Small Bridge Construction
Does anyone know if there is any guidance for small-span (2-10m) bridge construction (to take the load of pedestrians and motorbikes) ideally using local materials (i.e. timber and bamboo)? I am working on a project in Liberia, but any sub-sahran Africa guidance or general developmenet good practice would be useful.
An old pre(war) colonial engineer's year book should contain all that information in the form of strength tables , timber is default , bamboo that would be only in colonial manuals I guess. Or Wikipedia/Google of course. Triangles are always used in load bearing constructions, their stiffness is limited by the used material.
Kind regards, Marc
Hello Martin, I am not aware of a publication that gives the guidance you have requested. However I have fairly wide experience in bridge construction and will help if I can. Maximum applied vertical loading on a pedestrian/cycle bridge generally occurs when people gather on the bridge to observe something of interest. This can amount to 3 - 6kN/m2. Maximum horizontal loading on an edge barrier or parapet generally relates to people all leaning along one side, again to observe something of interest. On a very small span this edge feature can be supported independently of the spanning bridge. For a significant span over a watercourse the effect of floating debris (branches/trees) may dominate. Do you have any typical sizes of available components (e.g. diameter of bamboo poles)? Where many components are fixed together to form a spanning structure the method of joining them may be the determing factor. In the absence of proprietary fixing components a locally sourced tying system may be the most appropriate solution. Local expertise will identify what is available and what is most effective. I hope this is of some assistance. I am happy to respond to further and more specific queries. Regards, Giles Waley
Best and most useful two documents I have used are "FIELD ENGINEERING - An introduction to development work and construction in rural areas" compiled and edited by Peter Stern and others ... published by Intermediate Technology Publications (now called ITDG Publishing https://itdgpublishing.org.uk/ ), whole section on bridges and very practical. never ravel without that book. Another seminal work is Indian Practical Engineer's Hand Bool .... you will find in almost any developing country in the world to buy or borrow ... I have bought copies in Islamabad (Rp 255), and in Kabul. Has a section on foot bridges. Again would not travel without it, and frequently gift it t one of my local engineers at end of assignment.
As we are RedR forum lets not forget the obvious ... the RedR manual "Engineering in Emergencies" by Jan Davis and Bobby Lambert (available also on CD) which has a section on Small Emergency Timber bridges.
Much more comprehensive and detailed, including design charts, detail drawings, but very useful (though generally for vehicular bridges, it has a section on emergency bridges) is the UK Government Transport Research Laboratory document - Overseas Road Note 9 "A Design manual for Small Bridges", which was funded by DFID... possibly more than you need, but available free on the internet as a UK aid publication ... I always have a soft copy on my computer and it has been invaluable. [Any problems firstname.lastname@example.org]
Beware that most published strength values on bamboo are misleading. See our paper on this, which gives safe design values for most species: "Structural use of bamboo: Part 3: Design Values; The Structural Engineer, December 2016, Kaminski et al".
Be very cautious of durability as well, for both timber and bamboo. Very few species of timber are actually properly naturally durable against termites and rot, and no bamboo species are, so in nearly all cases you want to provide some protection against insects and rot. For rot, you simply need to keep it dry (i.e. elevated above ground, good drip details, good roof and overhang), and for insects it should be treated (e.g. with boron). See "Timber as a construction material in humanitarian operations" (www.humanitariantimber.org), and "Structural use of bamboo: Part 2: Durability and preservation; The Structural Engineer, October 2016, Kaminski et al".
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