Footing

Footing#

The Footing class models the section of a spread footing or raft — a one-way slab bearing directly on the ground.

It is a OneWaySlab in every respect but the rules the design codes write differently for a member on the ground: the minimum longitudinal reinforcement, the spacing the bars are detailed at, and the thickness the section is expected to have. Everything else — geometry, materials, reinforcement given as diameter and spacing, flexure and shear checks through a Node — is the slab’s, unchanged.

Warning

Sectional design only. Mento does no geotechnical calculation.

Footing answers one question: what reinforcement does this section need for the forces it is given. It knows nothing about the soil under it and does not check bearing pressure, settlement, sliding, overturning, uplift, or the plan dimensions the footing needs to spread its load. It does not size the footing.

Those are the engineer’s, and they come first: the plan size and the thickness are decided from the soil, and only then is the resulting section handed to Footing. Punching shear is a separate check — see PunchingSlab.

Key Concepts#

  • Geometry: a strip of the footing, given as a width and a height (the total thickness). Results are the totals across that width, so a 1 m strip reads directly as per-metre values.

  • Forces: the sectional forces at the face being designed — the moment and shear the soil pressure produces in the strip. Mento does not derive them from the soil; they are an input.

  • Reinforcement: bar diameter and spacing, one direction at a time. A footing spans both ways, so it is designed as two strips, one per direction.

  • Design code: taken from the Concrete object, as everywhere else in mento. ACI 318-19, CIRSOC 201-25 and EN 1992-2004 are all supported.

Usage#

1. Creating a Footing#

from mento import Concrete_ACI_318_19, Footing, SteelBar, MPa, cm, mm, m

concrete = Concrete_ACI_318_19(name="H25", f_c=25 * MPa)
steel = SteelBar(name="ADN 420", f_y=420 * MPa)

# A 1 m strip of a footing 60 cm thick
footing = Footing(label="Z1", concrete=concrete, steel_bar=steel,
                  width=1 * m, height=60 * cm, c_c=50 * mm)

2. Checking and Designing#

Exactly as for a slab: forces are attached through a Node, which drives the check and the design.

from mento import Forces, Node, kN, kNm

node = Node(section=footing, forces=[Forces(label="ELU 1", M_y=120 * kNm, V_z=90 * kN)])

node.design()          # flexure + shear
node.check_flexure()   # per-combination table
node.check_shear()

Reinforcement can also be assigned by hand with set_slab_longitudinal_rebar_bot() and set_slab_longitudinal_rebar_top(), and read back afterwards, the same way as for a OneWaySlab. See Slab for the full reinforcement and results API.

What differs from a slab#

Three rules, and the design applies all of them for you:

  • Minimum reinforcement. A member on the ground is exempt from the flexural minimum written for a member spanning between supports, and each code puts a different rule in its place. Footing returns the largest applicable minimum already applied, so the reinforcement it reports needs no correction afterwards. A face the moment puts in tension takes the minimum; a face it does not is left unreinforced — whether a footing carries top steel is yours to decide, since only you see both orthogonal sections of the element. The distribution reinforcement across the span is the same question asked of a different section, and is designed as one.

  • Bar spacing. Kept between 100 and 300 mm. A design answers a heavier demand with a larger bar rather than with bars closer than 100 mm, and both bounds appear in the flexure check table.

  • One mat. A footing is placed as a single grid, not as two independently detailed faces, so a design ends on one module: the top either at the bottom’s spacing or at exactly twice it, so one top bar lands on every second bottom bar. Within that, mento picks the module and both diameters that give the least steel covering what each face needs — e.g. Ø20 c/15 abajo con Ø20 c/30 arriba. A footing reinforced on one face only keeps its single grid.

  • Ø10 minimum. A footing mesh is not detailed with the thinnest bar in the catalogue, so the search does not reach below Ø10 to shave the lightly loaded face.

  • Depth. A section shallower than the code asks of a footing on soil warns when it is built. Under ACI 318-19 and CIRSOC 201-25 the clause is on the effective depth of the bottom reinforcement — §13.3.1.2 asks at least 150 mm (6 in.) — so what is measured is the depth left to the bars and not the overall thickness:

    Footing(label="Z2", concrete=concrete, steel_bar=steel,
        width=1 * m, height=18 * cm, c_c=50 * mm)
# UserWarning: Footing Z2 leaves 12.5 cm of effective depth to the bottom
# bars, below the 150 mm ACI 318-19 asks of a footing on soil. It is
# designed as given.

    Under EN 1992-1-1 there is no such clause and the 250 mm of practice is compared against the overall thickness instead. Either way it is advice, not a limit: the section is still designed as given, because the depth is the engineer’s to choose. The check is repeated with the actual bars at the end of design and when checking flexure or shear.

For the clauses these come from, the equations, and the readings mento takes where the codes leave room for judgement, see Footing.