Design Of Composite Columns Research Articles

An improved constitutive model for steel tube-confined ultra-high-strength concrete

This study aims to accurately simulate the compressive behavior of ultra-high-strength concrete (UHSC) under various confinement conditions through conventional triaxial and equi-biaxial compressive tests. The triaxial compressive tests revealed that the stress and dilation angle of the concrete varies with changes in axial plastic strain and confining pressure, which is not well-considered in existing mechanical models for UHSC material. To address this limitation, an improved constitutive model based on the concrete plastic damage (CDP) model is proposed in this paper. The improved model incorporates the confining pressure in the flow rule and the hardening/softening rule, in addition to the axial plastic strain. The tensile and compressive meridians of the yield surface are calibrated based on the triaxial and biaxial compressive test results. Furthermore, the study proposes formulas for expressing the stress–strain relation and the lateral-to-axial plastic strain relation of UHSC under different confining pressures. These formulas help generate the hardening/softening rule and the dilation angle evolution of the flow rule, respectively. To verify the proposed constitutive model, the study simulated tests on UHSC-filled steel tube (UHSCFST) stub columns under compression using the finite element method. The simulated results of the peak bearing capacity of the tested UHSCFST stub column specimens are satisfactory, and the interaction between the steel tube and the concrete is reasonably predicted. The proposed improved constitutive model has the potential to accurately simulate the compressive behavior of UHSC under different confinement conditions, which can aid in the design of composite columns made with UHSC materials.

Optimum design of steel columns filled with concrete via genetic algorithm: environmental impact and cost analysis

The use of concrete-filled tubular columns as part of structural systems has steadily increased throughout the years. The growing demand for structural elements of this nature is a direct result of the possibility to use various cross-section shapes that have increased strength, along with resistance to fire and other corrosive agents. The main objective of this article is to present the formulation for optimizing the design of composite columns in accordance with prescriptions from ABNT NBR 16239: 2013, considering financial cost and CO2 emission during manufacturing as objective functions. A Genetic Algorithm was used to solve three examples of composite tubular columns subjected to combined bending and compression, considering major axis and unsymmetrical bending. The financial cost in Brazilian currency and the CO2 emission in kilograms attributed to manufacturing concrete-filled composite columns were calculated and the optimization procedure was implemented on composite columns featuring CHS, RHS and SHS steel members. This study also considers the different concrete strengths and the optional inclusion of longitudinal rebar. For the cases analyzed, the financially and environmentally optimum design corresponds to a CHS composite column with no longitudinal rebar and the highest concrete strength tested, except when unsymmetrical bending is applied, in which case the optimum solution includes longitudinal rebar. Furthermore, results indicate that structural steel has the highest impact on the CO2 emission of the optimal designs. For the column with longitudinal rebar, the reinforcement steel presents the second highest financial impact, while concrete is responsible for the highest influence on CO2 emission.

Application of Eurocode 4 design provisions and development of new predictive models for eccentrically loaded CFST elliptical columns

The present study mainly aims to propose design models for the prediction of the peak strength of the concrete-filled steel tubular (CFST) elliptical columns subjected to eccentric compressive loading. At that point, the study has gain importance since the design of specifically such elliptical columns is not covered by current design codes. For example, the current provisions of Eurocode 4 provide a simplified method for the design of composite columns having circular or rectangular/square cross-sections. To this aim, first of all, the applicability and assessment of Eurocode 4 to the CFST elliptical columns under eccentric loading conditions were performed. Following, a data repository covering the experimental results of eccentrically loaded CFST elliptical columns (having different geometry, material property, load eccentricity in y and z axes, and longitudinal reinforcement ratio) was constituted. This data repository was used in the evaluation of the design guidance in Eurocode 4. Besides, the same experimental results were employed to develop two new numerical models based on gene expression programming (GEP) and artificial neural network (ANN) approaches. Finally, a sensitive statistical analysis was made to compare the prediction performances of Eurocode 4, GEP, and ANN models against experimental results. It was observed that Eurocode 4 design provisions had sufficiently accurate prediction performance, however, the developed GEP and ANN design models yielded consistent and robust prediction performance than the current design provisions by providing a much smaller mean absolute percent error of 7.5 and 1.4, respectively.

