When designing a structure’s foundation, it’s important to take into account the soil’s bearing capacity when designing the footing. The lower the bearing capacity of the soil, the wider the footing needs to be to offset the reduced capacity.
If the structure is going to reside on expansive soil or soil with low bearing capacity, additional foundation support may need to be designed to prevent structural damage. Deep foundations can minimize or eliminate structure settlements or problems caused by expansive or weak soil.
Minimum Widths of Concrete & Masonry Footings (inches)
Conventional wood frame construction
| Load-Bearing Value of Soils (psf) | 1,500 | 2,000 | 2,500 | 3,000 | 3,500 | 4,000 |
|---|---|---|---|---|---|---|
| 1-story | 16 | 12 | 10 | 8 | 7 | 6 |
| 2-story | 19 | 15 | 12 | 10 | 8 | 7 |
| 3-story | 22 | 17 | 14 | 11 | 10 | 9 |
4″ brick veneer over wood frame or 8″ hollow concrete masonry
| Load-Bearing Value of Soils (psf) | 1,500 | 2,000 | 2,500 | 3,000 | 3,500 | 4,000 |
|---|---|---|---|---|---|---|
| 1-story | 19 | 15 | 12 | 10 | 8 | 7 |
| 2-story | 25 | 19 | 15 | 13 | 11 | 10 |
| 3-story | 31 | 23 | 19 | 16 | 13 | 12 |
8″ solid or fully grouted masonry
| Load-Bearing Value of Soils (psf) | 1,500 | 2,000 | 2,500 | 3,000 | 3,500 | 4,000 |
|---|---|---|---|---|---|---|
| 1-story | 22 | 17 | 13 | 11 | 10 | 9 |
| 2-story | 31 | 23 | 19 | 16 | 13 | 12 |
| 3-story | 40 | 30 | 24 | 20 | 17 | 15 |
Table source: Table 403.1; CABO One and Two Family Dwelling Code; 1995
Additional Resources can be referenced on FEMA’s Foundation Analysis and Design.
Reinforced concrete footings are proportioned according to the provisions of ACI 318, Building Code Requirements for Structural Concrete. It is often opined that foundations should not yield, due to the high cost of foundation repair. However, nonlinear soil behavior is common in strong ground shaking, and it is traditional to design foundations for the reduced forces computed with the response modification factor, R, used for the superstructure. Neither the NEHRP Recommended Provisions nor earlier model building codes required the use of amplified forces for foundation design.
