Nonlinear El Equation for Slender Reinforced Concrete Columns

The ACI Building Code permits the use of a moment magnifier approach for the design of slender reinforced concrete columns. This approach was introduced into design practice to eliminate the need for extensive calculations, based on the solution to a differential equation, to compute second-order bending moments in columns. The moment magnifier approach is influenced by the critical buckling load P^sub c^. The computation of P^sub c^ is strongly influenced by the effective flexural stiffness EI, which varies due to the nonlinearity of the concrete stress-strain curve, creep, and cracking along the length of the unsupported column. This study was conducted to examine the influence of different variables on EI used for the design of slender, tied, rectangular reinforced concrete columns in braced frames under short-term loads and also to examine the existing ACI EI equations. Over 11,000 isolated square reinforced concrete columns, each with a different combination of specified properties of variables, in symmetrical single curvature bending, were simulated to generate the stiffness data. A new nonlinear design equation to compute EI of reinforced concrete columns was then developed from the simulated stiffness data and is proposed as an alternative to the existing ACI design equations for EI. [PUBLICATION ABSTRACT]