In Fema national US&R response system - Structrual collapse technicican training manual
"There are also vertical loads generated in a structure by earthquake shaking, but as mentioned previously, these forces rarely overload the vertical load resisting system. Earthquake induced vertical forces have caused damage to heavy concrete structures with high dead load compared to design live load. These vertical forces also increase the chance of collapse in concrete frame buildings due to either increased or decreased compression forces in the columns. (Increased compression that overloads columns or decreased compression that reduces column bending strength.)"
I didn't understand the second part of last sentence in paranthesis, where it says "....or decreases compression that reduces column bending strength".... what does this mean and why?