Lower slenderness limits for rectangular reinforced concrete columns

When designing a reinforced concrete column, most codes of practice specify that second order effects must be taken into account when the column slenderness exceeds a lower limit, usually associated to a certain loss of column carrying capacity. Such lower slenderness limits depend on the strength, stiffness, boundary conditions, and forces acting on the column. In this paper, simplified analytical expressions are presented for the lower slenderness limit of symmetrically reinforced rectangular columns. The limits, associated with a chosen (5 and 10%) loss of bearing capacity compared to nonslender columns, are derived based on general mechanics of concrete structures. The proposed formulae take into account the most important parameters governing the behavior of slender concrete columns, such as axial load, first order end eccentricities, ratio between permanent and total load, creep coefficient, amount and distribution of the reinforcement, and also different loading paths. Similar limits can readily be derived for section shapes other than the rectangular shape considered here. The results of the proposed formulae fit very well with those obtained by a nonlinear and time dependent analysis model previously developed and checked.