Manual of Military Training Part 91

=1112. Spar bridges.=--This name is applied to bridges built of round timbers lashed together. Intermediate points of support are provided by inclined frames acting as struts to transmit weight from the middle of the bridge to the banks. The single-lock and double-lock bridges with two and three spans of 15 ft., respectively, are the ones of most utility.

The first step in constructing a spar bridge is to measure the gap to be bridged and select the position of the footings on either bank.

Determine the distance from each footing to the middle point of the roadway if a single-lock, or the two corresponding points of a double-lock bridge. Next determine and mark on each spar except the diagonals the places where other spars cross it. The marking may be done with chalk, or with an ax. If possible a convenient notation should be adopted. As, for example, in marking with chalk, a ring around the spar where the edge of the crossing spar will come, and a diagonal cross on the part which will be hidden by the crossing spar.

A simple way to determine the length of spars is the following: Take two small lines somewhat longer than the width of the gap, double each and lash the bights together. Stretch them tightly across the gap so that the las.h.i.+ng comes at the middle as at _A_, Fig. 8. Release one end of each and stretch it to the footing on the same side as indicated by the dotted lines. Mark each line at the footing _C_ or _C'_, and at the position chosen for the abutment sill, _B_ or _B'_.

Cut the las.h.i.+ng and take each piece of rope to its own side. The distances _AB_ and _AB'_ are the lengths between the transoms, and with 2 ft. added give the length of road bearers required. The distances _AC_ and _AC'_ are the lengths of struts from b.u.t.t to top of transom, and with 3 ft added, give the total length of spars required.

For a double lock bridge, a piece of rope of a length equal to the length of the middle bay replaces the las.h.i.+ng. If the banks are not parallel, a measurement should be taken on each side of the bridge.

If desired, a section of the gap may be laid down on the ground in full size and the lengths of spars determined by laying them in place.

This method, though given as standard by all authorities, requires more time and more handling of material than the other and gives no better results.

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.

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=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, b.u.t.ts 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 b.u.t.ts 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 b.u.t.ts of the standards and the back and fore guys, _DD_ and _EE_, to the tips the fore guys are pa.s.sed across to the opposite bank. The guys of the _narrow_ frame should be _inside_ the guys and standards of the wide frame.

The frames are put into position one after the other, or simultaneously if there are enough men. A man is told off to each foot rope and one to each back guy to slack off as required, two turns being taken with each of these ropes around their respective pickets.

The other men raise the frame and launch it forward, a.s.sisted by the men at the fore guys, until the frame is balanced on the edge of the bank. The frame is then tilted until the b.u.t.ts rest on the footing, by slacking off the foot ropes and hauling on the fore guys, Fig. 5.

After the head of the frame has been hauled over beyond the perpendicular, it is lowered nearly into its final position by slacking off the back guys. When the two frames are in this position opposite each other, the narrow frame is further lowered until its standards rest upon the transom of the other. The wider (outer) frame is then lowered until the two lock into each other, the standards of each resting upon the transom of the other.

The center or fork transom, Figs. 3 and 4, is then pa.s.sed from sh.o.r.e and placed in the fork between the two frames. This forms the central support to receive a floor system of two bays, built as already described.

The estimated time for construction of such a bridge is about one hour if the material is available and in position on both sides of the stream. The construction of the roadway requires about twenty minutes; forming footings in masonry about one hour.

=1114. Construction of double-lock bridge=, Fig. 6.--Suitable for spans not exceeding 45 ft., and consisting of two inclined frames which lock into a connecting horizontal frame of two or more distance pieces, with cross transoms, dividing the gap to be bridged into three equal bays of about 15 ft. The force required is two or three noncommissioned officers and 25 to 50 men; the time for construction, except roadway, about two and one-half hours; extra time to be allowed for difficult footings.

The width of gap is measured, the position of footings determined, and the length of standards from b.u.t.t to transom determined and marked as before.

The inclined frames in this case are built of equal widths, launched as before, and held by guys just above their final position. Two stringers are launched out from each bank to the main transom. The distance pieces, Fig. 6, are put into position inside the standards, using tackle if necessary, and the road transoms are placed and lashed to the distance pieces at the places marked. Both frames are now lowered until they jam.

=1115. Roadway of spar bridge.=--For infantry in fours crowded the transoms should have a diam. of not less than 9 ins. for a span of 15 ft. Five stringers 2 ft. 3 ins. c. to c., and 6 ins. diam. at the tip will suffice. If the sticks vary in size, the larger ones should be notched down on the transom so as to bring the tops in the same plane.

