Arboriculture & Rigging

Bridge Building & Autumn Camp – Lamberhurst

After a well earned summer break the group asked if I was happy to help over the next year (academic) with another activity, and I had a few ideas, initially for a modest pedestrian bridge on the forestry training site to cross a small stream. However on a series of small tree felling courses I had recently opened up a ride to an area of sweet chestnut where tractor access was limited by another stream so this seemed a more useful project albeit perhaps a little ambitious. We wanted to bring this block of chestnut back into coppice rotation.

Scott Fraser bridge building exercise

I got going rapidly after the request was made to run an early October camp, and ordered a couple of large steel beams, each weighing in at around 200kg. I also needed to deck and frame other parts of the bridge and I was keen to use material from the estate, so I managed to whiz around and collect a few of the smaller oak stems, some had been cross cut to 3m on a medium tree course in error and I thought they would be ideal for deck planks. Nick Hilton of Woodwise forestry then kindly milled them up, getting through my long cutting list………..

Evening session– I was a little unsure what exactly to do for this session, but the Mondays sawmilling made the decision easy – with the ‘RSJ cladding oak beams’ weighing in at I guess around 300kg a method was needed to move them safely and with a high level of control, so I chose to look at mechanical advantage systems. I was introduced to these efficient methods when Ken Palmer of Arbormaster were doing a demonstration at Merrist Wood College in Surrey, perhaps 14 years ago and now they are regularly used in my contracting and also on some training courses, including rigging where simple systems can be used to lift in conjuction with a basic capstain lowering device. The scouts had looked at these before, but with larger blocks to move vehicles and I was able to add a little in terms of looking at how to estimate the advantage gained and also to capture progress. I also showed some video of the bridge site and the sawmilling filling in a bit of background and setting the scene for the jobs to be completed.

Camp– I helped set up camp, and could not believe how much kit could be fitted into a small trailer and indeed was actually needed for a 2 night camp.

With only a day and a half available an advance group was dispatched straight after breakfast on Saturday to the stream area for same basic leveling to accept the oak beams to act as bearers. I also set up a couple of high rigging anchor blocks either side of the stream, with the hope that the scouts would be able to use the GRCS to move the beams.

This worked out well with the bearers and then steels being slung (see photo) at their balance point and then lifted with the GRCS, easy for even the smallest scouts! A second 3:1 system was also used to control swing, and by mid afternoon the structure was in position and I set to work cutting the rebate to allow the oaks beams to slot into the RSJ web to hide it and given the profile of the oak (300 x x150mm) also realistically add strength. Disaster struck when the generator gave up and a plan B quickly emerged – lift the beams with the timber crane and cut them back at the workshop, then slot them in and no one would ever know………one front puncture and a spare generator and compressor later and we were back in business, ready to slot them back in position early Sunday ready for the deck to be positioned and fixed, no one would ever know………..

This deck laying process started off quite slowly and speeded up as the scouts got the hang of the GRCS and went at top speed as we later were able to manually lift the shorter beams as a group.

There was not quite time to fit the edge rails (hopefully will stop the trailer sliding off the edge) but the deck was completely finished and the group photo shows the team, with minutes to spare before camp had to be packed up.

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