What Is a Bailey Bridge?
A Bailey bridge is a type of pre-fabricated truss bridge designed by Sir Donald Bailey, an Englishman, during the Second World War. It is considered a great example of military engineering, built entirely by man-power, it met a great need. The old types of military bridge were unable to support the rapidly increasing weight of armored vehicles. Bailey was a civil servant in the British War Office who tinkered with model bridges as a hobby. He presented one such model to his chiefs, who saw some merit in the design.
The Bailey bridge proved to be the most useful and versatile military bridge of the war. It was built of panels and sections, or trusses, which could be combined to carry loads of up to 100 tons over spans up to 220 feet. The roadway of the bridge is supported by two main trusses pinned together. Each 10-foot section of bridge consists of two parallel trusses supporting the cross-members on which the surface is laid, and is known as a bay.
The capacity of the bridge may be increased by adding one or two extra trusses alongside the first, by adding extra trusses on top of the first two or three to make a second storey, or by both means. Therefore a single-truss, single-storey bridge is a single-single and you can have double-single, double-double and so on. The Bailey bridge was also adapted to float on a plywood pontoon. New versions of the Bailey bridge have been devised. The Mark 3 Bailey with a roadway 13 feet 9 1/2 inches wide is standard in the British Army.
Bailey bridges are also extensively used in civil engineering construction projects to provide temporary access across canals, rivers, railway lines, etc., which requires no special tools or heavy equipment for construction, the bridge elements are small enough to be carried in trucks, and the bridge is strong enough to carry tanks.
The success of the Bailey bridge was due to the simplicity of the fabrication and assembly of its modular components, combined with the ability to erect and deploy sections with a minimum of assistance from heavy equipment. Most, if not all, previous designs for military bridges required cranes to lift the pre-assembled bridge and lower it into place. The Bailey parts were made of standard steel alloys, and were simple enough that parts made at a number of different factories could be completely interchangeable.
Each individual part could be carried by a small number of men, enabling army engineers to move more easily and more quickly than before, in preparing the way for troops and material advancing behind them. Finally, the modular design allowed engineers to build each bridge to be as long and as strong as needed, doubling or tripling up on the supportive side panels, or on the roadbed sections.
The basic bridge consists of three main parts. The bridge’s strength is provided by the panels on the sides. The panels are 10-foot-long (3.0 m), 5-foot-high (1.5 m), cross-braced rectangles that each weigh 570 pounds (260 kg), and can be lifted by six men. The panel was constructed of welded steel. The top and bottom chord of each panel had interlocking male and female lugs into which engineers could inset panel connecting pins.
The floor of the bridge consists of a number of 19-foot-wide (5.8 m) transoms that run across the bridge, with 10-foot-long (3.0 m) stringers running between them on the bottom, forming a square. Transoms rest on the lower chord of the panels, and clamps hold them together. Stringers are placed on top of the completed structural frame, and wood planking is placed on top of the stringers to provide a roadbed. Ribands bolt the planking to the stringers. Later in the war, the wooden planking was covered by steel plates, which were more resistant to the damage caused by tank tracks.
Each unit constructed in this fashion creates a single 10-foot-long (3.0 m) section of bridge, with a 12-foot-wide (3.7 m) roadbed. After one section is complete it is typically 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.
For added strength several panels (and transoms) can be bolted on either side of the bridge, up to three. Another solution is to stack the panels vertically. With three panels across and two high, the Bailey Bridge can support tanks over a 200-foot span (61 m). Footways can be installed on the outside of the side-panels; the side-panels form an effective barrier between foot and vehicle traffic and allow pedestrians to safely use the bridge.
A useful feature of the Bailey bridge is its ability to be launched from one side of a gap. In this system the front-most portion of the bridge is angled up with wedges into a “launching nose” and most of the bridge is left without the roadbed and ribands. The bridge is placed on rollers and simply pushed across the gap, using manpower or a truck or tracked vehicle, at which point the roller is removed (with the help of jacks) and the ribands and roadbed installed, along with any additional panels and transoms that might be needed.
During World War II, Bailey bridge parts were made by companies with little previous experience of this kind of engineering. Although the parts were simple, they had to be precisely manufactured if they were fit each other correctly, so they were assembled into a test bridge at the factory to make sure of this. To do this efficiently, newly manufactured parts would be continuously added to the test bridge, while at the same time the far end of the test bridge was continuously dismantled and the parts dispatched to the end-users.