Concrete-Filled Double-Skin Steel Tubular Columns: Behavior and Design

By Mostafa Fahmi Hassanein and Mohamed Elchalakani

Concrete-Filled Double-Skin Steel Tubular Columns: Behavior and Design provides a thorough review of the recent advances on the behaviour and design of concrete-filled double-skin steel tubular (CFDST) columns. Drawing on their extensive knowledge and research, the authors cover topics such as different CFDST columns under axial compression, innovative techniques including the use of rubberised concrete, columns with different cross-sections, and steel material envelops and failure modes. This book is an overview of research carried out by this highly experienced and leading research group with specialist knowledge in the topic. It is an invaluable resource for researchers, graduates and post-graduate civil engineers and civil engineering designers.

• Provides a comprehensive overview of advances on the behaviour and design of concrete-filled double-skin steel tubular (CFDST) columns over the past decade
• Gives deep-dive explanation of important concepts such as the void ratio which makes these girders different from conventional concrete-filled steel tubular (CFST) columns
• Explains the failure modes of short and slender columns under compression, with detailed illustrations and photos from both real-life and virtual tests performed by the authors
• Presents in-depth analysis of the ultimate strengths of CFDST columns with different steel envelops and concrete infills
• Makes a detailed comparison with available international codes, such as Eurocode 3, and provides recommendations for future studies
• Discusses new innovative confining stress-based design for different types of CFDST short columns

• Language: English
• Publisher: Elsevier Science
• Release date: Feb 16, 2023
• ISBN: 9780443152290

Mostafa Fahmi Hassanein

Mostafa Fahmi Hassanein has completed his PhD at the age of 31 years from Tanta University, Egypt. Within his PhD study, he has participated in a doctoral steel course at Lulea University of technology, Sweden. He is currently "Professor of Structural Engineering" at the Department of Structural Engineering at Tanta University. His research focuses on the analysis and design of steel and composite structures, with the aim of improving the Design Codes and Standards that are currently used worldwide (e.g. EC3, EC4, AISC and AS 4100), to design more effective structures with minimised initial material costs and life-cycle costs. He has published more than 95 papers in international/Elsevier journals. His research works show his ability to collaborate with researchers from different disciplines and countries. He has served as a reviewer for different reputed international journals and conferences. He has also invited to the 8th European Solid Mechanics Conference (ESMC), Graz, Austria, 2012 as an "Invited Speaker". He has awarded the "State's Incentive Award in the Engineering Sciences" in 2015 from the Academy of Scientific Research and Technology, Egypt. Recently, he has awarded the "First Class Excellence Medal", from the Egyptian President in 2017. He is also a Consultant Engineer in the field of "Design of Steel Structures" in Egypt. He serves as an editorial board member for Thin-Walled Structures, ISSN No. 0263-8231, Elsevier. Based on his achievements, his biography has been accepted into Who's Who in the World, which is comprised of the top 3% of the professionals in the country. He also worked as a professor in the Southwest Petroleum University, Chengdu, China, between July 2019 and Jun 2020. More recently, he has named in Stanford University List for Best 2% Scientists Worldwide, 2020 and 2021.

Book preview

9780443152290_FC

Concrete-Filled Double-Skin Steel Tubular Columns

Behavior and Design

First Edition

Mostafa Fahmi Hassanein

Professor of Steel Structures, Department of Structural Engineering, Tanta University, Tanta, Gharbia, Egypt

Mohamed Elchalakani

Department of Civil Engineering, Faculty of Engineering and Mathematical Sciences, University of Western Australia, Perth, Australia

