Concrete Footer Calculations

Weight Dispersal

• Concrete footers disperse the weight of the structure they carry outward. Within the footing itself, the load is dispersed at a 45 degree angle. Within the soil beneath the footing, the load is dispersed at an even greater angle.To better understand the weight dispersal of your structure, measure the width of your concrete footing. This number is equal to the depth of soil it will take to reduce the load by one half. Go down the same depth again and the load has been reduced by two thirds.The type of soil located directly beneath the concrete footing is a very important factor in whether the footings will continue to settle once they have been set in place.

Soil Types

• The strongest soil type is bedrock, followed by gravel, course sand, fine sand, clay, silt and lastly, organic matter. All but the most dense soil types contain pockets of water and air. To prevent your concrete footers from shifting even on good quality soil, compact the soil where they will stand or add a 1 to 2 inch layer of gravel. The load bearing value of your soil should also be factored into the equation when you are calculating how many and what type of concrete footers you will require.

Footer Size

• The size and shape of your concrete footings can help determine how weight is distributed into the soil from the structure above. To calculate the appropriate size of your concrete footings, you will have to consider several factors including the load bearing value of your soil, the material and therefore the weight of the structure you are supporting, and the amount of stories the structure will have. For example, if your soil is rated at 2,500 pounds per square inch, and you are building a 2-story, brick facade over wood frame home, you will likely require 15 inch wide concrete footings. When constructing a new building the architect or engineer will not use an estimate, but will calculate the actual weight of the structure and the quality of the soil, and recommend the proper footing size.

Lifespan

• Concrete footings which have been either poured or constructed of block are meant to last 75 years or more.