Question

WHY BRIDGES ARE DECLARED UNSAFE AFTER LONG USE

Answer 1

It is best explained by wear and tear concept. The more a thing is used the more it is prone to degradation. Same goes for bridges , after being used for a long time they undergo a continuous cycle of stress and strain. the materials used for their construction loose their elasticity and get elastically fatigued. And hence they it may collapse and therefore is declared to be unsafe after long use.

Answer 2

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A bridge during its use undergoes alternating stresses and strains for a large number of times each day, depending upon the movement of vehicles on it. When a bridge is used for long time, it loses its elastic strength. Therefore, the amount of strain in the bridge for a given stress will become large and ultimately, the bridge may collapse. That is why the bridges are declared unsafe after long