[Brit (OP)]

My 13 year old son has to design a weight efficient bridge.
Where can he find out the weight of a bridge, and then work out the load it can bear?
Can anyone help or point us in the right direction please?
Many thanks.

[dentstudent]

Not sure, but I did do a summer school with the Open University, and one of the tasks was to build a bridge (perhaps 12" across, or 30 cms) out of wire which was then tested to destruction for load bearing. My design won [], and it was an arch with triangles structure. I'll see if I can find a picture of something similar...

So, a bit like this

http://www.go-explore-trans.org/images/2009/jan-feb/tynset_glulam_bridge.JPG

but if I recall, it was like half a bike wheel, with the traffic crossing along the bottom

[diagram=457_0]

No doubt there are better engineered structures, but this was quick, simple and very effective.

[Karsten]

My students are building model bridges in my classroom for 8 years now. I hope your son's teacher has talked about construction methods, bridge designs, and the pros and cons of certain methods and materials.

For a beam bridge it will be important to use triangles in the sides. It will also be important to make sure the road bed is made from sticks that reach "the other side". There is no point designing something that results in the collapse of several elements when only one breaks under the load. You need to avoid chain reactions. Next to construction methods (and maybe even more important), you need to consider workmanship. sloppily built bridges hold less no matter how well it is thought out. Tight glue joints are very important.

It is difficult to say more without knowing the exact rules/limitations of the project.

You can get a fine (and free) bridge designing software at http://bridgecontest.usma.edu/.

I would hate to think though that the exercise your son goes through is hands-off only. I cannot imagine good engineering of structures if the engineers never build anything.