The stringers should be long enough to overlap the transoms, and should be lashed together at each tip. The floor is held down by side rails over the outside stringers and lashed to them. If lumber can not be obtained, a floor may be made, of small spars, the interstices filled with brush, and the whole covered with loam or clay; Figs. 7 and 9.

Corduroy Roads

=1116. Corduroying= is done by laying logs crosswise of the road and touching each other. The result will be better if the logs are nearly of the same size. The b.u.t.ts and tips should alternate. If the logs are large the s.p.a.ces may be filled with smaller poles. The bottom tier of logs should be evenly bedded and should have a firm bearing at the ends and not ride on the middle. The filling poles, if used, should be cut and trimmed to lie close, packing them about the ends if necessary. If the soil is only moderately soft the logs need be no longer than the width of the road. In soft marsh it may be necessary to make them longer.

The logs may be utilized as the wearing surface. In fact this is usually the case. They make a rough surface, uncomfortable for pa.s.sengers and hard on wagons and loads, but the resistance to traction is much less than would be expected, and the roughness and slightly yielding surface make excellent footing for animals. Surface corduroy is perishable and can last but a short time. In marshes, where the logs can be placed below the ground-water level, they are preserved from decay, and if any suitable material can be found, to put a thin embankment over them, a good permanent road may be made.

Any tough, fibrous material may be used to temporarily harden the surface of a road. Hay or straw, tall weeds, corn and cane stalks have been used to good advantage. Such materials should be laid with the fibers crosswise of the road, and covered with a thin layer of earth, thrown on from the sides; except in sand, when it is better to dig a shallow trench across the road, fill it with the material and then dig another trench just in front of and in contact with the first and throw the sand from it back onto the material in the first trench, etc.

Brush work

=1117. A fascine= is a cylindrical bundle of brush, closely bound. The usual length is 18 ft. and the diam. 9 ins. when compressed. Lengths of 9 and 6 ft., which are sometimes used, are most conveniently obtained by sawing a standard fascine into 2 or 3 pieces. The weight of a fascine of partially seasoned material will average 140 lbs.

Fascines are made in a =cradle= which consists of five trestles. A =trestle= is made of two sticks about 6-1/2 ft. long and 3 ins. in diam, driven into the ground and lashed at the intersection as shown in Fig. 10. In making a cradle, plant the end trestles 16 ft. apart and parallel. Stretch a line from one to the other over the intersection, place the others 4 ft. apart and lash them so that each intersection comes fairly to the line.

=To build a fascine=, straight pieces of brush, 1 or 2 ins. at the b.u.t.t, are laid on, the b.u.t.ts projecting at the end 1 ft. beyond the trestle. Leaves should be stripped and unruly branches cut off, or partially cut through, so that they will lie close. The larger straighter brush should be laid on the outside, b.u.t.ts alternating in direction, and smaller stuff in the center. The general object is to so dispose the brush as to make the fascine of uniform size, strength, and stiffness from end to end.

When the cradle is nearly filled, the fascine is compressed or =choked= by the =fascine choker=, Fig. 11, which consists of 2 bars 4 ft. long, joined at 18 ins. from the ends by a chain 4 ft. long. The chain is marked at 14 ins. each way from the middle by inserting a ring or special link. To use, two men standing on opposite sides pa.s.s the chain under the brush, place the short ends of the handles on top and pa.s.s the bars, short end first, across to each other. They then bear down on the long ends until the marks on the chain come together.

Chokers may be improvised from sticks and rope or wire.

=Binding= will be done with a double turn of wire or tarred rope. It should be done in 12 places, 18 ins. apart, the end binders 3 ins.

outside the end trestles. To bind a fascine will require 66 ft. of wire.

Improvised binders may be made from rods of live brush, hickory or hazel is the best. Place the b.u.t.t under the foot and twist the rod to partially separate the fibers and make it flexible. A rod so prepared is called a =withe=. To use a withe, make a half turn and twist at the smaller end, Fig. 12; pa.s.s the withe around the brush and the large end through the eye. Draw taut and double the large end back, taking 2 half-hitches over its own standing part, Fig. 13.

When the fascine is choked and bound, saw the ends off square, 9 ins.

outside the end binders. After a cradle is made, 4 men can make 1 fascine per hour, with wire binding. Withes require 1 man more.

=A fascine revetment= is made by placing the fascines as shown in Fig.

14. The use of headers and anchors is absolutely necessary in loose soils only, but they greatly strengthen the revetment in any case. A fascine revetment =must always be crowned= with sods or bags.

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