Testing, modelling and design of high strength concrete-filled high strength steel tube (HCFHST) stub columns under combined compression and bending

Highlights

• Eccentric compression tests on high strength concrete-filled high strength steel tube (HCFHST) stub columns were conducted.

• The structural behaviour and resistances of HCFHST stub columns under combined loading were studied.

• The eccentric compression test results for HCFHST stub columns were discussed.

• FE models were developed to validate against the test results and then used to conduct parametric studies.

• The applicability of the existing design codes to HCFHST stub columns under combined loading was evaluated.

Abstract

This paper presents comprehensive experimental and numerical investigations into the structural behaviour and resistances of high strength concrete-filled high strength steel tube (HCFHST) stub columns under combined compression and bending. The combined loading tests were conducted on twelve stub column specimens fabricated from S700 high strength steel square hollow section tubes with two cross-section dimensions and concretes with three grades (C40, C80 and C110). The combined loading test setup, procedures and results, including the failure loads, load–mid-height lateral deflection curves and failure modes, were reported in detail. The failure loads and the evolution of the neutral axis of the HCFHST stub column specimens under combined loading were analysed. The development and distribution of the transverse and longitudinal strains of the outer S700 high strength steel tubes were discussed. The experimental programme was followed by a numerical modelling programme, where finite element models were initially developed and validated against the test results and then used to conduct parametric studies to generate further numerical data. The test and numerical data were used to evaluate the applicability of the relevant design rules, as set out in the European code, American specification and Australian/New Zealand standard, to HCFHST stub columns under combined loading. The evaluation results generally indicated that the European code and Australian/New Zealand standard slightly overestimate the failure loads for HCFHST stub columns under combined loading, while the American specification offers accurate, consistent and safe failure load predictions when applied to eccentrically loaded HCFHST stub columns.

Introduction

Concrete-filled steel tube (CFST) columns, comprising outer steel tubes and inner concrete cores, are widely used as main load-carrying components in long-span bridges and high-rise buildings. Recently, the use of high strength materials in CFST columns has become increasing prevalent, in order to accommodate the growing needs for constructing taller buildings and longer bridges. High strength steels are specified as those with material yield stresses greater than 460 MPa, while high strength concretes are defined as those with cylinder strengths greater than 60 MPa. In order to investigate the distinct structural behaviour and design of CFST columns fabricated with the use of high strength materials, experimental, numerical and analytical studies have been previously conducted, with a brief review on the experimental studies for those under combined loading (i.e. compression and bending moment) provided herein. Specifically, Lee et al. [1] conducted eccentric compression tests on CFST stub columns fabricated from high strength steels and normal strength concretes, investigated their structural behaviour and resistances under combined compression and bending, and assessed the current design codes based on the test data. Liu [2] and Du et al. [3] carried out a series of eccentric compression tests on normal strength concrete-filled high strength steel tube (NCFHST) long columns, investigated their global buckling behaviour and resistances, highlighted the inaccuracy of the current codified design rules, and proposed new accurate design approaches. Experimental investigations into high strength concrete-filled normal strength steel tube (HCFNST) stub and long columns under combined compression and bending have been conducted by Li et al. [4] and Portoles et al. [5], respectively, with the influence of concrete grades on the structural behaviour and failure loads discussed. With regard to high strength concrete-filled high strength steel tube (HCFHST) columns under combined compression and bending, Choi et al. [6] and Xiong et al. [7] conducted a series of tests on long members with various cross-section shapes, concrete grades and member lengths and highlighted the inaccuracy of the relevant codified design rules, while Varma et al. [8] performed monotonic and cyclic loading tests on HCFHST long columns to investigate their seismic behaviour and moment capacities under combined loading and verified that the current American code ACI 318–14 [9] provides reasonable moment capacity predictions. However, research into the structural behaviour of HCFHST stub columns under combined loading (where the second order effects and moments are not prominent) remained limited, with the only study carried out by Fujimoto et al. [10] in the early 2000s but without design analyses according to the current design codes. This thus prompts the present study, aiming at further investigating the structural behaviour and resistances of HCFHST stub columns under combined compression and bending and verifying the applicability of the established codified design rules.

In this paper, a testing programme was firstly conducted, including material tests to obtain the material properties of the outer high strength steel tubes and inner high strength concrete cores and eccentric compression tests to investigate the behaviour of HCFHST stub columns under combined compression and bending. Subsequently, a numerical modelling programme was conducted for the purpose of replicating the test results and generating further numerical data. The test and numerical data were adopted to evaluate the applicability of the design rules for CFST stub columns under combined compression and bending, as set out in the European code EN 1994–1-1 [11], American specification ANSI/AISC 360–16 [12] and Australian/New Zealand standard AS/NZS 5100 [13], to their HCFHST counterparts.