BRIDGES.
Weight.—Infantry in column of fours when crowded is allowed per lineal foot 560 pounds.
Cavalry under same conditions is allowed 700 pounds.
Field-artillery in column of sections weighs 400 pounds per lineal foot, so that a bridge built for infantry will carry it.
In calculating the strength of parts of bridges the live load must be doubled to bring it to dead load if weight is to be applied suddenly.
Formula for weight that may be borne safely by a beam supported at both ends and loaded in the centre:
| W = | bd2S | , |
| l |
W being the weight in pounds, l the distance between the points of support, d the depth, and b the breadth, all expressed in inches. S is a coefficient whose value for different kinds of wood is given in the following table:
| Ash | 2000 |
| Yellow Pine | 1100 |
| Beech | 1700 |
| Larch | 1300 |
| Birch | 1900 |
| Oak | 1600 |
Strength of Round Timber.—The strength of a circular pole is six tenths that of a square beam whose side is equal to the mean diameter of the pole; so that the equation becomes
| W = | 6dS | , |
| 10l |
in which d is the mean diameter.
The bridge must be capable of sustaining the weight when crowded; and while the load for infantry is usually distributed, the greatest strain is brought on the balks, or road-bearers, when the gun-wheels are at the centre of the bay; and a balk will only bear half the load concentrated at its centre that it will bear when distributed.
Roadway and Approaches.-The roadway may be of planks 1½ to 2 inches for ordinary traffic, or of poles, fascines, hurdles, etc. Litter or earth should be scattered over it to deaden the rattle of planks, which is apt to frighten the horses, and preserve the roadway. If heavy loads are to be hauled, planks should be laid longitudinally to form wheel-tracks. Eight feet in the clear will suffice for width of roadway, but nine feet is to be preferred. A hand-rail should be provided, especially if horses are to pass, and ribbands on either side. At least five balks (road-bearers) should be used on a bridge nine feet wide. The bays generally run from ten to fifteen feet.
A rise in the centre of the bridge, called the camber, is allowed for the subsequent settling. This allowance is usually one thirtieth of the bay. The approaches are very important; the exit must be fully as good as the entrance, to avoid crowding. If not, care must be taken not to admit troops at the entrance faster than they can pass off. Ramps at either end steeper than one on ten are inconvenient.
TRESTLES, PIERS, ETC.
Transoms.—They must be sufficiently strong to bear the weight that can be concentrated on one bay, reckoned as distributed load on the transom.
On a 9-foot roadway for light artillery, with bays of 15 feet, the transoms should not be less than 9 inches in diameter.
Trestles are used for bridges across shallow rivers having sound hard beds and not liable to sudden floods; and for gaps, etc. They are not suitable for deep muddy rivers. They may be used in streams 9 feet deep with velocity of 4 miles an hour; or in deeper streams if the current be less.
Two-legged Trestle ([Fig. 130]).—The standards, transom, and ledger are marked at the proper intervals, the heights of the first having been obtained by a careful inspection, and the lengths of the two latter depending on the width of the proposed roadway. The transoms should be about 3 feet longer than the width of the roadway in the clear, and the standards of the trestle should have a splay outward of six on one. Square lashings must be used. The braces are put on the frame with both butts and one tip on the same side, the second tip on the reverse side; the butts can be lashed simultaneously with the ledger and transom. The frame must then be squared by testing the diagonals, measuring from the butt of each standard, and the frame must be adjusted until these measurements are equal. The braces can then be lashed at the tips and crossing-point. If the timber is weak, both legs and transom can be doubled. Ledgers and diagonal braces can be of light material, as little strain is brought upon them; but they should be well lashed.
Fig. 130.
Placing Trestles.—They are most easily placed by hand; but with those with two legs only care must be taken that they do not fall over. Each frame should be fastened to either bank; cables or struts may be used.
Three-legged Trestles ([Fig. 131]), two of which are required for a single transom, have the advantage of standing without bracing, admit of ready adjustment, and utilize light material. They are, however, unsuitable for an uneven bottom, are extremely difficult to place, excepting by hand, and the legs require weighting in water. To make them, lash two legs together by a sheer lashing, open them out and then add the third leg; the trestle must then be up-ended, the feet placed on the angles of an equilateral triangle with sides about half the height, and three light ledgers attached.
Fig. 131.
Cribs, Gabions, and Fascines.—These are also used for supporting roadways, the two latter are particularly useful for short gaps.
