What is a presumptive bearing value?

A code-tabulated allowable foundation pressure assigned to a broad soil or rock class, usable without a site-specific geotechnical investigation where the building official permits it. IBC Table 1806.2 lists vertical bearing from 1,500 psf (clays and silts) up to 12,000 psf (crystalline bedrock), plus lateral bearing and sliding values. They are deliberately conservative — a geotechnical report almost always justifies more.

When can I use presumptive values instead of a geotech report?

IBC 1806.1 allows them where soil classification is determinable and no questionable conditions exist; the building official can require an investigation (IBC 1803) for questionable soils, expansive soils, high water table, deep fill, or larger structures. Many jurisdictions restrict presumptive values to light-frame or small structures. Mud, organic silt, unprepared fill, and peat get no presumptive capacity at all.

What are the IBC presumptive vertical bearing values?

Crystalline bedrock 12,000 psf; sedimentary and foliated rock 4,000 psf; sandy gravel or gravel (GW, GP) 3,000 psf; sand, silty sand, clayey sand, silty gravel, clayey gravel (SW, SP, SM, SC, GM, GC) 2,000 psf; clay, sandy clay, silty clay, clayey silt, silt, sandy silt (CL, ML, MH, CH) 1,500 psf — per IBC Table 1806.2.

How do the lateral bearing and sliding columns work?

Lateral bearing (psf per foot of depth below grade) resists horizontal loads on embedded elements like poles and foundation walls — it accumulates with depth. Lateral sliding resistance is either a friction coefficient times dead load (granular soils and rock) or a cohesion value capped at one-half the dead load (clays, 130 psf in the IBC table). IBC 1806.3 allows increases for isolated poles and specific conditions.

Can presumptive values be increased?

The classical one-third stress increase for wind or seismic load combinations applies only where the alternative basic load combinations of IBC 1605.2 are used — not with strength-design combinations. Some legacy codes allowed increases with footing width or depth; the IBC does not, beyond the specific provisions of 1806.3. When you need more capacity, the answer is a geotechnical investigation, not a bigger multiplier.

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Presumptive Bearing Values — Reference

By Michael Flynn, PE · water resources & dam safety engineer

Allowable foundation pressures by soil class per IBC Table 1806.2, for use where the code permits design without a site-specific geotechnical investigation. Confirm local amendments — several states modify this table.

IBC Table 1806.2 — Presumptive Load-Bearing Values

Class of material	Vertical bearing (psf)	Lateral bearing (psf/ft depth)	Lateral sliding
1. Crystalline bedrock	12,000	1,200	μ = 0.70
2. Sedimentary & foliated rock	4,000	400	μ = 0.35
3. Sandy gravel and/or gravel (GW, GP)	3,000	200	μ = 0.35
4. Sand, silty sand, clayey sand, silty gravel, clayey gravel (SW, SP, SM, SC, GM, GC)	2,000	150	μ = 0.25
5. Clay, sandy clay, silty clay, clayey silt, silt, sandy silt (CL, ML, MH, CH)	1,500	100	cohesion 130 psf

Sliding: friction coefficient μ applies to dead load for classes 1–4; for class 5, cohesion times contact area, not to exceed one-half the dead load. Mud, organic silt, organic clays, peat, and unprepared fill have no presumptive capacity — investigation required (IBC 1806.2 footnotes).

Presumptive means "absent better information." These values are screening-level and deliberately low. A geotechnical investigation routinely doubles or triples usable bearing on decent soils, pays for itself on anything bigger than light-frame construction, and is mandatory where soils are questionable, expansive, or saturated (IBC 1803). The values here are the floor, not the target.

Quick Screening Numbers

Situation	Working number
Continuous wall footing, 1,500 psf soil, 8-ft wall	≈ 16″–24″ wide
Column on 2,000 psf soil, 40 kip service load	≈ 4.5 ft × 4.5 ft
Minimum footing depth (frost, typical mid-Atlantic/Southeast)	12″–36″ per local code

Screening only — size from actual loads with the bearing capacity tool or the geotech report's allowable pressure.

Sources: 2021 International Building Code, §1806 and Table 1806.2 (ICC). Soil classifications per ASTM D2487 (USCS). Check state/local amendments — the table is frequently modified.

Computing ultimate capacity instead? Terzaghi bearing capacity → · Settlement? Consolidation tool.

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Pair with the unit weights card for dead-load takeoffs, the Terzaghi bearing capacity tool for computed capacity, and the Rankine earth pressure tool for wall design.

Presumptive Bearing Values — Reference

IBC Table 1806.2 — Presumptive Load-Bearing Values

Quick Screening Numbers

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