BRIDGE BUILDING.

PRELIMINARIES.

I. A bridge is a structure for sustaining the weight of carriages, animals, &c., during their transit over a stream, gulf or valley.

Bridges are constructed of various plans and dimensions, according to the circumstances and objects requiring their erection; and it is the purpose of this work, after a few remarks upon the general nature and principles of bridges, to attempt some analyses and comparisons of the respective qualities and merits of various general plans, with a view of deducing practical results, as to a judicious and economical choice and application of materials in the construction of these useful and important structures.

II. The force of gravity, on which the weight of bodies depends, acts in vertical lines, and consequently, a heavy body can only be prevented from falling to the earth, by a force equal and opposite to that with which gravity impels the body downward. This resisting force must not only act vertically upward, but the line of its action must pass through the centre of gravity of the body it sustains. All the forces in the world, acting parallel with, or perpendicular to, the vertical passing through its centre of gravity, could not prevent a

musket ball (concentrated to the point of its centre of gravity) from falling to the centre of the earth, unless it were a horizontal force capable of giving the ball a projection, such that the centrifugal tendency should equal or exceed gravity—a kind of force which could never be made available toward preventing people from falling into the water in crossing rivers; consequently, having no application in bridge building.

In fact, nothing but a continuous series of unyielding material particles, extending from an elevated body downward to the earth, can hold or sustain that body above the earth, by vertical and horizontal action. alone, either separately, or in combination.

III. Suppose a body, no matter how great or small, placed above the earth, with a deep void, or an inaccessible space beneath it. Attach as many cords to it as you please, strain them much or little only horizontally the body will fall, nevertheless. Thrust any number of rods, with whatever force you may, horizontally against it; still the body will fall. This is obvious from the fact that horizontal forces, acting at right angles with the direction of the force of gravity, have no more tendency to prevent, than to promote. the fall of the body.

Moreover, the space beneath being inaccessible, there is no foundation, or foot hold, upon which to rest a post or stud that may directly resist the action of gravity, while the lines of all other vertical forces or resistances, pass by the body without touching it.

In the case here supposed, the body can only be prevented from falling by oblique forces; that is, by forces whose lines of action are neither exactly horizontal, nor exactly perpendicular. Attach two cords to the

body, draw upon them obliquely upward and outward, in opposite directions, or from opposite sides of the void, with a certain stress, and the body will be sustained in its position. Apply two rods to it obliquely upward, of a proper degree of stiffness, in the same vertical plane, and on opposite sides of the perpendicular, a certain thrust exerted upon those rods, will prevent the descent of the body.

IV. Here, then, we have the elementary idea-the grand fundamental principle in bridge building. Whatever be the form of structure adopted, the elementary object to be accomplished is, to sustain a given weight in a given position, by a system of oblique forces, whose resultant shall pass through the centre of gravity of the body in a vertically upward direction, in circumstances where the weight can not be conveniently met by a simple force, in the same line with, and opposite to, that of gravity.

For a more clear illustration of this elementary idea, let us suppose a a', Fig. 1, to represent the banks of a river, or the abutments of a bridge; and gg', the line of transit for carriages, &c.; and, let us further suppose a load of a certain weight, w, to have arrived at a point centrally between a a'. The simplest method of sustaining the weight is, perhaps, either to erect two oblique braces aw. . a'w, or suspend two oblique chains or ties pw, p'w, from fixed supporting points a a', or p p'.

It is not necessary that the weight be at the angular point w, of the braces or chains, but it may be sustained by simple suspension at w' below, or simple support at w' above, and such obliquity may be given to the braces or chains as may be most economical; a consideration which will be taken into account hereafter.

V. Thus we see how a weight may be sustained centrally between the banks of a river, or the extremities of a bridge. But the structure must not only provide for the support of weight at this point, but also at every other point between a a', or g g'; and it is obvious that the same plan and arrangement will apply as well at any other point as at the centre, with only the variation of making the braces or chains of unequal length.

This, however, would require as many pairs of braces or chains as there were points between g g', a thing, of course, impracticable, since the oblique members would interfere with one another, and be confounded into a solid mass. We therefore resort to the transverse strength and stiffness of beams, - phenomena with which all have more or less acquaintance, and without digressing in this place to investigate their principles and causes, it will be assumed as a fact sustained by all experience, that, for sustaining weight between two supporting points upon nearly the same level, a simple beam affords the most convenient and economical means, until those points exceed a certain distance asunder, which distance will vary with circumstances;

but in bridge building, will seldom be less than 10 to 14 feet, where timber beams are employed. Hence, for bridges of a length of 12 to 14 feet, usually, nothing better can be employed than a structure supported by longitudinal beams, with their ends resting upon abutments or supports upon the sides of the stream.

Of course, no reference is here had to stone or brick arches. For, though these are advantageously used for short spans, and in deep valleys, where the expense of constructing high abutments for supporting a lighter superstructure, would exceed or approximate to that of constructing the arch, it is the purpose of this work to speak only of those lighter structures, composed mostly of wood and iron, and supported by abutments and piers of stone, or by piles, or frames of wood.

Having then adopted the use of beams for supporting weight upon short spaces, it is only necessary upon longer stretches, to provide support for a point once in 10 or 14 feet, by braces, &c., from the extremities; and for intermediate points, to depend on beams or joists extending from one to another of the principal points provided for as above.*

VI. For a span of 20 or 30 feet, it would seem that no better plan could be devised, than to support a transverse beam midway between abutments, by two pairs of braces or suspension chains, proceeding from points at or over the abutments, one pair upon each side of the road-way; this transverse beam affording support for longitudinal beams or joists extending

*It is susceptible of easy demonstration that the power of beams to sustain weight by lateral stiffness, forms no exception to the principle that oblique forces alone can sustain heavy bodies over inaccessible spaces. But this matter is deferred for the present.