Columns are primarily vertical structural members that support axial compressive loads from floors, beams, and roof systems, transferring them to the foundations. While primarily compression members, columns in building frames are almost always subjected to significant bending moments due to unbalanced floor loads, eccentric connections, or lateral forces (wind and earthquakes).

Columns are classified based on the type of transverse reinforcement used to confine the concrete core and prevent the longitudinal bars from buckling.

For a perfectly straight column loaded exactly at its geometric centroid with zero bending moment, the theoretical maximum nominal axial capacity ($P_o$) is the sum of the capacities of the concrete and the longitudinal steel ($A_{st}$).

Where $A_g$ is the gross area of the concrete cross-section.

However, a condition of "pure compression" is practically impossible due to construction tolerances (plumbness) and material heterogeneity. Therefore, the code caps the maximum design axial load ($P_{n,max}$) to account for accidental eccentricity.

Because columns must resist both axial load ($P_n$) and bending moment ($M_n$), their strength is defined by an interaction envelope. The Interaction Diagram plots the combinations of $P_n$ (y-axis) and $M_n$ (x-axis) that cause failure of the cross-section.

The code specifies strict limits on the amount and arrangement of longitudinal reinforcement ($A_{st}$) relative to the gross concrete area ($A_g$). The reinforcement ratio is defined as $\rho_g = A_{st} / A_g$.

Transverse reinforcement is critical to prevent the highly stressed longitudinal bars from buckling outward and spalling the concrete cover. It also provides shear resistance.

For a column to be classified and designed as a spiral column, the continuous helical reinforcement must meet strict volumetric requirements. The goal is that if the outer concrete cover spalls off, the increased strength of the confined core due to the spiral will more than compensate for the lost cover.

Composite columns combine a structural steel shape (like an I-beam or hollow tube) with reinforced concrete.

Corner columns in buildings often receive moments from beams framing into them from two orthogonal directions. This creates a state of biaxial bending, where the neutral axis is skewed across the section.

The exact analysis of a biaxially loaded column is complex, requiring the generation of a 3D interaction surface. A common simplified approach is the Bresler Reciprocal Load Equation:

Where:

This approximation is generally valid when $P_n \geq 0.10 P_o$. For smaller axial loads (where failure is tension-controlled and behavior is closer to pure biaxial bending), the PCA Load Contour Method is often preferred. This method defines a non-dimensional interaction surface at a constant axial load $P_n$ using the equation:

Where $M_{nox}$ and $M_{noy}$ are the uniaxial moment capacities, and $\alpha$ is a contour parameter that depends on column shape and reinforcement distribution (typically ranging from 1.15 to 1.5 for rectangular columns).

A column is classified as short if its strength is governed entirely by the capacity of its cross-section ($P_n, M_n$). It is classified as long (slender) if lateral deflections ($\Delta$) along its height become significant enough to induce secondary bending moments ($M_{secondary} = P \times \Delta$). This phenomenon is called the P-Delta effect.

The total moment ($M_c$) used for design must include the primary moment ($M_u$) from analysis plus the magnified secondary moment. The classification depends on the slenderness ratio $kL_u/r$.

The degree to which a frame can move laterally drastically impacts slenderness.