on it at the time. Earthquakes Earthquakes damage all structures, including bridges. Luckily, this kind of collapse is relatively infrequent, especially with modern bridges. Engineers have learned to design bridges in earthquake zones on areas that are much more resistant to movement. By defect Some bridge collapses are mysteries, and engineers only realise why after they conduct a complete investigation. In some cases, this could happen because inferior-quality material was used in the construction, or because of a defect in a key piece of the bridge. In other cases, the bridge was designed only to support a certain amount of weight and no more. Boat or train crash Both of these kinds of accidents are extremely rare, but boats and trains can cause a bridge to collapse for different reasons. With trains, it's the velocity of the impact that can bring a bridge down. With boats, it's the very large mass they have that can bring about the collapse, even if they are moving very slowly when it occurs. The best way to avoid bridge failures is to plan for them. Modern technologies that can detect structural weakness, safer working environments and better designs can all help to reduce these terrible accidents.