Bridge Concept Design for Mixia Village Objective: Construct a bridge (for villagers and livestocks crossing) over river Mixia. Limitations: • No assistance by machinery; • Construction by non-skillful labors; • Construction period 5 -7 days only; • Transportation of massive materials is difficult; • Mostly locally available materials: sands, stones, timbers & bamboos; and • Free from maintenance or villagers can manage to carry out long term maintenance works. Design considerations: Bridge geometry & foundation • Reconstruction of bridge at the same location may involve demolition of the existing bridge. During the construction period, villagers cannot cross the river; • The width of river at the current bridge is about 20m. Based on the given plan, the river width is about 16m at about 12m downstream. The proposed bridge shall be built at where as narrow as possible along the river bank. • Long span with large mid span bending moment is the major challenge of the proposed bridge. The attached bridge geometry makes use of a short mid span which minimizes the mid span bending moment. This part can either be made by galvanized steel tubes or even timbers/ bamboos as villagers are able to replace it when necessary; • To reduce span bending moment, the existing large boulder being used as central pier can be considered. However, according to the villagers, high water flow may even strong enough to push large boulders forward, thus the boulder may not be a reliable foundation.
Cementsand-large stones gabion as end supports
End supports can be countered balanced by making use the weight of the mixture of cements, sand and large stones by gabion with limited excavation works; and The foundation of piers can be constructed by mixing cement, sand and stones at a certain depth into river bed to prevent from being flowed away.
Choose of building materials • Structural elements must be long lasting, timbers and bamboos shall not be considered; • Steel scaffolding is commonly used as temporary works in Hong Kong. In fact, if it is properly maintained, it can last long comparing with timber and bamboos; • Galvanized steel tubes are chosen as the major structural components as they are relatively light weight and easily transported comparing with heavy duty steels. It reduces the risk in handling heavy materials. They also resist corrosion from river water and can be cut to desired length even on site to suit actual conditions; • Galvanized steel tubes shall be connected by swivel couplers as this system is simple to construct and easily handle even by non-skill labors within limited construction period;
The number of structural elements can be added up if finds necessary; and Timbers and bamboos are used as non-structural components like floor panels and hand railings as villagers are able to replace when necessary.
Cement-sandlarge stones gabion as end supports
Bamboo/timber floor (can be replaced when necessary)
Mid span can either be made by galvanized steels or bamboo (villagers can replaced when necessary by themselves) Bamboo hand railing
Galvanized steel tubes as main structural elements & connected by swivel couplers
Max water level
Cement -sand-stones foundation
Min water level
Bridge Concept Design for Mixia Village