behaviour and strength of fully encased composite columns

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BEHAVIOUR AND STRENGTH OF FULLY ENCASED
COMPOSITE COLUMNS

MD. SOEBUR RAHMAN

DOCTOR OF PHILOSOPHY
(CIVIL & STRUCTURAL)

DEPARTMENT OF CIVIL ENGINEERING
BANGLADESH UNIVERSITY OF ENGINEERING AND TECHNOLOGY
DHAKA, BANGLADESH

DECEMBER, 2016
BEHAVIOUR AND STRENGTH OF FULLY ENCASED
COMPOSITE COLUMNS

by

Md. Soebur Rahman

A thesis submitted to the Department of Civil Engineering of Bangladesh University of
Engineering and Technology, Dhaka, in partial fulfillment of the requirements for the degree
of
DOCTOR OF PHILOSOPHY
(CIVIL & STRUCTURAL)

DEPARTMENT OF CIVIL ENGINEERING
BANGLADESH UNIVERSITY OF ENGINEERING AND TECHNOLOGY
DHAKA, BANGLADESH

December, 2016
CERTIFICATE OF APPROVAL
The thesis titled “Behaviour and Strength of Fully Encased Composite Columns”, by
Md. Soebur Rahman, Student Number 0412044001 (F) Session: April/2012 has been
accepted as satisfactory in partial fulfillment of the requirements for the degree of Doctor
of Philosophy (Civil & Structural) on 04 December, 2016.

BOARD OF EXAMINERS

Dr. Mahbuba Begum Chairman
Professor (Supervisor)
Department of Civil Engineering
BUET, Dhaka-1000

Dr. Raquib Ahsan Member
Professor (Co-Supervisor)
Department of Civil Engineering

Dr. Abdul Muqtadir Member
Professor and Head (Ex-officio)
Department of Civil Engineering
BUET, Dhaka-1000

Dr. Syed Fakhrul Ameen Member
Professor
Department of Civil Engineering
BUET, Dhaka-1000

Dr. Ahsanul Kabir Member
Professor
Department of Civil Engineering
BUET, Dhaka-1000.

Dr. Md. Nazrul Islam Member
Professor (External )
Department of Civil Engineering
DUET, Gazipur.
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DECLARATION

Except for the contents where specific reference have been made to the work of others,
the studies embodied in this thesis is the result of investigation carried out by the author.
No part of this thesis has been submitted to any other University or other educational
establishment for a Degree, Diploma or other qualification (except for publication).

(Signature of the Student)
Md. Soebur Rahman

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ACKNOWLEDGEMENT
In the name of Allah, the most Gracious and the most Merciful
The author sincerely expresses his deepest gratitude to the Almighty.
First and foremost, the author would like to express thank to his supervisor Dr. Mahbuba
Begum, Professor, Department of Civil Engineering, BUET. It has been an honour to be
her first Ph.D. student. Her guidance on the research methods, deep knowledge,
motivation, encouragement and patience in all the stages of this research work has been
made the task of the author less difficult and made it possible to complete the thesis work.
The author also wishes to express his deepest gratitude to his co-supervisor Dr. Raquib
Ahsan, Professor, Department of Civil Engineering, BUET for his constant guidance,
invaluable suggestions, motivation in difficult times and affectionate encouragement,
which were extremely helpful in accomplishing this study.
The author also grateful to all the most respected members of Doctoral Committee for
their valuable and constructive advice and suggestions throughout this research works.
The author also takes the opportunity to pay his heartfelt thanks to all the staff members
of Concrete Laboratory and Strength of Materials Laboratory for their consistent support
and painstaking contributions to the research and experimental work.
The author also appreciatively remembers the assistance and encouragement of his
friends and well wishers and everyone related to carry out and complete this study.
Finally, the author wishes to express his deep gratitude to his family members, wife and
two daughter (Sumya and Safika) for their constant support, encouragement and sacrifice
throughout the research work.
.

