IRON BRIDGES.

Hawke's Bay Times, Volume II, Issue 57, 31 July 1862, Page 6 (Supplement)

IRON BRIDGES.

[From the Echon Examiner, June 28.] The number ana habits of the rivers of Nelson, and many other of the colony are serious obstacles in these early days of our existence of free communication. Very few of them will admit of abutments in the bed of the stream. Freshets twenty, forty, or sixty feet in height, bringing down logs of the largest size, debacles of ice and snow from the mountains, and the wandering propensities of the streams, make it necessary to bridge at one span, if at all, the full extent of the flood beds of rivers which are quite insignificant in their every day dimensions; and the great cost at which this is done is a prohibition, in many important cases, to doing it at all. The colony therefore owes a debt to those who will take trouble to procure or give information of economical methods of bridge building, and many of our readers in this town will be thankful at this moment for suggestions that may make the vote of £ 1,500 for a cart-bridge over the Maitai sufficient to carry out the purpose, in a safe and workmanlike way. Such a suggestion is made in a plan just placed in our hands by Mr. kitzGibbon, who has been in communication with several practical men in England on the subject of cheap bridges. The plan is from Mr. Dredge, the well known patentee of a peculiar suspension bridge. It does not profess to bo original and indeed, like a vast number of modern inventions is rather property of a national than of a personal kind. It has been adopted at Niagara for a railway bridge of 800 feet span, over which locomotives pass daily ; and has been reproduced in other places, among the rest at Chelsea, were Mr. P. Barlow, junior, has recommended it after a visit to Niagara to examine the bridge above named. It is a suspension bridge, stiffened by framed girders and tierods. The design before us is for a bridge 400 feet in span, with a sixteen feet roadway. I- consists of four wire cables, two on each side, about three to three-and-a-half inches diameter, resting on timber frames or towers sixty feet high, with iron capping: the extremities of the cables to landward are carried down to sunken masonry. The floor beams are supended in the usual way by ties about ten feet apart. Immediately below the'cable, ou’cach side, is a latticcdjimbcr framing, seven feet deep. The lattice bars are pieces of 11-inch plank, eight inches wide, fastened at the intersections with iron bolts. The tie or suspension rods pass through the centre of this latticed framing. It is proposed further to stay the structure by diagonal tie-rods fastened to moorings in the bed of the river, above and below the bridge. It will no doubt occur to some reader acquainted with mechanics, that there is a radical inconsistency in applying a rigid framing to a suspension bridge. A rope or chain stretched from point to point, with a weight hanging across it, alters its shape when the place of the weight is changed. Every washerwoman knows this by experience of the clothes-line. You must either cripple your rigid framing, or your chains or cables will be useless. There ai-e two answers to this objection: first, the lattice framing, seven feet deep and 400 feet long, is not more rigid that a three feet stick of green supplejack. It will yield considerably in its general form, but it is stiff enough to distribute the weight of a load passing across the bridge over a considerable number of the supension or tie rods, instead of allowing it all to fall on the one or two nearest : by this means the wave which occurs on a suspension bridge is much diminished. And, secondly the ties moored to the bed of the river take much of the freedom from the chains or cables, so that the stress is in lines quite different from those winch obtain in an orch’narv snsnension bridge, where the chains are practically free to assume any shape the position of the weight on the bridge may impress on them. It is not proposed that the wire cables should be twisted; they are to bo bundled of parallel wire, overlaid or served, as the sailors say, with a binding wire like the covered strings of the violin or piano. This form of rope is found practically equal in strength to a regular cablelaid rope, and is much cheaper, especially for colonial use, as it would cost less freight in the form of coils of wire than in the intractable form of wire cable. The rope is Jto be preserved by a mixture of tallow and tar applied hot.

It is estimated that bridges of the dimensions of this design, at any spot -within twenty miles, by a good road, of a port where the materials can be imported direct from England, may be mado at from £7 to £8 per running foot in span, including the cost of a machine for serving the cable. On this calculation the Maitai may be bridged for £1,400 at the utmost. The place is convenient, and, at the outside, the span need not exceed 200 feet to clear the highest floods. The amount voted for the purpose will probably cover the cost even if it should be thought that timber more durable than the excellent black birch on the hills around can he had by importation. The plan deserves the close attention of the local road boards and of the Provincial Government.