Plastic buckling and axial crushing of concrete-filled steel tubes: usage of multiple wood blocks

doi:10.1016/j.tws.2019.106487

Thin-Walled Structures

Volume 150, May 2020, 106487

https://doi.org/10.1016/j.tws.2019.106487 Get rights and content

Highlights

•
Plastic Transition Triangle (PTT) is defined for the first time to quantify the transition area of load-displacement.
•
A new index (E_i) is introduced to quantify the structural efficiency to evaluate capacity change against weight.
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The number of timber blocks in a certain volume of timber did not affect the capacity.

Abstract

This paper aims to lighten the weight of concrete-filled steel tubes using wood blocks. Different proportions of wood to concrete were used in a way that wood blocks were substituted with a fraction of concrete in small-scale columns. The behaviour of the composite members was compared with the reference specimens, i.e. fully concrete-filled tubes. Carbon Fibre Reinforced Polymer (CFRP) was also employed as a confining material for some specimens, the effect of which was investigated. The weight reduction that timber provided was evaluated against the axial capacity of each specimen. A new index is introduced to quantify the structural efficiency with a view to evaluating capacity change against the weight. A new parameter is defined in this paper to determine and quantify the transitional response of columns under compression between different peaks loads on load-displacement graphs. This parameter can further help reduce potential risks of failure/collapse at post-peak stages.

A key finding was that the number of timber blocks in specimens with the same volume of timber did not affect the axial capacity, which paves a path for designing timber blocks in multiple numbers. Despite a significant difference between the compression strength of timber and concrete, the capacity reduction provided by timber was not proportionate (was smaller) to the strength difference of the two materials, nor was proportionate to the weight reduction. This disproportion, which can be due to the interaction of all three materials, is structurally desirable especially for the seismic design where the total weight plays a key role. This can be attributed to the usage of stronger and thicker steel acting as a confining material, which is advised for similar structures based on the results of this study.

Introduction

Concrete-filled tubes (CFTs) have attracted significant attention in civil and structural engineering. These structures have been widely adopted by the construction industry, e.g. in buildings, bridges [1]. CFT is formed by filling a tube with filled materials such as concrete. Since the infilled material is circumferentially confined by the tube, it significantly improves the axial and bending capacity, ductility and energy absorption over conventional reinforced concrete and steel components. CFTs can also simplify the construction process by avoiding the formwork and minimising various installation processes. Due to these advantages, different CFTs have been extensively developed and investigated during the last decade.

In addition to the real-world structures, concrete-filled steel tubes (CFST) have gained significant interest as reported by different research papers in the area of structural engineering. Kwan et al. theoretically evaluated the confining stress and lateral strain of CFSTs [2]. They proposed a model using experimental data obtained in the literature in this field. Tao et al. investigated the bonding effect between the concrete and steel tubes in CFSTs [3]. In this reference, experimental methods were used to investigate the effect of different cross-sectional dimensions, types of steel, concrete and interface, and age of the concrete. Wang et al. investigated CFSTs under axial compression [4]. They developed a simplified model to predict the compressive strength of these structures based upon theoretical and experimental studies. Yuan et al. explored CFSTs with square sections under eccentric compression [5]. Finite element analysis and experimental studies were developed by them to evaluate the structural behaviour of their models. Lee et al. carried out tests under concentric axial compression on CFSTs with rectangular sections [6]. The CFSTs in this study were made by high-strength concrete and high-strength steel slender sections and investigated through finite element analysis and experimental modelling.

Different approaches have been developed to further improve the structural performance of CFSTs. Long et al. experimentally investigated the axial compressive behaviour of CFSTs with internal fibre reinforced polymer (FRP) tubes [7]. Hasan et al. experimentally studied the axial performance of CFSTs strengthened by steel reinforcing bars [8]. They found that reinforced CFSTs had higher capacity and ductility compared to the stiffened specimens. Ekmekyapar and Alhatmey looked at the performance of internally ring-stiffened high-performance CFSTs [9]. They focused on studying the post-fire resistance, and concluded that inner rings increased the contact between tubes and the concrete. Han et al. investigated the effect of defects on square concrete-filled tubes [10]. They compared the effect of long-term and short-term loading on the structural integrity of these structures. Li et al. investigated concrete-filled tubes made with seawater and sea sand [11]. They used FRP for strengthening these elements and developed a simplified theoretical model to find the load contribution carried by FRP.

In the last decade, research in this area has focused on reducing the weight of structural members aiming to improve the structural and seismic performance of different structures. Concrete-filled double-skin steel tubes (CFDST) are part of a different focus in CFT's research area, which is developed by pouring concrete between internal and external tubes. Since there is a hollow section in CFDST, it substantially reduces the total weight of the element compared to fully concrete-filled tubes. Hassanein and Kharoob provided a summary of previously developed compressive strength formulas used for capacity prediction of CFDSTs [12]. They also proposed a design formula for CFDSTs. Li et al. studied the axial compression behaviour of sea sand and seawater in CFDSTs [13]. Different parameters such as confinement factor, slenderness ratio, and void ratio were investigated. Wang et al. experimentally assessed the cyclic behaviour of CFDSTs [4]. They investigated the effect of hollow ratio for CFDSTs subjected to cyclic pure torsion. Li and Cai investigated the axial performance of CFDST using high-strength steel [14]. For the sandwiched concrete with confinement, a constitutive model was employed along with FE models, which predicted the results at a reasonable level. Li et al. proposed a load-strain model for CFDSTs under axial compression [15]. They evaluated the performance of their proposed model through numerical and experimental methodologies.

