we had to do one of these in like grade 5 for a school board challenge...
the bridge had a span of 30 cm...it was actually quite strong...we used lots of lasagna and made sort of a trussle bridge...in the end it could hold something like 10 litres of water, how ever much that weighs
i dunno if thats of any help but if youve got any questions ill try and remember from like 5 years ago