The Throw-away Bridge achievement entails building a bridge that successfully delivers both vehicles to the goal while still having at least one part break. Obviously there are any number of ways you could go about doing this, but this is the first (and simplest) one I encountered during normal play.

To get the achievement, you need to go the first bridge in the Eastern Mainland area. The design is as follows:
Make sure to choose the truck option when you test the bridge. The crossbeam on the right will snap when the trucks are mostly over the bridge, but the road stays stable long enough for both trucks to pass over it. Do it right and you should be able to complete it with 4,700 points:
And there you go, a shiny new achievement under your belt and a broken bridge!