EXPERIMENTAL STUDY ON BEHAVIOR OF UNPROTECTED FOAMED CONCRETE FILLED STEEL HOLLOW COLUMN UNDER FIRE

Reduction in self-weight and achievement of full fire resistance requirements are some of the important considerations in the design of high-rise structures. Lightweight concrete filled steel tube (CFST) column provides an alternative method to serve these purposes. Recent studies on lightweight CFST columns at ambient temperature have revealed that foamed concrete can be a beneficial and innovative alternative material. Hence, this study investigates the potential of using foamed concrete in circular hollow steel columns for improving fire resistance. A series of nine fire test on circular unfilled hollow and foamed concrete filled hollow section column were carried out. ISO 834 standard fire exposure test were carried out to investigate the structural response of these columns under fire. The main parameters considered are load level and foamed concrete density; foamed concrete density used are 1500 kg/m3 and 1800 kg/m3 at 15%, 20%, and 25% load level. All the columns tested are without any external fire protection, with concentrically applied load under fixed-fixed boundary conditions. The columns dimension was 2400 mm long, 139.7 mm diameter and steel tube thickness of 6 mm. The fire test result showed that foamed concrete increases the fire resistance of steel hollow column up to an additional 16 minutes. The improvement is more at load level above 15%, and the gain in fire resistance is about 71% when 1500 kg/m3 density foamed concrete is used. Generally, foamed concrete filled steel hollow column demonstrate a good structural fire behavior, based on the applied load and foamed concrete density. Also, inward local buckling was averted by filling the steel hollow column with foamed concrete. General method for composite column design in Eurocode 4 adopted to calculate the axial buckling load of 1500 kg/m3 foamed concrete filled columns. These type of columns can be used for structures like airports, schools, and stadiums; taking the advantage of exposed steel for aesthetic purpose and high fire resistance. It can also be used for high rise structures; taking advantage of high fire resistance and reduction in self-weight of a structure.

Non‐linear Analysis of Circular Composite Columns

AbstractComposite columns provide many advantages for the application in multi‐story and high‐rise buildings. However, EN 1994‐1‐1 does not provide rules for the design of circular concrete sections with fully encased steel profiles according to the simplified methods. Normally, the simplified method for the design of composite columns is based on a normal force‐moment interaction diagram which assumes plastic cross‐section resistance. However, the moment resistance is to be reduced by a reduction factor αM, considering among other the difference between strain‐limited and plastic moment resistance. This reduction factor is developed and calibrated for composite compression members with concrete encased sections, partially encased sections and concrete filled rectangular and circular tubes. Comparison of plastic moment resistance to results based on strain limitation show that the application for circular section with encased steel section but without a steel tube may lead to unsafe results.The paper discusses different approaches for the determination of the N‐M Interaction curve and the design on cross‐section level. The results obtained from these methods are then compared to each other for different dimensions of cross‐sections of circular columns; and for different ratios of reinforcements, various steel grades and for normal and high strength concretes. Furthermore, the investigation focuses on the validation of the reduction factor αM and shows for which section types the values according to Eurocode 4 cannot be applied.

