Concrete Pad Footing

How it calculates

The Concrete Pad Footing calculator designs square or rectangular reinforced concrete pad footings to AS 3600:2018 (Amendment 2). It checks bending, one-way shear, and punching shear in both plan directions, and verifies bearing pressure under service loads.

Geometry and inputs

The footing is modelled as a rectangular slab of plan dimensions X × Y and thickness D, with a rectangular column of dimensions X_c × Y_c centred on the footing. Soil depth above the footing (d_soil) and column offset from the centroid can also be specified.

Load distribution and bearing check

Applied column loads (axial force N, biaxial moments M_x and M_y, and shear) are resolved into a non-uniform bearing pressure distribution across the footing plan. The maximum bearing stress q_max is compared to the allowable bearing capacity q_a:

utilization = q_max / q_a ≤ 1.0

A limited stability check confirms that the footing remains in total compression (resultant load eccentricity within the kern) under working loads.

Flexural analysis (AS 3600:2018, Cl. 8.1)

Bending moments are calculated at the critical section for each direction (at the column face). Required flexural reinforcement A_st is determined from the ultimate moment demand. Moment capacity phi × M_u is checked in both the X and Y directions:

utilization = M / (phi × M_u) ≤ 1.0*

Minimum reinforcement per Cl. 8.1.6.1 is enforced: A_st,min = 0.19 × (D/d)^2 × (f'c / f_sy) × B × d

One-way (wide beam) shear (AS 3600:2018, Cl. 8.2.4.3)

Critical shear planes are taken at distance d from the column face in both plan directions. The simplified shear strength method (valid for f'c ≤ 65 MPa, no prestress, tension, or torsion) is applied:

utilization = V / (phi × V_u) ≤ 1.0*

Punching shear (AS 3600:2018, Cl. 9.3)

The critical perimeter for punching shear is taken at d/2 from the column face on all sides. Punching shear demand V* is the applied column load minus the upward soil reaction within the critical perimeter. Capacity phi × V_uo is a function of concrete strength, critical perimeter length, and effective depth.

utilization = V / (phi × V_uo) ≤ 1.0*

Assumptions

The column is rectangular and centred on the footing; the footing is subject to axial loads and uniaxial bending only. Overturning is not fully checked - only a limited stability check on working loads is performed. Concrete shear strength uses the simplified method (Cl. 8.2.4.3).