Cm factor for non-uniform moment diagram in RC columns

Generally, reinforced concrete columns are subjected to a non-uniform moment diagram due to unequal eccentricities at the two ends of the columns or to transverse loads along the columns. Most building codes and material specific codes allow the design of columns with the factor Cm, in order to simplify the general analysis of the column to approximate a column with equivalent uniform moment diagram. This paper presents expressions for Cm according to the type of load that is applied along the column. Two Cm expressions are proposed: the first one approaches the solution using a differential equation based on the elastic behavior of the columns; the second one is based on results obtained through a numerical simulation. This study is valid for hinged reinforced high and normal strength concrete columns, subjected to a sustained and short time load with compression and uniaxial bending. The proposed method can be used both to design and analyze RC columns with enough accuracy for engineering practice and involves higher precision and safety factor in comparison with current codes (Eurocode-2 (2004) [European Committee for Standardization. Eurocode 2: Design of concrete structures—Part 1: General rules and rules for buildings. EN 1992-1-1:2004] ACI-318 (08) [ACI Committee 318. Building code requirements for structural concrete (ACI 318–08). Detroit: American Concrete Institute; 2008]).

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