Fatigue in welds

[bljnv]

I am referring to BS 7608-1993 standard.
In section 3.1, it says the fatigue analysis procedure for welded structures is based on the assumption that only ranges of cyclic stress are considered for evaluating fatigue life i.e. mean stresses are neglected.
But for unwelded parts the mean stress correction is carried out while evaluating fatigue life.
I request the members to explain the why only range is considered for welded structures.
Thanks in advance.

 

[weldstan]

Would not range of loads in tension/tension or tension/compression be more critical in the evaluation, noting that welds have residual stresses, geometric changes, probable flaws and metallurgical differences, all of which are all conducive to lower fatigue life expectancy?