Measurig model bridge stress

Hello! One of my teachers asked me to help come up with an Arduino based approach to measuring the load carrying abilities of model bridges being made by other classes. We looked into load cells (which are way too expensive), hacking preexisting scales (which we have no idea how to do), and various other weight sensors for the task, but nothing looks like it will work. This is the main problem: every year, at least one person builds a bridge with a 400lb+ capacity. Load cells that can handle that are way out of the budget. One idea we’ve been toying with is using a piston with a pressure sensor in it to approximate the weight (we’d probably do a few tests and find a best fit equation for it). How feasible is this? I figure that there must be some reason I’ve never heard of anyone using something like this. Thank you!
~Eric

Spar4k fun has a number of sensors that might be of use. Model bridge competitions have been around for a long time, so searching for previous competitions might produce successful load measuring setups. Also, how the bridges will be force loaded will be important (sometimes they are sand bag loaded along the span). Perhaps placing the bridge on a large a large scale would be a practical option.

https://www.sparkfun.com/categories/23?page=all