Limbers.—Bridges supported on transoms lashed across the tops of limber-wheels may be used.
FRAME BRIDGES.
If a trestle bridge is impracticable, owing to depth of gap or stream, unevenness of bottom, or swiftness of current, a frame bridge may be used.
Fig. 132.
Single Lock.—This is suitable for spans not exceeding 30 feet, measured between the footing of the frames. The slope of the frames when in position must not exceed four on seven.
1st. Prepare footings 18 inches wider apart on one side than the other. They must be correctly squared or the frames will not lock.
2d. Measure the gap and lay out section of it on the ground with a line and pickets, allowing for camber.
3d. Lay out the standards in the section, and chalk-mark them at the proper positions for lashing ledgers and transoms.
4th. Lash the frames in position on either side of the gap; butts of standards towards the bank, ledger lashed on the top of standards, about two feet from the butts, but position varied according to nature of footings, transom lashed underneath. The splay of legs in a frame is not so great as in a two-legged trestle, one foot difference between transom and ledger generally sufficing; the transom of narrow frame 18 inches wider than the width of roadway in the clear between ribbands. Square the frames carefully before lashing the braces; see that the distance apart of the butts corresponds to the footings, broad frame to be 18 inches wider than the narrow. Drive pickets or large piece of timber (bollard) for guys and foot-ropes.
5th. Attach fore and back guys and foot-ropes to each frame, the latter fastened with a timber-hitch round the standards below the ledgers.
6th. Pass across the fore guys, passing those of the narrow frame between the horns of the broad frame.
7th. Launch and lock the frames.
8th. Send out temporarily two road-bearers and by their means and with the aid of two men working on the crutch of the bridge get the fork-transom in position.
9th. Send out remainder of road-bearers, resting the whole on the fork-transom.
10th. Place planks (chesses); rack down; put up hand-rail.
ESTIMATE OF TIMBER.
| Kind of Bridge. | No. of Spars. | Length in Ft. | Diam. in Ins. | Purpose. | |
| At Tip. | Mean. | ||||
| Single-lock (30-ft. Span). | 4 | 24 | 7 | Standards. | |
| 1 | 15 | 10 | Fork-transoms. | ||
| 2 | 15 | 6 | Transoms. | ||
| 4 | 15 | 4 to 6 | Ledgers and shore-transoms. | ||
| 4 | 20 | 3 | Diagonal braces. | ||
| 10 | 20 | 6 | Balks. | ||
| 4 | 20 | 3 | Ribbands. | ||
If the stores are at hand and ready for use, 1 non-commissioned officer and 16 men on each side should complete the bridge in a little more than an hour.
Double-lock Bridge.—This is used for spans of 30 to 45 feet. In laying out the section and making the spars for the double-lock bridge it must be remembered that the road-bearing transoms, TT, in this case are not the transoms lashed to the frames, but those which are subsequently lashed on the top of the distance-pieces.
Fig. 133.
The frames are of equal widths. In making the bridge the frames after being launched are held by the back guys a little higher than their ultimate position. The distance-pieces are then hauled across, the proper positions for the road-bearing transoms having been previously marked upon them; these transoms are then sent out and lashed in position, after which the back guys can be eased off and the bridge allowed to lock; the roadway is then laid.
ESTIMATE OF TIMBER.
| Kind of Bridge. | No. of Spars. | Length in Ft. | Diam. in Ins. | Purpose. | |
| At Tip. | Mean. | ||||
| Double-lock (40 to 45 Ft. Span.) | 4 | 20 | 7 | Standards. | |
| 2 | 25 | 10 | Distance-pieces. | ||
| 2 | 15 | 10 | Road-transoms. | ||
| 2 | 15 | 6 | Main transoms. | ||
| 4 | 15 | 4 to 6 | Ledgers and shore-transoms. | ||
| 4 | 20 | 3 | Braces. | ||
| 15 | 20 | 6 | Balks. | ||
| 6 | 20 | 3 | Ribbands. | ||
Fig. 134.
Single-sling Bridges, used to 50 feet.—The frames for a single-sling bridge have an upper (locking) and lower (road-bearing) transom. In laying out the section the frames must be made to lock at such a height as to have at least 9 feet of headway between the roadway and the upper transoms. As soon as the spars of the narrow frame are in position for lashing the distance from out to out of its standards at the upper transom must be measured, and the broad frame made of sufficient width to receive them. A snatch-block with a fall rove through it is lashed to the tip of each standard of the narrow frame. The frames having been locked, the fork-transom is hauled into position by means of the falls and men working in the crutch. The falls are then used to sling the centre transom temporarily into position. The slings are then arranged by men working in the crutch and astride the slung transom, one of the guys being generally used for the purpose; three or four turns are sufficient, care being taken that they do not ride. The slings, if too long, can be subsequently twisted up with handspikes to give the requisite camber to the roadway.
