Concrete-filled Steel Box Columns Research Articles

Seismic behavior of concrete-filled double-skin thin-walled steel box columns stiffened with PBL

Concrete-filled double-skin thin-walled steel box (CFDTSB) columns stiffened with PBL are a new type of columns formed by setting longitudinal ribs on an inner wall of outer and inner steel boxes of CFDTSB columns. Quasi-static tests on one CFDTSB column without PBL and two CFDTSB columns stiffened with PBL were carried out to investigate their seismic performance. The damage modes, hysteresis curves, envelope curves and strain distribution on outer steel boxes of different specimens were compared and analyzed. Then, FE models of CFDTSB columns stiffened with PBL and without PBL were established and parametric analysis was conducted to investigate the effects of cross-sectional hollow ratio, thickness of outer steel boxes and height of PBL ribs. At last, calculation methods for load bearing capacity of the two types of columns were proposed. It is found that, the arranging of PBL ribs significantly improves the local buckling of outer steel boxes, enables steel boxes to fully participate in bearing the axial load and bending moment, and enhances the interaction between steel boxes and sandwich concrete, so the bearing capacity, deformation capacity, and energy dissipation capacity of CFDTSB columns stiffened with PBL are improved a lot, compared with CFDTSB columns without PBL. That is to say, the seismic behavior of CFDTSB columns stiffened with PBL is good. The proposed method could consider the local buckling of steel boxes, steel-concrete interaction and effect of PBL ribs on them rationally, thus predicting the load-carrying capacity of the two types of columns.

Experimental study on axial compressive behavior of concrete-filled double-skin thin-walled steel box columns with and without PBL

To study the axial compressive behavior of concrete-filled double-skin thin-walled steel box (CFDTSB) columns and the effects of PBL ribs and cross-sectional hollow ratio, seven CFDTSB columns with or without PBL were tested under axial compression. By comparing the failure modes, initial stiffness, ultimate load and strength index, ultimate displacement and ductility, and strain distribution on the surface of the outer steel box of each specimen, the improvement of axial compressive behavior due to PBL ribs at different hollow ratios were investigated. The results showed that the greater of hollow ratio, the more obvious of steel box local buckling, and the weaker of interaction between steel box and concrete. PBL ribs effectively relieve the local buckling of steel boxes and enhance the participating degree of outer steel boxes in bearing axial forces; and improve the interaction between steel boxes and concrete and increase the confinement of steel boxes on concrete. In addition, the beneficial effect of PBL ribs is more obvious when the hollow ratio is larger. Therefore, compared with CFDTSB columns without PBL, CFDTSB columns stiffened with PBL have higher stiffness, axial compressive capacity, and deformation capacity. Based on that, axial compressive bearing capacity calculating formulas for CFDTSB columns with or without PBL were developed considering the local buckling and confinement of steel boxes and the effects of PBL ribs on them. The verification results proved that predicted values agree well with the experiment results.

Cyclic lateral load test and finite element analysis of high-strength concrete-filled steel box columns under high axial compression

This paper presents an experimental study of the seismic performance of high-strength concrete-filled box columns (CFBCs) under combined axial and cyclic lateral loads. Specimens were made of high-strength SM 570 M steel (with yield strengths between 520 and 580 MPa), and concrete with compressive strength (fc') greater than 80 MPa. Three parameters that affect the seismic performance of CFBCs were investigated: the width-to-thickness (b/t) ratio of the steel column, magnitude of the axial load, and the addition of concrete infill. The specimens, which were 280–420 mm in width and 2000 mm in height, were tested under combined axial (4058–10,090 kN) and cyclic lateral loads. Experimental results indicated that the lateral displacement ductility decreases significantly with an increase in either the axial load or b/t ratio. The addition of concrete infill inside a hollow steel box column does not improve the lateral displacement ductility of CFBCs under high axial load. Although the CFBC specimens satisfied the b/t requirement of a highly ductile member, as per AISC Seismic Provisions (2016), specimens under high axial load (40%Pn) failed at 4% drift, indicating that the requirement does not guarantee that CFBCs will sustain high axial load under significant drift (i.e., >3%). The Eurocode 4 (2009), AISC Specification (2010), and Architectural Institute of Japan (2014) reasonably estimate the flexural strength of high-strength CFBCs under axial load; however, ACI 318 (2011) does not. The finite element analysis program ABAQUS can be used to estimate the hysteretic behavior of specimens before significant strength degradation.

