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's compressive strength and, more importantly, its ductility. Ductility is the ability of a material or structure to undergo significant inelastic deformation without substantial loss of strength. In plastic hinge regions, which are deliberately designed to yield and undergo large deformations to dissipate seismic energy, this enhanced concrete ductility prevents brittle crushing and allows the column to maintain its load-carrying capacity even after cracking and yielding. This energy dissipation through ductile behavior is fundamental to a structure's ability to resist collapse during a major earthquake.

Second, the closely spaced transverse reinforcement is crucial for restraining the buckling of longitudinal reinforcement. Longitudinal reinforcement refers to the main steel bars running vertically within the column that resist axial forces and bending moments. As the concrete cover spalls away due to large deformations and the concrete core starts to deform, the longitudinal bars, now unbraced over a length, become susceptible to buckling under compression. Buckling is a sudden lateral instability that causes the bar to lose its compressive strength rapidly. Closely spaced ties or spirals act as lateral supports for these longitudinal bars, preventing them from buckling prematurely. This allows the longitudinal reinforcement to yield repeatedly in both tension and compression, fully developing its strength and contributing to the column's ability to undergo multiple cycles of large inelastic deformation and dissipate energy. Without this restraint, the longitudinal bars would buckle, leading to a sudden and brittle failure of the column, severely compromising the structure's integrity. Therefore, the close spacing ensures effective bracing over short lengths, maintaining the integrity and strength of the main reinforcement throughout the seismic event.

Together, the confinement of concrete and the prevention of longitudinal bar buckling, enabled by closely spaced transverse reinforcement in plastic hinge regions, ensure that reinforced concrete columns behave in a ductile manner, allowing the structure to deform significantly and dissipate seismic energy without catastrophic brittle failure, thereby enhancing life safety during severe earthquakes.