IBC Table 1806.2: Presumptive Load-Bearing Values of Foundation Soils
The International Building Code permits shallow foundations to bear directly on presumptive vertical soil values when professional laboratory geotechnical borings are not mandated:
| IBC Soil Class | Material Description | Allowable Bearing (q_a) | Lateral Bearing Pressure |
|---|---|---|---|
| Class 1 | Crystalline bedrock (Granite, Basalt, Gneiss) | 12,000 psf | 1,200 psf/ft |
| Class 2 | Sedimentary & foliated rock (Slate, Shale, Hard Limestone) | 4,000 psf | 400 psf/ft |
| Class 3 | Sandy gravel and/or gravel (GW and GP) | 3,000 psf | 200 psf/ft |
| Class 4 | Sand, silty sand, clayey sand (SW, SP, SM, SC) | 2,000 psf | 150 psf/ft |
| Class 5 | Clay, sandy clay, silty clay (CL, ML, MH, CH) | 1,500 psf | 100 psf/ft |
IRC Prescriptive Code Minimums vs. Geotechnical Load Equations
Residential building codes enforce a dual check. The footing width must satisfy both calculated soil bearing mechanics and prescriptive minimums:
Calculated Geotechnical Width (B_calc)
The physical soil mechanics equation requires B = (PLF / qa) × 12 in/ft. For heavy structures on weak Class 5 clay (1,500 psf), calculated width often expands to 24 to 32 inches, governing over the basic prescriptive charts.
IRC Table R403.1 Prescriptive Minimums
Even on strong bedrock (12,000 psf) where calculated width is just 4 inches, IRC Section R403.1.1 imposes a statutory absolute floor: footings shall never be less than 12 inches wide and 6 inches thick.
50-State Concrete Footing Cost Benchmarks (2026)
Turnkey contractor rates per linear foot for continuous residential strip footings including machine trenching, Grade 60 rebar, and ready-mix concrete.
| State Jurisdiction | Turnkey Cost / Linear Ft | Typical 140-Ft Foundation | Regional Labor Factor |
|---|---|---|---|
| Alabama | $32 - $58 | $5,800 | 0.88x |
| Arizona | $36 - $65 | $6,500 | 0.98x |
| California | $48 - $88 | $8,600 | 1.30x |
| Colorado | $40 - $72 | $7,100 | 1.06x |
| Florida | $34 - $62 | $6,100 | 0.92x |
| Georgia | $34 - $62 | $6,100 | 0.92x |
| Illinois | $39 - $70 | $7,000 | 1.05x |
| Indiana | $35 - $64 | $6,300 | 0.95x |
| Michigan | $37 - $68 | $6,700 | 1.01x |
| Missouri | $35 - $64 | $6,300 | 0.95x |
| New York | $47 - $86 | $8,400 | 1.28x |
| North Carolina | $34 - $62 | $6,100 | 0.92x |
| Ohio | $36 - $65 | $6,400 | 0.97x |
| Pennsylvania | $39 - $72 | $7,100 | 1.08x |
| Tennessee | $33 - $60 | $5,900 | 0.90x |
| Texas | $34 - $62 | $6,100 | 0.92x |
| Virginia | $37 - $68 | $6,700 | 1.01x |
| Washington | $46 - $84 | $8,200 | 1.24x |
Worked Builder Case Study: 2-Story Brick on Silty Clay (1,500 psf)
General contractor sizing continuous foundation footings for a 160 LF perimeter
Scenario: A builder in Indianapolis, Indiana is pouring a new foundation for a 2-story home with a full concrete basement and exterior brick veneer. The local soil is stiff silty clay (IBC Class 5, qa = 1,500 psf). Total calculated line load is 3,040 PLF (Roof: 720 PLF + Floors: 800 PLF + Brick: 720 PLF + 8" concrete basement wall: 800 PLF).
Engineering & Code Sizing:
- Geotechnical Sizing: B_calc = (3,040 / 1,500) × 12 = 24.32 inches → rounded to 26 inches wide.
- IRC Table Check: IRC Table R403.1(2) prescribes 26 inches width × 8 inches thickness.
- ACI 318 Projection Verification: The 8-inch stem wall leaves a projection of P = (26 - 8) / 2 = 9.0 inches. To satisfy the unreinforced projection rule (T ≥ P), the thickness must be increased from 8" to 10 inches thick.
- Material Takeoffs (160 LF): Requires 11.8 cubic yards of 3,000 PSI ready-mix concrete delivered and 3 continuous runs of Grade 60 #4 rebar (552 linear feet) on 3-inch dobies.
- Project Cost: Turnkey excavation, concrete, rebar, and forms total $7,850 ($49.00 per linear foot).
When Presumptive IBC Tables Are Prohibited
Section 1806.2 of the International Building Code strictly forbids the use of presumptive soil bearing values in specific ground conditions:
Frequently Asked Questions: Soil Bearing Capacity & Footings
What is presumptive soil bearing capacity under IBC Table 1806.2?
The International Building Code (IBC Table 1806.2) establishes default presumptive vertical load-bearing values when laboratory soil borings are not conducted: Class 1 Crystalline Bedrock (12,000 psf), Class 2 Sedimentary Rock (4,000 psf), Class 3 Sandy Gravel (3,000 psf), Class 4 Sand and Silty Sand (2,000 psf), and Class 5 Clay and Silty Clay (1,500 psf). Most residential building departments assume Class 4 or 5 (1,500 to 2,000 psf) in the absence of geotechnical reports.
How is continuous concrete footing width calculated?
Geotechnical footing width is calculated as B = (Total Line Load in PLF / Allowable Soil Bearing in psf) * 12 inches/foot. For example, a two-story home exerting 2,500 PLF on 1,500 psf clay requires (2,500 / 1,500) * 12 = 20.0 inches width. The governing width must also satisfy prescriptive minimums in IRC Tables R403.1(1) through R403.1(3) and cannot be less than 12 inches.
What is the ACI 318 projection rule for unreinforced concrete footings?
Under ACI 318-19 and IRC Section R403.1.1, the footing projection P is the distance the footing extends outward beyond the face of the stem wall: P = (Footing Width - Stem Wall Width) / 2. For plain (unreinforced) concrete footings, the footing thickness T must equal or exceed the projection (T >= P) to prevent diagonal tension shear failure. If a 26-inch footing supports an 8-inch stem wall, the projection is 9 inches, requiring at least a 10-inch thick footing unless transverse rebar is provided.
How much clear concrete cover is required over rebar in footings?
Per ACI 318-19 Table 20.6.1.3.1, concrete cast against and permanently exposed to earth (such as a foundation footing poured into a trench) requires a mandatory minimum of 3.0 inches (75 mm) of clear concrete cover over all reinforcing steel. Rebar must be elevated on concrete dobie blocks or plastic rebar chairs, never laid directly on dirt or rocks.
How much does it cost to pour continuous concrete footings in 2026?
In 2026, turnkey continuous strip footings cost $35 to $75 per linear foot ($52/LF national median) including trench machine excavation, Grade 60 rebar grid, and 3,000 to 4,000 PSI ready-mix concrete delivered ($160 to $210 per cubic yard). Adding wood edge formwork adds $6 to $12 per LF, while a concrete line pump truck adds a flat $450 to $750 fee.
Calculations adhere to International Building Code (IBC 2024 Table 1806.2), International Residential Code (IRC 2024 Section R403), ACI 318-19 (Building Code Requirements for Structural Concrete), and ASTM D1586 (Standard Test Method for Standard Penetration Test). Cost schedules reflect Q1 2026 RSMeans ready-mix and excavation indices.