Behaviour of rectangular concrete filled tubular flange girders under combined loading

AbstractSteel‐concrete composite construction can provide an efficient structural solution by utilizing the two component materials to create a single effective composite section. The high tensile strength and ductility of steel combined with the excellent compressive strength and robustness of concrete results in an effective composite cross‐section which can be used in a wide variety of applications. There are a number of different shapes of composite beam, and this paper is focussed on a relatively new structural solution called concrete filled tubular flange girders (CFTFGs). These comprise a steel beam in which the top flange plate is replaced with a concrete‐filled hollow section. The compression flange can be any shape (e.g. circular, rectangular, and pentagonal) and the focus in the current paper is on simply supported girders with a rectangular section for the compression flange (i.e. RCFTFGs). These members have very high lateral torsional buckling resistance compared with conventional steel I‐girders of similar depth, width and steel weight and are therefore capable of carrying very heavy loads over long spans. However, they are complex to analyse and their behaviour is governed by several inter‐related parameters which are investigated and discussed herein. Given the most common applications of these girders, such as in bridges and carparks for example, they are frequently exposed to a simultaneous combination of high axial loads and bending moments. Although current design codes offer rules for the design of composite columns under combined loading, the design of RCFTFGs, which are asymmetric in nature under the combined effects of tension and bending, are not explicitly included. This work aims to investigate the ultimate strength of RCFTFGs with stiffened webs under the combined effects of various levels of axial tension and positive (sagging) bending moment. Nonlinear three‐dimensional finite element analyses using the ABAQUS computer software package have been developed to facilitate the study. The model is validated using available experimental data and then employed to conduct an extensive parametric study. It is shown that the moment capacity of RCFTFGs is reduced in most situations under the presence of an axial tensile force in the steel beam. In addition, the tensile capacity of the RCFTFGs is limited by the axial capacity of the steel beam alone, with little contribution from the concrete infill. Several influential parameters are examined and conclusions are discussed. Based on the finite element analysis, the moment–axial force interaction relationship is presented and discussed.

Buckling behavior of circular steel tubes infilled with C170/185 ultra-high-strength concrete under fire

Modern building codes do not cover the use of ultra-high strength concrete (UHSC) in design of composite columns. This paper investigates the structural fire performance of UHSC filled steel tubular (CFST) columns. Experimental studies were carried out on 9 CFST columns with C170/185 concrete under standard ISO fire. The columns were subjected to concentrated or eccentric compressions and then heated to failure to record the fire resistance time. The simple calculation model in EC 4 was examined by predicting the fire resistance time and comparing them with the test results; the axial force–moment (N-M) interaction strength model for room temperature design was extended to predict the fire resistance. The test results showed that the CFST columns with UHSC exhibited similar fire performance to those with normal/high strength concrete with respect to thermal expansion, axial contraction and failure modes. Adding 0.1% polypropylene fibers in UHSC was found to be effective in preventing explosive spalling especially when it was heated rapidly under the standard fire. The N-M interaction strength model gave better predictions than the EC 4 model as it gave less scattered results when compared to the test data. EC 4 buckling curve “c” was found to be conservative in predicting the fire resistance time of CFST columns with UHSC subjected to pure compression. Parametric study showed that the fire resistance of CFST columns with UHSC reduced faster than those with normal strength concrete (NSC) and high strength concrete (HSC) in early stage of heating but slower at the later heating stage. Within the practical range of load level (i.e.,≤0.7), the CFST columns with UHSC employed by this study had better fire resistance than those columns infilled with NSC and HSC.

Assessment of high-strength concrete encased steel composite columns subject to axial compression

This paper investigates the axial load capacity of concrete encased steel composite stub columns with high strength concrete and steel materials. A total of 14 column specimens with varying material strengths and different steel section shapes were tested under concentric compression load. The test results revealed that the design methods in EN 1994-1-1 and JGJ 138-2016 overestimate the axial load capacity of high strength concrete encased steel columns. Adding a small percentage of steel fiber in the high strength concrete was found to improve the compression resistance of the composite section. A new test database consisting of 51 partially encased composite sections and 82 fully encased composite sections was established, covering a wide range of section geometric and material grades. Parametric study was then carried out to assess current design methods in predicting the axial capacity of such composite columns with respect to material strengths, steel contribution ratio, reinforcement ratio, section slenderness ratio, confined concrete area ratio, and concrete confinement efficiency. The effectiveness of concrete confinement in partially encased composite column was evaluated and a simplified method was proposed to compute the enhanced concrete strength based on regression analysis. For fully concrete encased composite columns, a concrete strength reduction factor was proposed to be used with EN 1994-1-1 to predict the compression resistance. Design recommendation was made considering the early cover spalling of high strength concrete and the material compatibility between steel and concrete in composite column design.

