Wind Turbine Foundation Design Calculator
Preliminary sizing and checks for onshore wind turbine gravity foundations: overturning, sliding, bearing capacity, and reinforcement demand.
Input Data
Turbine & Loading
Soil & Foundation
Results
Overturning stability
–Checks resisting moment vs. overturning moment at foundation base.
- Overturning moment MOT: – kNm
- Resisting moment MR: – kNm
- Safety factor SFOT: –
Sliding stability
–Checks horizontal wind load against frictional resistance at base.
- Horizontal load H: – kN
- Friction capacity μ·N: – kN
- Safety factor SFSL: –
Soil bearing pressure
–Checks maximum contact pressure vs. allowable bearing capacity.
- Max pressure qmax: – kPa
- Min pressure qmin: – kPa
- Allowable qallow: – kPa
Footing reinforcement (approx.)
–Estimates required steel ratio in the tension zone of the footing.
- Design bending moment MEd: – kNm/m
- Required steel ratio ρreq: – %
- Indicative bar area As,req: – cm²/m
Note: Results are for preliminary sizing only and do not replace a full code-compliant structural and geotechnical design.
How the wind turbine foundation calculator works
This tool provides a quick, transparent check of a typical onshore wind turbine gravity foundation. It estimates wind loads at the hub, transfers them to the foundation base, and checks:
- Overturning stability (resisting vs. overturning moment)
- Sliding stability (friction vs. horizontal load)
- Soil bearing pressure (qmax and qmin)
- Approximate reinforcement demand in the footing
1. Wind load on the rotor–nacelle assembly
The extreme wind load at hub height is approximated using a drag-type formula on the rotor swept area:
\( A = \pi \left(\dfrac{D}{2}\right)^2 \)
\( F_\text{wind} = \dfrac{1}{2}\,\rho\,C_d\,A\,V^2 \)
where:
- \(D\) = rotor diameter (m)
- \(A\) = swept area (m²)
- \(\rho\) = air density (default 1.25 kg/m³)
- \(C_d\) = drag coefficient (assumed 1.2 for conservative estimate)
- \(V\) = design wind speed at hub height (m/s)
The horizontal load at the hub is then multiplied by the load safety factor γF to obtain the design horizontal load \( H_d \).
2. Global vertical load and self-weight
The total vertical load at the foundation includes the self-weight of the turbine, tower, and concrete footing:
\( W_\text{turbine} = (m_\text{nacelle} + m_\text{tower}) \cdot g \)
\( W_\text{footing} = \gamma_c \, V_\text{footing} \)
\( N = (W_\text{turbine} + W_\text{footing}) \cdot \gamma_F \)
where \( \gamma_c \) is the concrete unit weight (kN/m³) and \( V_\text{footing} \) is the footing volume.
3. Overturning stability check
The overturning moment at the foundation base is calculated from the horizontal design load and the lever arm (hub height plus any additional eccentricity). The resisting moment is provided by the vertical load acting on the footing width:
\( M_\text{OT} = H_d \cdot h_\text{lever} \)
\( M_\text{R} \approx N \cdot \dfrac{B}{2} \)
\( SF_\text{OT} = \dfrac{M_\text{R}}{M_\text{OT}} \)
A typical target is \( SF_\text{OT} \ge 1.5 \)–2.0 for ultimate limit state, depending on the design code and project requirements.
4. Sliding stability check
Sliding resistance is provided by friction at the soil–concrete interface:
\( R_\text{fric} = \mu \, N \)
\( SF_\text{SL} = \dfrac{R_\text{fric}}{H_d} \)
Many codes require \( SF_\text{SL} \ge 1.5 \) for ultimate limit state. If the factor is too low, you can increase footing size, embedment depth, or improve the soil.
5. Soil bearing pressure under combined V–M
For a circular or square footing under vertical load \( N \) and overturning moment \( M \), the soil pressure distribution is assumed linear. The maximum and minimum pressures are:
\( A = B^2 \quad \text{(square footing approximation)} \)
\( e = \dfrac{M}{N} \)
\( q_\text{avg} = \dfrac{N}{A} \)
\( q_\text{max} = q_\text{avg} \left(1 + \dfrac{6e}{B}\right) \) \( q_\text{min} = q_\text{avg} \left(1 - \dfrac{6e}{B}\right) \)
The calculator compares \( q_\text{max} \) with the allowable bearing capacity. If \( q_\text{min} < 0 \), part of the footing is in uplift and the design should be refined.
6. Approximate footing reinforcement demand
To give a quick indication of reinforcement demand, the tool converts the overturning moment into an equivalent bending moment per meter width at the footing base and uses a simplified rectangular section design:
\( M_\text{Ed,per m} \approx \dfrac{M_\text{OT}}{B} \)
\( z \approx 0.9\,d \quad \text{with} \quad d = h - 0.1\,\text{m (cover + bar diameter)} \)
\( A_s = \dfrac{M_\text{Ed}}{z \cdot f_{yd}} \) \( \rho = \dfrac{A_s}{b \cdot d} \)
The output ρ (in %) and As (cm²/m) can be compared with minimum and maximum reinforcement ratios from your design code (e.g., Eurocode 2, ACI 318).
Design assumptions and limitations
- Onshore, shallow gravity foundation or simplified monopile equivalent.
- Static extreme wind load; dynamic and fatigue effects are not included.
- Uniform soil bearing capacity and friction coefficient at the base.
- No detailed check of punching shear, anchor cage, or crack width.
- No partial factors on material strengths beyond the user-specified γF.
Always verify the final design with project-specific geotechnical data, applicable standards (e.g., IEC 61400, Eurocode, ACI, AISC), and a qualified structural engineer.
Frequently asked questions
Can I use this calculator for early feasibility studies?
Yes. The tool is ideal for quick feasibility checks and comparing different hub heights, rotor diameters, or soil conditions. It helps you understand how sensitive the foundation size is to wind speed and soil capacity.
How should I choose the allowable bearing capacity?
Use values from a geotechnical investigation report for the specific site. If you only have ultimate bearing capacity, divide it by an appropriate factor of safety (often 2.5–3.0) to obtain an allowable value.
What if the overturning or sliding safety factor is too low?
You can increase footing diameter, increase embedment depth, improve soil properties (e.g., ground improvement, piles), or adjust the turbine layout. The calculator lets you iterate quickly by changing these parameters.
Does the monopile mode design the pile itself?
No. The monopile option only adjusts some assumptions (lever arm and stiffness) to give a rough indication of base reactions. Detailed monopile design requires specialized offshore analysis tools.