When you first launch Bridge Constructor you're briefly told how the game works, and what to look for when building a bridge.
Each island on the map has its own respective set of spans you'll need to build a bridge across. You have to solve every level before you can begin driving tankers across the bridges you've built.
A good bridge is completely green, like the one pictured here. You should have no issues driving the cars and trucks over it.
A horrible bridge, like this one, will fail almost immediately after you preview it. Notice nothing is driving over the bridge.
In-app purchases are capable of providing you with partial or complete blueprints for each level. Just keep an eye on how many coins you've earned, or you may end up paying out of pocket with real money.
The budget is one of the best ideas to the game, forcing you to use limited materials. But if you're not fond of it, you can get rid of it with an in-app purchase.