The multi-axial strength performance of composited structural B-C-W members subjected to shear forces

This paper presents a new method to compute the shear strength of composited structural B-C-W members. These B-C-W members, defined as concrete-filled steel box beams, columns and shear walls, consist of a slender rectangular steel plate box filled with concrete and inserted steel plates connecting the two long-side steel plates. These structural elements are intended to be used in structural members of super-tall buildings and nuclear safety-related structures. The concrete confined by the steel plate acts to be in a multi-axial stressed state: therefore, its shear strength was calculated on the basis of a concrete\'s failure criterion model. The shear strength of the steel plates on the long sides of the structural element was computed using the von Mises plastic strength theory without taking into account the buckling of the steel plate. The spacing and strength of the inserted plates to induce plate yielding before buckling was determined using elastic plate theory. Therefore, a predictive method to compute the shear strength of composited structural B-C-W members without considering the shear span ratio was obtained. A coefficient considering the influence of the shear span ratio was introduced into the formula to compute the anti-lateral bearing capacity of composited structural B-C-W members. Comparisons were made between the numerical results and the test results along with this method to predict the anti-lateral bearing capacity of concrete-filled steel box walls. Nonlinear static analysis of concrete-filled steel box walls was also conducted by using ABAQUS and the results agreed well with the experimental data.

CALCULATION OF STRENGTH OF CONCRETE FILLED STEEL BOX COLUMNS

The experimental - theoretical researches of concrete filled steel box columns (CFSBC) with pressed structure of concrete are executed. It is offered to make the CFSBC with preliminary pressed concrete core to improve their construction. The bearing capacity of centrally compressed samples of CFSBC has increased by 10 ÷ 15%. The received results are explained by the significant increase of the strength of concrete core in the preliminary compressed CFSBC samples due to the simultaneous display of three effects: long-term compression of concrete mix, preliminary lateral compression of concrete core and its work in the conditions of volume compression. The calculation of strength of CFSBC should be carried out on the basis of nonlinear deformation model of reinforced concrete. Three-axis tensely – deformed conditions are accepted for the analytical description of work of concrete core and external steel holder. Nonuniformity of stress state of concrete and bending of walls of external steel holder are taken into account. The basic dependences, which allow to carry out the approximate calculations of the strength of compressed CFSBC on limiting efforts, are also resulted.

Behavior of Eccentrically Loaded Plain and Steel Fiber Reinforced Concrete Filled Steel Box Columns

The present study investigates the behavior of steel fiber reinforced concrete filled steel box columns (SFRCFSBC) targeting to enhance their strength. A nonlinear finite element model using ANSYS program has been developed to investigate the structural behavior of the inspected columns. The results obtained from that model has been compared with those calculated using Euro code (EC4), AISC/LRFD (2005) and the Egyptian Code of Practice for Steel Construction (ECPSC/LRFD 2007). The comparison indicated that the results of the model have been evaluated to an acceptable limit of accuracy. A parametric study was carried out to investigate the effect of wall thickness, column slenderness and percentage of steel fiber in concrete on the ultimate strength of composite columns. Confinement of the concrete core provided by the steel case was also investigated. It can be concluded from the results that a considerable increase in compressive and flexural strength may be gained by increasing the steel fiber percentage up to 4%. The highest rate of increase in strength for long columns was about 20% by using steel fiber percentage between 0.5% and 1.0%, while for short and medium columns was about 10% by using steel fiber percentage between 1% and 2%.

