Design a Pole Footing (IBC 2021)

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Method & scope

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What it calculates

Column loads link directly from the beam or column calculation above, so changes propagate to the footing automatically. US structural engineers designing pier footings to ACI 318-19, including deck posts, mezzanine columns, and foundations in high-wind areas or unstable soils where uplift governs. The uplift resistance model accounts for skin friction below the active soil zone, giving a more complete picture on variable soil profiles than a simple bearing check alone. Input allowable skin friction, bearing capacity, and active soil movement zone depth to verify resistance against uplift and downward forces. Uplift safety factor summary (restrained vs unrestrained) and resistance tables replace blunt uplift fail messages with a fuller assessment.

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Calculation method

The Pole Footing (IBC 2021) calculator designs plain (unreinforced) concrete pier and pole footings to IBC 2021 and ACI 318-19. The pier is assumed to be continuously restrained against buckling by the surrounding soil, so unbraced length effects are neglected in the concrete compression checks.

Lateral embedment (IBC 2021, Cl. 1807.3)

The critical check for pole footings is lateral embedment, the depth needed to resist applied horizontal forces without excessive rotation. The IBC 2021 Cl. 1807.3 method uses:

• Lateral stress at 1/3 depth S_(1/3): the design lateral soil stress at one-third of the embedment depth
• Lateral stress at full depth S_d: the design lateral soil stress at the embedment depth

utilization = S_d / q_s ≤ 1.0 For nonconstrained piers (free to rotate at the top surface), the required embedment depth is solved iteratively from moment equilibrium. For constrained piers (fixed against lateral movement at grade by a floor slab or tie), a shorter depth may be required.

Vertical bearing

Gross vertical bearing pressure is the total downward service load divided by the pier base area: utilization = q_gross / q_a ≤ 1.0

Compression and bending capacity (ACI 318-19, Ch. 21)

The pier is checked as a plain concrete compression-bending member. Nominal axial capacity P_n and nominal moment capacity M_n are calculated using ACI 318-19 Chapter 21 provisions for plain concrete. The combined interaction is evaluated: utilization = (P_u / (phi × P_n)) + (M_u / (phi × M_n)) ≤ 1.0 The phi factor for plain concrete in combined loading is 0.60 per ACI 318-19.

Shear capacity (ACI 318-19, Ch. 21)

One-way shear demand at the critical section is compared to the plain concrete shear strength: utilization = V_u / (phi × V_c) ≤ 1.0

Uplift safety factor

Uplift is checked two ways. Without skin friction: FS_uplift = (W_pier + W_soil_above) / T_net ≥ required FS With skin friction below the active soil zone: FS_uplift_skin = (W_pier + W_soil_above + f_s × A_skin) / T_net ≥ required FS The restrained and unrestrained uplift safety factors are reported separately.

Downward capacity check

A downward capacity check verifies that the pier can carry the maximum compressive service load including self-weight: FS_down = (phi × P_n) / P_max ≥ 1.0

Concrete bearing at post-footing interface (ACI 318-19, Cl. 22.8)

Bearing stress at the post-footing interface is compared to the plain concrete bearing capacity. This check governs when concentrated point loads are transferred from a post or column into the top of the pier.

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How to use it

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Common questions

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Next steps