Numerical Analysis and Axial Bearing Capacity of Composite Columns with Recycled Aggregate Concrete-Filled Steel Tube and Profile Steel

The axial compressive behavior of composite columns with recycled aggregate concrete (RAC)-filled steel tube and profile steel was examined through numerical investigations. The constitutive models of the RAC material, profile steel, and steel tube were assigned to respective elements in the columns. The experimental results were used to verify the finite element (FE) model. Results indicated that the FE model accurately predicts the deformation, stress, and load–strain curves of the columns. Then, the effects of RAC strength, steel tube strength, profile steel strength, profile steel ratio, and diameter-to-thickness ratio on the axial compressive behavior of the columns were evaluated through a parametric study. The increase in RAC strength, steel tube strength, and profile steel strength significantly affected the axial bearing capacities of the columns. However, deformation ability and ductility decreased gradually as the RAC strength and steel strength increased. The increase in the diameter-to-thickness ratio had a negative effect on the axial bearing capacities of the long columns. Based on the experimental research, FE analysis, and ultimate analysis method, a formula for the axial bearing capacity of the composite columns was proposed. The validity of the proposed formula was verified through the comparison of the results. The research conclusions can provide a reference for the design of composite columns.

Multi-stopband negative stiffness composite column design for vibration absorption

This paper explores the basic mechanism of a multi-stopband negative stiffness composite column. The column consists of two parts: the base structure (concrete-filled steel tube (CFST)) and mass-spring-damper vibration absorbers (tungsten sphere-rubber). The local vibration absorbers create a wave “black hole” and guide waves into the energy-dissipating local vibration absorber under the resonant excited state. The efficiency of the multi-stopband negative stiffness composite column is verified via numerical testing. The results indicate that the resonance of the vibration absorber creates frequency stopbands that are influenced by the location, damping of the absorber, mass ratio, and stiffness ratio. Moreover, when the frequency of the local absorber is distributed around the natural frequency of the column, the vibration caused by earthquakes can be absorbed.

Uniaxial behaviour of sand concrete-filled steel tube columns

A major issue in the design of rectangular hollow section (RHS) steel columns is the local buckling ultimate limit state, which can seriously limit the overall resistance of the column. The use of concrete-filled steel tubes is frequently the solution that is adopted to overcome this problem, by enhancing the column resistance through the association of the resistances of the two materials. Major design codes contain rules for the design of composite columns, but the concrete's resistance is limited to the usual range of concrete classes. This paper presents the results from a series of experimental tests using low-cost/low-strength concrete, containing a small amount of cement and using sand as the only aggregate. This may be particularly advantageous from the logistic and economical points of view. The experimental results are presented, analysed and compared with those obtained for non-composite RHS steel columns. Finally, the Eurocodes design rules are used to assess their applicability to low-performance concrete.

Ultra-high Strength Concrete on Eccentrically Loaded Slender Circular Concrete-filled Dual Steel Columns

This paper presents an experimental study which was undertaken to investigate the effect of ultra-high strength concrete (UHSC) on the mechanical response of concrete-filled dual steel tubular columns (CFDST) subjected to eccentric loads. This campaign is a continuation of a previous program on axially loaded members and consists in testing 12 CFDST specimens and two concrete-filled tubular (CFST) columns which serve as references. The location of the dual-grade concretes (outer ring and inner core) and the steel thicknesses configuration were the parameters analysed. Therefore, in this program, two series can be distinguished: 6 columns with normal strength concrete in their outer rings and, conversely, 6 with ultra-high strength concrete. In addition, two different steel tube thicknesses configurations were considered: columns with thin outer steel tube and thick steel tube (thin-thick); and the opposite pattern (thick-thin). Moreover, the experiments on the CFST columns of reference served to investigate the effect of the extra inner steel tube and its filling in CFDST columns. Since the number of campaigns on eccentrically loaded slender CFDST columns is scarce, the results presented in this work become particularly relevant. The analysis of the results revealed that, for both series, the response of the specimens showed similar trend but the effect of UHSC differed from that observed for axially loaded columns. Finally, the experimental data were contrasted to the predictions given by Eurocode 4 for the design of composite columns resulting, in general, in slightly unsafe predictions.

