Calculator Inputs
Enter consistent SI units. Loads use kN, moments use kN-m, dimensions use meters, and bearing pressure uses kPa.
Formula Used
The calculator assumes a rigid footing group and linear reaction distribution.
P = factored vertical load
Base reaction = P / n
Mx design = Mx × moment factor + P × ey
My design = My × moment factor + P × ex
Ri = P / n + (Mx × yi / Σy²) + (My × xi / Σx²)
Soil pressure = Ri / footing area
Utilization = soil pressure / allowable bearing × 100
Negative reaction indicates possible uplift. High utilization needs review by a qualified designer.
How to Use This Calculator
- Enter service load components for the building.
- Set load factors according to your chosen design method.
- Enter grid spacing and footing count in both directions.
- Add moments and eccentricities if loads are not centered.
- Enter footing dimensions and allowable soil bearing pressure.
- Press the calculate button and review reactions above the form.
- Download the CSV or PDF report for project records.
Example Data Table
| Case |
Footing Grid |
Total Load kN |
Mx kN-m |
My kN-m |
Footing Size m |
Use |
| Small Frame |
2 × 2 |
950 |
80 |
60 |
1.8 × 1.8 |
Early sizing |
| Warehouse Bay |
3 × 2 |
1850 |
220 |
140 |
2.2 × 2.0 |
Bearing review |
| Irregular Layout |
4 × 3 |
3600 |
520 |
410 |
2.6 × 2.4 |
Moment check |
Footing Reaction Review Article
Why Footing Reactions Matter
A footing reaction check shows how building load reaches soil. It is an early review, but it can prevent costly design errors. The calculator uses a rigid base assumption. It spreads vertical load across a grid of footing points. It then shifts reactions with bending moments and eccentricity.
How Load Is Distributed
Reaction balance starts with total design load. Dead, live, roof, snow, wind, column, and footing self weight can be included. Each load may use its own factor. The uniform part is total load divided by footing count. The moment part is added by coordinate position. Footings farther from the center receive larger changes.
Uplift and Soil Pressure
The sign of each moment matters. A positive moment increases reaction on one side. It reduces reaction on the opposite side. If the reduced value becomes negative, uplift is shown. Uplift means that the assumed contact is not fully compressed. That condition needs careful engineering review.
Soil pressure is calculated by dividing each reaction by footing area. The highest pressure is compared with allowable bearing pressure. The result shows utilization, safety margin, and controlling footing. This helps compare several layouts quickly. Wider footings reduce pressure. More footings can reduce average reaction. Larger moments can still overload edge supports.
Eccentric Load Paths
Eccentric load paths often control low rise buildings. Wind, seismic force, sloped columns, or irregular framing can move the resultant load. This tool lets you enter direct moments and eccentric distances. It also reports equivalent eccentricity, so the load path is easier to inspect.
Practical Design Notes
Use the output as a screening aid. It does not replace code design, geotechnical reports, or structural drawings. Real foundations may need settlement checks, sliding checks, punching shear checks, reinforcement design, frost depth review, and drainage planning. Soil chemistry can also affect durability. Sulfates, chlorides, acidity, and groundwater exposure may require special concrete choices.
Start with service loads for soil bearing discussions. Use factored loads for strength comparisons when your design method requires them. Keep units consistent. Review any uplift, high utilization, or large eccentricity before final sizing. Document assumptions carefully. Save the table with project notes. Share the chart with team members. Recheck loads when plans, grids, soil reports, or material specifications change during later review.
FAQs
1. What does footing reaction mean?
Footing reaction is the vertical force transferred from the structure into each footing. It depends on total load, footing layout, moment, eccentricity, and assumed stiffness.
2. Why can one footing carry more load?
Moments and eccentric loads shift reactions across the footing group. Edge or corner footings may receive higher load when the resultant force moves away from the center.
3. What does uplift mean?
Uplift means the calculated reaction is negative. The footing may lose compression under that load case. This needs anchorage, redesign, or a different load combination review.
4. Is allowable bearing pressure service or factored?
Many soil reports give service allowable pressure. Some design methods use factored resistance. Match your input with the method required by your local code and engineer.
5. Can this replace a structural design?
No. It is a screening calculator. Final design may require settlement, punching shear, sliding, overturning, reinforcement, durability, and construction detailing checks.
6. Why include footing self weight?
Footing self weight increases soil pressure and stabilizing vertical load. Include it when checking bearing pressure and overturning effects at foundation level.
7. Which units should I use?
Use kN for loads, kN-m for moments, meters for dimensions, kN/m³ for density, and kPa for soil pressure. Keep all inputs consistent.
8. How does soil chemistry affect footings?
Soil chemistry can affect concrete durability. Sulfates, chlorides, low pH, and groundwater exposure may require special concrete, coatings, drainage, or material protection.