Pole footing

A pole, post or column embedded in earth, or in a concrete footing in earth, resisting lateral load by soil bearing: the depth of embedment of IBC 2024 Section 1807.3, with the presumptive soil values of Section 1806. The method is allowable stress only, because the Table 1806.2 values are allowable values. Loads combine with ASCE 7-22 2.4.1.

Inputs of the pole footing #

• Restraint at the ground surface: nonconstrained (1807.3.2.1, Eq. 18-1) or constrained by a rigid floor or slab-on-ground (1807.3.2.2, Eq. 18-2 and 18-3). Whether a floor truly restrains the post is your judgment; the floor or slab itself is not designed here.
• Installation: a hole backfilled with concrete, a column embedded in a poured concrete footing, clean sand, or controlled low-strength material (1807.3.3).
• Footing and post: shape and size of each, and the provided embedment d. With a concrete or CLSM installation, Width b for lateral bearing chooses the footing or the post. The code defines b as the diameter of a round post or footing, or the diagonal of a square one (the program takes the diagonal itself). The California DSA Bulletin 09-06 lists conditions for taking b as the footing; the choice and those conditions are yours. Sand backfill always uses the post, because the sand is part of the soil.
• Backfill concrete f'c, concrete unit weight, and whether the footing weight counts in the bearing check.
• Soil: a class of Table 1806.2, or entered values from a geotechnical report.
• Isolated pole: twice the lateral bearing (1806.3.4). Off by default. It applies to isolated poles such as flagpoles or signs, and to poles of buildings not adversely affected by a 1/2 in. motion at the ground surface under short-term lateral loads. That judgment is yours; the program never assumes it.
• 15× cap of 1806.3.3: 15 times the tabular value (prefilled, the usual reading, and the conservative one with the doubling) or 15 times the doubled value.
• Increase for W or E combinations: 1.0 prefilled. IBC 1806.1 allows one-third more only with the alternative combinations of IBC 1605.2, which are not computed here; a geotechnical report may state its own increase.
• Loads per load type, unfactored: lateral force P at its height h above grade, moment M at grade in the same sense as P·h, vertical load V (downward positive; the weight of the post is yours to enter as D). Wind and seismic act both ways. A lateral force in a gravity load type keeps the sign you enter.
• Factor of safety, uplift, and the code options on brittle materials (1807.3.1).

Auto-design (editor toolbar) varies the embedment d (1/2-ft steps) and the footing diameter or side (2-in. steps); the post, soil, restraint, installation, loads and increases are kept. Objectives: least volume (footing area × embedment), or shallowest embedment and then least volume.

Checks performed #

The result also gives the lateral force (or the moment at grade, when constrained) that the provided depth allows.

The drawings (screen, report, DXF and PDF sheet) show the elevation with the post, grade or the floor, the footing or hole, the lateral forces at their heights, the dimensions and the depths where S1 and S3 are taken, and the footing plan. Moments and vertical loads are listed in the sheet notes. Reinforcement and the post section are not drawn, because the calculation does not design them.

Limitations #

• The post or pole itself, the footing reinforcement, the transfer of forces into the concrete and the floor or slab at grade are not designed. A note gives the moment at grade for the column calculation. Wood poles must meet the AWPA U1 treatment of 1807.3.1 item 3, which is not checked.
• The IBC method only: no ASABE EP 486.3 and no other approved method (1807.3.2), no deflection calculation, no sloping ground, no layered soil, no water table. Friction under a footing (1806.3.1) is not added to the lateral resistance.
• Eq. 18-1 needs a lateral force: a load combination with a moment at grade and no lateral force is not feasible in the nonconstrained case.
• The 1605.2 alternative combinations are not computed. Local amendments are not applied (the City of Phoenix, for example, limits presumptive values and the ratio of depth to width without a geotechnical report; the California DSA has its own requirements).

Verification examples #

Verification status: 26 values compared in 6 examples (6 published, 0 hand calculations from the cited equations): 26 within tolerance, 0 recorded deviations. Code basis: partial. Only some provisions were read in the target edition; the rest rests on older editions, examples or summaries.

Published worked examples (backend/tests/test_verify_pole_footing.py), all on earlier IBC editions with the same equations. The published forces are the design level, so they are entered as load type L (factor 1.0 in D + L).

PDHonline Course S235 (J. W. Andrew, IBC 2006, pp. 3 to 5): a sign on a 32 in. footing in sandy clay, nonconstrained, twice the lateral bearing, 1,200 lb at 16 ft, 775 lb vertical.

CalcBook, Pole Foundation Design (IBC 2021, web page, Steps 1 to 4): a 6 kip vehicle load at 2.25 ft on a 2 ft footing in clay, constrained, no increase.

Maplesoft, Maple Flow flagpole footing (IBC 2018, pp. 2 to 3): 30 kips at 3 ft on a 4.5 ft footing, 350 psf/ft, constrained, isolated-pole factor 2. The published values are in Pa and m.

Connecticut DAS, Soils and Foundations (2016 OEDM Career Development, IBC 2012, p. 20): a 12 in. utility pole with 9 ft of embedment in sand, 200 lb at 40 ft. The slide stops after one pass of the equation.

Hand calculations of the 12 ft limit, the 15× cap, the 2× factor, square diagonals, uplift and Metric input are in backend/tests/test_pole_footing.py. The text of IBC 2024 Sections 1807.3.2.1, 1806.3.3 and 1806.3.4 was read in the 2025 California Building Code, which is based on it. Equations 18-2 and 18-3, Section 1806.1 and Table 1806.2 were read only in the secondary sources above and in earlier editions; see the project's documents-needed list.