Question

Why the bridges are declared unsafe after long use?

Answer 1

Bridges are declared unsafe after long use due to following reason -

After it's long use, the bridge continuously undergoes through alternating strains. As a result, the elastic strength of the bridge gets reduced. As this process goes on, the bridge develops elastic fatigue and a permanent change occurs in it's structure. This permanent change leads to it's collapse. In order to avoid the collapse of the bridge, they are declared unsafe and are put out of use.

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Answer 2

Due to the action of sunlight and rainwater and longtime exposure to them leads to their corrosion which may cause the bridge to become weak and one day it might fall. So they are declared unsafe.