Fig. 135.
A Treble-sling Bridge has three slung transoms, one being slung from the standards on either side between the fork and road-bearing transoms. The process of construction is similar to that of a single-sling. If the frames are very long and heavy, it is well to brace them above as well as below the road-bearing transoms to prevent their getting racked out of shape in launching. The upper braces, if they interfere with the headway, can be removed after the frames are locked. The great length of the standards in single-and treble-sling bridges, and the heavy transverse strains brought upon them by the weight of the roadway, will generally necessitate their being stiffened by ties which should be carried over vertical frames anchored to earth anchorages, or stout bollards on the bank, and twisted up taut with a handspike. Can be used for spans up to 70 feet.
ESTIMATE OF TIMBER.
| Kind of Bridge. | No. of Spars. | Length in Ft. | Diam. in Ins. | Purpose. | |
| At Tip. | Mean. | ||||
| Single-sling (50 to 60 Ft. Span). | 4 | 37½ | 6 | Standards. | |
| 3 | 15 | 10 | Road-transoms. | ||
| 3 | 15 | 6 | Top and fork transoms. | ||
| 4 | 15 | 4 to 6 | Ledgers and shore-transoms. | ||
| 4[10] | 18 | 3 | Braces. | ||
| 10 | 30 | 6 | Balks. | ||
| 4 | 30 | 3 | Ribbands. | ||
For a treble-sling bridge standards from 50 to 60 feet long will be required; also additional spars for transoms, road-bearers, and vertical frames.
THE STRINGER BRIDGE.
If the span does not exceed 25 feet, abutments good, and proper timber near at hand, this bridge can be easily built.
TABLE OF TOOLS AND MATERIALS FOR SINGLE-AND DOUBLE-LOCK AND UNSTIFFENED SINGLE-SLING BRIDGES.
| Descriptions. | Single- lock. | Double- lock. | Single- sling. |
| Tools. | |||
| Axes, pick | 4 | 4 | 4 |
| Blocks, snatch | 2 | 2 | |
| Chisel, brick | 4 | 4 | 4 |
| Hammers, striking | 4 | 4 | 4 |
| Handspikes | 2 | 2 | 6 |
| Levers, field-service | 1 | 1 | 1 |
| Mauls | 2 | 2 | 2 |
| Measuring-rods | 1 | 1 | 1 |
| Shovels | 4 | 4 | 6 |
| Measuring-tapes | 1 | 1 | 1 |
| Materials. | |||
| Chalk, pieces | 2 | 2 | 4 |
| Lines, Hambro', 150 ft. | 1 | 1 | 1 |
| Planks for chesses, 10 ft. × 12 in. × 1½ in. | According to span | ||
| Pickets, 5 ft. long, for bollards | 10 | 10 | 10 |
| Pickets, bundles of tracing | 1 | 1 | 1 |
| Racksticks and lashing | According to span | ||
| Foot-ropes, 3 in., 9 fathoms | 4 | 4 | 4 |
| Guys, 3 in., 20 to 30 fathoms | 8 | 8 | 8 |
| Ropes, 2 in., 18 fathoms each | 2 | ||
| Ropes, 2 in., 8 fathoms, for transom-lashings | 4 | 8 | 8 |
| Ropes, 1½ in., 5 fathoms, for ledger- and brace-lashings | 12 | 14 | 14 |
| Ropes, 1 in., 3 fathoms, for road-bearers | 10 | 20 | 20 |
| Tapes, tracing, 150 ft. | 1 | 1 | 2 |
| Yarn, spun, lbs. | 7 | 7 | 7 |
Six stringers and fifty flooring-poles, 6 inches in diameter and 12 feet long, will be required. Two axemen to each stringer will fell and prepare them in a few minutes, and while they are being carried and placed in position the axemen can prepare the poles. Stringers should have 4 feet extra length. Their diameter will depend on the weight to be borne, how distributed, and kind of wood. It is determined by the formulas already given. A good rough rule for calculating the live load which can be borne by rectangular timber of given length and scantling is:
For larch safe distributed live load in cwts. (112 lbs.)
| = | bd2 | , |
| L |
in which b = breadth in inches, d = depth in inches, L = length of span in feet.
