Open-ground storey buildings are vulnerable in earthquakes for three reasons: 1) The ground storey is relatively flexible and weak compared to the stiffer upper stories, acting like a "soft story" that undergoes much larger displacements. 2) This causes the building to swing like an inverted pendulum during shaking, severely stressing the ground story columns. 3) If the columns are not strong or ductile enough, they can be damaged or cause collapse, as seen in past earthquakes where many such buildings failed. Strengthening existing buildings and providing wall continuity in new designs can reduce this vulnerability.