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How to Build a Colobridge

Some Background

After a successful campaign spanning two years of publicity, research, speed monitoring and colobridge construction, Colobus Conservation decreased road mortality of the colobus in Diani from 23 roadkills in 1996 (Sept-Dec) to 2 roadkills in 1997 and 8 roadkills in 1998. Interestingly, in more recent years there has been an increase in the number of road injuries and mortalities however this is likely a manifestation of the number of reports that we receive coupled with the increase in traffic as Diani becomes more developed.

In general, colobus use the bridges when there is a continuum of trees either side of the road whereas sykes monkeys will use the bridges whether there is forest or bush either side of the road.  Vervets being more terrestrial, use the bridges rarely and baboons do not use the bridges presumably due to their terrestrialism and large body size.

The following is a step-by-step guide to constructing a colobridge.

Preliminary Research

1. Weekly research should be undertaken to verify sites of animal crossings. This should combine data on the exact locations of previous roadkills as well as previous and up-to-date sightings of troop crossings. Notes should include group composition (i.e. number of individuals, males, females, etc.) as well as direction crossed (west to east, east to west etc.), time of crossings and location.

2. Contact Ministry of Public Works and Housing (or respective government agency) for permission to build bridges, as well as investigate legal distance limits for placing the poles from the roadside. Minimum height of the bridges in Diani is five metres from the road surface.

3. Local publicity should be paramount throughout the planning phase to ensure local interest and support. 

Please note that speed bumps should be preferred over a colobridge due to financial and time obligations to build and maintain bridges. 



Colobridge Drawing

Stage 1

1.T-bar and anchor welding -- Welders should prepare two T-bars (fig. 1) and two anchors (fig. 2). angle line and flat bar is used for this purpose. T-bars: six holes should be drilled on each T-bar, two at each end, to attach the bridge and anchor line and two to attach the T-bar to the Cassuarina pole. Two 1-inch holes should also be drilled for bolting structure to pole.

2. Cassuarina Pole Preparation -- Paint both poles in Penotex. Drill two 1-inch hols on top of each pole (fig. 1). These will be used to attach the T-bar supports for the bridge. The t-bars should be attached tot he Cassurina poles BEFORE placing them in the holes.

3. Groundworks -- Dig four holes, two for the poles 2 metres deep and 1 metre diameter (depending on the site) and two for the anchors 1 metre deep and 1 metre diameter. Place the pole support upright with wooden support and nails until concrete is poured into the holes and allowed to set. The anchor ballasts should also be set in concrete during this time.

Colobridge Figure 1

Colobridge Figure 2


Stage 2

First measure the length that the bridge should be. You will need two 'runners' made from 3/16 inch galvanised wire stretching across the road. A 30 metre bridge, for example, will require at least 60 metres of wire. For easy construction, wrap the galvanized wire around a tree (fig. 3). The rungs are threaded on the galvanized wire like beads on a necklace. The order is as follows:

  1. Thread two side runners of 30 cm PVC conduit pipe – one on each side.
  2. Thread on one rung of bridge (comprising of galvanized 1/8 inch) chain link threaded through either 30 cm (¾ inch) rubber hose or 45 cm hard pressure pipe. See below*)
  3. Follow this with another two (30 cm PVC conduit pipe) side runners and so on.
  4. The pattern of RUNGS on the bridge is as follows -

Colobridge Figure 3

The design should follow the same pattern of one hard pressure pipe (45 cm) with chain link threaded through it and fed along both runners. Then two softer (30 cm) rubber pipes with chain link of the same length are threaded through them and fed along both runners (fig.4).

Colobridge Figure 4

Note: The pressure pipe rungs will prevent the bridge from folding in on itself as primates cross, while the rubber pipe rungs prove a more suitable gripping surface for hands and feet.

To hold all runners in place and prevent materials from slipping off during transportation and erection, put two 3/16 inch galvanized rope grips on each end of the galvanized wire.

Stage 3

I. Bridge attachment -- Attach one side of the bridge to one T-bar using ¼ inch thimbles, ¼ inch D-shackles an d3/16 inch wire rope grips (fig. 5). 3/8 inch Turnbuckles need to be attached at one end of the bridge to allow for tensioning at a later date (fig.6)
Colobridge Figure 5

II. Anchoring -- Anchor attachment is also required for each Cassuarina pole and during bridge connection to T-bars, one should also attach the anchor lines (fig. 5).

Attach a ¼ inch galvanized wire on the same T-bar, descend down to the anchor with a 1 inch turnbuckle (fig.5).

III. Tensioning -- To achieve a well-tensioned bridge, first pull the galvanized runners tight manually, then follow this tensioning using a car winch before fixing it. The main 1" turnbuckles on each anchor ballast should be tightened fully. PLEASE NOTE - YOU MUST LEAVE THE BRIDGE 3/8"TURNBUCKLES LOOSE as this will allow you to tension the bridge in the future if this is necessary -- and it usually is!

Colobridge Figure 6


A final note on bridge maintenance, as we consider this part of our core program here at Colobus Conservation.

It is estimated that in its lifetime, a colobridge will be used 32,600 times. In order to ensure its services are used to the maximum, the bridge must be kept in good condition for as long as possible. All the bridges are continuously checked to ensure that they have not come apart and that the support poles have not rotted or been invaded by termites.

Weekly bridge maintenance work is undertaken by Team Colobus. They repair any damage, no matter how small in order to prevent rapid dilapidation of the bridge. If a runner is askew, they bring the bridge down, repair it on the spot, and hoist it back up. If the poles are old and rotting, then they have to take down the whole bridge and put in new poles. This process takes a few days and is almost like building the bridge from scratch.