THEORITICAL AND EXPERIMENTAL STUDY FOR THE BEHAVIOR OF SHORT COMPOSITE COLUMNS TESTED UNDER STATIC CENTRIC OR ECCENTRIC LOADS

Steel concrete composite structures are widely used in many structural applications such as beams, slabs, walls and columns. Composite structures provide not only great reduction for the element size and weight but also high structural efficiency. Because of the little data and disagreement between the R.C. and steel Egyptian Codes of Practice, an experimental and theoretical investigation were conducted to study the behavior of concrete filled steel box columns. The experimental program consists of six square filled steel box columns and two square R.C. columns (reference elements). The experimental variables are the load position (centric or eccentric) and the distribution of the mechanical shear connectors (nails). Test results are presented and discussed in comparison with the estimated values predicated from the Egyptian R.C. and steel codes of practice as well as the values obtained from the finite element program (ANSYS 12).

A New Method to Assess Ductility of CFT Box Columns with Binding Bars under Axial Compression

A new method based on material properties instead of experimental data was proposed to assess the ductility of concrete-filled steel box columns with binding bars and those without binding bars. Comparison between ductility coefficients based on experimental data and the calculated values by the proposed method shows good agreement.

Experimental Research and Finite Element Analysis of Concrete-Filled Steel Box Columns with Longitudinal Stiffeners

For further study of mechanical properties of concrete-filled steel box columns (CFSBCs) with longitudinal stiffeners, axially loading tests of CFSBCs with longitudinal stiffeners was conducted to obtain their ultimate bearing capacity and failure modes. The test results were compared with those of hollow steel box columns with longitudinal stiffeners. Cross section of the test specimen was scaled from a chord member of Dongjiang Bridge. The experimental results show that failure mode of CFSBCs with longitudinal stiffeners is local buckling of steel plates, which is different from that of concrete-filled thin wall steel tube columns with longitudinal stiffeners. Although longitudinal stiffeners can prevent global buckling of steel plates, the effect is less obvious than that of concrete-filled thin wall steel tube columns. Meanwhile, three-dimensional finite element models (FEM) of the specimens were modeled using computer program ANSYS to obtain bearing capacities and load-strain curves. The FEM results coincide quite well with the test results. Further, influence of width to thickness ratio on mechanical behavior of CFSBCs was analyzed using FEM.

Ductility of Concrete-Filled Steel Box Columns with Binding Bars Subjected to Axial Compression

Ductility of concrete-filled steel box columns with binding bars and those without binding bars were discussed based on the experimental study. Two current methods were used to assess the ductility of concrete-filled steel box columns with binding bars and those without binding bars. Results show that binding bars can increase ductility of concrete-filled steel box columns. Ductility of concrete-filled steel box columns with binding bars at closer spacing is considerably better than that of concrete-filled steel box columns without binding bars.

Strength of Concrete-Filled Steel Box Columns with Buckling Effects

This paper studies the ultimate strength and ductility of short concrete-filled thin-walled steel box columns with local buckling effects using a nonlinear fibre element analysis technique. New effective width formulas for determining the ultimate strengths of steel plates with geometric imperfections and residual stresses in concrete-filled thin-walled steel box columns are incorporated in the fibre element analysis program to account for the effects of local buckling. The progressive local and post-local buckling is simulated by gradually redistributing the normal stresses within the steel box. Performance indices are employed to quantitatively evaluate the section and ductility performance of composite columns. The effects of local buckling, steel ratios and steel yield strengths on the strength and ductility of concrete-filled steel box columns are investigated. It is demonstrated that local buckling significantly reduces the ultimate strength of thin-walled high-strength steel box columns filled with concrete. The 4% limit on the steel ratio or the 0.2 limit on the steel contribution ratio imposed on composite column sections in current design codes leads to the use of very slender steel plates in concrete-filled steel box columns. It is suggested that the section performance index of concrete-filled steel box columns should be as high as 0.5 for a better structural performance. The use of high strength steels in concrete-filled steel box columns significantly increases the ultimate strengths of the columns but reduces their ductility.

