COUNTRY BRIDGES AND CULVERTS.
STRENGTH OF BRIDGES.
Bridge building is a profession of itself, and some of the great
bridges of the world are justly regarded as among the highest
achievements of mechanical science and skill. But it is proposed to
speak in this work only of the cheap and simple structures for spanning
small streams. The measure of the strength of a bridge is that of its
weakest part. Hence, the strength of a plain wooden bridge resting upon
timber stringers or chords, is equivalent to the sustaining power
of the timbers in the middle of the span. The longer the span, other
things being equal, the less its strength. The following table shows
the sustaining power of sound spruce timber, of the dimensions given,
at a point midway between the supports:
========+=============================================================
| WIDTH AND THICKNESS OF TIMBER.
LENGTH |--------------+--------------+---------------+---------------
OF SPAN.|6 by 8 inches.|6 by 9 inches.|6 by 10 inches.|6 by 12 inches.
--------+--------------+--------------+---------------+---------------
Feet. | Pounds. | Pounds. | Pounds. | Pounds.
--------+--------------+--------------+---------------+---------------
10 | 2,800 | 2,692 | 4,500 | 6,480
12 | 2,400 | 3,042 | 3,750 | 5,400
14 | 2,058 | 2,604 | 3,216 | 4,632
16 | 1,800 | 2,280 | 2,808 | 4,050
========+==============+==============+===============+===============
A stick of timber twenty feet between supports, will bear a load in its
center only one half as great as a timber of the same dimensions, ten
feet between supports. Thus four timbers six by twelve inches, in a
span of sixteen feet, would bear a load of eight tons; in a twelve foot
span, the same timbers would support a weight of nearly twelve tons.
BRACES AND TRUSSES.
The above is the initial strength of the timbers which support the
weight of the superstructure, and any load that it may have to sustain.
But in bridge building these timbers are reinforced by trusses or
braces, which add greatly to the sustaining power of the bridge.
[Illustration: Fig. 275.—A SIMPLE FORM OF BRIDGE SPAN.]
[Illustration: Fig. 276.—A STRONGER SPAN.]
[Illustration: Fig. 277.—A SHORT BRIDGE.]
Figure 275 shows the simplest form of a self-supporting bridge, which
will answer for spans of from ten to fifteen feet in length. The
braces, _c_, _c_, reach from near the end of the sill to about four
feet above the center. The truss rod, _d_, is one inch in diameter
for short bridges up to two inches for longer spans; it is provided
with an iron washer at the top. The rod passes through the sill, and
a cross-sill, _e_, which passes under the main sills, thus adding
firmness to the whole structure. Logs, _f_, _f_, are placed against
the ends of the sills to keep them in place, and where the wheels will
first strike them instead of the floor plank, thus greatly equalizing
the pressure. Figure 276 represents a modification of the above. The
two truss-rods and braces give the structure greater strength and
solidity, adapting it for spans eighteen feet in length. For the latter
length, sills should be of good material, ten inches wide and fourteen
inches deep, with three middle sills of about the same size.
[Illustration: Fig. 278.—A BOLT TRUSS.]
[Illustration: Fig. 279.—BRIDGE BRACED FROM BELOW.]
Figure 277 is a more improved style of bridge, the truss serving both
to support the structure, and as a parapet. The top railing is of the
same width as the sill, about one foot. The lower side may be cut away,
giving the bridge a more finished appearance. The railing at the center
is six inches thick, and three inches at the ends. The tie, _h_, is
full width and four inches thick. A bridge of this kind will answer for
heavy traffic, even if twenty feet in length. The bolt truss, in figure
278, is adapted for a span of twenty-five feet. This makes a bridge of
great firmness. Each set of truss-rods support a cross-sill. The road
planks are laid crosswise of the bridge. The middle sills are sometimes
half an inch lower than those along the sides, and should be four or
five in number. The ends of the planks fit closely against the inside
of the truss sills, thereby keeping the planks securely in place.
A common method of bracing is from below as shown in figure 279. This
is not usually a good practice, as the braces are liable to be carried
away by ice or floods.
ABUTMENTS, PIERS AND RAILINGS.
[Illustration: Fig. 280.—END OF A BRIDGE.]
If the sills of a bridge are laid directly upon the dry walls of
an abutment, or upon a heavy plank, the jar of passing teams soon
displaces some of the stones, and brings undue strain upon certain
portions of the wall. To avoid this, abutments are best made of cut
stones, and laid in cement. A wooden bent for the support of the ends
of the bridge may be made as shown in figure 280. The whole should
be constructed of heavy timber, pinned together. A coat of white
lead should cover the interior surface of all joints. The number and
position of the posts of the wooden abutment are seen in the engraving.
