If you notice boards or supports turning red, they will fail very soon. Use Construction mode to fix red sections before they collapse. Locate weak areas by allowing your soldiers to cross the bridge. Reinforce any areas that appear to be susceptible.
Boulders
Boulders will target specific areas of a bridge. Remember their trajectories, then strengthen the target areas. Build a roof to project your soldiers and horses from boulders. Make sure the roofs have good support, with triangular structures.
Easy "Monumental Construction" achievement
Try to get as many smaller pieces as possible while maintaining the stability of the bridge. It will not matter how much gold you spend, as long as you remain under the maximum. For example, consider doing this in Chapter 2, Bridge 4.
Increased stability
Create triangular structures for increased stability. When making bases, create arc-shapes from triangles. They will provide extra support on long bridges. The middle area of a bridge is usually the weakest. Use stronger materials (logs and stone supports) to supplement middle sections.
Saving money
Use a large amount regular wood instead of stone to save money. Use triangles within triangles construction for best results.
Avoiding collapses...