Why are allowable stresses in the WRC calculation under AD 2000 so low?

AD 2000 refers to the WRC bulletins regarding the calculation of local loads on nozzles (Merkblatt S 3/6).

In the procedure, it is indicated to calculate a utilization factor of the nozzle by relating its design pressure with the maximum allowable pressure.

This coefficient is then used to reduce the allowable values ​​used in the WRC context. There is a note with the reference to the norm regarding this operation.

Basically, the standard tells us that the material used to resist pressure must not be considered again in terms of resistance to local loads: to increase the allorable value it is necessary to intervene on the pressure calculation of the opening, thus increasing the maximum allowable pressure to reduce the coefficient that leads to lower material allowables.