Lock Trestle Bridge
Crossing a ravine is always a challenge. This bridge is similar to the seesaw bridge (see sheet I3), but is suitable for wider gullies and can be adapted to suit those that have irregular banks.
The number and size of spars required will depend on the actual gully size but the equipment listed will suit a ravine approximately 7 metres wide and 2 metres deep.
- 4 x spars 3.6 metres (12 ft) long
- 4 x spars 2.5 metres (8 ft) long
- 8 x spars 2.0 metres (6 ft) long
- 20 x spars 1.2 metres (4 ft) long]
- 58 lashings lengths, 4 large pegs or pickets
Construct 2 trestles, each one using 4 x 2.0 metre spars and 2 x 2.5 metre spars (a).
The top sections must be able to interlock and be narrower than the base for stability.
The actual height of the trestles is not critical as the deck of the bridge can have a slight fall either up or down to the support.
Once the width of the trestles is known, work can start on the two ladders which form the bridge deck. Each ladder will use 2 x 3.6 metre spars and 10 x 1.2 metre spars.
The first ladder should be narrow enough to just fit between the top of the trestles, once they are interlocked. The second ladder should fit inside the first ladder.
All the cross spars need to be on the same side of the uprights.
Stand the trestles in the gully with the tops interlocked. It may be necessary to foot them into the ground to ensure that they are stable.
The ladders are then lowered into position and pegs driven in alongside the first rung of the ladder, on the bank, to stop them slipping.
If it is not possible to stand the trestles in the centre of the gully, or if the banks are too wide apart for the available spars to span it, the bridge can be adapted as follows:
As for single lock trestle bridge plus
- 2 x spars 3.6 metre (12 ft) long
- 7 x spars 1.2 metre (4 ft) long
- spars for sheer legs (height will depend on gully and point where support is required)
The trestles are built and placed in the bottom of the gully, towards one bank.
The first ladder is also built as before and placed in position, leaning on the nearest bank.
A second ladder is constructed with 8 rungs on the top of the 3.6 metre spars, a gap where the ninth rung would have been and the tenth rung lashed underneath the long spars.
This ladder must be narrow enough to fit inside the first ladder.
A third ladder is built which must be narrow enough to fit inside the second ladder. This has only eight rungs, all lashed on the same side and from one end, thus leaving a section of the 3.6 metre spars without rungs at one end.
Two pairs of sheer legs need to be constructed to support the end of ladder 2, with cross bars between them to hold them at the correct distance apart.
Place the end of ladder 2, with the normally spaced rungs, onto the trestle in the gully. The other end should rest on the sheer legs. The rungs must be on the top of the ladder.
Feed the end of ladder 3 where there are no rungs over the one lashed underneath ladder 2 and lock them under the next two rungs.
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