Temporary structures should not be designed with the characteristic values of wind velocity or velocity pressure from the Eurocode 1.
Indeed, these characteristic values are determined considering a return period of 50 years, which does not suit the provisional nature of such temporary constructions. Therefore, temporary structures will often be designed with a smaller return period than the usual 50 years.
Recommended return periods (n) are given in EN1991-1-6:
This return period value is then used to compute the probability factor (cprob) as per formula (4.2) from EN1991-1-4:
- K is the shape parameter defined in the national appendix. The recommended value is K = 0,2
- p = 1/n (with n being the return period, in years)
- n is the exponent defined in the national appendix. The recommended value is n = 0,5
Be careful, in the cprob formula, n (the exponent) should not be mistaken with n (return period).
Of course, considering a 50-year return period in the above formula leads to cprob = 1,0.
Therefore, do not expect any benefit from the cprob factor for structures in use for more than a year:
Wind velocity for a return period of n years is then given by:2
Assume the case of a temporary structure in use from September to November (3 months) in a region where where characteristic wind velocity (for a return period of 50 years) is 24 m/s.
This temporary structure will be designed for a return period of 5 years:
The corresponding probability factor will be:
Resulting I the following wind velocity:
This results in a 15% reduction on wind velocity.
Advance Design results
Advance Design lets the user set the desired return period as well as the shape parameter and the probability exponent.
The corresponding wind velocity is then automatically computed:
Considering the appropriate return period when designing a temporary structure can lead to a significant reduction of wind velocity, thus scaling down the corresponding wind forces.
Not to mention the other parameters, such as the season factor cseason, that can diminish wind action even more.