Footings transfer building loads into soil or rock, but a local bearing number is only one input. Actual foundation behavior depends on soil classification, density, moisture, groundwater, fill control, settlement, frost, scour, slope stability, load path, footing structure, and adopted-code review.
IBC Chapter 18 includes presumptive bearing context, and those rows are useful for early planning. They do not classify your site soil, approve fill, replace settlement analysis, generate structural loads, or authorize construction. This guide explains how to use local IBC-style prompts while keeping the source gaps visible until a geotechnical engineer, structural engineer, contractor, inspector, and AHJ have the project-specific data.
Local IBC-Style Bearing Prompts
A practical planning snapshot often uses rows such as crystalline bedrock at 12,000 PSF, sedimentary or foliated rock at 4,000 PSF, sandy gravel or gravel at 3,000 PSF, sand or silty sand at 2,000 PSF, and clay or silty clay at 1,500 PSF. Treat these as local prompts unless the adopted code edition, local amendments, soil classification, bearing elevation, and AHJ direction are confirmed.
The prompt row is not a field classification. Topsoil, organics, uncontrolled fill, wet or soft clay, loose sand, high groundwater, expansive soil, frost-susceptible soil, scour, liquefaction, and slope effects can make a row unusable or incomplete. A current geotechnical report or AHJ-reviewed value should override a local planning row.
Soil Bearing Capacity Estimator
Estimate presumptive soil bearing capacity per IBC Table 1806.2 and size spread footings.
Bearing Area Arithmetic
The basic screen is simple: footing bearing area equals entered service load divided by the selected allowable bearing prompt. A 30,000 lb service load with a 2,000 PSF prompt needs 15 square feet of bearing area before rounding and before project-specific additions.
That arithmetic is not a footing design. The load must come from the project load path and applicable load combinations. The footing still needs self-weight, overburden, surcharge, groundwater, buoyancy, eccentricity, sliding, overturning, frost, drainage, concrete thickness, shear, flexure, reinforcement, dowels, cover, and constructability review where applicable.
When the Prompt Is Not Enough
Bring in project-specific geotechnical review when the structure is larger or heavily loaded, soil is variable, fill is present, groundwater is high, the site slopes, settlement matters, clay is expansive or highly plastic, frost or scour is a concern, nearby excavations or surcharges exist, or the building official requires it.
A geotechnical report can document borings, test pits, soil classification, density, moisture, groundwater, laboratory data, bearing recommendations, settlement estimates, earthwork requirements, undercut, compaction, and construction observation. Those items are outside a local bearing-area screen.