Buildings can be placed on bridge pillar section. If pillar is destroyed, placed buildings explode too, however this does not apply for overlay buildings (walls, sandbags, crates). Pillar cell with placed overlay will remain intact while the rest of pillar is destroyed. Selling overlay and repairing bridge afterwards will raise that intact cell by additional 4 levels.
Conditions:
destroying bridge pillar with already placed overlay
Steps To Reproduce:
build overlay on bridge pillar
destroy pillar
sell overlay
repair bridge
optional - repeat step 1
Expected Behaviour:
destroyed bridge pillar should also remove all placed overlays on top
Additional Files:
this is older gif footage from modded game, but result is same in vanilla
TuTuc0
commented
3 years ago 
Description:
Buildings can be placed on bridge pillar section. If pillar is destroyed, placed buildings explode too, however this does not apply for overlay buildings (walls, sandbags, crates). Pillar cell with placed overlay will remain intact while the rest of pillar is destroyed. Selling overlay and repairing bridge afterwards will raise that intact cell by additional 4 levels.
Conditions:
destroying bridge pillar with already placed overlay
Steps To Reproduce:
Expected Behaviour:
destroyed bridge pillar should also remove all placed overlays on top
Additional Files:
this is older gif footage from modded game, but result is same in vanilla
Gif captured by Tuco