What specific type of structural deficiency is a 'soft story,' and why is it a major seismic concern?

A 'soft story' is a specific type of structural deficiency characterized by a story in a building that has significantly less stiffness than the stories above or below it. This often occurs in the first story of a building due to large open spaces, such as parking garages or retail spaces, where fewer walls or columns are present to resist lateral forces. The lack of stiffness in the soft story causes it to undergo a disproportionately large amount of deformation during an earthquake. This concentration of deformation can lead to excessive stress on the columns and connections in that story, potentially exceeding their capacity and causing them to fail. Because the soft story is the weakest link, it becomes the primary location of energy dissipation. The upper stories, being stiffer, tend to move as a single unit, transferring all the lateral force demand to the soft story. This can lead to a sidesway mechanism, where the building collapses due to the failure of the columns in the soft story. Therefore, a soft story is a major seismic concern because it concentrates deformation and forces in one location, potentially leading to instability and collapse of the entire structure.