Question

why are bridges considered unsafe after long use

Answer 1

Because bridges undergoes through alternate strains.

bridges undergoes through alternate stress.

bridges lose their elastic property.

because of sunlight, rain water and longtime exposure lead bridges to go unsafe because this leads to corrosion.

these are the reason of collapse and to avoid this we need to use bridge rarely because bridges are unsafe after long use.

Answer 2

 If the bridge is used for a long time, due to alternate cycles of stress and strain the bridge gradually loses its elastic property and finally reaches to a condition called elastic fatigue. Hence, at this stage, the strain produced for a given stress will be very large and ultimately the bridge may collapse which may be very dangerous.