Sizing for soil pressure
The plan size comes from service loads (ACI 318-19 13.3.1.1). The gross pressure P/A + w ± 6M/(BL²) must not exceed the allowable gross bearing pressure, where w is the weight of the footing and backfill. Keep e = M/P ≤ L/6 so the whole base stays in compression. Strength design then uses factored loads and the net factored pressure: the footing's own weight causes no shear or moment.
One-way and punching shear
One-way shear is checked at d from the column face in each direction. Footings are exempt from the size effect (λs = 1, 13.2.6.2). Punching is checked on a perimeter at d/2 around the column. The shear is Pu minus the pressure inside the perimeter, plus the fraction γv of the unbalanced moment transferred by eccentric shear (8.4.4.2), and it is limited by the smallest of the three Table 22.6.5.2 expressions. Shear usually governs the thickness.
Flexure and detailing
Moments are taken at the column face, with the trapezoidal pressure where there is a moment. Steel is at least 0.0018bh. In rectangular footings, the fraction 2/(β + 1) of the short-direction steel is concentrated in a central band one footing-width wide (13.3.3.3). Bars must develop from the column face, and the column load must pass into the footing in bearing (22.8), with dowels of at least 0.005Ag.
Worked example
Column 500 × 500, PD = 1000, PL = 600 kN, MD = 50, ML = 30 kN·m, qa = 250 kPa, footing 3 × 3 × 0.6 m at 1.5 m, f′c 28, Ø20 bars. Service pressure 226 / 191 kPa ≤ 250. Pu = 2160 kN and qu = 240 kPa on average. Punching: d = 505 mm, Vu = 1918 kN, vu = 1.00 MPa ≤ φvc = 0.75 × 0.33√28 = 1.31 MPa. Moments at the face: 603 kN·m (L, with moment) and 563 kN·m (B). Both need less steel than the 0.0018 × 600 × 3000 = 3240 mm² minimum, so 11Ø20 each way.
FAQ
Why is the minimum steel governing?
Footings are thick for shear, so the flexural steel needed is often below 0.0018bh. Increasing the bar size beyond the minimum adds nothing.
Does the tool cover biaxial moment?
No – the moment acts in one direction only. For biaxial moment, check the corner pressure P/A ± 6Mx/(BL²) ± 6My/(LB²) separately.