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### Design base period, design service life, return period - do you distinguish clearly?

2023-04-28
Design base period, design service life, and return period are three-time concepts often encountered by structural engineers. Although the Unified Standard for Reliability Design of Engineering Structures
"Standards" (referred to as "Standards") Chapter 2 "Terms" lists the definitions of the design reference period and the design service life, but what is the difference between them, it is estimated that many people are still a little confused.

1. Return period
Before we get into the discussion, let's review the "return period." In our previous article, once in 50 years = once in 50 years? ——As mentioned in the fourth common sense of wind speed that structural engineers should know, the return period of a load refers to "the average time interval between the occurrence or occurrence of an event", and the return period measured in "years" and the annual exceedance of the load Probability is inversely proportional. For example, for wind loads with a return period of 50 years, the annual exceedance probability is 2%; for wind loads with a return period of 100 years, the annual exceedance probability is 1%.

For the wind load whose annual exceeding probability is p, the probability of not exceeding the wind speed in a certain year is 1-p, and the probability of not exceeding the wind speed in N years is (1-p) to the Nth power. Therefore, the exceeding probability of the wind speed in N years can be calculated by the following formula:

According to this formula: for the wind load in the 50-year return period, the annual exceeding probability is p=2%, and the exceeding probability within 50 years is:

The 100-year Transcendence Probability increases to:

And the probability of surpassing in 200 years will reach:

2. Design base period
From the above example, we can find that for variable loads, it is meaningless to only mention the exceeding probability without mentioning the corresponding time length. After all, people will die in the long run, the probability of exceeding variable loads will be close to 100%, and buildings will collapse (unless they are demolished before they collapse). Therefore, to unify the measurement standard, it is necessary to specify a unified time scale as the time parameter for variable load values. This time scale is the "design reference period".

Article 3.1.3 of "Code for Loading of Building Structures" stipulates that a "50-year design reference period shall be adopted when determining the representative value of variable loads." This is a mandatory provision. The reason why it is mandatory is that "there is no rule, there is no square circle", without setting a time basis, it is meaningless to discuss the probability of exceeding the load and the reliability index (probability of failure) of the structure.