Every floor is a row of nodes, each carrying a share of the floor's mass (weight = m·g). That weight travels down through the column segment directly beneath it toward the ground. Each column segment and each connecting beam has a load capacity (proportional to the material-strength slider, and slightly higher for lower floors, as in a real design).
Knock out a ground support and the weight that used to flow straight down through it has nowhere to go — it is redistributed sideways through the beams to the neighbouring columns at that floor, exactly as the alternate-load-path method used in real progressive-collapse analysis works. If a beam or column can't carry the extra load, it fails too, and the load it was carrying redistributes again. This can cascade — a genuine progressive collapse — or the structure can arrest the load within a floor or two and stay standing.
- Line colour — green (low load) through yellow to red (near/over capacity) for every surviving beam and column.
- Debris — any node that loses its full support path to the ground (checked by flood-fill connectivity every step) breaks free and falls under gravity.
- Material strength — scales every capacity at once; low values turn a single knocked-out support into a full progressive collapse, high values let the structure arrest the damage after one or two floors.