Flexural stiffness of partially encased composite columns in major axis bending

Abstract

Partially encased composite (PEC) column is a comparatively new type of composite column which consists of a thin-walled welded I shaped steel section with transverse links welded between the opposing flanges. The space between the flanges and the web is filled with concrete. Previously no substantial study has been done on effective flexural stiffness (EI) of PEC columns in major or minor axis of steel section. The influences of several key parameters on the slenderness behaviour of this column are yet to be investigated. Moreover, the ACI equations for EI, currently in use were developed for RC columns subjected to high axial loads and were simply modified, without any further investigation, for use in general composite column design. This study made an attempt to judge the applicability of this equation for the flexural stiffness of PEC columns in which steel shapes are partially encased by concrete. To this end the effective flexural stiffness of partially encased composite columns are evaluated theoretically using the slender column strength curve and the cross-sectional strength curve. Newmark’s iterative procedure was implemented to evaluate the second-order deflection of slender columns which was eventually used to calculate the second-order moment for slender columns. Straincompatibility and force equilibrium equations were used to construct the cross-sectional strength curve. An extensive parametric study has been conducted in this research in order to observe the effects of four geometric and two material variables on the flexural stiffness EI of slender PEC columns subjected to short-term loads and equal end moments causing symmetrical single-curvature bending about the major axis of the encased steel section. A number of 1,200 parametric data regarding EI were generated in this study. This data have been compared to the existing flexural stiffness equation in ACI code. It has been found that the existing ACI equation gives satisfactorily close results at low eccentricities. But at high eccentricities, ACI equation does not give satisfactory results. A regression analysis has been conducted and a design equation was proposed to calculate the flexural stiffness of PEC columns subjected to major axis bending. This proposed equation includes the parameters L/d and e/d, that were proven to be most significant factors affecting the behaviour of PEC columns under major axis bending. The reliability of the proposed EI equation was tested against all the parametric data and was found to be satisfactory.