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Design life means the minimum duration a structure is expected to last. The longer is the design life; the higher is the cost of a project. Therefore, in choosing the design life for a structure, engineers should consider the design life which generates a economical project without sacrificing the required function.
In selection of return period of certain design conditions, winds, waves, etc., one should consider the consequences of exceedance. In fact, there are normally no extreme maximum values of these design conditions and its selection is based on the probability of exceedance which is related to return period.
Therefore, design life may not be equal to return period of design conditions because their selections are based on different considerations.
This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.