Unlabelled Image

Table of Contents

Cover image

Title page

Copyright

About the authors

Acknowledgments

1: Introduction

Abstract

1.1: General

1.2: Objectives

1.3: Book organization

2: Development of CFDST columns

Abstract

2.1: Introduction

2.2: Advantages of CFDST columns

2.3: Erection of CFDST columns

2.4: Types of CFDST columns

2.5: Experimental studies

2.6: Finite element studies

2.7: Structural behavior

2.8: Failure modes of CFDST columns

2.9: The mechanism of the inner tube of CFDST columns

2.10: Formulas for compressive strength

2.11: Conclusions

References

3: CFDST short columns formed from carbon steels

Abstract

3.1: Introduction

3.2: Circular-circular CFDST columns

3.3: Circular-square CFDST columns

3.4: Square-square CFDST columns

3.5: Square-circular CFDST columns

3.6: New confining stress-based design for circular-circular CFDST columns

3.7: Conclusions

Appendix

References

Further reading

4: CFDST short columns formed from stainless steel outer tubes

Abstract

4.1: Introduction

4.2: Finite element models

4.3: Comparisons with the experimental results

4.4: CFSST columns

4.5: CFDST columns

4.6: CFDT columns

4.7: Summary and conclusions

References

5: CFDST slender columns formed from stainless steel outer tubes

Abstract

5.1: Introduction

5.2: Nonlinear finite element analysis

5.3: Validation of the FE model

5.4: CFSST columns

5.5: CFDST columns

5.6: CFDT columns

5.7: Conclusions

References

6: Rubberized CFDST short columns

Abstract

6.1: Introduction

6.2: Square RuCFDST short columns

6.3: Circular RuCFDST short columns

6.4: New confining stress-based design

6.5: Conclusions

Appendix I: Progressive axial loading of specimen SHS-O2I2-30

References

7: Future research

Abstract

7.1: Recommendations

7.2: Trends for future relevant works

Index

Copyright

Elsevier

Radarweg 29, PO Box 211, 1000 AE Amsterdam, Netherlands

The Boulevard, Langford Lane, Kidlington, Oxford OX5 1GB, United Kingdom

50 Hampshire Street, 5th Floor, Cambridge, MA 02139, United States

Copyright © 2023 Elsevier Inc. All rights reserved.

No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Details on how to seek permission, further information about the Publisher’s permissions policies and our arrangements with organizations such as the Copyright Clearance Center and the Copyright Licensing Agency, can be found at our website: www.elsevier.com/permissions.

This book and the individual contributions contained in it are protected under copyright by the Publisher (other than as may be noted herein).

Notices

Knowledge and best practice in this field are constantly changing. As new research and experience broaden our understanding, changes in research methods, professional practices, or medical treatment may become necessary.

Practitioners and researchers must always rely on their own experience and knowledge in evaluating and using any information, methods, compounds, or experiments described herein. In using such information or methods they should be mindful of their own safety and the safety of others, including parties for whom they have a professional responsibility.

To the fullest extent of the law, neither the Publisher nor the authors, contributors, or editors, assume any liability for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions, or ideas contained in the material herein.

ISBN: 978-0-443-15228-3

For information on all Elsevier publications visit our website at https://www.elsevier.com/books-and-journals

Unlabelled Image

Publisher: Matthew Deans

Acquisitions Editor: Glyn Jones

Editorial Project Manager: John Leonard

Production Project Manager: Prem Kumar Kaliamoorthi

Cover Designer: Matthew Limbert

Typeset by STRAIVE, India

About the authors

Mostafa Fahmi Hassanein, Professor of Steel Structures, Department of Structural Engineering, Tanta University, Tanta, Gharbia, Egypt

Dr. Mostafa Fahmi Hassanein completed his PhD at Tanta University, Egypt. During his study for this, he participated in a doctoral steel course at Lulea University of Technology, Sweden. He is currently Professor of Steel Structures at the Department of Structural Engineering at Tanta University. His research focuses on the analysis and design of steel and composite structures, with the aim of improving the design codes and standards that are currently used worldwide (e.g., EC3, EC4, AISC, and AS 4100), to design more effective structures with minimized initial material costs and life-cycle costs. He has published more than 95 papers in international journals. His research works show his ability to collaborate with researchers from different disciplines and countries. He has served as a reviewer for various highly regarded international journals and conferences. In 2012 he attended the 8th European Solid Mechanics Conference (ESMC) in Graz, Austria as an Invited Speaker. In 2015, the Academy of Scientific Research and Technology in Egypt awarded him the State’s Incentive Award in the Engineering Sciences, and in 2017, he received the First Class Excellence Medal from the Egyptian President. Dr. Hassanein is also a Consultant Engineer in the field of design of steel structures in Egypt. He serves as an editorial board member for Thin-Walled Structures (ISSN No. 0263-8231, Elsevier). Based on his achievements, his biography has been accepted into Who’s Who in the World, which is comprised of the top 3% of the professionals in Egypt. He also worked as a Professor at Southwest Petroleum University in Chengdu, China between July 2019 and June 2020. More recently, he was named in Stanford University’s list of the world’s top 2% of scientists, in 2020, 2021, and 2022.

