Right, the steel beams you see in car parks: These are 'Cellular' beams, which are a more modern version of 'Castellated' beams. These beams are 'I' shaped in section. The web section is what carries the shear, but shear flow is happy to travel in a diagonal direction in the steel left between those holes. They remove the circles of steel to save weight, but it doesn't add strength, its just removing material that isn't particularly stressed.
These steel beams act like trusses (trusses have diagonals members, which is essentially what those steel beams become once the holes are cut out).
This ONLY works for members who are in vertical bending (the car park floors load the beams vertically). If you take a member (such as your a, b or c pillar) and subject it to axial compression (if you roll over) with holes cut out, its buckling capacity will be massively reduced (it'll just crumple like tin foil). Buckling capacity relies directly on cross sectional area and bending strength about the weakest axis. Holes will reduce both these factors.
Even if you don't roll over, the a,b and c pillars are not I sections - they are more like box sections and are more subjected to bending action in lots of different directions (as part of the rigidity of the car), which means the pillars will be much much weaker with holes cut out.
The static stress isn't reduced by removing material. The forces in the member stays the same, it just finds a different path through the material. Stress on an a,b or c pillar from axial force will be enormously higher when material is removed as simply put there is less cross sectional area for the stress to be distributed over.
Jesus Christ, please don't cut holes in your pillars.
That, my friend, is understanding structural mechanics...
