Question: why do bridges collapse?

Zoe Lonsdale answered on 13 Mar 2017:

Many reasons. They may have deteriorated over time through weathering, ground movement, lack of maintenance. There may have been an error in design – this is unusual but it happens very occasionally and more usually in countries where the control of design is less stringent or there is none existent. Design errors could be from an external influence that was not considered adequately e.g ground water, sink holes, ground conditions. There could have been a lack of consideration for environmental impact e.g. an assumption on a maximum wind load that was then exceeded in a storm. There are many different reasons. Design is centered around assumptions in given conditions that are based on knowledge and experience, research, testing, statistics and data and then assumptions are applied and always with a factor of safety.

There could also have been errors made during construction. If a bridge has not been constructed in line with the drawings or any amendments needed have not been checked, then there is a risk of failure. Changes almost always have to be made but they must be checked and verified. There is always redundancy built into structures but its not infinite.