Mohamed Elchalakani, Department of Civil Engineering, Faculty of Engineering and Mathematical Sciences, University of Western Australia, Perth, Australia

Dr. Mohamed Elchalakani is an Associate Professor and the Director of the Structural Laboratory at the University of Western Australia. He is the author of two important books Single Skin and Double Skin Concrete Filled Tubular Structures—Analysis and Design and Geopolymer Concrete Structures with Steel and FRP Reinforcements—Analysis and Design, both published by Elsevier. He is a committee member of Australian Standard BD-023: Structural Steel. He is included in Stanford University’s current list of the top 2% of scientists in the world, where he is ranked in the top 0.7% among academics in civil engineering worldwide. His ResearchGate score is 40.1, which is in the top 97.5% of its users worldwide. He holds a few patents on building integrated energy storing systems. He serves on the editorial board of the Journal of Structural Engineering, published by the American Society of Civil Engineers, Scientific Reports, published by Nature, the Australian Journal of Civil Engineering, and Structures, a journal published by Elsevier. Dr. Elchalakani’s current Google Scholar citations are in excess of 5000 with an h-index of 40 where he authored or coauthored more than 250 technical papers. He is a Chartered Professional Engineer in Australia (CPEng) and a Registered Building Practitioner (RBP), and is also registered as a National Professional Engineer (NPE) in Australia, Asia, and Egypt. Dr. Elchalakani has received several awards, including the Holman Medal for the most outstanding PhD thesis and the Hunt Award for Excellence in Research in Engineering. He also received the prestigious Japanese Society for Promotion of Science Fellowship. His total research funds today are in excess of $1.5 million from competitive grants, consulting, and industry-funded projects.

Acknowledgments

The authors are grateful to Amany Refat Elsisy, Heba and Abdelrahman M.F. Hassanein, who helped to prepare and review the manuscript for this book. They would also like to thank Laila, Aya, Yaseen, and Farouk Elchalakani for reviewing all of the chapters.

The authors are grateful for the advice on composite structures received from eminent researchers in civil engineering from different parts of the world including the following: Prof. Nuno Silvestre from Universidade de Lisboa, Portugal; Prof. Leroy Gardner from Imperial College London, UK; Prof. Omnia Kharoob from Tanta University, Egypt; Associate Prof. Qing Liang of Victoria University, Australia; Dr. Vipulkumar Patel of La Trope University, Australia; Prof. Sherif El-Tawil from Michigan University, USA; Prof. Alaa Morsy from Arab Academy for Science, Technology, & Maritime Transport, Egypt; Prof. Metwali Abu Hamad from Cairo University, Egypt; Prof. Sherif Safar and Prof. Ezz-Eldin Sayed Ahmed from the American University in Cairo, Egypt; Prof. Xiao-Ling Zhao from the University of New South Wales (UNSW)/Monash University, Australia; Prof. Gangadhara Prusty and Prof. Serkan Saydam from UNSW, Australia; Prof. Nie Shidong, Dr. Shagea Alqawzai, Prof. Kang Chen, Prof. Le Shen, and Dr. Miao Ding from Chongqing University, China; Associate Prof. Nor Hafizah Ramli Sulong and Dr. Sabrina Fawzia from Queensland University of Technology (QUT), Australia; Associate Prof. Zainah Binti Ibrahim from the University of Malaya, Malaysia; Prof. Allan Manalo from the University of Southern Queensland (USQ), Australia; Prof. Hua Yang, Prof. Lanhui Guo, and Prof. Wei Zhou from the Harbin Institute of Technology, China; Associate Prof. Muhamad Hadi from Wollongong University, Australia; Dr. Mohamed Ali from the University of Adelaide, Australia; Prof. Emad Gad, Prof. Riadh Al-Mahaidi, and Prof. Jay Sanjayan from Swinburne University of Technology, Australia; Prof. Yong-Bo Shao from Southwest Petroleum University, Sichuan, China; Prof. Dilum Fernando and Dr. Chris Becket from the University of Edinburgh, UK; Prof. Hong Hao, Dr. Thong Pham, and Dr. Wensu Chen from Curtin University, Australia; Prof. Jingsi Hu from Hunan University, China; Prof. Brian Uy and Dr. Michael Bambach from the University of Sydney, Australia; Dr. Afaq Ahmed from the University of Engineering and Technology, Taxila, Pakistan; Prof. Sherif Yehia from the American University in Sharjah, United Arab Emirates; and finally Prof. Ali Karrech and Dr. Minhao Dong from the University of Western Australia, Australia, and Prof. Tianyu Xie from the South China University of Technology, China.

