This section discusses the design and construction of footbridges under nine metres in length.Any bridge longer than this should be designed by an engineer. Most footbridges in the countryside are on public rights of way, and their maintenance is the responsibility of the highway authority who should take appropriate advice on the design of any footbridge.

A major source of information on this subject, used for parts of this section, is ‘Footbridges in the Countryside’ (revised 1989, Countryside Commission for Scotland).

Consents

  1. Consent from the Highway Authority for a bridge on a public right of way.
  2. Consent from the area office of the EnvironmentAgency (England and Wales) or River Purification Board (Scotland), plus anyone with navigation rights.
  3. The Town and Country Planning (Scotland) Acts apply to footbridges, which may need planning permission.
  4. The Health and Safety at Work Executive have the right to inspect to ensure that maintenance work is being done correctly.
  5. The owner ’s insurers may need to be advised.

Site selection

In the choice of site, look for the following: –

  1. Shortest span.
  2. Solid banks which will provide strong foundations. Test with a soil auger or by digging test pits. Avoid bends where erosion occurs.
    Positioning a bridge
  3. Banks which will give clearance from flooding. The area office of the Environment Agency (England and Wales) or River Purification Board (Scotland) will advise on flood levels.
  4. A site which fits the desire line of the path.
  5. Easy access for plant and materials.

Survey

  1. Section across gap. Preferably this should be surveyed using a Dumpy level or similar. If this is not possible, use the following method.
    Working out levels
    You will need a surveyor’s tape or line, a steel tape, a spirit level, and three ranging poles or similar straight poles. You will also need some means of setting the line horizontal, such as a clinometer, sextant or home-made level. The latter can be made out of about 600mm length of clear flexible plastic tube such as caravan plumbing. Partly fill with water, and bind into a circle with insulating tape.
    Set a ranging pole firmly in the ground on either side of the gap. Check that they are vertical using the spirit level. One person then uses the levelling instrument against a fixed mark on the nearer pole while another person moves a marker against the further pole according to the instructions of the first person. Mark the level, and then measure the same distance down (x) on both ranging poles, and set the tape or line taut.
    Measuring the section
    Using a ranging pole set vertically, measure heights D and E, and read off or mark distances C, A and B. Measure A1 as a check. Replace the two ranging poles on the banks with marker posts so that the exact line can be found again.
  2. Sketch or photograph the elevations of the banks.
    Elevation of the banks
  3. Draw a sketch map showing details of the stream bed such as shoals or points where erosion is occurring. Include bank features and obstacles such as trees or rocks.
  4. Note the nearest vehicle access. The type of access to the site is of great importance in deciding on the bridge design. The difficulty of access may eliminate designs which use long, heavy beams, and may require a lattice girder design, which is not suitable for construction by voluntary labour (see Countryside Commission for Scotland, revised 1989).
  5. Look for evidence of flood such as twigs and debris in bankside trees, and deposits of river gravel. Note the height of these above the marker post.
  6. Consider whether or not the bridge should be a ‘feature’ in the landscape. In flat agricultural areas with deep drainage ditches, handrails of bridges provide useful waymarks.

Handrails as waymarks

In scenic or historic landscapes it may be desirable to site the bridge out of view.

Hiding a bridge

In less interesting scenes, an unusual bridge can provide a focus.

An unusual bridge as a focus

Loading

The loading on a bridge includes the following:

  1. The dead load of the bridge structure.
  2. The weight of bridge users plus the dynamic effect of their movement. The forces exerted by users leaning or falling against the handrail. Use is calculated for either normal or crowd loading, as shown.
    Bridge loading
  3. The horizontal effect of the wind on the bridge and the user, and the vertical effect from suction or pressure on the decking.
    Other bridge loading
  4. The vertical pressure from weight of lying snow and ice.
  5. Water pressure on the bridge during flood, plus pressure from floating debris.

These factors are given for interest only, as specialised knowledge is needed to design a structure for the calculated loadings. All the designs given in this chapter are suitable for normal pedestrian loading. Any bridge that may attract crowd loading, for example near a car park used by coaches, and bridges on bridleways, should be designed by an engineer.

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