Load combination of permanent and variable loads

This paper concentrates on the combination of permanent and variable loads in the structural probability theory and its implementation in codes. In the current codes, the permanent and variable loads are sometimes combined independently, and sometimes they are combined dependently. We propose that, for the safest outcome in the standardized load estimation, the actual permanent and variable loads should be combined dependently without any load reduction. The load reduction arising from the independent combination leads to an unsafe design. For example, when the permanent and variable loads are both equal to 1, the combination load is 2 if the dependent combination is applied. However, the value predicted by the model for independent load combination is only ca 1.8. Although the load formation processes are independent, the dependent combination is applied since the load formation and the load combination are different processes. To support our view, we present arguments and examples based on probability theory, physics and statics and relate them with the current codes.

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