Cordage. How Preserved. Strength. Blocks and Tackle. Knots, Hitches,
etc. Lashings.
Bridges, Trestles, Piers, etc. Single-sling and Treble-sling
Bridges. Stringer Bridges. Fords. Flying and Floating Bridges. Rafts
and Casks.
CORDAGE.
Ropes are distinguished as to size by their circumference in inches.
Their length is given in fathoms.
Hemp rope when new stretches freely, and deteriorates very much after a
few months' wear. It is about one third stronger than manilla rope.
To ascertain the strain in pounds a rope will bear without breaking,
square the circumference and multiply the result by the tabular unit in
the following table:
---------+--------------+-------------------+-------------------
Kind. |Circumference.| White. | Tarred.
| +---------+---------+---------+---------
| |3-strand.|4-strand.|3-strand.|4-strand.
---------+--------------+---------+---------+---------+---------
Hemp {| 2.5 to 6 in. | 1140 | 1330 | 850 | 1000
{| 6 " 8 " | 1090 | 1360 | 825 | 940
Manilla {| 2.5 to 6 in. | 810 | 950 | .... | ....
{| 6 " 12 " | 760 | 835 | .... | ....
---------+--------------+---------+---------+---------+---------
For ropes in daily use reduce the tabular unit one third to meet
reduction in strength by wear and exposure.
A safe general rule for all ropes is, one fourth the square of the
circumference gives the breaking weight in short tons.
TABLE OF WEIGHTS THAT MANILLA ROPE IN DAILY USE WILL SUSTAIN.
------------------+-----+-----+-----+-----+-----+-----+-----+-----+
Circumference, in.| 1 | 1¼ | 1½ | 1¾ | 2 | 2¼ | 2½ | 2¾ |
------------------+-----+-----+-----+-----+-----+-----+-----+-----+
Weight, lbs. | 540| 844| 1215| 1654| 2160| 2734| 3375| 4084|
==================+=====+=====+=====+=====+=====+=====+=====+=====+
Circumference, in.| 4¼ | 4½ | 4¾ | 5 | 5¼ | 5½ | 5¾ | 6 |
------------------+-----+-----+-----+-----+-----+-----+-----+-----+
Weight, lbs. | 9753|10935|12184|13500|14884|16635|17954|18252|
------------------+-----+-----+-----+-----+-----+-----+-----+-----+
[Part 2 of Table.]
------------------+-----+-----+-----+-----+-----
Circumference, in.| 3 | 3¼ | 3½ | 3¾ | 4
------------------+-----+-----+-----+-----+-----
Weight, lbs. | 4860| 5704| 6415| 7594| 8640
==================+=====+=====+=====+=====+=====
Circumference, in.| 6¼ | 6½ | 6¾ | 7 | 8
------------------+-----+-----+-----+-----+-----
Weight, lbs. |19805|21421|23100|24843|32448
------------------+-----+-----+-----+-----+-----
=To Preserve White Rope.=--Dip when dry into a bath containing 20 grms.
(about seven tenths of an ounce) of sulphate of copper per litre (a
little over one quart) of water; soak for four days and dry. Then soak
in a solution of 100 grms. (3.5 oz.) of soap per litre of water.
=Iron-wire Rope.=--Its breaking weight in tons is about equal to the
square of the circumference in inches.
=Steel-wire Rope.=--It is from 2 to 2½ times stronger than iron-wire
rope. Wire rope can be safely worked in field service to one half its
breaking weight.
=To Preserve Wire Rope.=--Apply linseed-oil with a piece of sheepskin,
wool inside; or mix the oil with equal parts of Spanish brown and
lampblack. If used underground or in water, take mineral or vegetable
tar, add one bushel of fresh-slaked lime to barrel of tar, boil it
well, and then saturate the rope with the boiling tar.
=Chains.=--Chains, when used in place of ropes, should be examined
to see that every link is free from flaws and not too much worn. The
chain should have no kinks in it, and each return, when more than one
is used, should bear its proper strain. Do not work a chain over a
windlass of small diameter; the iron of the links breaks by bending,
not by tensile strength. Lubricate freely.
SAFE WORKING STRAIN FOR FIELD SERVICE, AND WEIGHT IN FATHOMS.
