Optimization is usually carried out for the most unfavorable combination of loads and leads in a result, that has to correspond economically as well as statically the requirements for each case. The design load case is defined as the combination of unfavorable performing loads with a specific occurrence probability.

In the existing standards, different partial safety coefficients are assigned to the different loads in order to take risks into account which are not estimable. However, it becomes clear that identical safety coefficients not simultaneously mean an identical safety may separate that deviations from the resistance as well as on the loads, only be considered as a total.

In this way, the partial safety coefficients only consider indirectly possible material damages or mistakes in the construction process as well as earthquakes or other unforeseen load actions and stand in direct connection with the dispersion of the resistance and the actions.

Probability can be increased by more severe requirements on design, for example greater partial safety coefficients that a building would survive a specific period without damage. This probability, called survival probability in the following, is an objective measure of reliability of structures.

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