Width
The width is measured between the handrails. When calculating the width of decking and the distance apart of the main beams, allowance must therefore be made for the thickness of the handrails.
Below are the standards recommended in ‘Footbridges in the Countryside’ (Countryside Commission for Scotland, revised 1989). For bridges receiving occasional pedestrian use a width of 750mm may be considered adequate. However, as the same size and number of beams should be used as for a 900mm width bridge, the only saving is on the amount of decking required. Single beam bridges are narrower than 750mm, and should only be used at remote sites.
Main beams
From the tables below, read off the required size and numbers of main beams for the measured bridge span.
| Sawn timber: single beam | Span | |
| 300 x 300mm | Up to 9m | |
| Sawn timber: two/three beams | ||
| 750 – 900mm wide deck | ||
| No. of beams | Span | |
| 150 x 75mm | 3 | 3.0m |
| 200 x 100mm | 3 | 4.5m |
| 250 x 150mm | 3 | 6.5m |
| 250 x 200mm | 2 | 6.25m |
| 300 x 225mm | 2 | 7.75m |
| 350 x 250mm | 2 | 9.3m |
| 1200mm wide deck | ||
| No. of beams | Span | |
| 150 x 75mm | 3 | 2.75m |
| 200 x 100mm | 3 | 4.0m |
| 250 x 100mm | 3 | 5.75m |
| 250 x 200mm | 3 | 7.5m |
| 300 x 225mm | 3 | 8.0m |
| 350 x 250mm | 3 | 9.6m |
| Timber should be softwood Grade GS or MGS Species SC3 (see p204), pressure impregnated with creosote to 100Kg/m3. | ||
| Log | ||
| Average diameter | 750 – 900mm wide deck | |
| No. of logs | Span | |
| 250mm | 2 | 5.0m |
| 300mm | 2 | 6.7m |
| 350mm | 2 | 8.0m |
| 400mm | 2 | 9.75m |
| 1200mm wide deck | ||
| No. of logs | Span | |
| 250mm | 3 | 5.5m |
| 300mm | 3 | 7.0m |
| 350mm | 3 | 8.5m |
| 400mm | 3 | 10.0m |
Logs should be of Douglas fir, larch or Scots pine. They must be straight, and of the diameter specified for the middle third of the log length. Remove bark and branches, but do not trim flush. Soak all cut areas with at least three coats of creosote, as soon as possible after felling. Although this will not soak in far, it eliminates fungal growth on the cut ends, and reduces end cracking by slowing the rate of drying out.
The log should not be used if it has knots larger than 100mm, signs of fungal decay, or a marked spiral grain slope. Also reject logs that have surface cracks of 0.2mm width at time of felling (check with feeler gauge) or 3mm after seasoning. The total depth of cracks in any section must not exceed one third the diameter of the log.
Logs are best used at inaccessible sites which have suitable trees nearby. It is not worth transporting logs to a treatment plant, because of the cost, so they should only be used where a working life of about 10 years is acceptable. Telegraph poles are already treated, have a much longer life, and are therefore worth transporting some distance.
Steel
| Universal beam section | Span |
|---|---|
| 152 x 89 x 17.09 kg/m | Up to 6.25m |
| 203 x 133 x 25 kg/m | 6.25 to 8.50m |
| 254 x 146 x 31 kg/m | 8.50 to 10.40m |
For designs of steel beam footbridges see ‘Footbridges in the Countryside’ (Countryside Commission for Scotland, revised 1989).
Decking
The gap between the deckboards should be as shown below:
For normal pedestrian use of decking up to 1200mm width, either of the two following sizes of timber are suitable:
150 x 50mm softwood SC3 or SC4
100 x 36mm hardwood HS
Softwood should be pressure impregnated with creosote to 100kg/m3. These deckboards are thick enough to withstand many years’ use before replacement becomes necessary. As explained here, it is recommended that deckboards overlap the outer edge of the main beam by about 50mm. Always pre-drill, and attach using no.12 x 90mm sherardised screws. A strip of bituminous felt 20mm wider than the main beam, laid between decking and beam, will help protect the beam from rot.
