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Directions: First, select method A, B, or C at the top of the calculator: method "A" analyzes rectangular (tied) or circular (spiral) reinforced concrete columns — that is, calculates capacity or safe design (factored) load — given dimensions, number and size of rebars, concrete strength and steel yield stress; method "B" calculates rebar size and number given gross dimensions and design (factored) load; and method "C" calculates approximate gross area given design (factored) load and steel ratio. The area determined in method "C" can then be inserted into method "B" in order to calculate number and size of rebars. This calculator is based on the strength design method of ACI 318-08. Reinforcement ratio (steel area / gross column area) must be between 1% and 8%; column analysis (method "A") also checks bar fit based on assumptions about aggregate size and tie or spiral diameter, and assuming 1-1/2 in. cover.
All methods assume that columns are axially loaded, and are not slender enough for buckling to reduce the axial load capacity.
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More detailed explanations and examples can be found in my text.