--------------+-----------------------+-------------------------
Circumference.| Wire Ropes |Round-link Crane-chain,
| Hawser-laid. | Length of Link not
| |Exceeding Five Diameters
| | of the Iron.
+-------+-------+-------+--------+-------+--------
| Iron. | Steel.| | | |
| Safe | Safe |Weight |Diameter| Safe |Weight
|Working|Working| per | of Iron|Working| per
|Strain.|Strain.|Fathom.|of Link.|Strain.|Fathom.
--------------+-------+-------+-------+--------+-------+---------
Inches. | Cwts. | Cwts. | Lbs. | Inches.| Cwts. | Lbs.
¾ | -- | -- | -- | 3/16| 8 | 2.1
1 | 10 | -- | 0.94| ¼ | 15 | 4.88
1¼ | 13½ | -- | 1.5 | 5/16| 22 | 7.26
1½ | 21½ | 62½ | 2.5 | ⅜ | 32 | 10.57
2 | 40 | 112 | 3.5 | 7/16| 45 | 13.0
2¼ | 50 | -- | 4.5 | ½ | 65 | 16.0
2½ | 60 | 195 | 5.75| 9/16| 75 | 21.0
2¾ | 77 | -- | 6.5 | ⅝ | 92 | 25.0
3 | 92 | 245 | 7.5 | 11/16| 112 | 29.0
3¼ | 109 | -- | 8.5 | ¾ | 135 | 36.0
3½ | 125 | 275 | 10.75| 13/16| 157 | 43.0
4 | 157½ | 450 | 13.25| ⅞ | 182 | 46.5
4½ | 210 | 545 | 17.75| 15/16| 210 | 58.0
5 | 248 | 669 | 21.5 | 1 | 240 | 63.5
--------------+-------+-------+-------+--------+-------+---------
BLOCKS, TACKLE, ETC.
=Blocks= are of two kinds, _made_ and _mortised_. A made block consists
of four parts: _shell_, _sheave_, _strap_, and _bush_. A mortised block
is made of a single block of wood, mortised out to receive a sheave.
Blocks are single, double, or treble, according to the number of
sheaves.
A =Tackle= is a purchase formed by reeving a rope through two or more
blocks for the purpose of hoisting.
A =Whip= is a purchase made by a rope rove through one single block.
A =Gun-tackle Purchase= is a rope rove through two single blocks, and
made fast to the strap of the upper block. The parts of all tackles
between the fasts and sheave are called the _standing parts_. The parts
between the sheaves are the _running parts_, and the part which is
taken hold of in hoisting is called the _fall_.
A =Whip upon Whip= is where the block of one whip is made fast to the
fall of another.
A =Luff-tackle Purchase= is a single and a double block, the end of the
rope being fast to the upper part of the single block, and the fall
coming from the double block. A luff-tackle upon the fall of another
luff-tackle is called luff upon luff.
=A Watch-tackle= or =Tail-tackle= is a luff-tackle purchase, with a
hook in the end of the single block, and a tail to the upper end of the
double block.
=A Single Burton= is composed of two single blocks, with a hook in
the bight of the running part. Reeve the end of rope through the upper
block, and make it fast to the strap of the fly-block. Then make fast
your hook to the bight of the rope, and reeve the other end through the
fly-block for a fall. The hook is made fast by passing the bight of the
rope through the eye of the hook and over the whole. This is a very
quick-working tackle and a strong purchase. Used for hoisting entirely.
When a very heavy weight is to be raised, the standing part should be
attached to the slings by a fisherman's bend instead of to the block.
The _size_ of blocks is expressed by the _length_ of the _shell_ in
inches.
_Tackles_ are also designated by the number of sheaves employed, as
twofold (two single blocks), threefold (double and single block), etc.
The bight of a hook is the middle of the bend of the hook part.
Rope should always be stopped up, either with the end or with rope-yarn
stops, to prevent its getting into a snarl. When using ropes for
hauling, they should never be dragged upon the ground.
Before reeving a rope in a block the turn should be carefully taken
out, to prevent twisting when the weight is lifted. This is done
by stretching the rope out to its full length, and turning it in
the opposite direction to that in which it is laid up until all the
stiffness disappears.
