Calculator Inputs
Use service-load style footing checks. Enter eccentricities in the same plan directions as the footing width and length.
Example Data Table
| Item | Example Value |
|---|---|
| Foundation Type | Rectangular Footing |
| Vertical Project Load | 900 kN |
| Footing + Overburden + Other Load | 120 kN |
| Width, B | 2.50 m |
| Length, L | 3.50 m |
| Allowable Bearing Capacity | 250 kPa |
| Eccentricity eB / eL | 0.08 m / 0.10 m |
| Groundwater Factor / Adjustment Factor | 0.95 / 0.98 |
| Embedment Surcharge | 18 kPa |
| Illustrative Bearing Ratio | 0.568 |
| Illustrative Utilization | 56.75% |
Formula Used
1) Actual footing areaA = B × L
2) Effective dimensions under eccentric loadingB′ = B − 2eBL′ = L − 2eL
3) Effective bearing areaA′ = B′ × L′
4) Total vertical loadP = Project Load + Additional Load
5) Gross stress on effective areaqgross = P / A′
6) Net bearing stressqnet = qgross − surcharge
7) Adjusted allowable capacityqallow,adj = qallow × groundwater factor × adjustment factor
8) Soil bearing ratioBearing Ratio = qgross / qallow,adj
9) UtilizationUtilization (%) = Bearing Ratio × 100
10) Approximate corner stress checkqmax = (P / A) × (1 + 6eB/B + 6eL/L)qmin = (P / A) × (1 − 6eB/B − 6eL/L)
How to Use This Calculator
- Enter the service load carried by the footing.
- Add footing weight, overburden weight, or any extra vertical load.
- Enter footing width and length in meters.
- Provide the allowable soil bearing capacity from the geotechnical report.
- Enter eccentricity values along the width and length directions.
- Enter surcharge at the base if you want a net stress view.
- Adjust the groundwater and project factors when needed.
- Press the calculate button and review utilization, reserve, and stress checks.
FAQs
1. What does soil bearing ratio mean here?
Here it means the ratio between applied gross bearing stress and adjusted allowable soil bearing capacity. A value below 1.00 usually indicates the footing pressure remains within the selected allowable limit.
2. Why does eccentricity reduce the effective footing area?
Eccentric loading shifts contact pressure toward one side of the footing. To screen the design conservatively, the calculator uses reduced effective dimensions, which usually raises the computed bearing stress.
3. What is the difference between gross and net stress?
Gross stress uses total footing load over the contact area. Net stress subtracts base surcharge or overburden stress, which can help when comparing serviceability-style checks or settlement-related screening values.
4. When should I change the groundwater factor?
Change it when groundwater or reduced effective stress conditions justify a lower allowable value. If your report already provides a final allowable capacity, you can often leave the factor at 1.00.
5. Why does the page show qmax and qmin?
They provide an approximate corner-pressure check on the full footing area. qmin below zero suggests possible loss of contact or tension risk, which needs a more careful engineering review.
6. Can I use this for square footings?
Yes. Enter identical width and length values. The calculator still works, and the effective area logic remains valid as long as the eccentricity values are entered correctly for each plan direction.
7. Does this replace a full geotechnical design check?
No. It is a screening and design-support tool. Final foundation design still needs project geotechnical data, settlement review, code compliance, detailing checks, and professional engineering judgment.
8. What does a utilization above 100% mean?
It means the calculated effective bearing stress exceeds the adjusted allowable capacity entered in the form. You may need a larger footing, lower eccentricity, smaller load, or revised soil parameters.