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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 (f ) and yield strength of structural steel were used
cu
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.
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TABLE OF CONTENTS
ACKNOWLEDGEMENT iv
ABSTRACT v
TABLE OF CONTENTS vi
LIST OF FIGURES xi
LIST OF TABLES xiv
LIST OF SYMBOLS xvi
LIST OF ABBREVIATIONS xix
CHAPTER 1 INTRODUCTION
1.1 General 1
1.2 Objectives and Scope of the Study 3
1.3 Organization of the Thesis 5
CHAPTER 2 LITERATURE REVIEW
2.1 Introduction 7
2.2 Types of Composite Columns 8
2.3 Research on Steel-Encased Concrete Columns 9
2.3.1 Experimental investigations 9
2.3.2 Numerical and analytical investigations 15
2.3.3 Comparison of codes 18
2.4 Conclusions 22
CHAPTER 3 REVIEW OF DESIGN CODES ON COMPOSITE COLUMNS
3.1 Introduction 23
3.2 ACI-318 (2014) 23
3.2.1 Axial compressive strength 23
3.2.2 Flexural and axial loads 25
3.3 AISC-LRFD (2010) 26
3.3.1 Axial compressive strength 26
3.3.2 Axial loads and flexure (P-M) 28
3.4 Euro Code 4 (2005) 31
3.4.1 Resistance of cross sections 31
3.4.2 Axial load and bending moment (P-M) 32
3.5 Material Properties and Detailing Criteria 38
3.6 Conclusions 41
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CHAPTER 4 EXPERIMENTAL INVESTIGATIONS OF FEC COLUMNS
4.1 Introduction 42
4.2 Test Program 42
4.2.1 Description of test specimens 42
4.2.2 Explanation of test parameters 45
4.3 Column Fabrication 46
4.3.1 Steel section fabrication 46
4.3.2 Concrete mix design 47
4.3.3 Concrete placement 48
4.4 Material Properties 49
4.4.1 I-Shaped structural steel 49
4.4.2 Steel reinforcement 50
4.4.3 Concrete 51
4.5 Test Setup and Data Acquisition System 53
4.5.1 Setup and instrumentation of concentrically loaded FEC columns 54
4.5.2 Setup and instrumentation of eccentrically loaded FEC columns 55
4.6 Observations and Failure Mode 55
4.6.1 Failure of concentrically loaded columns 56
4.6.1.1 Column in Group SCN4A 57
4.6.1.2 Column in Group SCN4B 59
4.6.1.3 Column in Group SCH6A 61
4.6.1.4 Column in Group SCH6B 63
4.6.2 Failure of eccentrically loaded columns 64
4.6.2.1 Column Group SCN4E 64
4.6.2.2 Column Group SCH6E 65
4.7 Load versus Deformation Relationship 66
4.7.1 Concentrically loaded columns 66
4.7.2 Eccentrically loaded columns 70
4.8 Conclusions 71
CHAPTER 5 FINITE ELEMENT MODEL OF FEC COLUMNS
5.1 Introduction 73
5.2 Properties of Test Specimens 73
5.2.1 Test specimens from current study 74
5.2.1.1 Normal strength concrete FEC columns 74
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5.2.1.2 High strength concrete FEC columns 75
5.2.2 Test specimens from published literature 76
5.3 Geometric Properties of the Finite Element Model 83
5.3.1 Element selection 83
5.3.2 Mesh description 84
5.3.3 Modeling of steel-concrete interactions 85
5.3.4 End boundary conditions 85
5.4 Material Properties 86
5.4.1 Steel 86
5.4.2 Concrete 87
5.4.2.1 Stress-Strain relationship for concrete in compression 89
5.4.2.2 Stress-Strain relationship for concrete in tension 92
5.5 Load Application and Solution Strategy 93
5.5.1 Newton Raphson and Modified Newton Raphson Methods 93
5.5.2 The Riks Method 94
5.6 Conclusions 96
CHAPTER 6 COMPARISON OF NUMERICAL RESULTS WITH EXPERIMENTAL
DATA
6.1 Introduction 97
6.2 Performance of Finite Element Model 97
6.2.1 Axial load versus axial deformation 97
6.2.1.1 Test specimens from current study 98
6.2.1.2 Test specimens from published literature 102
6.2.2 Axial capacity and axial strain 107
6.2.2.1 Test specimens from current study 107
6.2.2.2 Test specimens from published literature 108
6.2.3 Failure Modes 112
6.2.3.1 Test specimens from current study 112
6.2.3.2 Test specimens from published literature 114
6.3 Contributions of Steel and Concrete in the Capacity of FEC Columns 119
6.4 Effect of Concrete Strength on Axial Capacity of FEC Column 121
6.5 Conclusions 121
CHAPTER 7 PARAMETRIC STUDY
7.1 Introduction 123
viii
7.2 Design of Parametric Study 124
7.2.1 Percentage of I-shaped structural steel 124
7.2.2 Column slenderness ratio, L/D 126
7.2.3 Load eccentricity ratio, e/D 126
7.2.4 Concrete compressive strength, f 126
cu
7.2.5 Transverse reinforcement spacing-to-depth ratio, s/D 126
7.3 Material Properties of Parametric Columns 128
7.4 Results and Discussion 129
7.4.1 Effect of structural steel percentages 129
7.4.1.1 Load versus axial deformation response 130
7.4.1.2 Axial capacity of FEC columns 131
7.4.1.3 Ductility index for FEC columns 133
7.4.1.4 Modes of failure 134
7.4.2 Effect of overall column slenderness ratio 135
7.4.2.1 Load versus axial deformation response 136
7.4.2.2 Peak load and corresponding moment 137
7.4.2.3 Load versus lateral displacement response 138
7.4.2.4 Load versus moment response 139
7.4.2.5 Modes of failure 140
7.4.3 Effect of load eccentricity ratio 142
7.4.3.1 Load versus average axial deformation response 142
7.4.3.2 Peak load and corresponding moment 143
7.4.3.3 Load versus lateral displacement responses 144
7.4.3.4 Axial load versus moment 145
7.4.4 Effect of concrete compressive strength 146
7.4.4.1 Load versus average axial deformation 147
7.4.4.2 Peak load and corresponding moment 148
7.4.4.3 Behaviuor of FEC columns with UHSM 149
7.4.5 Effect of transverse reinforcement spacing 150
7.4.5.1 Load versus axial deformation 151
7.4.5.2 Peak load for different tie spacing 152
7.4.5.3 Effect of tie spacing with UHSM 152
7.5 Conclusions 154
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