Fences, Gates and Bridges: A Practical Manual

CHAPTER XIV.

Chapter 282,198 wordsPublic domain

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.

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.

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.

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.

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.

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.

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.

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.

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.