Ghanbari-Ghazihahani et al. investigated the structural performance of rectangular tubes with timber infill and carbon fibre reinforced polymer (CFRP) confinement [16]. This study aimed to reduce the weight, and improve the ductility, and economic and environmental features of the new sections. They showed that the capacity of CFRP-confined tubes improved by 175% compared to the unreinforced specimens. Ghazijahani et al. further investigated the effect of softwood and different cross-sectional areas in tubular sections under axial compression [17,18]. They showed that tubes with timber infill not only reduced the weight, but also maintained the same crushing performance as fully concrete-filled specimens. In addition, the CFTs with timber infill had significant energy absorption to the mass ratios. Nabati et al. experimentally investigated the effect of CFRP on the axial behaviour of CFSTs with different timber cores [19]. According to their experimental results, they developed a formula to obtain the axial capacity of CFTs with timber infill. Jiao et al. investigated the hardwood cores inside the concrete, covered by high strength square steel tubes [20]. They concluded that the weight reduction caused an increase in the strength per unit mass values of timber-concrete-filled specimens.

This research continues to investigate CFT columns in which wood blocks are designed to provide weight reduction at the nominal values of 20% and 30% compared to the concrete-filled tubes, along with maintaining a reasonable strength. The prime motive of this research was initially to evaluate the difference between solid timber (as infill) and the same volume of the timber with different numbers (multiple pieces). The principal reason behind designing multiple timber infills is to examine the possibility of reducing the embedded energy of construction and installation by making smaller pieces of timber to be embraced by concrete and steel. The results suggest an auspicious outcome of using multiple pieces, which will be elaborated in the following sections.

Section snippets

Specimens and their design

One of the main contributions of this paper is to determine the effect of the interaction between concrete and timber when the variable is the number of timber pieces inside the concrete, given that the volumetric proportion of timber to concrete is the same for each group. For example, for 20% weight reduction (provided by a constant volume of timber) we designed two groups of specimens as seen in Fig. 1 and Table 1. GR(I) comprises the specimens with timber blocks entirely fitting inside of

Set-up and experimentation

Square hollow steel (SHS) tubes were used in this study with the nominal outer-side dimension of 50 × 50 mm and the thickness of 3 mm (Fig. 1). Compression tests were performed using an AVERY machine (Fig. 2). The specimens were fixed (but with a simple contact) within the base plates of the setup. The loading speed was low enough to be able to call the experiments quasi-static testing. Some of the specimens were repeated as seen in Fig. 1, e.g. SC1, SC2, SCT-0.3-4(1), SCT-0.3-4(2),

Deformational response and failure modes

Fig. 4 shows failure of the specimens as the load increased and the deformations developed. Due to the low length-to-side (L/B) ratio of the specimens, column buckling was not seen in this experimental program. The elephant-foot buckling dominated the failure at the start, as clear deformational lobes were formed on the body of the steel tubes. The onset of these lobes was at various locations across the length, which was due mostly to the different stiffness of various sections along the

Plastic Transition Triangle (PTT)

PTT is defined in this paper to determine and quantify the transitional response of columns under compression from Initial Peak Load (IPL) to Initial Trough Load (ITL) and Second Peak Load (SPL). It should be noted that the reason why multiple peak points are seen on the load-displacement graphs can be attributed to the fact that as the specimens experience crushing, the materials experience a compaction process throughout loading. This compaction increases the mechanical properties of the

Axial capacity: post-yield's critical points

Obtaining the axial capacity was one of the main objectives of this study, especially when the weight reduction of composite sections (including timber) is taken into account compared to SC specimens. It should be noted that the strength ratio of concrete to timber was about 2.3 (60.48 MPa over 26.44 MPa) in this study. Thus, the timber is regarded softwood while the concrete is in the range of high-strength. Fig. 10 compares IPL of different specimens. As expected, IPLs of SC1 and SC2 were the

Energy absorption

Energy absorption was calculated here as the area under the load-displacement graphs up to the maximum displacement of 40 mm, which is commonly attainable for all specimens. The specific absorbed energy (SAE) is defined as the energy absorption to the weight of each specimen which highlights lighter-weight specimens with high energy absorption. Radar charts of Fig. 14 give both values in which energy is calculated in Joule (J) and SAE in Joule/Gram (J/g). The energy absorption of SC specimens

Conclusion

The present research aimed to extend the idea of using timber inside concrete in short columns. The ratio of the compressive strength of the concrete (high-strength) over timber (softwood) was about 2.3. PTT on the load-displacement graph was introduced to quantify the transitional response between different critical points. SC and SCT-0.3-4-FRP specimens possess lower absolute values of ξ _1-2 and ξ _2-3 indicating a more stable response within the PTT area.

IPLs of SC specimens were the highest

Acknowledgment

This project is part of the first author's University Research Fellowship (Beacon of Enlightenment) as the sole Chief Investigator of the project, funded by Deputy Vice-Chancellor (Research) of the University of Adelaide. The support of the University is appreciated.

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Full length articlePlastic buckling and axial crushing of concrete-filled steel tubes: usage of multiple wood blocks

Highlights

Abstract

Introduction

Section snippets

Specimens and their design

Set-up and experimentation

Deformational response and failure modes

Plastic Transition Triangle (PTT)

Axial capacity: post-yield's critical points

Energy absorption

Conclusion

Acknowledgment

J. Constr. Steel Res.

J. Constr. Steel Res.

J. Constr. Steel Res.

Thin-Walled Struct.

Thin-Walled Struct.

J. Constr. Steel Res.

Thin-Walled Struct.

Mar. Struct.

Eng. Struct.

J. Constr. Steel Res.

Full length article
Plastic buckling and axial crushing of concrete-filled steel tubes: usage of multiple wood blocks