Cm-factor for RC slender columns under unequal eccentricities and skew angle loads at the ends

Abstract

Reinforced concrete slender columns, in general, are subjected to a non-uniform bending moment law, due to the occurrence of unequal eccentricities and skew angles at the ends. Design codes suggest calculating an equivalent moment factor Cm in each bending direction with the aim of reducing the analysis of the column to another equivalent one with equal eccentricities and angles at the ends.This paper presents a new equation to compute the equivalent moment factor Cm derived from the numerical model. This model has been calibrated and checked with 255 experimental tests from the literature. The experiments correspond to RC slender columns, pinned–pinned and subjected to short-term ultimate loads and unequal eccentricities at the ends, for both uniaxial bending and biaxial bending. In HSC slender columns, a factor k3 is proposed to be applied along the concrete cover region in order to take into account the concrete cover spalling off.From the comparison with the current formula of the design codes (ACI-318 and Eurocode 2), the proposed Cm increases the safety factor for low and medium slenderness of the column (λg<20), low levels of axial load (N/Ncr<0.50), high-strength concrete (50<fc<100MPa) and double curvature bending (β<0). The proposed formula allows reinforced concrete slender columns to be both checked and designed with enough accuracy for practice engineering, and is simple to apply.