Pole Footing (IBC 2021)

How it calculates

The Pole Footing (IBC 2021) calculator designs plain (unreinforced) concrete pier and pole footings to IBC 2021 and ACI 318-19. Soil checks use service (ASD) load combinations. Concrete strength checks use factored (strength) load combinations.

Lateral embedment

The calculator back-calculates the lateral soil stress S' from the IBC embedment equations, using your actual embedment depth d. It compares S' to the allowable lateral soil stress S_a:

utilization = S' / S_a ≤ 1.0

• Nonconstrained piers (free at grade): S' is taken at one-third of the embedment depth, and S_a = S × min(d, 12 ft) / 3.
• Constrained piers (restrained at grade, for example by a slab): S' is taken at the embedment depth, and S_a = S × min(d, 12 ft).

S is the allowable lateral pressure per unit depth. Applied moments are converted to an effective lateral load, V_s + M_s / h, using the height of load application. A required embedment depth is reported for guidance, with no pass/fail.

Vertical bearing

Gross bearing pressure at the pier tip is the governing ASD axial load over the gross pier area. In Combined mode, downward skin friction is subtracted first. The check is skipped in Shaft Friction Only mode.

utilization = q_s / q_a ≤ 1.0

Compression and bending capacity

The pier is checked as a plain concrete member under factored loads. The interaction ratio is the larger of a compression-face and a tension-face check:

compression face = M_u / (phi × M_n,c) + P_u,max / (phi × P_n)

tension face = (M_u / S_m - P_u,min / A_g) / (phi × 5 × lambda × √f'c)

The compression capacity term uses the total pier length as a conservative length. The section modulus S_m is reduced for an embedded post. The phi factor for plain concrete is 0.60.

Shear capacity

Factored shear at the ground surface is compared to the plain concrete shear strength. Capacity uses a concrete shear stress of 2 × lambda × √f'c. It applies to half the net area for embedded posts, or three-quarters of the gross area for a bracket:

utilization = V_u / (phi × V_n) ≤ 1.0

Uplift safety factor

When the uplift check is on, each ASD combination with net uplift is listed. Resistance is skin friction below the active zone, plus pier weight if you choose to include it:

FS_u = (Q_s + W_f) / P_uplift ≥ FS_u,min

Q_s = uplift skin friction × π × b × (d - active zone depth). W_f is the pier self-weight, factored by the ASD dead load factor or unfactored, as selected.

Downward capacity check

Each ASD combination with downward axial load is listed. Resistance depends on the Axial Capacity Mode: End Bearing Only, Shaft Friction Only, or Combined. Pier self-weight is deducted when self-weight is considered:

FS_dn = (Q_p + Q_s,down - W_f) / P ≥ FS_dn,min

Q_p is end bearing (q_a × A_g). Q_s,down is downward skin friction below the active zone.

Concrete bearing at the post-footing interface

The factored axial load at the post is compared to the plain concrete bearing strength. Bearing strength increases by up to 2 times where the bearing area is smaller than the pier. This check runs only when no moment is applied to the post. Otherwise the calculator asks you to check bearing separately.