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.