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.
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.
* * *
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||
|60 D. S.||
||2.5 tons - 3 bays of decking|
|70 D. S.||
||5.1 tons - 6 -do-|
|80 D. S.||
||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.
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.