The construction of a frame is shown in [Fig. 1], and the system of marking in [Fig. 2]. The arrangement of frames to form a single lock bridge is shown in [Figs. 3] and [4], and a double lock bridge in [Fig 6].
| Fig. 1 | Fig. 2 |
Plate I
[1113]. Construction of single-lock bridges, [Figs. 3], [4], and [5].—Suitable for spans of 30 ft. or less. The two frames lock together at the center of the span; their slope must not be more than 4 on 7. The bridge can be erected by two or three noncommissioned officers and 20 men, one-half on each side of the gap. Heavy spars require more men.
The footings at A and B must be firm, horizontal if possible, and at right angles to the axis of the bridge. In a masonry pier they may be cut out. In firm soil a simple trench will suffice. In yielding soil a plank or sill must be laid in the trench. The frames are made of such length as to give a slight camber to the bridge, which may be increased to allow for probable settlement of the footings. The inside dimension of one frame is made slightly greater than the outside dimension of the other, so that one frame may fall inside of the other when hauled into position. For a 9 ft. roadway the standards of the narrow (inside) frame should be 9 ft. 6 ins. apart at the transom and 10 ft. 6 ins. at the ledger, in the clear, and the other (outside) frame 1 ft. 6 ins. wider throughout.
A frame is constructed on each bank. The standards are laid on the ground in prolongation of the bridge, butts toward the bank. The ledgers are lashed on above and the transoms beneath the standards at the positions marked. The diagonal braces are lashed to the standards, two butts and one tip above the latter, and to each other. Before the braces are lashed the frame must be square by checking the measurements of the diagonals.
If necessary, pickets for the foot and guy ropes are driven, the former about 2 paces from the bank and 4 paces on each side of the axis of the bridge; the latter about 20 paces from the bank and 10 paces on each side of the axis. The foot ropes, CC, [Fig. 5], are secured by timber hitches to the butts of the standards and the back and fore guys, DD and EE, to the tips the fore guys are passed across to the opposite bank. The guys of the narrow frame should be inside the guys and standards of the wide frame.