LAUNCHING THE BAILEY BRIDGE

The British "Bailey bridge" is a knock-down steel bridge, which can be transported in a truck train and erected where needed. With the change of some bolt-head sizes only, it has been adopted by the U.S. Army. The bridge is a "through" type with the roadway carried between the main girders. These girders are formed of panels pinned together to make 10-foot bays. The strength of the girder of 60-foot span can be increased from 20 tons up to better than 80 tons by making it a double-truss, double-story type. The basic unit is a welded, flat-lattice girder 10 feet long, 5 feet 1 inch high, and 7 inches wide, made of high-tensile structural steel, weighing 570 pounds, which can be carried by six men.

TANK CROSSING DOUBLE-TRUSS, SINGLE-STORY BRIDGE

The "launching"--or actual placing of the bridge out across a stream--is usually done by pulling the completed structure out and across the stream over rollers by means of a "launching nose" or extension, made by pinning on to the end of the bridge the necessary number of 10-foot bays, which extend out ahead of the bridge. The pulling is done by means of suitable cables made fast on the opposite side of the stream. If necessary, of course, the free end of the bridge can be floated, derricked, or cantilevered out across the gap.

The following account from British sources of a method of launching a bridge without the use of the nose extension is of practical value to engineers.

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To reduce the problem of jacking down to one end of the bridge only, it has been found possible to launch a "DS"--double truss, single story--counter-weight instead of the launching nose.

In preparing to launch by this method careful attention to the height of the rollers must be given as there is no nose into which a link can be placed. Launching should be done on a level plane, allowance being made in the calculation for the sag of the bridge and for the base of the end posts projecting 6 inches below the bottom chord as well as for any difference in bank height. A normal layout of stores with stringers placed further back from the rocking rollers than is usual, is satisfactory.

In construction, a normal double-story bridge, less decking, is built to the requisite length. A Single-story skeleton tail is added, the bridge being kept as near the point of balance as possible by frequent booming out. The last bay is decked and counter-weight is added according to the table:

Length of bridge Length of tail Counter-weight 60 D. S. 50 feet 2.5 tons - 3 bays of decking 70 D. S. 60 feet 5.1 tons - 6 -do- 80 D. S. 70 feet 5.55 tons - 7 -do-

Counter-weights are calculated with the center line of rollers on center line of base-plate.

On completion, four men are sent to the far bank to position the bearing under the end posts; remainder of the party launch the bridge, pushing downward and outward.

When the bridge has reached the far bank, and the head has been lowered to rest on the base plates, two jacks are placed on the end of the tail and two jacks one bay back from the bridge. Jacking proceeds until the panel pins connecting all but the last bay of the tail can be driven out with a sledge hammer. The tail is jacked down on to the plain rollers and pushed back out of the way. The bridge is now jacked up on the near bank, the rollers are removed and the base plates are positioned. Packing is placed under the remaining bay of the tail. By jacking down on to the packing, the remainder of the tail can be removed. End posts are fitted and the jacking down is completed.

Decking can be fitted and the far ramps placed while the jacking is in hand. The advantages gained from this method of construction are:

(1) A shorter span of bridge than would normally be necessary, can frequently be used.

(2) Jacking down one side only and dismantling the tail after the bridge has been opened to traffic, speeds the time of completion.

(3) Construction is possible with reduced working numbers and equipment.

Note

An American official source suggests that the losses of small tools, which are difficult to replace, will be considerably reduced if bridge erectors, when working over water, will carry them slung around their necks with string. The hide-faced hammers may be adapted for stringing by drilling a hole, or cutting a groove in the handle, near the hand end.