Behaviour and strength of fully encased composite columns

Abstract

This study presents experimental as well as extensive numerical investigations on fully encased composite (FEC) columns under concentric and eccentric axial loads. The experimental program consisted of thirteen (13) FEC columns of two different sizes with various percentages of structural steel and concrete strength. These FEC columns were tested for concentrically and eccentrically applied axial loads to observe the failure behaviour, the ultimate load carrying capacity and axial deformation at the ultimate load. Numerical simulations were conducted on FEC columns under axial compression and bending using ABAQUS, finite element code. Both geometric and material nonlinearities were included in the FE model. A concrete damage plasticity model capable of predicting both compressive and tensile failures, was used to simulate the concrete material behaviour. Riks solution strategy was implemented to trace a stable peak and post peak response of FEC columns under various conditions of loading. To validate the model, simulations were conducted for both concentrically and eccentrically loaded FEC test specimens from current study and test specimens from published literatures, encompassing a wide variety of geometries and material properties. Comparisons were made between the FE predictions and experimental results in terms of peak load and corresponding strain, load versus deformation curves and failure modes of the FEC columns. In general, the FE model was able to predict the strength and load versus displacement behaviour of FEC columns with a good accuracy. A parametric study was conducted using the numerical model to investigate the influences of geometric and material properties of FEC columns subjected to axial compression and bending about strong axis of the steel section. The geometric variables were percentage of structural steel, column slenderness (L/D), eccentricity ratio (e/D) and spacing of ties (s/D). The compressive strength of concrete (fcu) and yield strength of structural steel were used as the material variables in the parametric study. The strength of the materials were varied from normal to ultra-high strength. In general, L/D ratio, e/D ratio, strength of steel and concrete were found to greatly influence the overall capacity and ductility of FEC columns. The effects of ultra-high strength concrete (120 MPa) and ultra-high strength steel of 913 MPa on the FEC column behaviour was also explored. Use of ultra-high strength structural steel in FEC column increased the overall capacity by 40% accompanied by a reduction in the ductility by 17 %. However the ductility was regained when the tie spacing was reduced by 50%. Finally, the experimental as well as the numerical results were compared with the code (ACI 2014, AISC-LRFD 2010 and Euro code 4) predicted results. The equations given by the three codes can safely predicte the capcity of FEC columns constructed with UHSM (concrete 120 MPa and structural steel 913 MPa) for concentric axial load. For concentrically loaded FEC columns the material limits specified in these codes may be extended to cover the range of ultra-high strength materials. However, the simplified plastic stress distribution proposed in AISC-LRFD (2010) was found to be unsafe for predicting the load and moment capacities of eccentrically loaded FEC columns with ultra-high strength structural steel and concrete.