Shallow foundation — pad / strip footing
The calculator checks a rectangular pad or strip footing at the ultimate limit state: eccentricity of the resultant, bearing resistance of the foundation base in drained and undrained conditions, and sliding. It applies design approach 1, combinations 1 and 2, to characteristic loads.
Footing type
A pad footing is a rectangle B × L where B is the shorter dimension. A strip footing is calculated per 1 m length: loads are entered in kN/m and kNm/m and the horizontal force acts perpendicular to the axis of the strip.
Geometry
Axis x runs along B, axis y along L. D is the smallest vertical distance from the finished ground level to the foundation base.
Positive directions as in the diagram: a positive moment and a positive horizontal force both shift the resultant in the positive direction of the axis; at the base My0 = My + Hx·t and Mx0 = Mx + Hy·t. For Mx this is the opposite of the right-hand rule.
Loads at the top of the footing
| Component | permanent G | variable Q | Unit |
|---|---|---|---|
| N Vertical force | kN | ||
| Hx Horizontal force in direction x | kN | ||
| Hy Horizontal force in direction y | kN | ||
| Mx Moment about axis x | kNm | ||
| My Moment about axis y | kNm | ||
Characteristic values: one set of permanent effects G and one set of variable effects Q from one variable load situation. Vertical forces are compressions without the self-weight of the footing, which the calculator adds automatically; include the weight of backfill above the footing in NG.
Foundation soil
Undrained conditions are checked for fine-grained soils. With “No”, cu,k is not used.
Characteristic values from the geotechnical investigation. Enter γ1 and γ2 for the actual conditions, including the effect of groundwater: γ1 as the average above the foundation base (q = γ1·D), γ2 below the base to a depth of 2.5·Bef.
Further parameters
Show advanced settings
The self-weight of the footing Gf = B·L·t·γb (for a strip per 1 m length) is added to the permanent load.
| Set | γG,sup | γG,inf | γQ,sup | γQ,inf |
|---|---|---|---|---|
| A1 | 1.35 | 1.00 | 1.50 | 0.00 |
| A2 | 1.00 | 1.00 | 1.30 | 0.00 |
| Set | γφ | γc | γcu |
|---|---|---|---|
| M1 | 1.00 | 1.00 | 1.00 |
| M2 | 1.25 | 1.25 | 1.40 |
| Set | γR,v | γR,h |
|---|---|---|
| R1 | 1.00 | 1.00 |
The partial factors are fixed: combination 1 = A1 + M1 + R1, combination 2 = A2 + M2 + R1. Further φu = 0, Spd = 0 and a horizontal foundation base α = 0.
- Enter the loads as characteristic values at the top of the footing: one set of permanent effects G and one set of variable effects Q. All Q components must belong to one variable load situation; the calculator neither combines several variable actions nor applies ψ factors. The calculator adds the footing's self-weight automatically; include the weight of backfill above the footing in NG.
- In each combination the calculator checks every load scenario in which all permanent effects, including the self-weight of the footing, share one factor γG (unfavourable γG,sup or favourable γG,inf) and all variable effects share one factor γQ (unfavourable γQ,sup or favourable γQ,inf = 0, i.e. no Q). Each check is governed by the scenario with the highest utilisation; the card shows the governing combination and scenario, and “Show calculation” lists the other scenarios.
- A strip footing is calculated per 1 m length as the limiting case of a very long rectangular footing: all shape factors equal 1 and the horizontal force acts perpendicular to the axis of the strip footing. This is a derived assumption of this version of the calculator.
- Moments are carried to the foundation base algebraically as M + H·t, using the positive directions in the diagram: a positive moment and a positive horizontal force both shift the resultant in the positive direction of the axis. The eccentricity keeps its sign; the effective dimensions use its absolute value and are ordered so that B' ≤ L'.
- Enter the unit weights γ1 and γ2 so that they match the actual conditions, including the effect of groundwater; the calculator has no separate groundwater level input. The overburden pressure is q = γ1·D.
- For frost protection the minimum foundation depth in the Czech Republic is 0.80 m, more in mountain areas. Fine-grained soils of classes F7 and F8, which are sensitive to drying and shrinkage, need 1.60 m; temporary structures may be founded at 0.40 m if climatic effects cannot harm them. The calculator only warns when D < 0.80 m; the verdict is not affected.
- The calculator does not check settlement, overall stability of the footing or slope, uplift, an inclined foundation base, layered subsoil, reinforcement or punching shear.
- This is not a substitute for a complete structural design.