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PART 8: Pioneering Bridges

Simple Bridges   

Monkey Bridge
The monkey bridge is constructed using two sheer legs and bridged with a hawser and handrails secured using pickets. this an easy structure to build as most of the bridge is constructed using rope.

How to build
Assuming you have access to both banks, construct two sheer legs with the crossing of the spars being a little above half way. The top portion of the spar should allow you to position the handrails at about shoulder height. The distance between the tips of the spars should be 1 metre apart.

The sheer legs are then raised into position and the base of the spars butted into the ground and guyed into position.

Old canvas or sacking should be placed in the diagonal to reduce friction and wear on main foot rope.
The main rope (75mm) is passed over the sheer legs and the rope aligned and hand strained so that the position of anchors can be determined.~

A 3-2-1 picket anchor should be placed in a straight line to the 'foot rope' on each side of the river bank. If only one set of pulleys is available then a log and picket anchor is placed on one bank and a 3-2-1 picket anchor on the other to which your pulleys are attached.

Hand rails are attached using clove hitches and are carried down to the ground like guys.

Securing ropes are tied between the handrails and foot rope using clove hitches.

Test structure before using.

Pulleys will need to be checked at regular intervals to take up slackening of ropes with use.   

Trestle Bridge
The trestle can be used as a basic building block for a whole range of bridges. The bridge illustrated is the locking trestle bridge which can either have a see-saw bridging platform or a fixed ramp on each side.

Equipment required
2 No. 6 metre spars
8 No. 4 metre spars
2 No. 3 metre spars
3 No. 2.5 metre spars
Spars for handrail supports and rungs for bridge
Lashing ropes
Handrail ropes

How to build
Construct two trestles. It will be necessary to construct these according to the depth of the river and river bank.
These trestles are placed opposite each other on the banks of the river and allowed to fall together.
A ladder type construction is created using the 6 metre spars and small poles and handrail supports.
A 2.5 metre spar is placed between the diagonal created by the joined trestles. This is not fixed as it is a rolling pivot for the ladder structure.
The ladder is fixed to this rolling spar.
A large log placed on each bank will protect the bank from damage caused by the see-saw action of the bridge.  

Draw bridge
The draw bridge is similar in construction to the trestle bridge. You need a trestle frame, a ladder structure - drawbridge- some tackle and picket anchor and a large log.

Equipment required
2 No. 6 metre spars
6 No. 4 metre spars
1 No. 3 metre spar
1 No. 2.5 metre spar
Small poles for ladder
A large log
Pickets
Pulley and 50mm rope
Lashing ropes

How to build
Construct the trestle frame, bottom brace position will be determined by height of log.
Make a ladder structure using 6 metre spars and small poles as rungs.
Fix 4 metre braces from trestle frame to ladder and fix handrails. the handrails can be of rope or spars if you prefer.
Place 3-2-1 picket anchor in the ground and rrig up a pulley to it.  

Aerial Runway Code

Aerial runways are potentially highly dangerous, and remain so even when all reasonable precautions have been taken, both in their construction and use. The aim should be, therefore, to eliminate avoidable risk and thereafter to exercise the highest degree of responsibility in the use of the apparatus.

Equipment
Main Hawser
This should be of hemp (manila or sisal) or Polypropylene. The recommended size is 75mm circumference, and in no case should rope under 60mm circumference be used. If the hawser has been out of use for any length of time - even if stored in ideal conditions the rope should be checked by twisting the lay of the rope and checking that there is no trace of mildew. The rope should also be checked its entire length for wear and tear.

Other cordage
All other cordage whether lashing or securing ropes should be of good quality and should be checked in the same way. A safe working rule is to use nothing under 30mm in the main structure.

Block and tackle

A luff tackle, comprising one single and one double 150mm block reeved up together with 50mm circumference rope, will provide a mechanical advantage of four to one. The travelling block will be a single sheave block to suit the size of the hawser and of greatest importance it must have a fixed or swivel ring or eye. Under no circumstances can a block be used with a hook, even with 'mousing' applied. Other small blocks will be needed to suit the towing rope. All blocks should be fully inspected before they are used and should also be oiled.

Pickets and spars
Where wooden pickets are used they must be in good condition free from splits and rot, and of adequate length and thickness. 120 cm X 75mm butt should be regarded as the minimum. In use they should be driven with a maul ( not a sledge hammer) for two -thirds of their length at right angles to the strain , which for obvious reason should be secured close to the ground.
Spars should be equally sound 125 mm butt should be regarded as the minimum irrespective of length

Seat
The seat may consist of a bosuns chair or a discarded tyre suspended from the travelling block. If a seat is to be used the seat should be bored and the rope supporting the chair passed through the bored holes rather than using a scaffolding hitch which may slip. The supporting ropes should be 30mm in circumference.

Protection
Old sacking or canvas should be used to act as saddling in the crutches of the sheer legs and around the tree to protect the bark.

Holdfast
Where a tree is available, make sure that it is 'up to the job' and that with suitable protection it will not suffer any ill effects. At the head of the run, use the main trunk of the tree in preference to a branch.

Angle of the slope
No exact formula is possible as runways are notoriously idiosyncratic, but a safe plan is to test your apparatus by giving it a 'dry run' with a heavy log in place of the passenger to ensure that the chair travels at a reasonable speed and comes naturally to rest a few feet short of the shearlegs. This should be regarded as standard procedure. If the speed of descent is thought to be too great it can be reduced by reducing the strain on the tackle. An additional precaution is a braking device which can be fitted to the hawser, consisting of a rope grommet with two slack lines anchored to pickets on either side of the run.

Embarking and disembarking
When the take - off at the head of the run is at some height above ground level ( e.g. from the branches of a tree) passengers should embark at the foot of the run, where safety and comfort can easily be checked, before being drawn up by the towline. An additional crossbar on the shear legs will facilitate mounting and dismounting.

Finally, while Aerial Runways remain the most popular and adventurous of the pioneering projects open to Scouts, they can be dangerous, even when all the precautions are taken. It follows that the greatest care should be exercised, and common sense - that invaluable part of the Leaders stock and trade, applied at every stage to ensure that unnecessary or avoidable hazards are not introduced. While the Aerial Runway is in operation a responsible adult should be in charge at all times.

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Have a look at our Pioneering section.