Combined Footing Calculator

ACI 318-19 combined footing design for two columns with adjustable spacing. Checks bearing pressure, one-way shear, two-way (punching) shear, and flexure. Interactive diagram shows resultant location and kern limit.

What It Does

This tool designs a rectangular combined footing supporting two columns. It calculates:

• Resultant load location based on column loads and spacing
• Bearing pressure with eccentricity checks (kern limit)
• One-way shear (wide-beam action) at critical sections
• Two-way shear (punching shear) around each column
• Flexure (bending moment) in longitudinal direction
• Reinforcement requirements based on calculated moments

The calculator provides immediate feedback on pass/fail status for each check with utilization ratios.

Inputs Explained

Column Loads

• P1, P2 (service loads): Unfactored column loads from structural analysis. Use factored loads (Pu) for strength checks.

• Pu1, Pu2 (factored loads): Column loads multiplied by load factors (typically 1.2 for dead, 1.6 for live per ACI 318).

Column Dimensions

• col1Width, col2Width: Column width (shorter dimension).
• col1Depth, col2Depth: Column depth (longer dimension).

Footing Geometry

• columnSpacing: Center-to-center distance between columns. Controls eccentricity of resultant load.

• footingWidth: Footing width perpendicular to the line joining columns (shorter dimension).

• footingLength: Footing length along the line joining columns (longer dimension).

• footingDepth: Total thickness from top of concrete to bottom of footing.

Material Properties

• fc: Concrete compressive strength (e.g., 4000 psi, 28 MPa). Affects shear and flexure capacity.

• fy: Reinforcement yield strength (typically 60,000 psi or 420 MPa).

• qAllowable: Allowable soil bearing pressure from geotechnical report.

Other Inputs

• cover: Concrete cover to reinforcement (typically 3 in. or 75 mm).
• barDia: Rebar diameter for reinforcement design.

Assumptions

1. Full soil contact: Resultant falls within kern (e < L/6). No uplift is considered.

2. Linear pressure variation: Soil pressure varies linearly across footing base based on eccentricity.

3. Rectangular footing: Footing shape is rectangular with columns along the longitudinal axis.

4. ACI 318-19 provisions: Checks follow ACI 318-19 strength design method.

5. Reinforcement in one direction: Longitudinal flexure is checked. Transverse reinforcement may require separate design.

6. No column moments: Columns are assumed to transmit only vertical loads. Additional moments are not considered.

How to Use

1. Enter column loads: Input service loads (P) and factored loads (Pu) from your structural analysis.

2. Define geometry: Enter column dimensions, spacing, and footing dimensions.

3. Specify materials: Enter concrete strength, rebar yield, and allowable soil bearing.

4. Review results: Check pass/fail status for each limit state. Utilization < 1.0 indicates passing.

5. Adjust design: If a check fails, increase footing dimensions or thickness and recalculate.

6. Export results: Use CSV export to document calculations for review.

Design Checks

Bearing Pressure

Bearing pressure is calculated using the resultant load (P1 + P2) and its eccentricity from the footing center. When eccentricity exceeds L/6 (kern limit), pressure becomes concentrated at one edge.

Formula: q = P/A × (1 ± 6e/L)

Where:

• P = total load (P1 + P2)
• A = footing area (L × B)
• e = eccentricity of resultant
• L = footing length (direction of eccentricity)

One-Way Shear

One-way (wide-beam) shear is checked at a distance d from the column face. The critical section extends across the full footing width.

Formula: Vc = 2 × lambda × sqrt(fc') × b × d

Where:

• lambda = lightweight concrete factor (1.0 for normal weight)
• fc' = 0.75 × fc (for shear)
• b = footing width
• d = effective depth (total depth - cover - barDia/2)

Two-Way Shear

Two-way (punching) shear is checked on a perimeter located at d/2 from the column face. Each column is checked separately.

Formula: Vc = 4 × lambda × sqrt(fc') × bo × d

Where:

• bo = critical perimeter (average of column dimensions + d)
• Other variables as defined above

Flexure

Flexure is checked in the longitudinal direction at the column face. The critical section is where maximum moment occurs.

Formula: As = Mu / (phi × fy × (d - a/2))

Where:

• Mu = factored moment at critical section
• phi = 0.9 for flexure
• a = depth of stress block (As × fy / (0.85 × fc × b))

Notes

• Resultant location: The diagram shows the resultant location relative to footing center. If outside the kern (L/6), consider increasing footing length.

• Shear capacity: Both one-way and two-way shear must be satisfied. The lower capacity governs.

• Reinforcement design: Longitudinal reinforcement is provided based on calculated moment. Transverse reinforcement may need separate design for local strip action.

• Minimum reinforcement: ACI 318 minimum temperature and shrinkage reinforcement is not explicitly checked in this tool.

• Educational use only: This tool provides screening calculations. Final design requires qualified engineer verification per ACI 318-19 and project requirements.