Nonlinear analysis of concrete-filled thin-walled steel box columns with local buckling effects

Local buckling of steel plates reduces the ultimate loads of concrete-filled thin-walled steel box columns under axial compression. The effects of local buckling have not been considered in advanced analysis methods that lead to the overestimates of the ultimate loads of composite columns and frames. This paper presents a nonlinear fiber element analysis method for predicting the ultimate strengths and behavior of short concrete-filled thin-walled steel box columns with local buckling effects. The fiber element method considers nonlinear constitutive models for confined concrete and structural steel. Effective width formulas for steel plates with geometric imperfections and residual stresses are incorporated in the fiber element analysis program to account for local buckling effects. The progressive local and post-local buckling is simulated by gradually redistributing the normal stresses within the steel plates. Two performance indices are proposed for evaluating the section and ductility performance of concrete-filled steel box columns. The computational technique developed is used to investigate the effects of the width-to-thickness ratios and concrete compressive strengths on the ultimate strength and ductility of concrete-filled steel box columns. It is demonstrated that the nonlinear fiber element method developed predicts well the ultimate loads and behavior of concrete-filled thin-walled steel box columns and can be implemented in advanced analysis programs for the nonlinear analysis of composite frames.

Seismic demand predictions of concrete-filled steel box columns

Steel columns partially filled with concrete at base have found their wide application in modern highway bridge systems in areas where severe earthquakes are likely to occur. The assessment of capacities in terms of ductility and ultimate strength as well as the estimation of demands of such structures are vital in any seismic design methodologies. This study presents demand prediction procedures for partially concrete-filled steel box columns. To this end, two methods are proposed: (1) a single degree of freedom system analysis procedure that uses a bilinear or trilinear force–displacement hysteretic model derived from results of a static pushover analysis; and (2) a finite element analysis procedure involving beam-column elements and individual cyclic stress–strain relations of concrete and steel (i.e. a fiber analysis procedure). Both procedures are applied to a number of specimens tested at Nagoya University by pseudo-dynamic testing procedure, and comparisons are made between test and analytical results. The results show that the maximum displacement demands computed from the fiber analysis and the method that involves with the trilinear hysteretic model exhibit good agreement with test results. Predicted residual displacement demands using both procedures do not agree well with test results, and alternatively, an empirical equation is proposed for the residual displacement demand in terms of the maximum displacement demand.

An analytical model for thin-walled steel box columns with concrete in-fill

Concrete-filled steel box columns are widely used as a major structural element in tall buildings as they provide efficient structural performance in resisting axial compression. Local buckling of steel plates is a common failure criteria when plate slenderness ratio is large in box sections. A method using an effective width principle is proposed in this paper to predict the behaviour and load carrying capacity of thin-walled steel tubes with concrete in-fill. The present investigation addresses columns pinned at their ends and subjected to biaxial loading. Columns tested by other researchers have been analysed using the proposed method and the predicted results are compared with the corresponding test results. The proposed model is also verified against Eurocode 4. The comparison shows that the method could predict the ultimate load with sufficient accuracy.

Static Long-Term Effects in Short Concrete-Filled Steel Box Columns under Sustained Loading

Concrete-filled steel columns have been widely used over the last decade, which has been brought about by a renaissance of using structural steel in construction worldwide. Considerable research efforts have been conducted on these column types into the short-term effects for strength, and other extensive research has been conducted in relation to seismic effects. However, only limited research has been conducted on the long-term performance of these columns under sustained loading. This paper presents a series of experiments designed to ascertain the creep and shrinkage characteristics of high-strength concrete in composite columns, and also those of high-strength concrete specimens. These values were compared with existing experimental results. It is shown that the creep and shrinkage characteristics of concrete inside a steel column are much lower than those for exposed concrete. The static strength after long-term loading was also determined in this paper. A predictive model based on previously developed ACI provisions for both the service load and ultimate load behavior of the concrete specimens and the composite columns is calibrated with the experiments, and recommendations for design are suggested.