Finally, we wish to thank our families for their support and understanding during the many years that we have been undertaking research on composite structures at Tanta University and the University of Western Australia during the preparation of this book.

In the name of Allah, the Entirely Merciful, the Especially Merciful. [All] praise is [due] to Allah, Lord of the Worlds—the Entirely Merciful, the Especially Merciful, Sovereign of the Day of Recompense. It is You we worship and You we ask for help. Guide us to the straight path—the path of those upon whom You have bestowed favor, not of those who have evoked [Your] anger or of those who are astray.

The Holy Quran, Surat Al-Fatihah

1: Introduction

Abstract

Concrete-filled double skin tubular (CFDST) columns have the advantage of being able to resist forces compared to conventional concrete-filled steel tubular (CFST) columns. A CFDST column consists of two concentric steel tubes with concrete sandwiched between them. This book focuses on the compressive strength of CFDST columns and is based on published studies of the last two decades, and it has been recognized that intensive research is still needed for the development of CFDST columns with different cross sections under different parameters. This chapter provides a general introduction to concrete-filled double skin tubular columns. This is followed by the objectives of this book and its organization.

Keywords

Concrete-filled double skin steel tubes; Experimental test; Finite element analysis; Short column; Slender column; Axial compression; Ultimate strength; Design

1.1: General

Concrete-filled double skin tubular (CFDST) columns have the advantage of being able to resist forces compared to conventional concrete-filled steel tubular (CFST) columns. Other important advantages of CFDST columns include their high strength and bending stiffness, higher fire resistance, and favorable construction ability. A CFDST column consists of two concentric steel tubes with concrete sandwiched between them. Unfortunately, there are no significant applications of this new structural column worldwide, partly due to the lack of design provisions in different design manuals and international standards. Accordingly, this book focuses on the compressive strength of CFDST columns and is based on published studies of the last two decades. CFDST columns are the currently recommended structural members for modern buildings.

However, a review of the research carried out on CFDST columns under axial compression is still needed with emphasis on experimental and finite element (FE) studies. Experimental and FE data have been collected and compiled in a comprehensive table using different parameters found in the literature. The review also highlights, based on up-to-date results, the effects of confinement of concrete, initial imperfection and residual stresses, concrete compaction, hollow ratio, thickness ratio, long-term sustained loading, axial partial compressive loading, preloading on steel tubes, steel fiber-reinforced concrete, and external confinement on the behavior of CFDST columns. Specific emphasis is placed on various design methods of CFDST columns with different cross sections. Generally, it has been recognized that intensive research is still needed for the development of CFDST columns with different cross sections under different parameters.

Based on the earlier introduction, it seems that research on CFDST columns with different configurations and materials has been expanded to include many variables. Indeed, the authors have contributed to the development of such elements, as will be seen in this book, which emphasizes on providing designers and researchers with the recent developments in CFDST columns.

