Footings

What Rischio checks in a footing and against which limits, how a footing model is built, and where to find each method, equation and warning.

Rischio checks square, rectangular and strip footings on layered soil for bearing capacity, eccentricity and elastic settlement, with every step in the calculation report. It is not a structural design tool: punching shear and reinforcement are outside it, as are consolidation settlement and seismic loading.

What it checks

Check	Rule	Limit
Bearing capacity The ground under the footing giving way $FS = \frac{q_u}{q_{max}}$	$FS = \frac{q_u}{q_{max}}$	$\ge 3.0$
Eccentricity along the width The load inside the middle third, across the width $e_B$	$e_B$	$\le \frac{B}{6}$
Eccentricity along the length The load inside the middle third, along the length $e_L$	$e_L$	$\le \frac{L}{6}$
Settlement The footing's elastic settlement, at its centre $S_e$	$S_e$	$\le 25 \text{ mm}$

The limits are Rischio’s defaults; each result shows its margin, and the check with the least margin governs.

How a model is built

A footing is square or rectangular ( $B \times L$ ) or a strip (width $B$ ), with its base at depth $D_f$ . It carries a vertical load $V$ , a horizontal load $H$ and the moments $M_B$ and $M_L$ ; a strip has no $M_L$ . The soil is a stack of horizontal strata, each with $\gamma$ , $c$ and $\phi$ , $\gamma_{sat}$ where it goes below the water table, and optionally $E_s$ and $\mu_s$ ; the water table is optional. Inputs describes each of them in full.

In these pages

Inputs: every field of a model.
Method and assumptions: units and conventions, the assumptions, and the method in order.
- Bearing capacity: eccentricity and the effective area, the contact pressure, the general bearing capacity equation and the factor of safety.
- Settlement: the elastic settlement at the centre of the base, and what isn’t included.
Equations: every step of the calculation, in the order of the report.
Warnings: what each warning means.
Verified examples: worked examples from Das & Sivakugan, with the book’s answer beside Rischio’s.