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Design  a  Footbridge

This page has been made as a starting point for school projects.  Possible projects could be -

A   Choose the bridges that you think best for the given situation.  Give your reasons, and explain what materials you could use, how you would fix them, and how you would ensure reliability.  And, very important, explain how you would assemble the bridge, including any falsework you would need.

B   Similar to A, but choose the least suitable designs, and explain the reasons for your choice.

C   Choose a few bridges, and explain how the forces go in each part, and how they are taken into the ground.

D   Think of some designs that are not included here, and explain how they work.

Here is the actual situation that you have to consider.  A visitor to this web-site had to build a footbridge across a creek.  His budget was very small, and only two other people were available to help with the construction.  The first diagram below shows an approximate cross section.

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The diagrams below show some of the possible solutions to the problems of building a bridge.  The inclusion of a design does not imply that it is suitable, and in fact some are definitely poor designs for this project.  The diagrams do not include any foundations that might be necessary.  The dimensions and shapes are purely schematic, and methods of joining are not shown.  

Look at these designs and try to work out the types of force that might occur in the parts.  As a clue, compression members (struts) are in red, tension members (ties) are in blue, while beams (which suffer both compression and tension) are in grey.  Consider also the types of materials that might be usable for the various parts, and the possible ways of joining them.  Look at the diagrams also in the light of different conditions that might apply.  Some are listed below.

Ground - could be soil, clay, soft rock, or hard rock.

Load - Could be just one person (the owner of the land), or could be up to ten people walking or thirty people standing.

Water - could be a slow, reliable stream, or liable to rising up to the top of the banks and flowing at high speed.

Appearance - May not matter, or may be important because of the location.  Don't forget that in a small bridge, the handrails can affect the appearance quite strongly.  They can even have structural function.

Beam bridges

Cantilever Bridges

Cable Stayed Bridges

Note that parts of decks are in compression.

Suspension Bridges

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Truss Bridges

and more complicated trusses