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October 7, 2016

56m suspension bridge at Manjedal Activity Centre, Western Australia

Greg from West Australian Scout Pioneering wrote in with photos and a description of the awesome bridge, supported by A frames on each end and a raft in the centre. Here's Gregs description:

Photo courtesy Greg Barbera


Project : Build a Rope bridge for the West Australian Cub Scout event – Enduro 
Location: Manjedal Activities Centre (M.A.C) – Lake Jones 
Depth: Varies from 4 meters to 15 meters Span: 56 m (40m over water) 
Water Temp: Very cold 
Weather: everything except snow 
Duration: 16 hrs to build
Photos courtesy Greg Barbera

To help celebrate the 100th Anniversary of Cubs, the Organizing team decided to go with a “Back to the Future” theme. They also want to surprise the cubs by having an activity based around the traditional methods of pioneering that have been with the movement for over 100yrs. The West Australian Scout Pioneering Crew (WASP) were asked to build a bridge across Lake Jones that would be sturdy enough to withstand a very large number of Cubs and Leaders using it.
Over the two days of Enduro approximately 800 Cubs and a small number of leaders made the crossing. To ensure the build met the requirements of pioneering in Australia, the walk rope was set at a height of 1.8m from the ground so that helmet & harness would not be required (anything over 2m). This meant that we had to install some form of support in the middle of the arc to keep the ropes & cubs out of the water. Several suggestions where made of a tower, but owing to the erratic shape of the bottom of the lake there would be no guarantee that a level section could be found. It was decided that a pontoon would work providing it could be securely moored and be able to support the bridge as it pass over it. It was decided that a 4m x 4 m pontoon made out 100mm dia. pine logs with 4 x 200ltr blue plastic drums and 6 x 140ltr white drums would both support it and stabilize the middle section. In addition 2 vertical poles where fitted so the bridge ropes could be attached to the pontoon using Carabiners, this would still allow the ropes to be tensioned without moving the pontoon from its moored position. 
We also fitted an arrival and departure deck which only covered the area under the bridge, a walkway was also fitted for a leader to use to assist those passing through the middle section. On the trial build of the pontoon we fitted guy ropes to the vertical posts but found that caused problems and a trip hazard for the leader, so this time we used 3 spars on each vertical and lashed them in place, to reduce the amount of rope required we made the first lashing a figure 8 and pulled the side poles out to create the angle brace. Then fitted one on the outside and lashed a horizontal spar in place so the walk rope could also be clipped onto the pontoon. For this span we fitted each line with 2:2 rope block combination and used the pontoon as a means to keep the lines tensioned as the combined weight totalled approx 350 kg. The main ropes where 20mm dia. sisal and to assist the hand rails to support the walk rope we fitted short lengths of rope which we refer to as stringers. Normally when we fit pulleys on to the main ropes to take up the tension, we used the prussic loop to attach the block, for this one we used the “Dog & Tail hitch”. This hitch does the job well and is easier to move when it is not under load. 
On the shore we used “A” frame assemblies that are self supported by guy ropes and thus allowed the main ropes to be tensioned freely. Carabiners were also tied to the frames to support the hand lines. To stop the main line travelling along the horizontal bar, we used a short length or rope and tied a clove hitch on both sides. Normally we would use a log & picket anchorage for our rope bridges, but in this case the ground on both sides was too soft to get a good footing. On one side the soil was too moist and on the other side there was a very large ant hill, which meant we had to contend with hollow ground owing to the nest. We therefore bound the trees with hessian to reduce any damage to them. 
To reduce the risk of the Cubs endangering themselves if they fell off, each person making the crossing was required to wear a life jacket and we had a rescue craft on standby (canoe). Out of the 800 cubs only 2 ended up in the water and 7 were rescued by the safety vessel as they held onto the hand rails.

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