1.2: Objectives

The aim of this book is to deepen the understanding of the behavior of CFDST columns. Thus, the main objective is to develop different design procedures for CFDST columns with different lengths (i.e., short, intermediate-length, and long) and materials used in construction. Furthermore, the goals are extended to study the effect of concrete confinement on different configurations. Moreover, the effect of using rubberized concrete on the behavior of CFDST short columns is considered.

1.3: Book organization

This book contains an introduction besides six chapters. Chapter one is concerned with the introduction of this book. Chapter two describes the development of CFDST columns. Chapter three focuses on the compressive strength and behavior of CFDST short columns formed from carbon steels. Chapter four discusses the behavior and strength of CFDST columns formed from stainless steel outer tubes by considering the results provided by the authors. Chapter five describes the overall buckling behavior and strength of CFDST slender columns formed from stainless steel outer tubes. In chapter six, the effect of using rubberized concrete in forming CFDST short columns is thoroughly described. Finally, chapter seven concludes this book by providing conclusions, recommendations, and further topics to be investigated.

2: Development of CFDST columns

Abstract

Concrete-filled double skin tubular (CFDST) columns have the advantage of being able to resist forces compared to conventional concrete-filled steel tubular (CFST) columns. Other important advantages of CFDST columns include their high strength and bending stiffness, higher fire resistance, and favorable construction ability. A CFDST column consists of two concentric steel tubes with concrete filled between them. Unfortunately, there are no significant applications of this new structural column worldwide, partly due to the lack of design provisions in different design manuals. Accordingly, this chapter focuses on the compressive strength of CFDST columns and is based on published studies of the last two decades. CFDST columns are the currently recommended structural members used in the construction of modern buildings. In this chapter, a review of the research carried out on CFDST columns under axial compression is provided, with emphasis on experimental and finite element (FE) studies. Experimental and FE data have been collected and compiled in a comprehensive table using different parameters found in the literature. This review also highlights, based on up-to-date results, the effects of confinement of concrete, initial imperfection and residual stresses, concrete compaction, hollow ratio, thickness ratio, long-term sustained loading, axial partial compressive loading, preloading on steel tubes, steel fiber-reinforced concrete, and external confinement on the behavior of CFDST columns. Specific emphasis is placed on various design methods of CFDST columns with different cross sections. Generally, it has been recognized that intensive research is still needed for the development of CFDST columns with different cross sections under different parameters.

Keywords

Concrete-filled double skin steel tubes; Experimental test; Finite element analysis; Short column; Slender column; Axial compression; Ultimate strength; Design

Notations

Roman letters

ADS cross-sectional area of a CFDST column

Asi cross-sectional area of the inner steel tubes of a CFDST column

Aso,As cross-sectional area of the outer steel tubes of a CFDST column

Asc cross-sectional area of the sandwiched concrete

Ac,no min al nominal cross-sectional area of concrete, given by π(D − 2te)²/4

Ak area of the hollow part

Ap partial bearing area of the compressive load

B depth of the outer tube of a rectangular CFDST column, outer minor axis width of an outer round-end rectangular or elliptical outer tube of a CFDST column

b depth of the inner tube of a rectangular CFDST column, outer minor axis width of an inner round-end rectangular or elliptical inner tube of a CFDST column

D diameter of the outer tube of a circular CFDST column

d diameter of the inner steel tube of a circular CFDST column

e/r load eccentricity ratio

(EI)e effective elastic flexural stiffness of a CFDST column

fy yield strength

fsyi yield strength of the inner tube

fsyo yield strength of the outer tube

fc′ compressive strength of the concrete cylinder (unconfined concrete strength)

fck characteristic concrete strength (0.67fcu)

fcu characteristic cube strength of concrete

fcc′ compressive strength of confined concrete

frp,se′ lateral confining pressure on the sandwiched concrete provided by the outer tube

fyi,corner yield strength of the corners of the inner tubes

fyi,flat yield strength of the flat portions of the inner tubes

fyo,corner yield strength of the corners of the outer tubes

fyo,flat yield strength of the flat portions of the outer tubes

IDS moment of inertia of the CFDST section

KL,Le effective buckling length

kp strength index that provides the influence of the preload on the column strength

kbc bearing capacity factor of a partially loaded CFDST column

Nosc,u compressive capacity of the outer tube with the sandwiched concrete

Ni,u compressive capacity of the inner tube computed as (Asifsyi)

