Bridge Design for Nkuv, Cameroon

Ashley Waples

Summary:

The Design for a bridge to be built in Nkuv, Cameroon is in response to a request by the Hope College Chapter of Engineers Without Borders. The problem presented by this organization, which is currently working to bring safe drinking water to Nkuv, was to provide a safer way to cross a river into the village than the log currently in place. The bridge needs to span the width of the river, forty feet, and be completely free spanning. It also needs to be designed with locally available products, within safety standards, and not to allow vehicles to cross the bridge.
In the process of designing the bridge, several concepts were considered. Primarily, different types of bridges were analyzed to decide upon the best design. These types of bridges included a standard truss bridge, a timber arch bridge, a modular bridge with steel and timber, a reinforced concrete bridge, a suspended bridge, and a land-filled bridge. The two most practical choices were the arch bridge and the truss bridge, but it was finally concluded that the truss bridge would be most efficient to design and build. This design allows for the length of beams available in the village and also is more within the skill level of the workers who will build the bridge.
The overall design of the bridge is a warren truss without vertical supports. This is a simple truss design that makes use of isosceles triangles. The bridge will be composed of three trusses each spanning the forty-foot width and have decking laid across them perpendicularly. The width of the river is designed to be four feet, which will not allow vehicles to cross. Simple handrails are attached to both sides of the bridge, which will lay on top of two concrete bases and piles on each end of the river. The concrete base supports will also be laid with steps up to the top of the truss bridge which is three feet high. The piles are necessary to add stability to the unknown soil composition on the banks of the river. These will have rolled steel spiraled into the piles to add stability. Each of the trusses have five horizontal members on top and bottom, eight feet in length and diagonal members five feet in length attached at the joints. This can be seen in the drawing on the next page. The material of the truss beams will be a soft pine sawn wood of the approximate dimension of 2x4s. The pin connections and each joint are still currently under design, but those will most likely also be made of timber and be connected with bolts.
Analysis on the bridge has concluded that it is indeed more then strong enough to support the desired loading. With the assumption of 100 psf loading to the bridge, analysis was run on each truss using the program Visual Analysis. With the assumption that the middle truss would carry twice the loading of the two outer trusses, the area supported by each pin was found and its load was calculated. This load was found to be 1600 lbf on each pin for a total of 8000 lbf on the middle truss and 16000 lbf on the entire bridge. Although this seems high for a footbridge, it allows for a large safety factor. With this type of loading, analysis showed a maximum deflection of -1.87 inches in the y-direction and -0.42 inches in the x-direction. It also showed a maximum tensile axial load of 14138.9 lbs in a bottom member and a maximum compressive load of 13061 lbs in a top member of the center truss. This is well within the modulus of elasticity of the soft pine used when the corresponding maximum stress is calculated. Therefore, even with the maximum assumed loading, the bridge is more than strong enough to support the need of the village.