Blocks should be overhauled very often to see that the sheaves are
working properly on the pins, and that they work smoothly. If they do
not, turn the pins end for end and rub a little black lead (graphite)
on them to lubricate them; also on the sides of the sheaves where they
rub against the shell.
When hoisting with tackles, they should never be allowed to twist. If
they show a tendency to do so, insert a bar in the block or sling, and
use it as a lever to hold it straight.
It frequently happens that the men cannot apply their full strength in
the direction in which it would be most effective. In such cases hook a
single block to some object about two feet above the ground and reeve
the end of the fall through it, so that the men can add their strength
to their weight, and more men can apply themselves.
Never trust the suspension of a weight to holding it by the unaided
strength of men. If it is possible to get a turn around any fixed
object, even in raising or hauling a weight, it is best to take a turn,
as all that is gained is then saved.
Always select such blocks that the fall will run freely through them,
and not ride upon the edges of the sheaves. If it does, it will be
certain to cut. The rope should not quite fill the groove on the
sheave. In this way excessive friction is avoided.
The power gained by using tackle is as follows:
Two single blocks, or gun-tackle: nearly doubled.
Luff-tackle, double and single block: doubled. If the double block is
movable, trebled.
Two double blocks: power × 3⅓.
Double and treble blocks: power × 4.
Two treble blocks: power × 4½.
Whip upon whip (single Burton): trebled.
When one tackle is applied to another, the power obtained is found by
multiplying their respective values together.
No advantage is gained by using a greater number of sheaves than two
treble blocks in one fall.
The power is equal to the weight divided by the number of ropes
(standing parts) attached to the lower block, or by twice the number of
rising pulleys (sheaves).
KNOTS, HITCHES, ETC.
=Thumb-knot= (Fig. 108).--Used to prevent the end from unfraying, or to
prevent its slipping through a fall.
=Figure-eight Knot= (Fig. 109).--Used to prevent the end from
unfraying, or to prevent its slipping through a fall.
[Illustration: FIG. 108.]
[Illustration: FIG. 109.]
[Illustration: FIG. 110.]
[Illustration: FIG. 111.]
[Illustration: FIG. 112.]
=Reef-knot= (Fig. 110).--For joining two ends of rope or chain together.
=Singlesheet Bend= (Fig. 111).--For joining dry ropes of unequal size.
=Doublesheet Bend= (Fig. 112).--Same for wet ropes.
=Draw-knot= (Fig. 113).--Same when it may be required to cast them
loose.
=Running-knot= (Fig. 114).--To form a loop that will draw taut.
[Illustration: FIG. 113.]
[Illustration: FIG. 114.]
=Bowline= (Fig. 115).--To form a loop at the end of a rope which will
not slip.
=Half-Hitch.=--For securing the loose ends of lashings.
=Two Half-hitches= (Fig. 116).--For making fast a rope-end to an
object.
=Clove-hitch= (Fig. 117).--Two half-hitches, used for commencement
and finish of lashings; making fast the end or the bight of a rope to
any object.
[Illustration: FIG. 115.]
[Illustration: FIG. 116.]
[Illustration: FIG. 117.]
[Illustration: FIG. 118.]
=Timber-hitch= (Fig. 118).--Used on pieces of timber, or in making fast
to spars, where the weight will keep the rope taut.
=Round Turn= and =Two Half-hitches= (Fig. 119).--For making fast a rope
so that the strain shall not jam the hitches.
=Fisherman's Bend= (Fig. 120).--For making fast where there is give and
take motion, as when a boat is at anchor.
[Illustration: FIG. 119.]
[Illustration: FIG. 120.]
[Illustration: FIG. 121.]
=Lever-hitch= (Fig. 121).--For drawing pickets, fixing rounds of
rope-ladder, bars to, or loops on, drag-ropes.
=Man's Harness-Hitch= (also Fig. 121).--Loop to pass over shoulder. To
fix a rope with a weight on it rapidly to a block.
[Illustration: FIG. 122.]
[Illustration: FIG. 123.]
=Cat's-paw= (Fig. 122).--At the end or in the middle of a rope for
hooking on a block.
=Blackwall Hitch= (Fig. 123).--Simple hitch with pliable rope and
fixed weight. Used to transfer the strain from one rope to another, or
to secure a tackle to a rope for a horizontal pull.
=Stopper Hitch= (Fig. 124).--To shift the strain off a rope temporarily.