Np preload applied on the outer steel hollow section

Nus ultimate strength of the outer steel tubular column

Ppl,Rd plastic resistance to axial compression taking into account the concrete confinement

Pcr, Pe elastic critical buckling load

PAISC ultimate axial capacity of a CFST column according to the AISC [1]

Pul axial compressive strength of a CFDST column

Puo axial compressive strength of a CFDST short column

Pu,Tao design strength as proposed by Tao et al. [2]

Pu,Has1 design strength as proposed by Hassanein et al. [3]

Pu,Has2 design strength as proposed by Hassanein et al. [4].

Pso section capacities of the outer steel tube

Pc section capacities of the concrete

Psi section capacities of the inner steel tube

Pu,Zha ultimate load for the CFDST columns (SHS inner and CHS outer) proposed by Elchalakani et al. [5]

Pcorner corner capacities of steel tubes

Pflat flat portion capacities of steel tubes

Pu,Li bearing capacity of CFDST sections with preload on the outer steel tube suggested by Li et al. [6]

Pu,Yang bearing capacity of partially loaded CFDST sections (see Fig. 2.7) with preload on the outer steel tube, as expressed Yang by et al. [7]

t, to thickness of the outer steel tubes of a CFDST column

ti thickness of the inner steel tubes of a CFDST column

to thickness of the steel tube

ta top endplate thickness (not the ring-bearing plate)

rexti external radius of the inner steel tube of a CFDST column

rexto external radius of the outer steel tube of a CFDST column

rinti internal radius of the inner steel tube of a CFDST column

rinto internal radius of the outer steel tube of a CFDST column

Greek letters

α steel ratio calculated as α = Aso/Asc

αn nominal steel ratio calculated as αn = Ase/Ac, no min al

β partial compression area ratio, which is greater than unity

ζ confinement factor calculated as ((Asofsyo)/(Ac, no min alfck))

φo stability ratio according to GBJ17-88 [8]

Ω solid ratio, Asc/(Asc + Ak)

γc strength reduction factor

γse factor used to account for the effect of strain hardening on the strength of outer steel

γsi factor used to account for the effect of strain hardening on the strength of inner steel

γss factor used to account for the effect of strain hardening on the strength of stainless steel

λ column slenderness ratio

λp limiting slenderness ratio of a short column

λr limiting slenderness ratio of an intermediate-length column

si47_e  column slenderness parameter (relative slenderness)

σ0.2 0.2% proof stress of stainless steel material

σ3i lateral passive pressure provided by the inner tube of a CFDST column to the sandwiched concrete in the radial direction

σ3o lateral passive pressure provided by the outer tube of a CFDST column to the sandwiched concrete in the radial direction

χ hollow section ratio, given by d/(D − 2te), or reduction factor for relative buckling mode in terms of the relevant relative slenderness calculated using European strut curves

νe Poisson’s ratios of a steel tube with concrete infill

νs Poisson’s ratios of a steel tube without concrete infill

ηp preload ratio

Abbreviations

CFDST concrete-filled double skin tubular

CFST concrete-filled steel tubular

CHS circular hollow section

FE finite element

NA not available

RHS rectangular hollow section

SHS square hollow section

2.1: Introduction

Historically, the concept of double skin composite construction was devised for use in submerged tube tunnels [9]. A graph presenting a cross section of double skin composite construction is shown in Fig. 2.1[10]. This cross section was used for the first time in the Kobe Minatojima Submerged Tunnel in Japan [11]. Recently, concrete-filled double skin tubular (CFDST) columns have been under consideration as load-bearing elements in construction projects. A CFDST column consists of two concentric steel cylinders with concrete filled between them. Currently, CFDST columns are recommended structural members because they have several advantages over conventional concrete-filled steel tubular (CFST) columns or reinforced concrete or structural steel columns [12].

