Bearing capacity¶
The ultimate load q_lim is the pressure that brings the foundation soil to failure. Loadcap NX computes it with several methods in parallel, for each ULS combination, and compares the outcomes.
The three-term equation¶
All methods start from the classic form of the ultimate load:
q_lim = c·N_c·s_c·d_c·i_c + q·N_q·s_q·d_q·i_q + ½·γ·B·N_γ·s_γ·d_γ·i_γ
where:
- N_c, N_q, N_γ — bearing-capacity factors, functions of φ′;
- s — shape factors (depend on B/L);
- d — depth factors (depend on D/B);
- i — load inclination factors;
- q = γ·H_F — surcharge at the founding plane (see embedment height);
- c — cohesion (c′ drained, c_u undrained).
The methods¶
| Method | Year | Notes |
|---|---|---|
| Terzaghi | 1955 | Historical formulation, not-too-deep foundations |
| Meyerhof | 1963 | General shape, depth and inclination factors |
| Hansen | 1970 | Extends Meyerhof (inclined base and ground) |
| Vesic | 1975 | Widely adopted N_γ factors; includes punching |
| Brinch-Hansen | — | EC7/EC8 formulation |
| Meyerhof-Hanna | 1978 | Two-layer soils (weak over strong or vice versa) |
| Richards | 1993 | Bearing capacity in seismic conditions |
Enable the methods to compare in the Calculation methods card. The governing result is the one with the lowest design resistance across all methods and combinations.
Why several methods
Different methods adopt different N_γ factors and corrections: comparing them gives a measure of the scatter of the result. Meyerhof-Hanna in particular is essential when a weak layer overlies a strong one.
Eccentricity and effective area¶
With a moment, the load is eccentric. The eccentricity is e = M / N. Loadcap
adopts Meyerhof's effective area criterion, reducing the dimensions:
B′ = B − 2·e_B, L′ = L − 2·e_L.
Overturning
If the eccentricity exceeds B/6 the reaction becomes partialised; beyond
B/2 the foundation overturns and Loadcap flags it. Large eccentricities
strongly reduce the effective area and hence the bearing capacity.
Check and safety factor¶
For each method and combination:
- Design resistance:
R_d = q_lim / γ_R,v; - Service stress at the founding plane
E_d; - Safety factor:
FS = q_lim / E_d.
The combination is verified when the design resistance is greater than the
action (equivalent to FS ≥ γ_R,v).
Sliding check¶
Additional check against sliding of the foundation under the horizontal action. It is triggered when a shear exists (H_x or H_y). The design resistance is:
R_d = (N·tan δ + a·A′ + E_pd) / γ_R,o
with δ the soil-foundation friction angle, a the soil-foundation adhesion,
A′ the effective area and E_pd the lateral passive thrust. The action is
V_d = √(H_x² + H_y²); the check is satisfied when FS = R_d·γ_R,o / V_d ≥
γ_R,o.
In the Sliding check card (above the Water table) you set:
- Soil-foundation adhesion a [kN/m²];
- Soil-foundation friction angle δ [°];
- Passive thrust fraction [%].
Zero values = automatic derivation
Leaving adhesion and δ at 0, Loadcap derives them from the layer below the founding plane (a = layer cohesion, δ = φ′). A passive thrust fraction of 0 excludes the lateral passive thrust (active only for strip and pad footings). In undrained analysis δ is set to 0.
Punching check¶
For foundations on low-stiffness soils, Loadcap performs the punching check after Vesic, comparing the rigidity index I_r with its critical value I_crit. If I_r < I_crit, punching failure occurs and is reported in the results.
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