Final answer:
In Michigan, footings are required to be placed 42 inches below grade, which is below the frost line to prevent structural damage caused by soil expansion and contraction due to freeze-thaw cycles.
Step-by-step explanation:
In Michigan, footings should be placed 42 inches below grade to be beneath the frost line. This depth ensures that the footings are not affected by the seasonal freeze and thaw cycles that can cause soil to expand and contract, which can lead to structural damage.
It is important for construction to meet these standards to ensure the stability and longevity of a structure.
The minimum depth of a footing depends on the ground's bearing capacity. The depth of a footing on soil should be at least 150 mm above the bottom reinforcement. For footings on piles, the minimum depth should be at least 300 mm.
Footings should extend to a minimum depth of 12 inches below previously undisturbed soil. Footings also must extend at least 12 inches below the frost line (the depth to which the ground freezes in winter) or must be frost-protected.