Fig. 2.1

Fig. 2.1 An example of a submerged tube tunnel cross section [10] .

This chapter presents the state-of-the-art knowledge on CFDST columns, including experimental and finite element (FE) studies. A summary of the real or virtual (FE) experiments reported in the literature is presented in a tabular form. The report includes the behavior of short and slender CFDST columns. A detailed discussion on the effects of concrete confinement, initial imperfection and residual stresses, concrete compaction, hollow ratio, thickness ratio, long-term sustained loading, axial partial compressive loading, preloading on steel tubes, steel fiber-reinforced concrete, and external confinement on the behavior of CFDST columns is presented. The failure modes of CFDST columns are briefly outlined, followed by the design methods.

2.2: Advantages of CFDST columns

Steel-concrete composite columns have widespread usage as load-bearing constituents in construction. Therefore, considerable research efforts have been devoted to investigate CFST columns (see Fig. 2.2). Using different rigorous analysis methods (see, for example, Johansson [13] and Chitawadagi et al. [14]), simplified design approaches for CFST columns have been developed. These design approaches have been included in modern codes such as the EC4 [15] and the AISC [1]. Nevertheless, CFST columns have some disadvantages [12], namely, (1) the outer steel bears the largest part of the external load than does the concrete core under axial compression per the same cross-sectional area because of its higher stiffness under composite action; (2) the neutral axis at the central concrete makes an insignificant contribution to flexural strength; (3) the central concrete makes an insignificant contribution to torsional strength; (4) the initial elastic dilation of the concrete under compression is small, and, thus, the confining pressure provided by the steel tube to the concrete is relatively low during the elastic stage [16,17]; and (5) due to the heavy self-weight of the concrete, any improvement in the strength-to-weight ratio of CFST columns is limited.

Fig. 2.2

Fig. 2.2 A typical concrete-filled steel tubular (CFST) column.

From the earlier paragraph, it is clear that the central part of the concrete core of a CFST column can be effectively replaced by another smaller, hollow steel tube with similar axial, flexural, and torsional strengths maintained. This form of column construction is known as a CFDST column. Fig. 2.3 displays the basic forms of the cross-sectional representatives of CFDST columns, where CHS, SHS, and RHS stand for circular, square, and rectangular hollow sections, respectively. However, a CFDST column has several advantages over a CFST column, which could be summarized as:

•It has higher axial, flexural, and torsional strengths [12,16–18] compared to those of a CFST column. Additionally, its strength-to-weight ratio is significantly improved by replacing the central concrete with a steel tube of a much smaller cross-sectional area. Moreover, the inner tube expands laterally under compression loading, and, hence, the confining pressure provided to the concrete increases [19]. Consequently, in a CFDST column, the initial confining pressure builds up more rapidly than it does in a CFST column so that both elastic strength and stiffness are enhanced.

•A CFDST column contains less concrete, which creates a more sustainable environment by reducing the embodied energy levels of the column.

•The cavity inside the inner tube provides a dry atmosphere for possible catering of facilities or utilities such as power cables, telecommunication lines, and drainage pipes. Therefore, CFDST columns are chiefly used in maritime structures.

Fig. 2.3

Fig. 2.3 Cross-sectional types of CFDST columns. (A) Circular CFDST: CHS inner and CHS outer. (B) Circular CFDST: SHS inner and CHS outer. (C) Square CFDST: CHS inner and SHS outer. (D) Square CFDST: SHS inner and SHS outer. (E) Rectangular CFDST: RHS inner and RHS outer. (F) Rounded-end rectangular CFDST. (G) Elliptical CFDST.

2.3: Erection of CFDST columns

During construction, the inner tube of a CFDST column is first erected. This is then followed by the erection of the outer tube. After the erection of both tubes, concrete is poured in between them. Fig. 2.4 shows the erection process of CFDST columns used in transmission towers [20]. Plates and bolts are used to fix the position of the tubes while the flanges on both ends of each tube are used to connect it with its extension.

Fig. 2.4

Fig. 2.4 Erection of CFDST columns (A) during construction and (B) concrete placement.

