Behaviour of partially encased composite columns with high strength concrete

Highlights

• FE model capable of simulating the response of partially encased composite columns.

• Complex behaviour due to interaction between concrete and thin-walled steel section.

• Constitutive model for high strength concrete, including post-peak softening branch.

• Load-deformation response predicted accurately for concentric and eccentric loading.

• Parametric study identifies key differences in behaviour when using HS concrete.

Abstract

The behaviour of partially encased composite columns under eccentric and concentric axial loading tends to be complex because of the interaction of the concrete with the thin-walled steel section. When constructed with high strength concrete, developing numerical simulations of the response of these columns under load is particularly challenging. In this paper, a finite element model capable of simulating this response is presented. A constitutive model that has the capability of predicting the overall stress–strain behaviour of high strength concrete, including the post-peak softening branch and residual strength, is selected. The finite element model has been validated by conducting simulations of the behaviour of partially encased composite test columns with high strength concrete. The load-deformation response, ultimate capacity, and failure mode are predicted accurately by the model for both concentric and eccentric axial loading conditions. Finally, a parametric study is presented that uses the validated finite element model to identify the key differences in strength and failure behaviour of partially encased composite columns when using high strength instead of normal strength concrete.

Introduction

In recent years, high strength concrete has been used widely in buildings, bridges and other structures. Use of high strength concrete in columns can significantly reduce the size of the column and consequently reduce the dead load on the foundation system. Moreover, the available floor space for a building can be increased due to the reduction in column size. Partially encased composite (PEC) columns are one of the recent developments in steel-concrete composite construction. This new composite system is composed of very thin steel plates that are welded together into an H-shape, as shown in Fig. 1. Transverse links are installed between the flange tips to inhibit the occurrence of local buckling, and concrete is then poured between the web and the flange plates.

Extensive experimental research has been conducted on thin-walled PEC columns with built-up sections by several research groups [1], [2], [3], [4], [5], [6], [7], [8] to investigate their behaviour under various loading conditions. A series of tests on PEC columns with normal strength concrete under monotonic concentric and eccentric axial loads have been performed by Filion [1], Tremblay et al. [2], Chicoine et al. [3], [4], Bouchereau and Toupin [5] and Prickett and Driver [6]. The results of these experimental investigations on PEC columns indicated that the behaviour of this composite column is significantly affected by the local instability of the thin steel flanges. The failure of the composite columns occurred by a combination local buckling of the steel flanges between the transverse links, yielding of the steel and crushing of the concrete. Experiments on short PEC columns with high performance concrete under pure axial compression, as well as combined axial and flexural compression have been carried out by Prickett and Driver [6] under monotonic conditions. Behaviour of PEC columns under axial compression and cyclic horizontal loads has been studied by Bouchereau and Toupin [5] and Chen et al. [7]. Tests of beam-to-column connections were also performed for beams framing into the weak axis of the PEC column by Muise [8].

Numerical simulations on partially encased composite columns have been performed by Maranda [9], Chicoine et al. [10], Begum et al. [11] and Chen et al. [7]. Maranda [9] and Chicoine et al. [10] modelled one-quarter of the column cross-section with a length of one link spacing. The model developed by these researchers provided a very good representation of the capacity and load versus displacement response of short PEC test specimens with normal strength concrete [2], [3] up to the ultimate load. However, the researchers identified significant challenges in simulating the local instability of the thin flanges and the triaxial behaviour of the partially confined concrete in the column. Begum et al. [11] were able to overcome these challenges in the finite element model through the implementation of a dynamic explicit formulation along with a damage plasticity model for concrete and a contact pair algorithm at the steel-concrete interface. These researchers implemented the model for reproducing the test results of a large number of normal strength concrete PEC test columns [3], [5], [6] encompassing a wide variety of geometric properties and loading conditions. The model has been shown to predict the behaviour of tested PEC columns accurately when they are constructed with normal strength concrete [11]. However, the suitability of this model for predicting the response of PEC columns with high strength concrete had not been explored previously. Moreover, the reference test database for PEC columns with high strength concrete is not as rich as the database for those with normal strength concrete. The primary difference in modelling PEC columns with high strength concrete as opposed to normal strength concrete lies in the unique nature of the constitutive relationship of high strength concrete. Though the effects of high strength concrete on the behaviour of fully encased composite columns are relatively well-understood through tests as well as numerical simulations [12], the results cannot be used directly for PEC columns where the effect of confinement on the concrete is negligible [3], [4], [5], [6]. Therefore, the effects of high strength concrete on the strength and ductility of PEC columns with various geometric properties needs to be explored through parametric investigations.