Fig. 114-Represents an elevation A of a sharp lengths, depending on the curvature required; these pieces abut end to end, the joints being in the direction of the radii of curvature, and the pieces composing the different courses break joints with each other. The courses may be connected either by jibs and keys of hard wood; or by iron bolts. This method is very suitable for all light framework where the pressure borne is not great. Wooden arches are chiefly used for bridges and roofs. They serve as intermediate points of support for the framing on which the roadway rests in the one case, and the roof covering in the other. In bridges the roadway may lie either above the arch, or below it; in either case vertical posts, iron rods, or bridles connect the horizontal beams with the arch. 553. The greatest strain in wooden arches takes place near the springing line; this part should, therefore, when practicable, be relieved of the pressure that it would directly receive from the beams above it by inclined struts, so arranged as to throw this pressure upon the lateral supports of the arch. The pieces which compose a wooden arch may be bent into any curve. The one, however, usually adopted is an arc of a circle, as the most simple for the mechanical construction of the framing, and presenting all desirable strength. CHAPTER V. BRIDGES. I. CLASSIFICATION. II. STONE BRIDGES. III. WOODEN BRIDGES. IV. CAST-IRON BRIDGES. V. WROUGHTIRON TRUSS BRIDGES. VI. TUBULAR BRIDGES. VII. SUSPENSION BRIDGES. VIII. SWING BRIDGES. IX. AQUEDUCT BRIDGES. Ι. CLASSIFICATION. 554. A bridge is a structure for supporting a roadway over a body or stream of water, or over a depression in the earth. If the structure is over a depression in which there is usually no water, it is called a viaduct. If the structure supports a water-way, it is called an aqueduct, and if the aqueduct is over a river, it is sometimes called an aqueduct-bridge. Bridges may be classed according to their mechanical features; in which case we have— 1. Arches. 2. Trussed bridges. 3. Tubular bridges. 4. Suspension bridges. They may also be classed according to the materials which compose them; as Stone, Wood, and Iron. The former is more convenient for the purposes of analysis, but the latter will be used in this work. II. STONE BRIDGES. 555. A stone bridge consists of a roadway which rests upon one or more arches, usually of a cylindrical form, the abutments and piers of the arches being of sufficient height and strength to secure them and the roadway from the effects of an extraordinary rise in the water-course. 556. The general location of a bridge will depend upon the approaches, and the particular locality may be modified by the character of the banks, the soil or subsoil, and the bends in the stream. High embankments and deep excavations will naturally be avoided, if possible. The faces of the piers and abutments should be nearly or quite parallel to the thread of the stream. 557. Survey. With whatever considerations the locality may have been selected, a careful survey must be made not only of it, but also of the water-course and its environs for some distance above and below the point which the bridge will occupy, to enable the engineer to judge of the probable effects which the bridge, when erected, may have upon the natural regimen of the water-course. The object of the survey will be to ascertain thoroughly the natural features of the surface, the nature of the subsoil of the bed and banks of the water-course, and the character of the water-course at its different phases of high and low water, and of freshets. This information will be embodied in a topographical map; in cross and longitudinal sections of the water-course and the substrata of its bed and banks, as ascertained by soundings and borings; and in a descriptive memoir which, besides the usual state of the water-course, should exhibit an account of its changes, occasioned either by permanent or by accidental causes, as from the effects of extraordinary freshets, or from the construction of bridges, dams, and other artificial changes either in the bed or banks. 558. Water-way. When the natural water-way of a river is obstructed by any artificial means, the contraction, if considerable, will cause the water, above the point where the obstruction is placed, to rise higher than the level of that below it, and produce a fall, with an increased velocity due to it, in the current between the two levels. These causes, during heavy freshets, may be productive of serious injury to agriculture, from the overflowing of the banks of the water-course ;-may endanger if not entirely suspend navigation, during the seasons of freshets;-and expose any structure which, like a bridge, forms the obstruction, to ruin, from the increased action of the current upon the soil around its foundations. If, on the contrary, the natural water-way is enlarged at the point where the structure is placed, with the view of preventing these consequences, the velocity of the current, during the ordinary stages of the water, will be decreased, and this will occasion deposits to be formed at the point, which, by gradually filling up the bed, might, on a sudden rise of the water, prove a more serious obstruction than the structure itself; particularly if the main body of the water should happen to be diverted by the deposit from its ordinary channels, and form new ones of greater depth around the foundations of the structure. The water-way left by the structure should, for the reasons above, be so regulated that no considerable change shall be occasioned in the velocity of the current through it during the most unfavorable stages of the water. 559. For the purpose of deciding upon the most suitable velocity for the current through the contracted water-way formed by the structure, the velocity of the current and its effects upon the soil of the banks and bed of the natural waterway should be carefully noted at those seasons when the water is highest; selecting, in preference, for these observations, those points above and below the one which the bridge is to occupy, where the natural water-way is most contracted. 560. The velocity of the current at any point may be ascertained by the simple process of allowing a light ball, or float of some material, like white wax, or camphor, whose specific gravity is somewhat less than that of water, to be carried along by the current of the middle thread of the water-course, and noting the time of its passage between two fixed stations. 561. From the velocity at the surface, ascertained in this way, the average, or mean velocity of the water, which flows through the cross-section of any water-way between the stations where the observations are taken, is nearly four-fifths of the velocity at the surface. Having the mean velocity of the natural water-way, that of the artificial water-way will be obtained from the following expression, in which 8 and v represent, respectively, the area and mean velocity of the artificial water-way; S and V, the same data of the natural water-way; and m a constant quantity, which, as determined from various experiments, may be represented by the mixed number 1,097. With regard to the effect of the increased velocity on the bed, there are no experiments which directly apply to the cases usually met with. The following table is drawn up from experiments made in a confined channel, the bottom and sides of the channel being formed of rough boards : 562, Bays. As a general rule, there should be an odd number of bays, whenever the width of the water-way is too great to be spanned by a single arch. Local circumstances may require a departure from this rule; but when departed from, it will be at the cost of architectural effect; since no secondary feature can occupy the central point in any architectural composition without impairing the beauty of the structure to the eye; and as the arches are the main features of a stone bridge, the central point ought to be occupied by one of them. The width of the bays will depend mainly upon the character of the current, the nature of the soil upon which the foundations rest, and the kind of material that can be obtained for the masonry. For streams with a gentle current, which are not subject to heavy freshets, narrow bays, or those of a medium size may be adopted, because, even a considerable diminution of the natural water-way will not greatly affect the velocity under the bridge, and the foundations therefore will not be liable to be undermined. The difficulty, moreover, of laying the foundations in streams of this character is generally inconsiderable. For streams with a rapid current, and which are, moreover, subject to great freshets, wide bays will be most suitable, in order, by procuring a wide water-way, to diminish the danger to the points of support, in placing as few in the stream as practicable. 563. Classification of Arches. Arches are classed, according to their concave surface, as: cylindrical, conical, conoidal, warped, annular, groined, cloistered, and domes. A right arch is one in which the axis is perpendicular to the face; and an oblique arch is one in which the axis is not perpendicular to the face. A rampant arch is one in which the axis is not in a horizontal plane. 564. Surfaces of the Arch. The soffit is the inner concave surface. |