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August 26, 2016

Suspension Bridge from Feb 1937 Scouter magazine

I found this suspension bridge article written by L.C. Sands in the February 1937 Scouter magazine:

No special credit is claimed for having succeeded in building the bridge described here, as it involves a principle in common use, but as the writer has not seen anything similar described in any of the books on pioneering, he considers that other Scouters might possibly like to try it out, and doubtless improve upon it. Its chief advantages are that it needs only simple materials, is easy to construct, and is one which could be crossed with ease by many people who might hesitate to venture on a monkey bridge. Any measurements stated here may be altered to suit local conditions.

Two Towers were constructed of four spars about 9 ft. long, with 1 1/2–2 in. butts, and seven Scout staves 5 ft-long. First the side trestles were made by lashing two spars together as near the top as possible with a sheer lashing, and then the ledger was put on with square lashings about oft-from the bottom of the spars. These two trestles were then connected together to form the tower by lashing a strong pole across the top in the V’s made by each pair of spars, and by lashing poles adjacent to or slightly above the ledgers of the original treetles. The base of the finished tower was about 4 ft. square. About 2 ft. from the ground further poles were lashed to both back and front of the tower: the one (A in the diagram) nearest the bridge proper bore the two hawsers of the roadway, while both bore the planking.

The Roadway was made with two 2 in. ropes to which were attached twelve Scout staves at intervals of about 1 yd., by means of lever hitches a few inches from the ends of the staves. This method is probably not the best for strength, but it is quick, although about 1 ft. of the rope is used up in making each lever hitch. When this is swung into position, the ropes at one end should pass over the front pole (A) on the tower, under the back one (B), and be made fast to pickets or some convenient tree if available. The other ends should be similarly secured, but if possible block and tackle should be used to tighten them at one end, as was used in this case.

Suspension lines were made from 1 in. cord throughout, twelve in all three for each side of each half of the bridge. These lines were attached to the pole forming the road-bearers of the bridge, and to passed over the top pole of their respective towers to pickets or suitable trees. In the case of the lines attached to the two centre poles (E and F.)loops should be made in the cords about 2 feet from the ends fastened to the roadbearers. The two loops in these two lines from E and F (see diagram) were then drawn towards one another by another cord until the angles between the roadway and the portions of the cords below the loops were almost right angles, and then made fast. This other cord was then continued in either direction, taking a turn round each of the suspension lines in passing to the towers, to form a handrail. The reason for so dealing with the middle suspension cords was to increase the angle between the suspension lines and the roadway, so as to get a better suspension of the centre part of the bridge This proved the most satisfactory of several methods tried.

The other cords from C. D. G. and H go straight from these points over the top poles of the towers to their respective pickets. These suspension cords should all be fastened on to the roadbearers at D, E, F, G and H before the suspension part of the bridge is swung into position across the gap to be bridged. All that then remains to be done is to pass the cords over the top bars of the towers and fasten to the pickets. Before use the suspension cords should be adjusted so that the centre of the bridge is slightly higher the the ends. This method of suspension was found to be the most satisfactory and easiest to make as any particular cord can be readily tightened up. Of course extra strength would be gained by adding similar cords to some - the other road-bearers, e.g., those between C-10, and G. H.

To complete the bridge some form of planking should be laid across the road-bearers and lashed to them. In this case, some fifteen inches (in width) of planking put on for the whole length, and this in itself was a considerable weight. The length of the actual bridge between the towers was 38 ft.

This bridge proved very satisfactory in practice. After it had been used a number of times the drop in the centre, while an average-sized adult was standing on it in the middle, was only 9 in, and it may be added that the writer rode his bicycle across it.

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