Question

bridges are declared unsafe after long use why

Answer 1

Because they get weak due to water effect and wind

Answer 2

Bridges are declared unsafe after long use because:
1. The structure may become unstable due to the heavy weight it bears every day.
2. The bridges may become weak due to its exposure to weather.
3. If the bridge is supported by pillars, any damage to those will be a cause for the bridges to be closed.