorating, undersized bridge in this location, uses high-performance materials, pre-fabricated components, and innovative construction techniques., Up to six-foot diameter steel pipe piles were driven into the soil to support the foundations. A cofferdam, or watertight enclosure, provided a temporary dry area to build the concrete foundations., Concrete pile caps and pier pedestals were formed on the top of the piles. , Prefabricated segments of the V-piers were transported to the site by barge and lifted into place by cranes., The segments were placed, alternating left and right to keep the pier balanced., The tips of the V-piers were connected and held together by slender concrete tie beams containing tensioned steel cables., Curved steel girders completed the arch appearance and a concrete deck slab provided the final roadway surface.