Strength of short concrete filled high strength steel box columns

The use of concrete filled steel box columns has been consistently applied in the design of tall buildings as they provide considerable economy in comparison with conventional steel columns. Their use allows the adoption of steel or composite floor systems combined with economically constructed columns. The use of these columns also has considerable advantages over reinforced concrete columns as they allow higher percentages of reinforcement to be adopted. In basements of tall buildings where car park space is of premium cost, a reduction in the column size can provide significant economic benefits. The use of high strength steel can be applied in these situations. This paper provides an extensive set of experiments on high strength steel box columns filled with concrete. A numerical model is presented and calibrated successfully with these tests. Furthermore, comparisons with the Eurocode 4 model for composite columns are also undertaken in this paper and this is found to be unconservative in its prediction of axial and combined strength. A mixed analysis technique is therefore presented, which treats the concrete as rigid plastic and the steel as linear elastic. This model is calibrated well with the numerical model presented and both of these models are found to be conservative in predicting the test results.

Theoretical study on the post-local buckling of steel plates in concrete-filled box columns

This paper presents a theoretical study on the post-local buckling behaviour of steel plates in welded steel box columns filled with concrete, by using the finite element method. The effects of various geometric imperfections, residual stresses and width-to-thickness ratios on the post-local buckling characteristics of steel plates in composite columns are investigated. A novel method is developed for evaluating the initial local buckling loads and post-local buckling reserve strength of steel plates with imperfections associated with a theoretical analysis. Two effective width formulas are proposed for the design of clamped steel plates restrained by concrete and are employed in the ultimate strength calculation of short concrete-filled steel box columns. The proposed design models are compared with existing experimental results with a good agreement.

Strength of Concrete Filled Steel Box Columns Incorporating Local Buckling

Concrete filled steel box columns have recently experienced a renaissance in their use throughout the world. This has occurred due to the significant advantages that the construction method can provide. This paper deals with the strength behavior of short columns under the combined actions of axial compression and bending moment. The paper addresses the effect of steel plate slenderness limits on this behavior. An extensive set of experiments has been carried out and a numerical model developed elsewhere is augmented and calibrated with these results. A simple model for the determination of the strength-interaction diagram is also verified against both the test results and the numerical model developed in this paper. This model, based on the rigid plastic method of analysis, is existent in international codes of practice, but does not account for the effects of local buckling, which are found to be significant with large plate slenderness values, particularly for large values of axial force. Thus some suggested modifications are proposed to allow for the inclusion of slender plated columns in design.

Experimental study on ultimate strength and ductility of concrete filled steel columns under strong earthquake

After the shock of the Hyogo-Ken Nambu Earthquake, a newly revised Japanese Seismic Design Method for Bridge Structures requires that steel bridge piers should not suffer serious damage due to such strong earthquakes. As one of the suitable methods to meet this requirement, a method of inserting an additional steel tube into the inside of a steel bridge pier can be considered. In this method, the ductility of the bridge pier is significantly enhanced, if the cross section is designed in such a manner that the axial compressive load, caused by the dead load of the superstructure, is mainly carried by the inner steel tube and the local buckling of the outer steel pier as well as the column buckling of the inner steel tube are prevented by the encased concrete between them. To investigate the ultimate strength and ductility of these concrete-filled steel box columns under strong earthquakes, an experimental study is executed.

Parametric Study on the Structural Behaviour of Steel Plates in Concrete-Filled Fabricated Thin-Walled Box Columns

A theoretical investigation of the post-local buckling behaviour and ultimate strength of steel plates in concrete-filled fabricated box columns is presented. The finite element method is used to predict the post-local buckling response of plates restrained by concrete. Geometric imperfections and residual stresses are considered in the analysis. The model developed is initially compared with a classical solution for clamped plates with initial imperfections. An extensive parametric study is then carried out to investigate the minimum local buckling coefficient and the effects of geometric imperfections and residual stresses on the post-local buckling strength of clamped steel plates. An effective width model is proposed for the design of clamped steel plates, and this can be used in the determination of the ultimate strength of short concrete-filled box columns. Independent test results of concrete-filled steel box columns are compared with the theoretical predictions by using the proposed design model. It is confirmed that the proposed design model predicts the behaviour of real concrete-filled steel box columns extremely well.