Why does seismic detailing for reinforced concrete structures specifically require closely spaced transverse reinforcement (ties or spirals) in column plastic hinge regions?

Seismic detailing for reinforced concrete structures specifically requires closely spaced transverse reinforcement, such as ties or spirals, in column plastic hinge regions primarily for two critical reasons: to provide confinement to the concrete core and to prevent the buckling of longitudinal reinforcement, both of which are essential for achieving ductile behavior under severe earthquake loading. First, these closely spaced ties or spirals provide confinement to the concrete core. The concrete core refers to the concrete section enclosed by the transverse reinforcement. Under axial compression and bending moments, as experienced during an earthquake, unconfined concrete will eventually crush and spall off its outer cover, leading to a rapid loss of strength and stiffness. However, the closely spaced transverse reinforcement laterally restrains the concrete core, putting it under a state of triaxial compression. This confinement significantly enhances the concrete....