Your weapon is a spear doing 100 dmg , 30% of it is piercing.  

Strength and %dmg modifiers are applied before the 70% - 30% split.  
Assuming you have 500 str and +100% dmg the game will get this ;  

100*(1+500/500)*(1+100%)+500*0.04 = 420 dmg  

THEN the 70% - 30% split comes ; 294 physical + 126 piercing dmg  
And finally dex and +%piercing damage modify the piercing part.  

As you can see , str and +%dmg increase physical and piercing dmg by an equal amount.  
Just a note ; flat +damage modifiers are added after the split.