Moveable and transporter bridges

The moveable and transporter bridges are two of the oldest types of bridges known to humankind. The bascule or draw span was developed by Europeans during the middle Ages. Revival of construction of moveable bridges started during the late 19th century. Reliable electric motors and techniques for counterbalancing the massive weights of the bascule lift, or swing spans marked the beginning of modern moveable-bridge construction.

The Lacey V Murrow Bridge

Completion of Tower Bridge over the Thames in London (1894), a 260ft (79m) roller-bearing trunnion bascule and the best known bascule bridge in the world, and Van Buren Street Bridge in Chicago, the first rolling lift bridge in the USA, marks the efficient solution to problems of lifting and locking mechanisms. In 1914, the world's largest double-leaf bascule was completed, spanning 336ft (102m). The Saint Charles Airline Railway Bridge (1919) in Chicago was at 260ft (79m) the longest single-leaf bascule when it was completed. In 1927, the world's longest single-span swing bridge, 525ft (160m) was built, over the Mississippi. One of the most interesting and unusual moveable bridges is the Lacey V Murrow Bridge (1940), whose design reached back to the pontoons built by Roman legions.

The tower bridge

The depth and breadth of the lake precluded the construction of conventional piers on pilings, cantilever, or suspension spans, so bridge engineers designed a floating bridge supported by hollow concrete pontoons to connect Seattle and Mercer Island. Equally unique was the retractable floating draw span for ocean-going ships in the lake.

Finally, bridges have been widely used since ancient times and have evolved through the years. They started as simple tree trunks laid across streams. Stone slabs were used and eventually wooden frames and finally steel and concrete. Bridges were first constructed based on experience and trial and error, but as engineer's understanding of the forces acting on beams grew, they formed new and accurate methods of calculation. Bridges today are all designed by computers with architectural software before the first stone is laid. As time goes on and technology progresses construction techniques will improve. New materials will replace the old ones and bridge construction will continue developing.

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