Design of composite columns with cross‐sections not covered by Eurocode 4

AbstractDedicated to Prof. Dr. Akimitsu Kurita on his 70th birthdayThe use of composite columns with special types of sections e.g. concrete filled tubes with massive inner cores became very popular in the last years. These sections are not in the scope of the simplified design method of Eurocode 4. The paper deals with a design proposal for columns with those sections based on the general design method and it shows how this method can also be used extending the scope of the simplified design method in Eurocode 4.

A general method for designing non-symmetrical composite steel and concrete columns

The study of non-symmetrical composite columns is complex and European and North American standards only approach this issue superficially. In this text, a new proposal is presented for non-symmetrical composite column design subject to biaxial bending and axial compression, based on the compatibility of the deformation method, which considers the nonlinearity of the material with constitutive models according to Brazilian standards and geometric nonlinearity, as well as the equilibrium of the column in the displaced position. As a strategy for solving the general method, use is made of the "fiber element method", and for solving problems involving nonlinear equilibrium equations, the Newton-Raphson method.

Influence of Ultra-high Strength Concrete on Circular Concrete-filled Dual Steel Columns

This paper presents the results of an experimental program where the load-bearing capacity at room temperature of 14 slender circular concrete-filled dual steel tubular (CFDST) columns under axial load was analysed. In this study, two specimens were prepared as ordinary concrete-filled steel tubular (CFST) columns to be used as references to evaluate the mechanical contribution of the inner steel tube and its infill in the CFDST columns. In addition, the effect of two types of concrete: normal strength concrete (NSC) and ultra-high strength concrete (UHSC) was assessed. Besides, in order to study the influence of the steel share between the inner and outer tube, different cross-sectional configurations were considered. Since the number of experimental results available in the literature on slender CFDST columns is scarce, this work provides novel results to this research field. The different influence of the steel distribution in the response of the specimens of each series was observed, with no influence in the case of columns with ultra-high strength concrete in the outer ring. Finally, the current provisions of Eurocode 4 for the design of composite columns were assessed by means of the results of these tests, being necessary more test data to extract solid conclusions about their accuracy and reliability.

Statistical calibration of safety factors for flexural stiffness of composite columns

Composite column design is strongly influenced by the computation of the critical buckling load, which is very sensitive to the effective flexural stiffness (EI) of the column. Because of this, the behaviour of a composite column under lateral loading and its response to deflection is largely determined by the EI of the member. Thus, prediction models used for composite member design should accurately mirror this behaviour. However, EI varies due to several design parameters, and the implementation of high-strength materials, which are not considered by the current composite design codes of practice. The reliability of the design methods from six codes of practice (i.e., AS 5100, AS/NZS 2327, Eurocode 4, AISC 2010, ACI 318, and AIJ) for composite columns is studied in this paper. Also, the reliability of these codes of practice against a serviceability limit state criterion are estimated based on the combined use of the test-based statistical procedure proposed by Johnson and Huang (1997) and Monte Carlo simulations. The composite columns database includes 100 tests of circular concrete-filled tubes, rectangular concrete-filled tubes, and concrete-encased steel composite columns. A summary of the reliability analysis procedure and the evaluated reliability indices are provided. The reasons for the reliability analysis results are discussed to provide useful insight and supporting information for a possible revision of available codes of practice.