Handrails
Handrails must always be strong enough to withstand people leaning on or falling against them. The number of handrails will depend on the situation and the expected use.
A single handrail on one side only gives minimum protection. In hazardous situations two or three rails on both sides should be used, with wire mesh if there is a risk to the safety of children or animals. The height should be as shown below.
The spacing of the handrail posts within the limits indicated above give a pleasing proportion to the handrail, which is the most conspicuous part of the bridge. However for structural reasons, raked handrail posts should be a maximum of one metre apart.
To design the handrails:
- Decide on cantilever or raked fixing for the handrail posts (see below).
- Decide on the required height and number of rails for the situation. For cantilevered posts, using the diagram above and the known total bridge span, choose a suitable spacing between handrail posts, as measured between post centres. The spacing must be consistent.
- Using the following table, read off the required timber size in the chosen timber grade.
| Spacing | Post sizes a x c | ||
|---|---|---|---|
| Softwood | Softwood | Hardwood | |
| SC4 | SC5 | Keruing HS | |
| 1000 | 100 x 75 | 100 x 75 | 100 x 75 |
| 1250 | 100 x 100 | 100 x 75 | 100 x 75 |
| 1500 | 100 x 100 | 100 x 100 | 100 x 75 |
| 1750 | 120 x 100 | 100 x 100 | 100 x 100 |
| 2000 | 120 x 100 | 100 x 100 | 100 x 100 |
| 2250 | 150 x 100 | 120 x 100 | 100 x 100 |
| 2500 | 150 x 100 | 120 x 100 | 120 x 100 |
- Using the table below, read off the required rail size for the post spacing.
| Rail size b x d | Spans between posts | ||
|---|---|---|---|
| Softwood | Softwood | Hardwood | |
| SC3 | SC4 | Keruing HS | |
| 50 x 75 | 1400 | 1500 | 1750 |
| 50 x 100 | 1600 | 1700 | 1900 |
| 50 x 150 | 1800 | 2000 | 2250 |
| 75 x 50 | 1900 | 2100 | 2500 |
| 75x75 | 2200 | 2400 | 2700 |
Handrails and posts should be pressure treated with water- borne or organic solvent preservative. Do not use creosote where it can get onto the hands or clothes of walkers.
Different methods of fixing the handrails to the posts are shown below. Only one rail to be spliced at any one post.
Handrail post fixing
Bridges with main beams of sawn timber should have cantilevered handrail posts, which are strong and simple to construct. Bridges with log or steel beams cannot easily have posts attached in a cantilever, and require a raked fixing.
Cantilever
The strongest fixing is made by using a 20mm mild steel tie, with two 76mm diameter x 6mm washers and nuts. The tie should be positioned just above the mid point of the beam. Before tightening the nuts, skew nail a 100mm x 75mm strut into position as shown.
On single beam bridges, use 12mm x 150mm coach screws, in either of the methods shown below. The U bracket is made of 25mm x 2mm galvanised steel strap which is pre-drilled before plating.
Raked
On log and steel main beams, the handrail posts must be supported by a raker to two deckboards extended to a distance equivalent to half the height of the handrail post.
The handrail posts should be not more than one metre apart. A wider spacing requires sizes of post and raker which will be too large to be safely supported by the extended deckboards.
The bottom end of the raker is cut to shape and tightly butted against a 100 x 50 x 300mm long stop, secured to each deckboard with two 12mm x 90mm galvanised screws in pre-drilled holes.
The handrail post and raker are joined at the top with a 12mm x 140mm galvanised bolt with two 50mm x 3mm galvanised washers.
Toeboards
A 75mm x 50mm toeboard can be used instead of, or as well as, a handrail. It acts as a safety feature to delineate the edge, and discourages vandalism to the deckboards. Using no.12 x 90mm sherardised screws, attach the toeboard to alternate deckboards, so that its outer edge is directly above the outer edge of the main beam.
