Strength and deformation behaviour of eccentrically loaded square concrete filled steel tubular columns

Abstract

Concrete filled steel tubular (CFST) column, comprising a hollow steel tube infilled with concrete with or without additional reinforcements or steel section, has been widely used in high rise building construction. The main advantage of CFST column is that the local buckling of the outer steel tube is delayed or even prevented by the concrete core while the inner concrete core is confined by the steel tube providing enhancement in strength and ductility under high compressive load. Extensive experimental and numerical studies have been carried out by several researchers on concentrically and eccentrically loaded CFST columns with various geometric and material properties. Most of this research work has been performed on CFST columns constructed with available standard tube shapes. However, limited research has been found on CFST columns in built-up steel sections. Current design rules for CFST columns are specified in AISC-LRFD (2010), Eurocode 4 (1994), ACI 318R (2014), British standard BS 5400 (2005) and Canadian Standard Association CSA (2009). But the design of eccentrically loaded CFST column is highly conservative in the available design codes due to the lacking of experimental research. CFST column is a new system for the construction industry of Bangladesh. In the upcoming version of Bangladesh National Building Code (BNBC 2017), the design guidelines for CFST columns are included which is adopted from AISC 2005 specifications. The applicability of these design provisions in the construction environment of Bangladesh needs to be explored. To this end, an attempt has been made in this study to investigate the strength and failure behaviour of the eccentrically loaded square CFST columns constructed with built-up steel section and locally available materials. This paper presents an experimental investigation on the behaviour of eccentrically loaded CFST columns regarding four parameters: concrete compressive strength (fc/): 27 Mpa to 44 Mpa; cross sectional slenderness ratio (B/t): 25 to 42; global slenderness ratio (L/B): 3 to 10 and load eccentricity ratio (e/B): 0 to 0.45. Total eleven CFST columns with square cross section were tested under uniaxial eccentric compression. The influences of these parameters on the failure mode, load-strain response, peak load, ultimate moment, mid-height deflection and performance indexes of the square CFST column were investigated. Finally, the design approaches adopted in (Eurocode 4 and AISC-LRFD 2010) were reviewed and applied to calculate the ultimate axial strength and moment of the tests columns. Subsequently, the predicted values were compared with the experimental results obtained from the experiments. Based on the results, it was observed that concrete compressive strength (fc/), cross sectional slenderness ratio (B/t), global slenderness ratio (L/B) and load eccentricity ratio (e/B) have significant effects on the load and deformation behavior of eccentrically loaded CFST columns. Stiffness and ultimate capacity of the tested column decreased with the increase of cross-sectional width to tube thickness ratio and load eccentricity ratio, whilst they increased with the increase of concrete compressive strength and the decrease of global slenderness ratio of the specimen. On the other hand, Axial strain at peak load and ductility index of the tested specimen decreased with the increase of concrete compressive strength, B/t ratio and L/B ratio but increased with the increase of e/B ratio of the specimen. However, Ultimate bending moment of the tested column increased with the increase of fc/ and e/B ratio, but decreased with the increase of B/t ratio and L/B ratio of the specimen. Eurocode 4 (2005) somewhat overestimated the ultimate axial strengths, but underestimated the ultimate bending moments of the tested square CFST columns in built-up steel sections, whilst AISC-LFRD (2010) presented the best and safe prediction for both of them. Eurocode 4 (2005) predicted higher capacity than the experimental results about 6%. In general, both codes showed good agreement with the experimental results.