2.4: Types of CFDST columns

2.4.1: According to length

The relationship between typical strength (Pul) and the slenderness ratio (λ) for columns under axial compression is presented in Fig. 2.5. For a CFDST column, λ is defined as:

si54_e    (2.1)

where Le, IDS, and ADS are the effective buckling length, the sectional moment of inertia, and the cross-sectional area of the CFDST column, respectively. As can be seen, the curve is divided into three failure stages: plastic, elastic-plastic, and elastic instability. Accordingly, slender columns may be grouped into intermediate-length columns, which fail by elastic-plastic buckling, and long columns, which fail by elastic buckling (see Fig. 2.5). A review of the available literature shows that there are no existing values for λp and λr for CFDST columns with both carbon steel jackets, as shown in Fig. 2.3. However, in their research, Hassanein and Kharoob [19] provided values for λp and λr for circular CFDST columns with stainless steel jackets. Based on their study [19], circular CFDST columns with stainless steel jackets with slenderness ratios λ greater than si55_e are considered slender. On the other hand, the slenderness limit delineating between intermediate-length and long columns (λr) for circular CFDST columns is si56_e [19], where σ0.2 is the proof stress of the outer stainless steel tube. Hence, it can be concluded that intensive research is required to find the proper values of λp and λr for CFDST columns with different cross sections (Fig. 2.3), with both carbon and stainless steel jackets.

Fig. 2.5

Fig. 2.5 Column strength-slenderness ratio relationship.

2.4.2: According to straightness

Studies on CFDST columns involve investigations of straight, inclined, and tapered columns, as can be seen in Fig. 2.6. Nowadays, inclined and tapered columns are used as load transfer members in some particular structures [21]. The cross-sectional areas of straight columns are equal along their column length, and they are also equal from the bottom to the top for inclined columns but with a certain inclination angle. On the other hand, tapered columns are characterized by a gradual reduction in their cross-sectional area from the bottom to the top due to the tapered angle.

Fig. 2.6

Fig. 2.6 Typologies of CFDST columns. (A) Straight column. (B) Inclined column. (C) Tapered column.

2.5: Experimental studies

In the last two decades, several experimental (Exp) tests have been performed on CFDST columns with carbon steel tubes, loaded on the entire section, as summarized earlier by Han et al. [22] in 2004 and by Zhao and Han [23] in 2006. In recent years (i.e., after 2006), several tests have been conducted, but they have not yet included in one publication providing the added information and the required areas to be investigated. Accordingly, this section provides information about the experimental tests conducted on CFDST columns.

Tests on CFDST short columns with outer and inner CHSs were carried out by Wei et al. [16], Tao et al. [2], and Uenaka et al. [24]. Consequently, simplified formulas for the strengths of short columns were suggested [2,24]. Circular CFDST columns with inner SHS and outer CHS tubes were investigated by Elchalakani et al. [5], and they proposed a strength predictor for these columns. Han et al. [22] explored CFDST columns with inner CHS and outer SHS tubes. They also proposed equations [22]. In addition, tests on CFDST short columns with outer and inner RHS tubes were carried out by Tao and Han [25], and, based on the test results, a simplified formula for the strength of short columns was suggested. Additionally, a simplified formula for the strength of square CFDST short columns (with square inner tubes) was proposed based on the tests conducted by Zhao and Grzebieta [18].

The studies also extended to the fire performance of CFDST columns exposed to standard fire, as reported by Lu et al. [26]. Long-term loading was well-investigated by Han et al. [27]. The research conducted by Han et al. [21] on CFDST columns included two columns with stainless steel jackets on different cross-sectional types (cross sections shown in Fig. 2.3A, C, F, and G) and column typologies (straight, inclined, and tapered). More recently, experiments on tapered CFDST columns have been conducted by Li et al. [20], whereas partially loaded columns (Fig. 2.7) have been considered by Yang et al. [7]. Finally, Ho and Dong [12,28] used external steel rings to improve the strength, stiffness, and ductility of CFDST short columns.

Fig. 2.7

Fig. 2.7