Thanks for pointing out Bridge Builder. It's a great game. I don't have the egineering background you have, so it's quite funny to see how I finish the levels. I managed to get past 10, but my bridge still collapsed. If you get the train far enough across the bridge before it collapses, the front part of the train can pull the last car off the bridge and complete. I think they should probably include a spec that the bridge must be intact to complete the level. The budget on 10 is just too darn low.
I was impressed with my abilities on level 7 and 8. I only spent 75% and ~62% of my budget respectively. Now on to playing level 11.
Finally got past level 15 after reading the hints and then realizing, counter-weights, duh! After seeing the 1st and 2nd place bridges, it made me be able to build my level 10 bridge w/o having it break. Decided to send in my level 10 bridge that breaks, but works to the authors. I think they need to add in a new parameter, weight of the bar. Right now I think they only add a constant for each point as apposed to how much "material" is there. Oh well, at least I get to say I completed the game, and would give my right arm for a copy. (Can't give my left arm, as that's the arm I write with!)