1. Calculate the total number of linear feet of your foundation. For example, if your foundation is 30 feet wide and 40 feet long, then add all the sides together to get the total
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Concrete is good in compression, poor in tension. Steel is good in tension, poor in tension. By the addition of reinforcing steel into concrete, you take the strengths of one material
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I am assuming you are trying to calculate the amount of concrete needed. You take the volume of the form  multiply length times width times depth, in feet, subtract the volume of
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Rebar or reinforcing steel and a solid base will help your concrete fight
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