Safety

Bridge building is potentially dangerous, because of having to handle heavy and bulky main beams. The safest way is to use lifting plant such as a lorry-mounted crane, but many sites will not have suitable access. Below are some methods of moving and positioning main beams without using machinery.

A 6m timber beam weighs approximately 165kg, and a 9m timber beam 433kg. A6m steel beam weighs approximately 150kg, and a 9m beam 279kg. Over a carrying distance of 200m or more, the load per person should be kept below 25kg, and carrying beams or slings used to spread the load equally. There is always the chance, especially on rough ground, that one or more workers will stumble or be in a position where they cannot take any load, thus increasing the load on the rest. It is advisable to move heavy objects in such a way that they can be put down quickly in a stable position. Where possible use rollers of suitable round timber.

The moving and positioning of main beams requires plenty of people, but with one person only to give the orders so there is no confusion. Once the beams are in position, the construction of the decking and handrails is best done by no more than three people. If too many people try to work at the same time on an unfinished bridge they will get in one another ’s way, with potential danger if there is a drop to the stream bed. Other volunteers can be involved in preparation of decking and handrails, or in clearing and surfacing the bridge approaches.

Carrying materials

This can be the hardest part of constructing a bridge. Special transport such as an Argocat or helicopter may have to be used for moving main beams into remote locations.

If main beams have to be moved without a machine, always try to avoid having to lift them, as this is when accidents can occur. Consider one of the other methods first. The Manual Handling Operations Regulations (Health and Safety Executive) gives guidance on safe practices.

‘Dollies’ are small carts with wide wheels, used on building sites. A ‘bomb dolly’ can take up to 500kg. Local builders may be able to lend one.

A 'dolly' for carrying loads

Timber skids can be nailed to the bottom of the beam so that it can be pulled along the ground without being damaged. Try and use horsepower, or failing that, volunteer-power.

A horse pulling a beam

If the main beam has to be lifted, use carrying beams which can be securely gripped. Always tie the main beam to the carrying beams so it cannot move. Fix blocks to the bottom of the beams so that the load can be put down without fingers being trapped.

Using blocks to protect fingers

Try and take some of the weight on shoulder slings. These can be made of old seat belts from scrap yards.

Using shoulder slings

Railway sleepers can be carried using metal pins pushed through existing holes in the sleeper.

Carrying a railway sleeper

Positioning the main beams

The procedure must be carefully planned. For shallow streams and ditches, it should be possible to manhandle the beam into position. The remaining beam or beams can then be slid along the one already in place. Beams which are too heavy to manhandle will have to be brought to the site by machine, and must be lifted into place by a lorry mounted crane or excavator arm.

To manhandle beams across deep gaps, use one of the following methods:

  1. Place one beam alongside the abutment, and using a heavyweight to counterbalance, slide it out across the gap. The next beam can then be pushed along it and into place. Use rollers as necessary to assist.
    Positioning the main beams
  2. Pull the beam across the gap, using a stout tree or post to belay the pulley. If neither are available, make an anchor of metal pins lashed together with rope.
    Using a pulley and rope
  3. Use shear legs and spars. This should be done by an expert.

Order of work

Design

This summarises the stages in designing a bridge.

  1. Gain consents
  2. Survey the site.
  3. Note materials available, resources, access and expected use.
  4. Choose bridge type and width.
  5. Consult the table of beam sizes
  6. Design abutment or end seat, and fixing for main beams.
  7. Calculate handrail post spacing and handrail size.
  8. Calculate spacing and number of deckboards.
  9. Make a complete materials and tools list.
  10. Decide on method of getting materials on site.

Construction

This summarises the stages in constructing a bridge.

  1. Construct abutment or end seat and scour protection. Allow time for concrete to set.
  2. Place damp-proof membrane on abutment/end seat.
  3. Position main beams. Wedge and secure struts.
  4. Lay bituminous felt on beams.
  5. Attach deckboards, except for those between the handrail posts.
  6. Fix handrail posts and tighten steel ties.
  7. Attach remaining deckboards.
  8. Attach handrails.
  9. Build steps and/or stile.
  10. Surface approach path.

Chapters