TABLE OF CONTENTS

ACKNOWLEDGEMENT v ABSTRACT vi TABLE OF CONTENTS vi LIST OF TABLES xi LIST OF FIGURES xiii LIST OF ABBREVIATION xvii NOTATION xviii

Chapter 1 INTRODUCTION 1.1 General 1 1.2 Objectives and Scope of the Study 4 1.3 Organization of the Thesis 5

Chapter 2 LITERATURE REVIEW 2.1 Introduction 7 2.2 Advantages of Concrete Filled Steel Tubular (CFST) Columns 8 2.3 Applications in Construction of Concrete Filled Steel Tubular (CFST) Columns 11 2.4 Current Development of Concrete Filled Steel Tubular (CFST) Columns 18 2.5 Experimental Investigation on Concentrically and Eccentrically Loaded CFST Columns 20 2.6 Summary 36

Chapter 3 EXPERIMENTAL PROGRAM 3.1 General 38 3.2 Description of Test Specimens 38 3.3 Explanation of Test Parameters 40 3.4 Test Column Fabrication 40 3.4.1 Steel section fabrication 41 3.4.2 Mixing, placing and curing of concrete 41 3.5 Material Properties 43 3.5.1 Steel 43 3.5.2 Concrete 45 3.6 Test Setup and Data Acquisition System 46

Chapter 4 RESULTS AND DISCUSSIONS 4.1 General 50 4.2 Failure Modes 50 4.3 Load-Strain Responses 56 4.3.1 Effect of concrete compressive strength (fc/) 57 4.3.2 Effect of cross-sectional slenderness ratio (B/t) 58 4.3.3 Effect of global slenderness ratio (L/B) 59 4.3.4 Effect of load eccentricity ratio (e/B) 59 4.4 Ultimate Load 60 4.4.1 Effect of concrete compressive strength (fc/) 61 4.4.2 Effect of cross-sectional slenderness ratio (B/t) 62 4.4.3 Effect of global slenderness ratio (L/B) 62 4.4.4 Effect of load eccentricity ratio (e/B) 63 4.5 Axial strain at peak load 64 4.5.1 Effect of concrete compressive strength (fc/) 64 4.5.2 Effect of cross-sectional slenderness ratio (B/t) 65 4.5.3 Effect of global slenderness ratio (L/B) 66 4.5.4 Effect of load eccentricity ratio (e/B) 67 4.6 Axial Load versus Mid-Height Deflection Relation 67 4.6.1 Effect of concrete compressive strength (fc/) 69 4.6.2 Effect of cross-sectional slenderness ratio (B/t) 70 4.6.3 Effect of global slenderness ratio (L/B) 71 4.6.4 Effect of load eccentricity ratio (e/B) 71 4.7 Ultimate Moment 72 4.7.1 Effect of concrete compressive strength (fc/) 73 4.7.2 Effect of cross-sectional slenderness ratio (B/t) 74 4.7.3 Effect of global slenderness ratio (L/B) 74 4.7.4 Effect of load eccentricity ratio (e/B) 75 4.8 Performance Indicies 76 4.8.1 Ductility index 76 4.8.2 Concrete contribution ratio 80 4.9 Summary 84

Chapter 5 DESIGN CODES AND COMPARISIONS 5.1 General 85 5.2 AISC-LRFD (2010) Formulae 85 5.2.1 Axial compressive strength 87 5.2.2 Axial loads and flexure (P-M) 89 5.3 Eurocode 4 (2005) Formulae 92 5.3.1 Resistance of cross sections 93 5.3.2 Axial load and bending moment 94 5.4 Limitations of Design Standards 97 5.5 Comparison of Results with Code Predictions 97 5.5.1 Eurocode 4 (2005) 98 5.5.2 American Institute of Steel Construction (AISC) 100 5.6 Comparison of Results with Axial Load-Bending Moment (P-M) Interaction Curves 103 5.6.1 Effect of concrete compressive strength (fc/) 103 5.6.2 Effect of cross-sectional slenderness ratio (B/t) 105 5.6.3 Effect of global slenderness ratio (L/B) 108 5.6.4 Effect of load eccentricity ratio (e/B) 109 5.7 Summary 111

Chapter 6 CONCLUSIONS AND RECOMMENDATIONS 6.1 General Conclusions 112 6.2 Recommendations for Future Study 113

REFERENCES 115

LIST OF TABLES