HOW TO BUILD A COLOBRIDGE
Some tips on creating your own cross-traffic bridges.
... back to the [main colobridge page].
Some Background
Wakuluzu: Friends of the Colobus Trust, Diani Beach, Kenya has produced these guidelines. After a successful campaign spanning two years of publicity, reserach, speed monitoring and colobridge construction, the Colobus Trust has decreased road mortality of the Angolan black-and-white colobus (Colobus angolensis palliatus) in Diani from 23 roadkills in 1996 (Sept-Dec) to 2 roadkills in 1997 and 8 roadkills in 1998.
Nine bridges have been constructed to date and four more are planned for 1999 as ongoing habitat degradation to disrupt more troops.* ALL BRIDGES ARE NOW BEING USED BY THE COLOBUS. You may find that other (more inquisitive) species of primate use the bridge first. In Diani, we find that Sykes and vervet monkeys are the first to set foot on the structures while Colobus require one-possibly-two months to cross. This may vary dependent upon colobine species. Other factors to consider which will also influence crossings are length of bridge, habitat type surrounding the bridge (i.e. Area of open exposure), and human activity in the area.
There now follows a step-by-step guide to constructing a colobridge.
* Since the production of this guide in 1998, the Trust has created a total of 19 colobridges, with two new ones planned for this summer, making the total 21.
Preliminary Research
[1.] Weekly research should be undertaken to verify sites of colobus crossings. This should combine data on the exact locations of previous roadkills as well as previous/up to date sightings of troops crossings. Notes should include group compositions (i.e. number of individuals, males, females, etc.) as well as direction crossed (west to east, east to west etc.), time of sightings and location.
[2.] If speed bumps are an option, areas on consideration should be ones where speeding traffic is the most prevalent AND where the colobus cross on a regular basis.
[3.] Contact Ministry of Public Works and Housing (or respective government agency) for permission to build bridges, as well as investigate legal limits you will have to place the poles from the roadside. Minimum height of the bridge in Diani is 7 metres and 5 metres from the road tarmac itself.
[4.] Local publicity, community involvement should be paramount throughout this phase to ensure local support, interest and participation during the following stages.
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 holes 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.
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:
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).
n.b. 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)
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!
MAINTENANCE (ADDITION FROM 2003)
A final note on bridge maintenance, as we consider this part of our core programmes here at the Trust.
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 the Trust team. 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.
