Step-by-step explanation:
1. Age of existing bridge: if bridge is too old and has crossed expected life span, then it is better to build new bridge.
2.Weight of traffic moving over bridge: If weight of traffic moving over bridge is more than the designed weight then new bridge has to be built.
3.Temperature: Temperature play a key role in developing thermal stress. If bridge is designed for withstanding certain temperature and during coarse of time the temperature has crossed the expected temperature, then new bridge has to be built.