Section 11.5.1/Wall Design for Axial Load and Out-of-Plane Bending
ACI 318-19 Section 11.5 covers the design of concrete walls as compression members, including the simplified empirical method and the rational design method for combined axial and lateral loads.
Concrete bearing walls can be designed by two methods. The simplified method (Section 11.5.3) limits the design axial strength to Pn = 0.55 x f'c x Ag x [1 - (kLc/32h)^2], where Ag is the gross area, Lc is the vertical distance between supports, h is the wall thickness, and k is the effective length factor (0.8 for walls braced top and bottom, 1.0 for unbraced, 2.0 for cantilever). This empirical equation applies only when the resultant of all factored loads falls within the middle third of the wall thickness (eccentricity not exceeding h/6). Minimum wall thickness is 1/25 of the supported height or 4 inches, whichever is greater. Minimum reinforcement is 0.0012 times the gross area for deformed bars No. 5 and smaller (vertical) and 0.0020 for horizontal bars. The rational design method (Section 11.5.2) uses the column design provisions of Chapter 10 and applies when eccentric loads, lateral loads, or slenderness effects exceed the simplified method's limits.
Why this section exists
Concrete walls commonly serve as both vertical load-bearing elements and lateral force-resisting elements. Many bearing walls carry relatively concentric loads (floor and roof reactions) and can be efficiently designed using the simplified empirical method, which avoids the complexity of a full column interaction diagram. The eccentricity and slenderness limits on the simplified method ensure it is only applied where the wall behavior is predictable. Walls with significant lateral loads (basement retaining walls, shear walls with overturning) or high slenderness ratios must use the rational method, which accounts for second-order (P-delta) effects and combined axial-bending interaction.
What plan reviewers look for
Plan reviewers check the wall schedule for wall thickness, height, reinforcement, and the design method. For the simplified method, they verify the eccentricity is within h/6, the slenderness ratio kLc/h is reasonable, and the minimum reinforcement is provided. They verify the wall thickness meets the 1/25 height minimum. For walls with lateral loads (earth pressure, wind, seismic), they verify the rational method is used with a full shear design check and moment capacity verification. They check bar development at wall-to-footing and wall-to-slab connections.