Practical Guide to Planning and Designing Wall Footings for Stability and Strength

When it comes to constructing walls, the design and analysis of wall footings are crucial for ensuring stability and strength. This practical guide follows the guidelines of ACI 318-19 (Building Code Requirements for Concrete and Commentary), focusing on simple principles and straightforward examples to help you understand the process.

Wall Footing Basics

The wall footing, often referred to as a strip footing, supports structural or nonstructural walls. Its design involves principles of beam action with slight modifications. The goal is to safely transmit and distribute loads to the soil while preventing excessive settlement, rotation, sliding, and overturning.

Analysis of Wall Footing

To understand the forces acting on a wall footing, consider Fig. 1, which depicts the critical sections for moment and shear force. The computation of bending moments involves factors such as the width of the footing, the width of the supported wall, and the ultimate bearing capacity of the soil.

Footing Size Determination

Determining the size of the footing involves considering unfactored loads and effective soil pressure. This pressure is computed based on allowable bearing pressure, ensuring safety factors prevent soil bearing capacity exceedance and settlement within acceptable limits.

Equations for Footing Size:

Equation 3: $A_{re q} = q ^{e} D + L$
Equation 4: $A_{req} = \max\left(\frac{D + L}{q_e}, \frac{1.3W}{E}$ \

Footing Depth

According to ACI 318-19, the overall depth of the foundation must satisfy the effective depth requirement, ensuring the bottom reinforcement depth is at least 150 mm. This holds true for sloped, stepped, or tapered foundations as well.

Reinforcement Area Calculation

The main reinforcement area ( $A_{s}$ ) is computed using Equation 5, involving the ultimate moment ( $M_{u}$ ), strength reduction factor ( $ϕ$ ), yield strength of steel ( $f_{y}$ ), effective depth ( $d$ ), and depth of the rectangular stress block ( $a$ ).

Minimum Reinforcement:

For steel grade < 420: Equation 6
For steel grade = 420: Equation 7

Bar Spacing/Placement

Determining the number of bars and their spacing involves dividing the reinforcement area by the area of one bar ( $A_{b}$ ). Main bar spacing is calculated using Equation 8, and distributed bar spacing is determined from the distributed reinforcement area.

Shear Strength of Concrete

Ensuring the design shear strength of concrete ( $V_{c}$ ) is equal or greater than the ultimate shear force is crucial. Equation 10 considers strength reduction factor, concrete compressive strength, width of the footing, and effective depth.

Summary of Design Procedure

Estimate footing thickness ( $h$ ) meeting shear requirements.
Compute weight of fill and weight of footing.
Calculate effective bearing capacity ( $q_{e}$ ).
Estimate required area ( $A_{re q}$ ).
Calculate design pressure ( $q_{u}$ ) based on factored loads.
Compute shear force and design shear strength of concrete.
Calculate maximum moment and reinforcement area.
Compute minimum reinforcement and maximum spacing.
Estimate main and distributed bar spacing.
Draw design draft.

Conclusion

This comprehensive guide provides a step-by-step approach to the analysis and design of wall footings, ensuring stability and strength in your construction projects.