Theoretical study of UHPCC composite column behaviors under axial compression

To improve the durability and service life of reinforced concrete column such as bridge piers, an advanced composite column made of Ultra High Performance Cementitious Composites (UHPCC) permanent form is proposed. Based on elasticity plasticity theory, axial compression behavior of the composite column was studied theoretically. The first circumferential cracking load and ultimate limit loading capacity are derived for the composite column. Short composite column compression tests and numerical simulations using FEM method were carried out to justify the theoretical formula. The effects of UHPCC tube thickness on the axial compression behavior were studied. Using the established theoretical model and numerical simulation, the large dimension composite columns are calculated and analyzed with different UHPCC tube thickness. These studies may provide a reference for advanced composite column design and application.

Behaviour and design of composite columns incorporating compact high-strength steel plates

Concrete filled steel tubular columns (CFSTCs) are finding increasing use in modern construction practice throughout the world. The efficiency of CFSTCs can be further improved if high-strength materials are used. High-strength steel provides attractive alternatives to normal-strength steel for multi-storey and high-rise construction applications. This paper presents an extensive experimental investigation into the axial load behaviour of square composite columns incorporating compact high-strength steel plates. The test parameters include the concrete strength (f′c=21–55N/mm2), depth-to-thickness ratios in the range of 16–40, as well as length-to-depth ratios in the range of 2.60–2.85. Furthermore, a simplified confining pressure versus depth-to-thickness ratio model, appropriate confined concrete constitutive models, and an accurate finite element model which incorporates the effects of initial local imperfections and residual stresses has been developed using the commercial program ABAQUS. The predictions of the behaviour, ultimate strengths, and failure modes are compared with the experimental results to verify the accuracy of the models developed. Additionally, comparisons with the prediction of axial load capacity by using the Australian Standards, Eurocode 4, and American Institute of Steel Construction code provisions for composite columns are also carried out.

Cost and CO2 Emission Optimization of Steel Reinforced Concrete Columns in High-Rise Buildings

The construction industry is a representative industry that consumes large amounts of energy and produces substantial pollution. The operation of a building accounts for a large portion of its total CO2 emissions. Most efforts are focused on improving the energy efficiency related to the operation of a building. The relative importance of the energy and CO2 emissions from the construction materials increases with the increasing number of low-energy buildings. To minimize the life-cycle energy use of a building, the energy consumed from both materials in the construction phase as well as the energy consumed from the operation of the building must be reduced. In this study, an optimal design method for composite columns in high-rise buildings using a genetic algorithm is proposed to reduce cost and CO2 emissions from the structural materials in the construction phase. The proposed optimal method minimizes the total cost, including the additional cost calculated based on CO2 emissions from composite columns, while satisfying the structural design criteria and constructability conditions. The proposed optimal method is applied to an actual 35-story building, and the effective use of structural materials for the sustainable design of composite columns is investigated. It is shown that using more concrete than steel section and using high-strength materials are economically and environmentally effective methods.

Fire design method for bar-reinforced circular and elliptical concrete filled tubular columns

A method for calculating the design axial buckling load in the fire situation of unreinforced axially loaded concrete filled circular hollow section columns was presented by the authors in a previous paper [1]. In the present paper, the method is extended to bar-reinforced columns of circular and elliptical cross-section, as a necessary continuation to complete the proposal. The method presented here is based on the guidelines of Clause 4.3.5.1 in Eurocode 4 Part 1.2 for the fire design of composite columns and is developed on the basis of the results of new parametric studies, with varying values of the outer diameter of the column, steel tube wall thickness, relative slenderness, percentage of reinforcement and fire exposure time. From the results of these parametric studies, appropriate expressions and tables for the different parts which integrate the design method are derived. The proposed method is valid for centrally loaded bar-reinforced circular and elliptical concrete filled tubular columns, with a maximum percentage of reinforcement of a 5% and makes allowance for columns with a high slenderness, extending the current limits of Eurocode 4 Part 1.2.