International Journal of Polymer Science

Volume 2018, Article ID 2385725, 13 pages

https://doi.org/10.1155/2018/2385725

## Experimental and Numerical Study on the Compression Behavior of Square Concrete-Filled Steel Tube Stub Columns with Steel Fiber-Reinforced High-Strength Concrete

^{1}Department of Architectural Engineering, Catholic Kwandong University, Gangneung, Gangwon-do, Republic of Korea^{2}Research Institute of Industrial Science, Hanyang University, Seoul, Republic of Korea^{3}Department Fire and Disaster Prevention Engineering, Kyungnam University, Changwon, Gyeongsangnam-do, Republic of Korea^{4}Department of Architecture, Sahmyook University, Seoul, Republic of Korea^{5}Department of Architectural Engineering, Hanyang University, Seoul, Republic of Korea^{6}Department of Architectural Engineering, Chungwoon University, Incheon, Republic of Korea

Correspondence should be addressed to Yun-Cheul Choi; rk.ca.noowgnuhc@iohccy12

Received 3 March 2018; Accepted 22 April 2018; Published 11 June 2018

Academic Editor: Aamer Bhutta

Copyright © 2018 Hyung-Suk Jung et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### Abstract

This study was conducted to evaluate the applicability of concrete-filled steel tube (CFT) columns made from high-performance construction materials. KBC2016, South Korea’s current building code, limits the maximum compressive strength of concrete at 70 MPa and the maximum yield strength of steel at 650 MPa. Similar restrictions to material properties are imposed on major composite structural design parameters in other countries worldwide. With the recent acceleration of the pace of development in the field of material technology, the compressive strength of commercial concrete has been greatly improved and the problem of low tensile strength, known to be the major limitation of concrete, is being successfully addressed by adding fiber reinforcement to concrete. Therefore, the focus of this study was to experimentally determine the strength and ductility enhancement effects, which depend on material composition. To this end, we performed concentric axial loading tests on CFT stub columns made from steel with a yield strength of 800 MPa and steel fiber-reinforced high-strength concrete. By measuring the strain at the yield point of CFT steel during the test, we could determine whether steel yields earlier than ultimate failure load of the member, which is a key design concept of composite structures. The analysis results revealed that the yield point of steel preceded that of concrete on the stress-strain curve by the concurrent action of the strain increase at the maximum strength, attributable to the high compressive strength and steel fiber reinforcement, and the strain increase induced by the confining stress of the steel tube. Additionally, we performed parametric study using ABAQUS to establish the broad applications of CFT using high-performance materials, with the width-to-thickness ratio as the main parameter. Parametric study was undertaken as experimental investigation was not feasible, and we reviewed the criteria for limiting the width-to-thickness ratio as specified in the current building code.

#### 1. Introduction

In line with the continuous trend toward high-rise buildings and long-span structures, it is becoming increasingly necessary to develop high-strength and high-performance materials. Development of high-performance construction materials is an essential factor enabling the construction industry to build ultrahigh buildings and ultra-long-span structures and can contribute to fundamentally solving the issue of securing usable floor space, caused by increased member sizes. In particular, for ultrahigh buildings requiring vertical members with large cross-sections, high-performance materials can allow this cross-section to be reduced. This is reflected in the recent trend of column cross-section design for large-scale structures based on the use of high-performance materials and increasing demand for composite cross-sections designed to maximize material performance.

However, current design codes [1–4] prescribe the upper and lower limits of material strength in the design of composite members, as well as systems combining structural steel and streel-reinforced concrete. This can be ascribed to the conservatism of design codes, which basically apply conservative requirements and criteria on the basis of various research reports when evaluating untested design elements [5–10]. The reason for such restrictions on material strengths is the change in strain at the maximum strength when the compressive strength of concrete exceeds 70 MPa in the ultimate strength calculation, according to existing research results. Furthermore, a maximum allowable yield strength is applied to members subjected to compressive loads to reflect the difficulty of inducing the yield of steel before that of concrete at a design yield strength exceeding 650 MPa. In cases where the material strength levels deviate from those of the test results from previous studies as reviewed by the building code, it is recommended to ensure security through separate testing or to reduce the design yield strength and compressive strength to the imposed limits. To err on the side of safety, it is general practice to opt for the latter method; however, this approach is tantamount to losing the advantages gained by using high-performance materials as described above. In order to leverage the advantages of high-performance materials, it is therefore of paramount importance to perform experiments to test the actual effects of high-performance materials when their strengths exceed the maximum strength allowed by the building code. This is especially important for high-strength steel, given the necessity to use it along with high-strength concrete (HSC) to enhance the performance of the compressive strength of concrete.

