Inputs
Metric unitsExample Data Table
Use this example to verify the calculator behavior and units.
| Item | Value | Units |
|---|---|---|
| Pier diameter | 600 | mm |
| Pier length | 6.00 | m |
| Concrete strength f'c | 30.0 | MPa |
| Steel yield fy | 500 | MPa |
| Reinforcement | 8 bars × 20 | mm |
| Strength factor ϕ | 0.65 | — |
| Unit skin friction fs | 45.0 | kPa |
| End bearing qb | 1,500 | kPa |
| Safety factor SF | 2.50 | — |
| Structural design strength ϕPn | 5,461.6 | kN |
| Geotechnical allowable Qall | 373.2 | kN |
| Governing axial capacity | 373.2 | kN |
Formula Used
1) Structural axial capacity (concrete + steel)
This calculator estimates nominal axial capacity using a common reinforced concrete expression for concentric compression:
- Ag = π D² / 4 (gross area)
- Ast = n × (π db² / 4) (steel area)
- Pn = 0.85 f'c (Ag − Ast) + fy Ast (nominal strength)
- Pdesign = ϕ × Pn × reduction (design strength)
Use an appropriate ϕ factor and detailing rules per your design standard.
2) Geotechnical axial capacity (shaft + base)
The geotechnical check uses an ultimate resistance model and divides by a safety factor:
- Qs = fs × (π D L) (shaft resistance)
- Qb = qb × (π D² / 4) (base resistance)
- Qult = Qs + Qb (ultimate capacity)
- Qallow = Qult / SF (allowable load)
Inputs fs and qb should come from geotechnical recommendations.
3) Governing capacity
In “Governing” mode, the calculator reports the minimum of the structural design strength and the geotechnical allowable load.
How to Use This Calculator
- Choose the calculation mode: governing, structural only, or geotechnical only.
- Enter the pier diameter and embedded length in metric units.
- Fill in concrete strength, steel yield, and reinforcement details.
- Enter the unit skin friction, end bearing pressure, and safety factor.
- Press Calculate to see results above the form.
- Use the download buttons to export a CSV or a PDF report.
Professional Notes for Pier Axial Capacity Checks
1) What this calculator is intended to provide
This tool delivers a preliminary axial capacity screen by combining a reinforced concrete compression check with a simple shaft-and-base soil resistance model. It is useful for early sizing, option studies, and quick verification of field assumptions before detailed design. Results are expressed in kN and clearly identify which check governs.
2) Structural inputs that drive the outcome
Concrete strength and reinforcement area typically control the structural design strength. For many projects, concrete strengths in the 25–40 MPa range and steel grades around 420–600 MPa are common. The strength factor ϕ is often selected between 0.65 and 0.75 depending on detailing and code provisions. Increasing diameter raises gross area rapidly, which can increase Pn even with modest reinforcement.
3) Geotechnical inputs and safety factors
The geotechnical allowable load is computed from Qult = Qs + Qb and then divided by your safety factor. Unit skin friction values can vary widely with soil type and construction method, while end bearing values depend strongly on the founding stratum. Many allowable-load designs apply safety factors in the 2.0–3.0 range; a higher SF reduces allowable capacity and can become the governing check.
4) Interpreting governing capacity
In “Governing” mode, the reported capacity is the minimum of structural design strength and geotechnical allowable load. It is common for geotechnical allowable capacity to govern for short piers in weaker soils, while structural strength may govern for heavily reinforced sections or for conservative soil parameters. Use the output table to compare ϕPn and Qallow side by side and document the basis.
5) Recommended documentation and checks
Record the source of fs, qb, and the chosen safety factor, and confirm whether negative skin friction, groundwater effects, or construction tolerances apply. For final design, verify load combinations, detailing limits, and any additional checks such as settlement, buckling, group effects, and lateral interaction with the pile cap.
FAQs
1) What does “Governing” mode mean?
It reports the smaller value between structural design strength and geotechnical allowable load, giving a conservative screening capacity for early sizing decisions.
2) Can I use this for tension capacity?
No. The structural expression and bearing term are for compression screening. Tension capacity requires uplift resistance, reinforcement development checks, and site-specific geotechnical uplift parameters.
3) Why is my geotechnical allowable much lower than ϕPn?
Soil resistance is often the limiting factor, especially with short embedment, low unit skin friction, or a high safety factor. Verify fs, qb, and SF from the geotechnical report.
4) What is the “Reduction factor” used for?
It applies an additional global multiplier to the structural design strength to reflect project policy, uncertainty, or simplified allowance for effects not explicitly modeled in this screen.
5) How should I choose ϕ?
Select ϕ according to your design standard and detailing conditions. Values commonly fall between 0.65 and 0.75 for axial compression, but requirements vary by code and confinement.
6) Does the calculator include settlement or group effects?
No. It focuses on axial capacity only. Settlement, pile/pier group efficiency, downdrag, and interaction with the superstructure should be checked separately for final design.
7) Are the CSV and PDF exports identical to the on-screen results?
Yes. Exports pull from the most recent successful calculation stored in the session, so the downloaded values match the results shown above the form.