The robust physics deployed in Bridge Construction Set let you build a unlimited variety of bridges. If they plummet into the river you know you need to go back to the drawing board. When test vehicles pass over your bridge and make it safely across you know you've succeeded.
In Bridge Construction Set you design and build bridges and then stress test them to see how your creations hold up under pressure. Bridge Construction Set is about building a bridge that doesn't break, although watching your bridge creation break and plunge a train into the watery depths below can be half the fun.