Example data table
| Scenario | Height | Wind | Area | Weights | Foundation | Allowable | Typical outcomes |
|---|---|---|---|---|---|---|---|
| Base case | 45 m | 42 m/s | 28 m2 | 180 + 35 kN | 4 x 4 m | 250 kPa | qmax below allowable, low eccentricity |
| Higher wind | 55 m | 55 m/s | 35 m2 | 220 + 45 kN | 5 x 5 m | 250 kPa | Higher moment, check uplift and qmax |
| Smaller footing | 40 m | 45 m/s | 25 m2 | 160 + 30 kN | 3 x 3 m | 200 kPa | qmax may exceed allowable, revise sizing |
Formula used
- Wind pressure: q = 0.613 * V^2, with V in m/s, then q * Kz * Kt * G.
- Wind force: Fw = q * Cd * A, where q is kPa (kN/m2) and A is m2.
- Overturning moment: M = Fw * h, using effective height h.
- Vertical load: V = Wtower + Wequip + Wlive + Wextra.
- Eccentricity: e = M / V.
- Bearing pressures: qmax/min = V/A +/- 6M/(B*L^2) for a rectangular footing.
How to use this calculator
- Select your unit system, then enter tower height and wind speed.
- Enter exposure, topography, gust, drag coefficient, and projected area.
- Provide tower weight, equipment weight, and any extra or live loads.
- Enter foundation length and width, plus allowable soil bearing pressure.
- Click Calculate to view results above the form.
- Download outputs using the CSV or PDF buttons for reports.
Tower foundation load planning guide
1) Loads that control tower foundations
Tower foundations are usually governed by overturning from wind, combined with gravity loads from the mast, platforms, feeders, and mounted equipment. This calculator estimates base shear, overturning moment, and the resulting soil pressure distribution so you can screen footing sizes early.
2) Wind inputs and practical ranges
For preliminary checks, wind speed is the biggest driver because pressure scales with the square of velocity. Typical design wind speeds can range from 30 to 60 m/s, depending on region and code. Exposure and gust factors commonly vary from 0.8 to 1.3, while drag coefficients for tower shapes often fall near 0.9 to 1.6. Enter the projected area that best represents the wind-facing silhouette.
3) Moment arm and eccentricity checks
Overturning moment is computed as M = Fw × h, where h is the effective height of the wind resultant. Eccentricity is e = M/V. A helpful rule for spread footings is to keep the resultant within the middle third, which means e should be less than or equal to L/6 for the governing axis. If e grows, edge pressures increase rapidly and tension can develop.
4) Bearing pressure and uplift screening
The tool reports average, maximum, and minimum bearing pressures using the rectangular footing relationship q = V/A ± 6M/(B·L²). Many sites show allowable bearing values between 100 and 300 kPa, but the geotechnical report is the final authority. If minimum pressure becomes negative, uplift is likely and the foundation may require increased dead weight, a larger footprint, or anchorage solutions.
5) Using results for preliminary sizing
Start with service-level factors (1.0) to understand expected behavior, then apply higher factors to compare demand growth. Increase footing length first when overturning is dominant, and increase width when overall bearing is high. Document key inputs and results using the CSV or PDF download for coordination and review.
FAQs
1) What does “projected area” mean?
Projected area is the wind-facing silhouette area of the tower and attachments. Include ladders, platforms, antennas, and cable trays if they meaningfully add wind exposure.
2) How do I choose the wind force height?
Use the centroid height of the wind-exposed area. If you do not know it, a conservative starting point is roughly half the tower height, then refine as geometry becomes available.
3) Why does wind speed affect results so strongly?
Wind pressure scales with V², so a 10% increase in wind speed produces roughly a 21% increase in pressure, raising both base shear and overturning moment.
4) What indicates a risk of uplift?
If the minimum bearing pressure is below zero, the footing edge is in tension in a simplified model. That is a strong signal to increase footing size, add weight, or use anchors/piles.
5) What bearing pressure value should I enter?
Use the allowable bearing pressure from the geotechnical report for the foundation depth and soil layer. In early studies, values between 100–300 kPa are common, but site-specific data governs.
6) Can I use this for final design?
No. This calculator is for preliminary sizing and screening. Final design should follow the governing structural and geotechnical standards, include load combinations, and account for settlement and lateral soil response.
7) How can I reduce excessive qmax?
Increase foundation footprint, reduce eccentricity by increasing length along the overturning axis, add dead weight, or reduce wind demand through refined area, shielding, or updated exposure assumptions.