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Footing size is determined by
a. Engineering data regarding the soil and structural loads
b. NC Building Code minimum footing widths
1. Single story, conventional wood frame = 16”
2. Single story, brick veneer over wood frame = 16”
3. Use Min. Width of Concrete or Masonry Footings chart for various
conditions
4. Assumes a soil with a bearing capacity of 2000 psf
c. Accepted “rule of thumb” is based on thickness of foundation wall
1. Width of footing = 2 x foundation wall thickness
2. Depth of footing = foundation wall thickness
3. Assumes firm, undisturbed soil or engineered fill
4. Minimum soil bearing capacity of 2000 psf
Footing Design and Construction
Steel Reinforcement may be added to footing to control tension forces
a. Two longitudinal rebars, in bottom portion of footing carry tensile loads
1. Prevents spreading apart of footing where cracks occur due to uneven
settlement
2. Tension forces occur only in bottom portion of footing
3. Top portion of footing is in compression
b. Bar size is based on 1/8”
Example: #4 bar = 4/8” (1/2)
c. Transverse bars may be tied to longitudinal bars at regular intervals to aid
in resisting cracking
d. “Chairs” hold bars in place.
Frost line
a. The frost line is the depth to which the ground will freeze.
1) Depth varies with geographic location.
2) Local codes specify the minimum footing depth.
b. The footing must be placed below the frost line.
1) When the ground freezes and thaws, it expands and contracts.
2) If footing is placed above the frost line, it will be pushed upward causing cracks in the foundation.
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