Formula used
- A = L × B
- V = fD(D + SW) + fL(Live) + fO(Other)
- q_avg = V / A (kPa)
- q_mx = 6·Mx / (L·B²), q_my = 6·My / (B·L²)
- q_max = q_avg + |q_mx| + |q_my|
- q_min = q_avg − |q_mx| − |q_my|
- e_x = My / V, e_y = Mx / V; kern check: |e_x| ≤ L/6 and |e_y| ≤ B/6
How to use this calculator
- Enter footing plan dimensions and site allowable bearing capacity.
- Add vertical loads and overturning moments from your analysis.
- Select a design mode, or apply custom factors if needed.
- Click Calculate to view pressures and checks above.
- Use CSV or PDF buttons to share results with stakeholders.
Example data table
| Case | L (m) | B (m) | V (kN) | Mx (kN·m) | My (kN·m) | qmax (kPa) | Status |
|---|---|---|---|---|---|---|---|
| A | 3.0 | 2.4 | 980 | 180 | 95 | ~205 | OK |
| B | 2.6 | 2.0 | 980 | 220 | 120 | ~285 | Check |
| C | 3.4 | 2.8 | 1100 | 240 | 160 | ~195 | OK |
Load inputs for tower foundations
This calculator groups vertical actions into dead, live, equipment, and other loads. A typical small telecom tower base can range from 700–1,300 kN service vertical load depending on steel mass, platforms, and mounted gear. Enter loads as kN to keep the bearing results directly in kPa (kN/m²). When self-weight is enabled, the footing weight is computed from thickness and concrete unit weight and added to the dead bucket.
Factored demand and safety intent
Select service mode for quick checks, or apply factored demand for design review. In ultimate mode, the calculator uses common factors to increase gravity demand and moments. Custom factors are included so your site can match local design requirements. For reporting, the output shows both service vertical load and the design vertical load used for bearing, keeping comparisons transparent for reviewers.
Bearing pressure distribution and limits
Bearing pressure is estimated using a linear stress distribution: average pressure plus moment-induced components. The calculator reports qavg, qmax, and qmin. A practical target is qmax ≤ allowable bearing from the geotechnical report. If qmin becomes negative, part of the base would be in tension, indicating uplift risk or loss of contact.
Eccentricity checks for stability
Eccentricities ex and ey are computed as moment divided by design vertical load. The kern check compares |ex| to L/6 and |ey| to B/6. Staying within the kern helps keep compression across the entire base. If eccentricity exceeds the kern, consider increasing dimensions, adding ballast, or revisiting load paths.
Example data for fast validation
Use the sample below to confirm your workflow after deployment, then replace with project values.
| Example | L (m) | B (m) | V (kN) | Mx (kN·m) | My (kN·m) | Allowable (kPa) |
|---|---|---|---|---|---|---|
| Quick check | 3.0 | 2.4 | 980 | 180 | 95 | 220 |
With these inputs, qmax typically remains near the allowable limit, and the sizing suggestion provides a minimum area estimate to guide early coordination with excavation and concrete teams.
FAQs
1) What should I enter as allowable bearing capacity?
Use the site geotechnical allowable bearing for the founding depth and soil type. Keep units in kPa. If you only have tons/ft², convert before entering to maintain correct pressure checks.
2) Why can qmin become negative?
Large overturning moments can shift compression to one side, making the opposite side lift off in the linear model. Negative qmin suggests uplift risk or partial contact and needs engineering review.
3) Do I need to include footing self-weight?
Include it for realistic gravity demand, especially for uplift-sensitive cases. The tool estimates weight from thickness, area, and concrete unit weight. Excluding it is acceptable for quick comparisons when loads are uncertain.
4) What do the kern limits mean?
The kern is the central zone where the resultant load keeps compression across the entire base. If |ex| ≤ L/6 and |ey| ≤ B/6, full contact is more likely under the simplified model.
5) How should I interpret the sizing suggestion?
It is a preliminary area estimate using V/allowable bearing and your current L/B ratio. Use it to start layout discussions, then refine with detailed stability, shear, punching, and reinforcement checks.
6) Which design mode should I use?
Service mode is helpful for immediate bearing and uplift screening. Ultimate or custom factors are better for design submittals. Always align factors with your governing code and project specifications.
7) Does this replace a structural foundation design?
No. It is an estimating and reporting tool for bearing pressures and eccentricity checks. Final foundation design must consider geotechnical recommendations, reinforcement, shear, settlement, construction tolerances, and applicable standards.