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Wooden and Iron Bridges.
1st. Of Wooden Bridges.-The abutments and piers are formed either of wood or stone; stone is generally preferable from being of a less perishable nature, and affording more resistance. The arrangement of the stone piers and abutments differs little or nothing from those of stone bridges. Wooden abutments may be formed by imbedding large pieces of timber in a vertical position, at intervals of a few feet, forming the facing to sustain the earth: or abutments may be formed by constructing crib-work, of large pieces of square timber laid horizontally upon each other. These are firmly connected, sometimes by iron bolts. Then the enclosed space, which is generally a triangle, is filled with earth or stones. The piers may be also made according to either methods, and then filled with stones. Sometimes wooden piers are made by driving beams vertically into the bed of the stream a few feet apart. These connected at top by a horizontal beam, well fastened to the uprights by bolts. Then other pieces are placed in a diagonal position, to brace the whole firmly.
The frames of wooden bridges may be composed of straight timber, or it may be those which form wooden arches. The usual form of making wooden arches consists in making the arch of several concentric courses of timber bent to a suitable curvature; and the different courses firmly united by keys of wood and hoops of iron. The sleepers which rest on the crown of the arch, are supported, between this and the extremities, by inclined stirrup pieces. (On this subject see Mr. Mahan's Elementary Course of C. Engineering.)
Of Iron Bridges.-Iron is an elastic substance, and is greatly affected by heat or cold, expanding with one, and contracting by the other. Therefore, when a heavy load acts upon an iron bridge, the whole is put in motion.
When it expands, the whole weight of the arch is raised, and the pressure on the abutments is compounded of the matter and velocity of the weight raised. This is not the case with a stone bridge: consequently the strength of the abutments of an iron bridge should be such, as not only to sustain the weight of the arch, but also the additional push arising from the above cause.
The following account is given of the iron bridge over New River, Bristol Harbour. The arch is a circular segment, of 100 feet span, with a rise of 15 feet: the width of the bridge 31 feet: the whole is of cast iron of the strongest grey metal; amounting to 150 tons, (viz.) 100 tons in the ribs, pillars, bearers, balustrade, &c., and 50 tons in the plates for the roadway. The arch consists of two concentric circle rings, firmly connected and bound together. Each of these is formed of 6 ribs, at 6 feet distance from each other, tied together by cross bars, at intervals of 91 feet. On the upper ring of each rib, stand a number of pillars, in an upright position, their tops formed like a T, as bearers to support the plates of the roadway. The whole is put together and secured in the firmest manner: so that, if any part of the bridge be injured, it may be taken out and replaced, without disturbing the mean body.
To make the arch uniformly strong throughout, it ought to be an arch of equilibration. To find the weight over every part of the curve, so as to put the arch in equilibrio, is the same thing as to find the vertical thickness of the arch in every part.
I would, says Dr. Hutton, connect every course of frames to those next above them, so as that the whole bridge may rise or settle together as one mass, by expansion or contraction. Yet I would not tie or bolt the frames together lengthways, but would simply make the edge of the side of each frame, fit into the groove of the next, going into each other 2 or 3 inches; by which means the
arch frames will always fit close together, in every degree of temperature, without tearing asunder at the ties. By this means the effect of any partial or local pressure or shock will be distributed among a great number of the adjacent parts, and so the effect be broken from the immediate place of action. Also will be obviated any dangerous effect from expansion or contraction.