For fir the load may be 4/3, for cedar 5/3, for beech, oak, and pitch-pine 2, and for teak 7/3 of load of larch.
This formula gives a theoretical factor of safety of about 3.
The stringers are placed in position by means of jumping-poles, each of which should be strong enough to bear the weight of the stringer and a little longer than the hypothenuse of the right-angled triangle, the base and perpendicular of which are respectively three fourths the width and depth of the chasm.
Place first stringer on abutment as a temporary wall-plate and chock it; slide the second and third over the chasm a little more than one fourth their length for a footway; push out one jumping-pole, butt first, until nearly balanced on wall-plate stringer, and pass the bight of a rope around it a little in advance of its centre of gravity, by means of which a man on the footway supports the butt while the pole is being slipped forward until it reaches its proper resting-place (while this is being done the ends of the stringers forming the footway are held down); then place second jumping-pole in position; draw back the two stringers forming footway; cross jumping-poles, lashing them about 2 feet above the level of the abutment, and attach guy-rope long enough to reach across the stream; place stringer in crutch and push forward until nearly balanced; then raise end and push so as to throw it forward and cause the other end to rest on the opposite abutment. Four men now cross over on the stringer, steadying themselves by means of the guy-rope held taut for the purpose, lift the stringer off the jumping-poles, and, with assistance from opposite side, roll the stringer in position. Pull back jumping-poles and place the other stringers.
The flooring is laid as for corduroy roads, and the bridge is finished by pinning on a ribband of poles to hold down the ends of the flooring and erecting hand-rails if required. It is well to cover the roadway with straw to lessen the jolting of carriages.
FORDS, FLYING AND FLOATING BRIDGES.
When reconnoitring a river with a view of effecting a passage, observe: the nature of the banks, the nature of the bed, position and depth of fords, strength of current, whether tidal or otherwise, probability and extent of floods.
Fords.—The following depths are fordable: For infantry, 3 ft.; for cavalry, 4 ft.; wagons containing ammunition, 2⅓ ft. Gravelly bottoms are best; sandy bottoms are bad, as the sand stirs up and increases depth of water. Fords should be clearly marked by long pickets driven into the bed of the river above and below the ford, their heads being connected by a strong rope. It is well to mark the pickets, in order that any rise of the water may be at once evident.
The depth of a river is generally most uniform in straight parts; at bends the depth will generally be greater at the concave bank, and less at the convex. For this reason a river which is not anywhere fordable straight across may be found passable in a slanting direction between two bends.
To measure the velocity of a stream, use a light rod weighted at the end so as to stand vertically in the water; note the distance it floats in a given number of seconds; then seven tenths the mean number of feet a second gives the number of miles an hour.
Ferries and Flying Bridges.—The simplest form of permanent ferry consists of ropes stretched across the river by means of which rafts can be hauled from bank to bank.
The flying bridge can be used if the velocity of the current is two miles an hour or more. The current moves the boat or raft across the stream by acting obliquely against its side, which should be kept at an angle of about 55 degrees with the current. The cable, whose length should be 1½ to 2 times the width of the river, and float if possible, can either be anchored in midstream (in which case the boat can swing between two landing-places), or two cables may be used, one anchored on either bank. Or a cable may be stretched from bank to bank as taut as possible and six feet above the water at the lowest point. The boat (a double-ender) is attached to travellers, which are small wheels grooved on the circumference to fit the cable on which they ride, maintained in their position by a counterpoise below, to which the stem-and stern-lines of the boat are attached. Long, narrow, deep boats with vertical sides, to which leeboards can be attached, are the best for the purpose, and straight reaches of a river the most suitable places for flying bridges.
Floating Bridges.—These can be made of boats, barrels, timber, etc.
Each pier must have enough available buoyancy to support the heaviest load that can be brought on one bay of the bridge. The length of the piers should be at least twice the breadth of the roadway, for steadiness, and they should be connected together at their extremities by tie-balks or lashings.
The waterway between the piers should never be less, and should if possible be more, than the width of those piers.
In barrel-pier bridges, or with boats having strong gunnels and frames, each balk bears on both gunnels of adjacent piers; in weak boats they bear on a central beam supported on the keel.
The roadway of floating bridges is similar to that already described.
