Which type of force is a column under a heavy load primarily experiencing?

A column supporting a load is under COMPRESSION — the force squeezes it from both ends (the load pressing down from above, the foundation pressing up from below). Different structural members experience different primary forces: (1) COMPRESSION — squeezing; columns, pillars, struts, the lower side of beams under load, foundations; (2) TENSION — pulling apart; cables, ropes, suspension bridge cables, the upper side of beams under load, tendons; (3) SHEAR — parallel forces causing sliding; bolts in joints, beams under perpendicular load (vertical shear), scissoring action; (4) BENDING — combination of tension on one side and compression on the other; beams under load (top compressed, bottom in tension); (5) TORSION — twisting force; drive shafts, screws being tightened, hurricanes on towers. Materials chosen for their force characteristics: STEEL — strong in both tension and compression (used widely); CONCRETE — strong in compression, weak in tension (must be reinforced with steel rebar to handle tension); STONE — strong in compression, weak in tension (used in arches that distribute load as compression); WOOD — moderate tension and compression; CABLE — strong in tension only. Reinforced concrete combines concrete (compression strength) with steel rebar (tension strength) — versatile for beams that experience both. Pre-stressed concrete pre-tensions the steel before pouring concrete, increasing tension resistance. Buildings, bridges, and other structures are designed to identify which members experience which forces and choose materials/dimensions accordingly. Columns can fail from EXCESSIVE COMPRESSION (crushing) or BUCKLING (sudden lateral collapse before reaching compressive strength) — long thin columns are more susceptible to buckling.