A log should be laid upon the wall at _m_, to relieve the bridge
from the shock of the passing wagons. A center pier should be avoided
as much as possible, as it offers serious obstruction in floods, and
ice, drift wood and other floating matter become piled against it,
seriously imperiling the entire structure. But in cases where the
length of the bridge is so great as to require one or more piers, they
may be constructed on the plan shown in figure 281, or in case the
bottom is so soft as to render the mudsill insecure, a line of piles
supporting a cross-timber, as in figure 282. A strong, reliable parapet
or railing should always be provided. The want of one may be the cause
of fatal accidents to persons and horses. Figure 283 gives a side view
of a good railing, and figure 284 shows the manner of bracing the posts
to the ends of the cross-beams. They should be thus braced at every
alternate post of the railing. The floor should be double, as shown
in figure 285, the lower planks laid diagonally, and the upper layer
crosswise.
[Illustration: Fig. 281.—FRAMED PIER.]
[Illustration: Fig. 282.—BRIDGE SUPPORTED BY PILES.]
[Illustration: Fig. 283. RAILING OF BRIDGE.]
[Illustration: Fig. 284. RAILING OF BRIDGE.]
[Illustration: Fig. 285.—PLANK FLOOR OF BRIDGE.]
BRIDGES FOR GULLIES.
For small gullies which cross roadways or lanes in farms, and are not
the beds of constant streams, but are occasionally filled with surface
water, a very simple bridge is sufficient. One like that shown in
figure 286 is as good as any. The sills, _a_, _a_, are sunk in a trench
dug against the bank and at least to the level of the bed of the creek.
The cross-sills, _b_, _b_, are not mortised into them, but simply laid
between them. The pressure is all from the outside, hence it will
force _a_, _a_, tighter against the ends _b_, _b_, which must be sunk
a little into the bed of the creek at its lowest point. The posts are
mortised into the sills, _a_, _a_, and plates, _c_, _c_, and _d_, _d_,
upon which the planks are laid. Props may be put against the lower
sides of the posts to hold the bridge against the stream.
[Illustration: Fig. 286.—FRAME FOR BRIDGE.]
[Illustration: Fig. 287.—CONVENIENT FARM BRIDGE.]
A cheap but practicable bridge is shown in figure 287. Two logs are
laid across the gully, their ends resting on the banks, and to them
puncheons or planks are spiked to form the bridge. Stout posts, well
propped and reaching above the highest water mark, are placed against
the lower side of the logs. If the creek rises, the bridge, being
free, will be raised on the surface of the water, while the posts will
prevent its being carried away. Should it not rise with the water, it
opposes so little surface to the current that the posts will hold it
fast.
ORNAMENTAL BRIDGES.
[Illustration: Fig. 288.—RUSTIC BRIDGE.]
[Illustration: Fig. 289.—A BRIDGE OF ROCKS.]
No feature adds more to the appearance of ornamental grounds than
tasteful bridges. A stream or narrow channel connecting two parts of
a small sheet of water, affords an opportunity for the introduction
of a bridge. In the absence of such features a bridge may be thrown
across a dry ravine. Whatever style may be adopted, should harmonize
with the general character of the surroundings. An elaborate bridge of
wood or masonry would be as much out of place on grounds unadorned by
other structures, as a rude rustic one would be near highly finished
summer-houses and other architectural features. On most grounds a neat
rustic bridge, something like the one shown in figure 288, would be in
good keeping with its environments. Such bridges may be made of red
cedar logs and branches, resting upon stone abutments. Where boulders
are abundant, a stone bridge, something like figure 289, may be built
at very little cost, and will last for generations. The pleasing effect
of rustic or other ornamental bridges is enhanced by training Virginia
creeper or other climbing plants upon them.
ROAD CULVERTS.
A culvert under a road is, in effect, a short bridge. The simplest form
of plank culvert, resting upon stone abutments, is shown in figure 290.
Such a structure is cheaply built, and serves a good purpose while the
wood-work remains sound. But the planks wear out and the timbers decay,
requiring frequent renewing. Where stone is abundant it is much cheaper
in the end to build wholly of stone, as in figure 291. After the
abutments are built, a course of flat stone, along each side, projects
inward from six to ten inches, as at _a_, _a_, which are covered with
a broad stone, _b_. Where the stream to be crossed is so narrow that a
row of single stones is sufficient to cover the opening, a culvert like
that seen in figure 292 is cheaply made. Such structures will remain
serviceable for a generation, if the foundations are not undermined by
the action of the water.
[Illustration: Fig. 290.—CULVERT WITH PLANK FLOOR.]
