in the years already spent
实际已使用年限 practical serviced life
由于绝大部分结构在承受地震作用时都已使用一定年限，其耐久性必然随着使用年限的增加发生损伤。
As the majority of structures to withstand earthquakes have been used a certain number of years, its durability inevitably increase as the useful life of damage occur.
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