Inputs
Formula used
Perimeter: p = pi*D
Shaft area: As = p*L
Base: Qb = qb*Ab
Ultimate: Qult = Qs + Qb
FoS = Qult / P
Allowable = Qult / FSdesign
How to use this calculator
- Enter pier diameter, embedded length, and applied service load.
- Select shaft and base resistance methods that match your data.
- Provide unit resistances directly, or use clay correlations shown.
- Set your design safety factor for allowable capacity evaluation.
- Optionally limit capacity using a structural resistance value.
- Press Calculate to view results and download reports.
Design intent and scope
This calculator estimates axial pier safety by comparing ultimate resistance with an applied service load. It combines shaft resistance along the embedded length and end bearing at the base. Inputs are kept unit-consistent: stresses in kPa, areas in m^2, and loads in kN. Results support preliminary sizing, quick sensitivity checks, and report-ready summaries.
Key inputs that control capacity
Diameter increases both base area and perimeter, so capacity often grows faster than linearly with D when end bearing is significant. Embedded length mainly increases shaft area, making L influential in friction-dominated designs. Applied load P directly reduces the computed factor of safety, so use the same load basis you use for acceptance.
Interpreting shaft and base resistance
For the shaft, you may enter a direct unit resistance fs or estimate it in clay using the alpha approach, fs = alpha*su. For the base, you may enter qb directly or estimate qb in undrained clay using qb = Nc*su. These correlations are simplified; select parameters from investigation data, testing, and method-specific guidance.
Safety factor and allowable capacity checks
Ultimate geotechnical capacity is Qult = Qs + Qb, where Qs = fs*As and Qb = qb*Ab. The calculator reports FoS = Qult/P and allowable capacity = Qult/FSdesign. Utilization is shown as P/allowable, which is convenient for quick pass-fail screening. Optional structural limiting applies Qult = min(Qult, Qstruct) when a structural resistance is provided.
Quality control and reporting outputs
Review intermediate values such as As, Ab, Qs, and Qb to catch unit mistakes and unrealistic strengths. Typical fs values for cohesive soils may range from tens to low hundreds of kPa, while qb may be several hundred to a few thousand kPa depending on strata and construction. Use the CSV for checking multiple cases and the PDF for submittals, keeping notes on assumptions, groundwater, and installation effects. Consider running upper and lower bound scenarios to bracket uncertainty, and archive each run with a project note for traceable decisions later internally.
FAQs
1) What is the difference between FoS and the design safety factor?
FoS is the calculated ratio Qult/P for your current inputs. The design safety factor is your acceptance target used to compute allowable capacity (Qult/FSdesign). The status compares FoS against that target.
2) Should I use service load or factored load for P?
Use one basis consistently. If your resistance model is an ultimate estimate, many workflows compare to service load using a global safety factor. If your project uses factored loads, adjust your factors and interpretation accordingly.
3) How do I choose alpha and Nc for clay methods?
Select values from your geotechnical report, test correlations, and the specific method you follow. Alpha typically varies with clay strength and interface roughness. Nc is often near 9 in undrained conditions, but may differ by guidance.
4) Why does structural limiting sometimes reduce Qult?
Geotechnical resistance can exceed what the concrete section, reinforcement, or connection can safely carry. When you provide a structural capacity, the calculator adopts the smaller of geotechnical and structural ultimate values for a conservative combined check.
5) What units should I use for fs and qb?
Enter fs and qb in kPa (which equals kN/m^2). The calculator multiplies these by area in m^2 to obtain kN. If you have values in MPa, multiply by 1000 before entering.
6) Can this handle layered soils, tension, or settlement checks?
This version uses single representative unit resistances and an axial compression safety check. For layered profiles, tension, and settlement, you should analyze strata-by-strata and include load-transfer or settlement methods outside this simplified tool.
Example data table
| Case | D (m) | L (m) | P (kN) | fs (kPa) | qb (kPa) | FoS |
|---|---|---|---|---|---|---|
| A | 0.60 | 10.0 | 1,200 | 60 | 800 | 1.13 |
| B | 0.80 | 12.0 | 1,800 | 75 | 1,000 | 1.54 |
| C | 1.00 | 15.0 | 2,500 | 90 | 1,200 | 2.07 |
Recent runs
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|---|---|---|---|
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