[Illustration: Fig. 291.—STONE CULVERT.]
[Illustration: Fig. 292.—CHEAPER STONE CULVERT.]
[Illustration: Fig. 293.—ARCHED CONCRETE CULVERT.]
[Illustration: Fig. 294.—ANGULAR CONCRETE CULVERT.]
Where flat stones enough cannot be easily procured, culverts may be
built of concrete. The abutments are first made, as in other cases;
then empty barrels or sugar hogsheads, according to the capacity of
the opening, are fitted in, or better still, a temporary arch is made
of rough, narrow boards. The concrete of cement, sand and gravel, is
then prepared and poured in, temporary supports of lumber having been
fixed across each end of the culvert to keep the concrete in place
until it hardens. Small stones may be mixed with the concrete as it
is poured into place, and the whole topped off with a row of them.
This protection of stones on the top is valuable, in case the covering
of earth is worn or wasted away at any time while it is in use. For a
longer culvert a flattened arch is made of concrete, as shown in figure
293. Light timbers are laid across, the ends resting lightly on the
abutments. Across the middle of these a round log is placed to support
the crown of the arch. Elastic split poles are sprung over all, and
upon these are nailed thin narrow boards, extending lengthwise of the
culvert. The ends being temporarily protected, the concrete is mixed
and poured on, as before. As soon as the concrete has become thoroughly
well “set,” the light cross-sticks are cut in two and the temporary
work removed. A cross-section, showing another form of concrete
culvert, and the method of construction, are shown in figure 294. Such
a culvert is more easily built than the last, but is not as strong. The
best and most durable culvert is of stone, with a regular half-round
arch. Such work can only be done properly by a regular mason, but in
the end it is cheaper, where the stone can be obtained, than any kind
of make-shift.
[Illustration]
Transcribers’ Notes
Italics are rendered between underscores (low line characters)
e.g. _italics_.
Small caps are rendered as ALL CAPS.
The term ‘barb wire’ appears 26 times. The term ‘barb-wire’ appears
11 times. Both variants have been have been left as printed. Other
hyphenation inconsistencies and various other errors have been
corrected as listed on the following table.
+------------------------------------+
| Transcribers’ Changes |
+----+---------------+---------------+
|Page| As printed | Changed to |
+----+---------------+---------------+
| 5 |Wire Stretchers|Wire-Stretchers|
| 6 |169 |170 |
| 10 |to-the |to the |
| 11 |hight |height |
| 12 |live-stock |livestock |
| 16 |can not |cannot |
| 20 |wheelbarrow |wheel-barrow |
| 23 |back furrow |back-furrow |
| 23 |wheelbarrow |wheel-barrow |
| 29 |strap hinge |strap-hinge |
| 34 |four inch |four-inch |
| 41 |half-inch |half inch |
| 45 |horse-shoe |horseshoe |
| 48 |three quarter |three-quarter |
| 49 |live stock |livestock |
| 50 |injured |injured. |
| 50 |end posts |end-posts |
| 52 |fence post |fence-post |
| 53 |once |Once |
| 58 |hard-wood |hard wood |
| 65 |iron |iron, |
| 68 |hedge-plant |hedge plant |
| 73 |if |is |
| 78 |any where |anywhere |
| 80 |Figure |figure |
| 85 |drift wood |driftwood |
| 85 |hoop iron |hoop-iron |
| 86 |tenpenny |ten-penny |
| 87 |drift wood |driftwood |
| 90 |crosspiece |cross-piece |
| 91 |drift-wood |driftwood |
| 92 |brush wood |brushwood |
|103 |post- |post |
|108 |extremeties |extremities |
|114 |one quarter |one-quarter |
|116 |ground piece |ground-piece |
|117 |cross-wise |crosswise |
|117 |Gate-ways |Gateways |
|118 |end posts |end-posts |
|120 |end piece |end-piece |
|122 |fence post |fence-post |
|122 |three quarter |three-quarter |
|122 |one half |one-half |
|133 |can not |cannot |
|146 |gate frame |gate-frame |
|149 |two inch |two-inch |
|153 |one quarter |one-quarter |
|153 |240 |240. |
|153 |end piece |end-piece |
|156 |hard-wood |hard wood |
|162 |passage-way |passageway |
|165 |wheelbarrow |wheel-barrow |
|165 |live stock |livestock |
|167 |end posts |end-posts |
|168 |gate frame |gate-frame |
|169 |cross piece |cross-piece |
|172 |watercourse |water-course |
|174 |land holder |land-holder |
|176 |live stock |livestock |
|178 |cross sill |cross-sill |
|179 |279 |279. |
|179 |truss rods |truss-rods |
+----+---------------+---------------+