As the assessment has to consider stress values within the welds, but only stress values within the joint plates are provided, the stresses of the elements need to be upscaled. For this a factor fw is used which describes the proportion of a weld cross section "a" to the sheet thickness "t", see the figure below.

The factor fw is calculated by:

The upscaling of membrane and bending stresses is done according to the following formulas: ,

In most weld joints this will be a conservative approach, since the force flow will not always go entirely through the weld.