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.
Generally speaking, the structures with relatively longer expected life spans are "not necessarily" more expensive than those with shorter expected life spans. Think of timber structures that in many cases/locations are more expensive than reinforced concrete ones while having shorter lives compared to them.
The return period for design events has a very loose relation with the expected life of the structure. Nuclear power plants are designed for seimic events with return periods of at least 10,000 years, while their expected life is at least two orders of magnitude shorter.
A longer design return period increases the upfront costs (design and construction) but reduces the long-term ones (seismically caused damage). Hence, with a suitable assumption on interest rates you could combine the two and find an optimum. In practice there other intervening factors, such as public sensitivity to the facility, and the final result is implicitly embedded in the design codes and licensing procedures.
Dear Aryan Shahabian ... Thanks for your answer...
Because of the expansive of maintenance... Engineer looks for longest life for their projects... also , there are some special structures like bridge, dam, ... water treatment plant, .. . etc.. and more need long life...
Agree with Joaquin Marti ... good answer ... Thanks so much..
You mention Nuclear power plants as an example... what about other structure type, ... like small house, .. administration building.... etc..
For seismic motions most building codes use return periods of 475 or 500 years (i.e. 10% probability over 50 years) for any permanent structure, whatever its expected life. Recently some codes have started using 2475 years, but with reduction coefficients that lead to a similar final result.
Of course the requirements are more stringent for structures that pose special risks (nuclear facilities, storage tanks for liquefied natural gas, etc.), carry out particular functions (schools, hospitals, etc.), have greater significance (historical heritage, etc.), and many other special cases.
You will also find that, for many facilities, more than one return period is used, each with a different set of associated requirementes (e.g. OBE and SSE).
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