WOODEN BRIDGES.
The superstructures of wooden bridges have been constructed in a variety of ways, but their abutments have usually been made of stone. In all, therefore, which regards the foundations and masonry of the abut ments, the process is precisely similar to what has been described for stone bridges, only the weight and hori zontal thrust of wooden bridges being much less, require a smaller proportion of masonry. Where the streams are narrow, it is only necessary to lay beams across, reach ing between the abutments at the distance from each other of four to five feet from middle to middle, and to have braces reaching from some part of the face of the abutments to the lower side of each beam. Besides these supports, the outer beams are usually trussed up by means of the side railing. Across the beams, planking of three or four inches in thickness is laid to receive the gravel. A plank is also fixed along the inside of each railing, to support the side of the roadway. The railing is secured by braces fixed on the outside, or by strong iron knees on the inside.
When the stream is wider than to be reached across by one length of beams, the most usual way has been to drive rows of piles at each length of beams, in lines pa rallel with the current of the river, at about four or five feet distant from each other, (middle to middle,) and also fixing braces up and clown the river to support each pier, or row of piles. These piles are driven and fixed in the same manner as described for the foundations of stone piers. They have usually diagonal braces cross ing each side of the row of piles in the form of an x, and which arc bolted together at each pile. Tenons arc formed on the top of each pile, upon which a cap is mor ticed ; and in order to shorten the bearings, pieces of timber are laid immediately under each beam, extend ing five to ten feet on each side of the cap. The rest of the structure is completed in the same manner as has been described for one bay of beams. A very perfect
model of this sort of bridge was constructed by Mr Pe ter Nicholson, upon the river Clyde at Glasgow. It is only a foot bridge, but the principle is excellent. See Plate CII.
Instead of driving piles for supports, frames are some times constructed on dry land, which are afterwards sunk in their proper situations, after the bottom has been made level to receive them. These upright frames have grated frames attached to their Lase, and which extend on each side of the upright frames. Through the spa ces formed by this excess of breadth, short piles arc dri ven to keep the grating and frames secure. These up right frames have their sides covered with planking ; and in order to add to their stability, the lower part is filled with gravel or small stones. Upon the edges of the frames which face the stream, triangular pieces of cast iron are fixed, to prevent ice or other matter float ing clown the river from injuring the bridge. Fender piles arc sometimes driven, so as to form a triangle at some little distance above, and opposite to each pier. This mode has been adopted with perfect success by Mr Telford on the Severn, 8 miles below Shrewsbury. These pilings and frames, in deep and rapid rivers, be ing not only difficult and expensive in the first instance, but notwithstanding all the precautions which can be taken, liable to injuries not easily remedied, wooden bridges, of great extent, have been constructed from bank to bank, without any intermediate supports. This has been performed in different ways with great inge nuity. When the banks rise considerably above the le vel of the river, it is usual to construct the supporting frames chiefly below the level of the roadway.
1. Merely by means of two principal rafters, whose lower ends stand on the abutments, and their points meet below the middle of the beam ; or sometimes the upper points pass the outer beams, and meet below the top of the side railing.