This bridge is a bit different from usual and can be adjusted to cross ravines etc of varying widths. If you need to join spars using a sheer lashing try not to get the joins opposite each other as this will create a weak point. A very long bridge will be heavy and a system of pulleys will be required to lift it.
- 2 x 5.0m (16ft) spars
- 6 large pickets
- 5 x 4.0m (12ft) spars
- 2 single pulley blocks
- 4 x 2.5m (8ft) spars
- 72 lashing lengths
- 6 x 2.0m (6ft) spars
- 2 x 15m (45ft) ropes
- 20 x 1.2m (4ft) spars
- 1 reasonably large log
- 9 x 1m (3ft) spars
- large tent pegs
- 2 x 7.0m ropes
Prepare a ladder using 2 x 5m (16 ft) spars and a selection of 9 x 1.0m (3ft) and 4 x 2.0m (6ft) cross members. The 2.0m treads should be placed one at each end with the other two evenly spaced along the length of the ladder. (Diagram A)
The ladder should be placed so that the treads rest on top of the 5.0m (16ft) spars and vertical 1.2m (4ft) spars lashed to the outside of the 5.0m (16ft) spars next to the 2.0m (6ft) cross-spars. These form the basis of the handrail. At one end it is important that these verticals extend approx. 0.3m (1 ft) below the ladder.
The handrail verticals are braced by lashing 1.2m spars to the top of each one and the outer end of each 2m (6ft) spar. (Diagram B)
The four end uprights are also braced along the length of the long spars using the remaining 1.2m (4ft) spars.
The ladder is completed by tying a thin line tightly between each of the vertical spars as a hand rail.
You may find that a diagonal bracing on the ladder is necessary.
Prepare 2 identical A frames using the 4 x 4.0m (12ft) spars and 2 x 2.5m (8ft) spars. The sheer lashing should be approx. 0.5m from the top of the 4m spars. Lash a second, 2.0m (6ft), cross spar about 1.0m down from the apex.
Fix the two pulleys to the 4.0m (12ft) spar which will join the top of the A frames and run the ropes through them.
Join the top of the two A frames using this spar. The cross spars need to be on the outside. Ensure that the distance between the frames is wider than the ladder. You may find it easier to do this with the structure lying on its side.
Raise the structure.
When you are certain that your A frames are in the correct position, lash a brace on the outside of each A frame to the short cross member, as struts. The lower ends of the struts should be lashed to large pegs driven into the ground. The A frames may need to be guyed.
Drive pegs in at the feet and lash the spars to them – this will also help stability.
The log should be placed between the A frames and pickets driven in to hold it in place. The feet of the end verticals on the bridge, which extend furthest below the ladder, rest against this log. When the drawbridge is raised, it will pivot at this point.
Use a round turn and two half hitches to tie the end of the lines which go through the pulleys to the far end of the ladder.
Place the end of the ladder in position against the log and, using your ropes etc, lower it over the ravine. A second log secured at the landing place on the far side will help to keep the bridge level.
The bridge is now ready for use. Either use a suitable tree or put in pickets beyond the A frames to which the lines can be tied, holding the bridge in position.
Check these lines and their fixings carefully if you plan to leave the drawbridge up.
Note: Since the design of the project the principles of Health and Safety have changed dramatically.
Caution should be exercised to assess the risks associated with this build and especially regarding possible falls from height, so handrails or belays and harnesses may be required.
To download the complete plan and design document please Click Here