[Illustration: FIG. 124.]
=Magnus Hitch= (Fig. 125).--For making fast to round spars.
[Illustration: FIG. 125.]
[Illustration: FIG. 126.]
[Illustration: FIG: 127.]
=Sheepshank= (Fig. 126).--To shorten a rope without cutting it.
LASHINGS.
=Square Lashing= (Fig. 127).--Make fast one end (the fixed if there
be one) of the lashing-rope to one of the objects to be lashed by a
timber-or clove-hitch, or by passing the eye-end of the rope under it,
bringing it up and passing the other end through the eye; then, after
hauling taut, pass it round the second object, round the first, up
again round the second, and down round the first, continuing this until
a sufficient number of returns are made, when the whole of the returns
are tightened by bringing the end round the returns between the objects
two or three times, and making fast with two half-hitches on either of
the first returns or one of the objects.
If the returns on one of the spars lie in succession outside the
returns first applied, they must lie in succession inside those first
applied on the other spar.
=Diagonal Lashing= (Fig. 128).--Begin with a timber-hitch or
running-bowline round both spars and draw them together; then take
three or four turns across each fork and finish with frapping-turns.
Wedges with well-rounded points are often useful for tightening
lashings.
[Illustration: FIG. 128.]
[Illustration: FIG. 129.]
=Sheer Lashing.=--The lashing is made fast to the lower spar, above
where they cross, with a timber-hitch; as many turns are taken around
both spars towards the feet as may be necessary to cover the cross,
and the end is then led around this lashing and both spars where
they cross and four or five frapping-turns taken so as to bring them
together, and the ends made fast around the upper spar above the cross.
=Gin Lashing= (Fig. 129).--The three legs of the gin are placed
so that the centre one is pointing in the opposite direction to that
of the others, all being parallel, the upper ends resting on a skid
and two inches apart. The lashing is commenced with a clove-hitch on
an outside spar and carried over and under loosely and without riding
six times. Two frapping-turns are taken in each interval and the whole
finished off with two half-hitches round one of the spars. Iron chain
is much better than rope for these fastenings.
=Fishing Spars.=--This consists in strengthening spars by lashing
others parallel to them. The fishing spars should be placed against the
spar to be fished so that they may take off as much strain and be in as
close contact as possible. Lashing-ropes should then be applied round
all of the spars until they are all firmly bound together.
=Frapping.=--The drawing together the several returns of a lashing
so as to tauten it. The rope used for frapping is generally the
running-end of the lashing itself; after having frapped, the end of the
rope is made fast by two half-hitches, one around all the returns, the
other on half only.
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_ = _b__d²__S_/_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_ = 6_d__S_/10_l_, 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.
[Illustration: 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.
[Illustration: 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.
[Illustration: 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 |No. of|Length|Diam. in Ins. | Purpose.
Bridge. |Spars.|in Ft.| |
| | +-------+------+
| | |At Tip.| Mean.|
------------+------+------+-------+------+---------------------------
Single-lock | 4 | 24 | 7 | |Standards.
(30-ft. | 1 | 15 | | 10 |Fork-transoms.
Span). | 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.
[Illustration: 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 |No. of|Length| Diam. in In. |
Bridge. |Spars.|in Ft.| |
| | +-------+------+ Purpose.
| | |At Tip.| Mean.|
-----------+------+------+-------+------+---------------------------
Double-lock| 4 | 20 | 7 | |Standards.
| 2 | 25 | | 10 |Distance-pieces.
(40 to 45 | 2 | 15 | | 10 |Road-transoms.
Ft. Span.) | 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.
-----------+------+------+-------+------+---------------------------
[Illustration: 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.
[Illustration: 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 |No. of|Length| Diam. in In. |
Bridge. |Spars.|in Ft.| |
| | +-------+------+ Purpose.
| | |At Tip.| Mean.|
------------+------+------+-------+------+---------------------------
Single-sling| 4 | 37½ | 6 | |Standards.
(50 to 60 | 3 | 15 | | 10 |Road-transoms.
Ft. Span). | 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-|Double-|Single-
| lock. | lock. | 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
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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.) =
_b__d²_/_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
5_c²__l_-_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 | Bung | Head | No. |Available
| |of Stave.| Diam. | Diam. |Gals.|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.