A Bailey bridge is a factory made truss bridge. It was made in small sections that could be easily moved and put together on site. It was developed by the British during World War II for military use. 

A Bailey bridge did not need special tools or heavy equipment to put into place. The wood and steel bridge parts were small and light enough to be carried in trucks and lifted into place by hand. The bridges were strong enough to carry tanks.  Bailey bridges continue to be used in building projects and to provide temporary crossings for people and vehicles. 

It was designed by a British engineer, Donald Bailey. The British army tested different bridge designs in 1941–1942, including a suspension bridge and a stepped arch bridge. The flat truss bridge was the most successful. An early test model of the Bailey flat truss bridge is still in place over Mother Siller's Channel in Dorset. The Bailey bridge was first used by the army in Tobruk in 1942. The US Army built many of the bridges for use after the D-Day landings in France.

Parts of the bridge

The basic Bailey bridge is made up of three main parts, the floor, the stringers and the side panels. The floor of the bridge is made up of a number of  that run across the bridge. Across the bottom of these, forming a square, are . The bridge's strength is provided by the panels on the sides. The panels are , , cross-braced rectangles. Each weighs , and can be lifted by six men.

The transoms rest on the lower part of the panels, and are held together by clamps. The stringers are placed on top of the completed structural frame, and large pieces of wood are placed on top of the stringers to provide a roadbed. Ribands bolt the planking to the stringers. Later in the war, the wood was covered by steel plates, to protect it from the damage caused by tank tracks.

Each unit built in this way is a single  section of bridge, with a  roadbed. After one section is complete it is pushed forward over rollers on the bridgehead, and another section built behind it. The two are then connected together with pins pounded into holes in the corners of the panels.

Modern Bailey bridges

Bailey bridges are in regular use throughout the world. Some examples include:
 The longest Bailey bridge was built in October 1975. This , two–lane bridge crossed the Derwent River at Hobart, Australia. It was opened around a year after the Tasman Bridge disaster destroyed the only river crossing, dividing the city in two. The Bailey bridge was in use until the Tasman Bridge was re-opened on 8 October 1977.
 A Bailey bridge between the Suru River and Dras River in Ladakh, India is the highest bridge in the world at an altitude of 5,602 meters (18,379 ft) above sea level. It was built in 1982 by the Indian Army.
 In the mid-1950s Lime Rock Park, an auto racing track in Lakeville, Connecticut, bought a Bailey bridge left over from WWII. The bridge meant vehicles could enter the center of the track while races were taking place. The bridge has been in continuous use since. It was moved to new, raised supports in the spring of 2008. This may be the only WWII Bailey bridge in regular daily public service in the U.S.
 An footbridge being built at Jawaharlal Nehru Stadium, one of the main venues for 2010 Commonwealth Games collapsed on September 21, 2010, a few weeks before the opening ceremony, injuring 27 people. The Indian Army built a Bailey bridge to replace it in four days, at a fraction of the cost.

Other uses
The Skylark launch tower at Woomera was built up of Bailey bridge parts.

References

Other websites 

 "Push Over Bridges...", October 1944, Popular Science detailed article about the WW2 Baily Bridge
 Homepage about Bailey bridges (many photos, information, links, ...) 
 Bailey Bridges in New Zealand
 Animated build of a modern Mabey Compact 200 Bridge (similar to the original Bailey Bridge)

Bridge types