The major problem associated with the use of high-strength steel for rectangular CFT is the strain capacity of concrete. In general, the strain capacity of concrete in CFT is greatly enhanced owing to the enlarged confinement zone in concrete compared with steel-reinforced concrete. According to EC2 [11], however, as the compressive strength of concrete increases, the local strain in specific sections decreases during maximum strength development, which may make it difficult to apply CFT to HSC.

There has been continuous research into steel-reinforced concrete since its development in the 1960s [12], and it is now a common construction material. The ultimate failure of concrete subjected to uniaxial compression is caused not only by compressive stress but also by cracks triggered by lateral expansion under compressive loads. In this process, steel fiber-reinforced concrete contributes to increasing the initial strength of crack generation by tensile force, whereas HSC contributes to preventing spalling through high density owing to the crosslinking effect of steel fiber [13–16].

With this background, this study was conducted to investigate the applicability of HSC for high-strength steel rectangular CFT. The study aimed to evaluate the possible contribution of steel fiber reinforcement to solving the problem of insufficient strain capacity of concrete that may result from the use of HSC. To this end, we performed experiments on rectangular CFT stub columns made from high-performance materials. Additionally, finite element analysis was performed to investigate the effect of CFT using high-performance materials depending on the width-to-thickness ratio, which is one of the important factors by which the yield strength is influenced in the design standards.

#### 2. Limitation of Material Strength and Axial Strength according to Code Provisions

According to South Korea’s current building code (KBC 2016) [1], the compressive strength of CFT columns can be calculated from the flexural buckling limit state depending on the slenderness ratio. Specification for Structural Steel Buildings of the American National Standards Institute (ANSI/AISC 360-16) [3] also sets forth an approach to calculating the strength of members taking account of the slenderness ratio. Specifically, when the ratio of the strength of section to the elastic critical buckling load, which represents the slenderness ratio of the member, is less than or equal to 2.25, (2) is used to calculate the compressive strength, and if it exceeds 2.25, (3) is to be used. This is identical to the calculation method presented in KBC 2016. For noncompact sections, the compressive strength of composite columns is to be calculated in two categories of local buckling and its absence. For slender members, since the slenderness of steel is determined by the width-to-thickness ratio, the compressive strength of the section is to be calculated according to the slenderness ratio of each component part.

The strength of compact sections, whose width-to-thickness ratio is smaller than , is to be calculated using
where is the nominal axial compressive strength of the section (*N*), is the superimposed strength of the section (*N*), is the yield strength of steel (MPa), is the area of steel cross section (mm2), is the compressive strength of concrete (MPa), is the area of concrete cross section (mm2), is the area of continuous rebar (mm2), and is the yield strength of the rebar (MPa).

The strength of the noncompact section, whose width-to-thickness ratio falls within the range of and , is to be calculated using
where is the slenderness ratio of the element, is the limiting width-to-thickness parameter for compact element , is the limiting width-to-thickness parameter for noncompact element . denotes the axial yield strength of the column (*N*) and is to be calculated using
where and denote the moduli of elasticity (MPa) of steel and concrete, respectively.

The nominal strength of the slender section, whose width-to-thickness ratio exceeds (lower limit) and smaller than or equal to (upper limit), is to be calculated using where is the critical stress, which is determined as follows in case of a rectangular cross-section: where is the width of element exposed to compression (mm) and is the thickness of plate (mm).

A strength reduction factor of 0.75 is to be applied and the percentage of steel in the total cross-sectional area must exceed 1%.

It was found that the same limits are applied to concrete compressive strength in KBC2016 [1] and ANSI/AISC 360-16 [3]: the lower and upper limits are 21 MPa and 70 MPa, respectively, for normal-weight concrete, and the upper limit for lightweight concrete is 42 MPa. As for the yield strength of steel, however, the two codes set forth different upper limits: KBC2016 [1] prescribes the design yield strength of structural steel used for calculating the strength of composite columns not to exceed 650 MPa and ANSI/AISC 360-16 [3] limits the maximum yield strength of structural steel and steel reinforcement to 525 MPa and 550 MPa, respectively.

#### 3. Compression Test of Rectangular CFT Stub Columns Using High-Performance Materials

##### 3.1. Experiment Design

To determine the applicability of rectangular CFT made from high-performance materials in view of the provisions of the design codes as reviewed above, compression testing was performed on rectangular CFT columns made from high-performance materials. A total of eight experiments were performed, with the compressive strength of concrete, type of steel, and content level of steel fiber as independent variables. The specifications of the specimens are shown in Figure 1, and those of each variable are outlined in Table 1.