Confinement Zone

A confinement zone in a column is a region where you require a smaller spacing of stirrups for higher ductility. Concrete is a very brittle material and it can easily split in tension. During earthquake the demand on reinforced concrete members increase than the capacity. We make this purposely to dissipate earthquake energy entering into the building in the form of hysteresis energy curves where a member goes through a number of cycles of reversible loading.

During earthquakes or lateral deformation cause because of any reason, column experience maximum moments at its ends. These end locations go under maximum non linearity in terms of stress and strains. Now our friend concrete cannot carry this much amount of strains in tension and so it requires a ventilator to breathe and carry on its work. That ventilator is the closely spaced ties. These ties in the region of high moment zone helps in the level of ductility we are looking for and it enhances the performance of concrete.

The requirements of confinement zone are very strict in codes as some serious failures have occurred in past.

As you can see, all the failures are in the end zones and so they care called the confinement zones in a concrete column.

Source: Quora and Google images.