How Footings Work
Soil Capacity, Structural Load, and Footing Surface Area are Critical
As every builder knows, the foundation is the first critical engineering decision made for a structure. Without adequate support, the structure can settle or suffer serious and sometimes catastrophic failures. This is true not only for continuous footings used in most single-family residential construction, but also for so-called “isolated” footings used under load-bearing posts commonly found in deck, post-frame, and porch construction. Fortunately, by their isolated nature, determining footing size is relatively straight forward.
Footings Spread the Weight of a Structure Over a Larger Surface
Because the bottom of a post has a relatively small, concentrated surface area, the structural load on the post can push through the soil, allowing settling and structural damage. Footings function by dispersing the weight on the post over a wider area, thereby reducing the number of pounds per square foot (psf) at any point on the soil surface. By increasing the surface area with a footing, more load can be supported by the same post without settling.
A footing, by itself, provides vertical support, keeping the weight of the structure from forcing the post further into the soil. Lateral (side to side) stability can be provided the backfilled soil or other means. Uplift resistance can be provided by various means, including cleats attached to the post.
Bearing Force
The downward bearing force acts as an axial load on the column and is a combination of live, dead, snow, and other loads on the building. Bearing force and soil load carrying capacity are the variables that determine footing size. The FootingPad under the column transfers the vertical load from the concentrated area of the column out to a larger area so that the pressure under the FootingPad is within the allowable limits of the soil.
Deck and Post-Frame Load Calculation Guides
Use these guides if you’d like to manually calculate your footing size by post. These guides also offer information on formulas associated with deck and post-frame building construction, including snow load weight.