18.3 Concrete Columns and Bearing Walls
Key Takeaways
- Concrete column design is an axial-flexural interaction problem even when loading is described as concentric; code axial limits, construction eccentricity, and frame moments remain relevant.
- Longitudinal bars provide axial and flexural resistance, while ties or spirals restrain bars and confine the core through different detailing and behavior.
- Slenderness and second-order effects depend on unsupported length, end and frame restraint, cracked effective stiffness, sustained load, and sway classification—not gross dimensions alone.
- Concrete and masonry bearing walls require material-specific ACI 318-14 or TMS 402/602-16 checks for axial load, eccentricity, out-of-plane bending, slenderness, openings, and reinforcement.
- Average `P/A_g` stress is a diagnostic value, not a replacement for interaction strength, minimum reinforcement, detailing, stability, or concentrated bearing checks.
Vertical concrete members carry gravity force, frame moment, lateral effects, and construction eccentricity into foundations. A bearing wall spreads load along length but can still bend, buckle, or concentrate force around openings. For July 2026, use the April 2024 PE Civil: Structural specification, current PE Civil Reference Handbook, ACI 318-14, and TMS 402/602-16 for masonry. Do not import April 2027 editions.
Concrete Columns: Axial Load Plus Moment
A reinforced concrete column combines a concrete compression region with longitudinal reinforcing bars in compression or tension. Strain compatibility and equilibrium generate a nominal axial-moment interaction curve. Near high compression, concrete crushing and code axial-strength limits govern; at lower axial load, flexural behavior becomes more tension-controlled. The strength-reduction factor and applicable design point depend on the ACI 318-14 strain and confinement context.
A load described as concentric does not justify using 0.85f'_cA_g as a design answer. Reinforcement displaces concrete area, longitudinal bars contribute force, transverse reinforcement affects confinement, and ACI places limits below an ideal perfectly concentric nominal maximum. Frame analysis, construction tolerances, beam moments, and minimum eccentric effects create bending. For biaxial columns, interaction must include moments about both axes.
Tied and Spiral Reinforcement
A tied column uses discrete closed ties and crossties to restrain longitudinal bars, confine the core, and support shear resistance. Tie spacing, bar support, hooks, and layout prevent longitudinal-bar buckling and hold the cage during construction.
A spiral column uses continuous helical reinforcement around a usually circular core. Properly detailed spiral confinement can sustain core integrity and deformation after cover spalling, which is reflected in ACI strength treatment. A loose spiral, inadequate volumetric ratio, poor anchorage, or excessive pitch does not earn that behavior. Neither ties nor spirals replace required longitudinal reinforcement, development, splices, or connection force transfer.
At lap splices, beam-column joints, corbels, and foundation interfaces, congestion and confinement can govern detailing. Column bars must develop force into the footing or next story. A strong isolated section with an inadequate splice or joint is not a complete vertical path.
Worked Eccentric-Load Stress Diagnostic
An 18 in square column carries service axial load P = 900 kips at eccentricity e = 3.0 in about one centroidal axis. Before performing an ACI strength interaction, calculate gross uncracked elastic edge stresses as a diagnostic.
A_g = 18(18) = 324 in^2
S = bh^2/6 = 18(18^2)/6 = 972 in^3
M = Pe = 900(3) = 2,700 kip-in
Uniform stress:
P/A_g = 900/324 = 2.778 ksi compression.
Bending stress magnitude:
M/S = 2,700/972 = 2.778 ksi
Therefore the extreme stresses are 5.556 ksi compression and zero. The eccentricity equals the rectangular kern boundary h/6 = 3.0 in in this linear uncracked model. Greater eccentricity would predict tension at one edge.
This is not an ACI capacity check. Concrete cracking, nonlinear compression, reinforcement force, factored load, φ, axial upper limits, slenderness, and interaction remain. The diagnostic does show why gross average stress P/A_g = 2.778 ksi hides the peak created by eccentricity.
Slenderness and Second-Order Effects
A slender column bows under axial load, increasing moment through P-delta; story sidesway produces P-Delta. ACI 318-14 permits specified moment-magnification or second-order analysis routes with effective stiffness reflecting cracking, reinforcement, sustained load, and creep. Use the chosen route consistently.
Unsupported length is measured between lateral supports effective in the buckling direction; physical column height may not equal that value. End restraint and whether the frame is sway or nonsway influence moment amplification. A wall can also be slender out of plane. Gross EI used without the required reductions can understate second-order effects. Braces, slabs, or walls counted as restraints need adequate stiffness, strength, and anchorage.
Concrete Bearing Walls
A concrete bearing wall carries distributed roof or floor reactions but also sees eccentric bearing, out-of-plane wind or soil pressure, in-plane lateral force, and moments at openings. ACI wall provisions control minimum thickness or reinforcement as applicable, effective height, axial-flexural strength, and detailing. Loads over a door or window must flow through lintel and jamb regions into the wall below. Concentrated beam reactions can require local bearing or reinforcement.
Do not treat a narrow wall pier automatically as a long uniform wall; its behavior may resemble a column and its boundary conditions matter. Precast wall panels require erection bracing, bearing, connections, and diaphragm attachment before the final load path exists.
Masonry Bearing Walls
Masonry uses units, mortar, grout, and reinforcement as an assemblage governed by TMS 402/602-16, not ACI concrete column equations. Check axial load with eccentricity, out-of-plane flexure, slenderness, effective height and thickness, reinforcement and grout placement, and connection to floors, roofs, and foundations. Openings create piers, jambs, and lintels; ungrouted cells or interrupted bond cannot be assumed to transfer reinforced forces.
Concrete and masonry may share equilibrium equations, but their design strengths, stiffness assumptions, construction tolerances, and detailing rules remain separate. A generic P/A comparison cannot replace either standard.
Vertical-Member Workflow
- Trace factored axial force and biaxial moments to the foundation.
- Identify material, section, ties or spiral, reinforcement, openings, and load eccentricity.
- Determine unsupported lengths, sway condition, and effective stiffness.
- Include required second-order effects.
- Apply the compatible ACI or TMS axial-flexural and shear checks.
- Verify reinforcement limits, confinement, splices, bearings, connections, and foundation transfer.
Finish by asking whether the design addressed peak stress and stability, not merely average compression.
An 18 in square section carries 900 kips at 3 in eccentricity. Using gross elastic properties, what is the maximum edge compressive stress?
Which statement best describes transverse reinforcement in concrete columns?
A reinforced masonry bearing wall is being checked for axial load and out-of-plane bending. Which design approach is appropriate?