Boats.—Open boats should not be immersed deeper than within one foot of the gunnel, and a still larger limit of safety will be required in rough water or a violent current. They should be placed "stem on" in the bridge so as to point against the current, and slightly down by the stern; if the river be tidal, they alternate stem and stern. Few boats, except heavy barges, are strong enough for balks to rest on their gunnels. Use a central transom resting on the thwarts and blocked up from underneath to bring the weight directly on the keel.
Buoyancy.—This may be determined by loading the boat with unarmed men to such a depth as is considered safe. For bridging purposes the number of men multiplied by 160 gives the available buoyancy in pounds.
If the number of men be divided by four, the result will be the central interval in feet at which the boats may be placed in the bridge to carry infantry in fours crowded.
RAFTS OF CASKS OR BARRELS.
Buoyancy of Casks.—The actual buoyancy is given by the formula 5c2l-W, in which c is the circumference of the cask in feet half-way between the bung and the extreme end; l is the length in feet, exclusive of projections, measured along a stave; and W is the weight of the cask in pounds.
The available buoyancy for bridging purposes may be taken at 9/10 of the actual buoyancy.
A safe buoyancy may be obtained by multiplying the content in gallons by 8⅓.
Knowing the buoyancy of one cask, the number to be united in a raft to sustain any desired load can be readily calculated.
Constructing the Float.—Having determined on the number of casks to be united in one float, take one third the number and place them transversely on two poles laid on the ground parallel to each other and one half the height of a cask apart; the casks to be distributed evenly over a distance equal to the width of the proposed bridge, bungs up. Lay another pair of poles on the top of the casks, and lash them to the first pair at the ends and between each two casks, and cut off any surplus pole that may project at the ends. Having made and launched three such floats, unite them by lashing a pole across the ends and middle of them, so as to make a large square raft. When enough such rafts are completed, they can be placed, and a roadway built on them. In placing the rafts the sides of the barrels should be towards the current.
Floats of casks, when in bridge, should always be rigidly connected with each other at their ends by stringers, which must be lashed to both gunnels of each float; the roadway stringers can then be laid and should rest squarely on both gunnels of each float and should be lashed, especially if there be any sway or for animal traffic.
TABLE OF WEIGHTS, DIMENSIONS, CAPACITY, ETC.
| Name. | Weight. | Length of Stave. | Bung Diam. | Head Diam. | No. Gals. | Available Buoyancy. |
| Lbs. | Inches. | Inches. | Inches. | Lbs. (About). | ||
| Tar | 50 | 30 | 20½ | 17½ | 30 | 250 |
| Pork | 60 | 30 | 22 | 18½ | 32 | 267 |
| Vinegar | 80 | 34 | 23½ | 20½ | 45 | 375 |
| Whiskey | 80 | 34 | 23½ | 20½ | 45 | 375 |
| Kerosene | 70 | 34 | 25 | 21 | 45 | 375 |
| Molasses | 70 | 34 | 25 | 21 | 52 | 433 |
American wine-barrels are about the same size and weight as whiskey-barrels.
Rafts.—Raft bridges may be used in a current not exceeding four miles an hour.
Cubic content of a log in feet is equal to the square of one fifth of the mean girth multiplied by twice its length.
Weight of timber (dry) per cubic foot: Ash 47 lbs., beech 43, elm 36, fir 32, oak 54, pine 40, poplar 24, sycamore 37, willow 25. Green timber is about one fourth heavier.
Flotation.—To obtain the flotation of a log, multiply its cubic content by the difference between its weight per cubic foot and the weight of a cubic foot of water (62½ lbs.). Take five sixths of the result for the available buoyancy.
To Form a Raft.—Place the logs side by side, the small ends alternating; strongly secure with rope and, if possible, by cross and diagonal pieces of scantling fastened by spikes or treenails. If it is to be used as a pier, the logs may be placed in two layers to avoid obstructing the waterway. A central transom must be used. The up-stream end of the raft may, with advantage, be convex.
Make-shift Anchors.—Two or more pickaxes lashed together; heavy weights, such as large stones or railway irons; the latter are best when bent.
Nets filled with stones are particularly good on rocky bottoms.
Protection.—Arrangements must be made up-stream to protect a floating bridge from damage from floating substances, either by a boat patrol, or by stretching a net, or some intercepting obstacle, across stream.
CHAPTER XIII.
Hasty Demolition. Gunpowder. Dynamite. How Used in Blasting. Guncotton. Rack-a-rock. Handling, Transportation, and Storage of High Explosives. Charges for Hasty Demolition. Where and How to Place Charges.