Safety Factors – IEC 61400-1 ed. 4:background document

The aim of this document is to describe the basis for the partial safety factors recommended in IEC61400-1 ed. 4.The scope is to• give the basis for selecting materials partial safety factors in ‘recognized design codes’ –taking into account inspections (e.g. in relation to fatigue)• give the basis for design assisted by testing – determination of characteristic values for material parameters and load bearing capacities on the basis of test results• give the basis for modifying load partial safety factors if compared to the ‘normal’ situation better / additional information (less uncertainty) is available for estimating loads; e.g. modification of safety factors depending on specific site conditions.Section 2 briefly describes the theoretical basis for calibration of partial safety factors using reliability based methods. The required reliability level is discussed in section 3. In section 4 three basic models for calculating the design value of the load bearing capacity is presented. Next, reliability-based calibration of material partial safety factors is described in the following cases: DLC 1.1 and 6.1 with extreme load; fatigue of welded steel details; DLC 2.1 and 2.2 with extreme load and faults; component / consequence class partial safety factor ϒc . Finally, also the load partial safety factor in typhoon conditions is considered. Section 5 how the uncertainty level influences the partial safety factors.Finally annexes are presented on Reliability and partial safety factors for tower buckling; Reliability of concrete structures for wind turbines; Safety factors for fatigue of welded details in steel structures for wind turbines; and an overview of the main changes in material partial factors in the CD IEC 61400-1: 2014 ed. 4 compared to IEC 61400-1: 2005 ed. 3