Concrete-encased Steel Research Articles

Axial compression behavior of prefabricated T-section thin concrete encased steel short columns

A T-section thin concrete encased steel (T-TCES) column is proposed for the residual buildings, avoiding the convex corners in rooms. The required concrete cover thickness for concrete encased steel members specified in the existing codes is quite large, which is the key problem and to be solved in the proposed column by reducing the concrete covers, facilitating the transportation and installation. To evaluate the mechanical performances of the proposed columns, especially the local stability of the internal T-section steel with the obviously reduced concrete covers, tests of nine short column specimens were conducted under axial monotonic loading. Also, the finite element (FE) analysis was conducted and validated. The effect of the proportion of steel sectional area, concrete strength, stirrup spacing and the concrete encasement thickness outside the flange on the failure modes, load-displacement responses, load-strain responses and characteristic loads were obtained. According to the test and FE analysis results, the peak load can be improve by increasing concrete strength or the proportion of steel sectional area or decreasing the stirrup spacing. The T-TCES columns with low-grade concrete or small stirrup spacing or high proportion of steel sectional area have a better composite action between the internal steel and concrete. Increasing the thickness of the concrete encasement outside the flange can prevent the T-TCES column from cracking. The concrete encasement and reinforcement bars can restrain the internal steel buckling, and improve the steel strength of the composite column compared with the steel column. The calculation methods for the peak and design resistances of the T-TCES column were proposed and proved to show satisfactory accuracy. The simplified model of the load-displacement responses and the calculation methods of the axial stiffness are proposed and validated against the test results.

Compression behavior of the concrete-filled double skin steel tube columns under hydrostatic pressure: Experimental and modeling study

This paper conducted an experimental study focusing on the axial compressive behavior of circular concrete-encased concrete-filled steel tubular (CFDST) columns composed of a circular inner steel tube under active confinement. The investigation aimed to explore the impact of concrete compressive strength grades and diameter-to-thickness ratio of steel tubes in the presence of hydrostatic pressure. The effects of varying outer diameter-to-thickness ratios (44, 45.6, 85.84, and 86) and concrete compressive strength grades (10 and 20 MPa) on the performance of CFDST columns were investigated in this paper. Additionally, the study considered the impact of two inner steel tube diameter-to-thickness ratios (20.5 and 39). The experimental results demonstrated that external pressure can significantly enhance both the load-carrying capacity and ductility of the specimens. The ultimate strength of the columns was improved by a minimum of 34% and a maximum of 92%. Finally, empirical formulas for predicting the axial compression capacity of CFDST under active confinement using gene expression programming were presented. The accuracy of the proposed model was then assessed by the artificial neural network, and the results indicate that both models perform well when compared to the empirical equations that take into account the hydrostatic pressure of wet concrete.

Investigation of the Buckling Capacity of Concrete Encased Steel H-Piles on Highway Bridge Structures

Investigation of the Buckling Capacity of Concrete Encased Steel H-Piles on Highway Bridge Structures

Cyclic axial compression stress-strain model for FRP-confined concrete-encased cross-shaped steel columns

FRP-confined concrete-encased cross-shaped steel columns (FCCSCs) are novel composite columns developed in recent years. For the bearing capacity design and seismic design of FCCSCs, the development of an accurate cyclic axial compression stress–strain model is necessary. Considering such a demand, this paper presents the cyclic axial stress–strain model for FCCSCs. The model is informed by physical observation and test measurements from a series of experimental investigations conducted by the authors and other researchers. The proposed cyclic model consists of two main components, namely (1) the monotonic stress–strain envelope curve and (2) the cyclic rules and equations to describe the unloading and reloading paths. Moreover, the hybrid confinement effect of FRP tube and cross-shaped steel is considered in the stress–strain model through the effective confinement stress model, in addition to their influence on the transition and ultimate point of the envelope curve. Finally, the accuracy of the proposed model is verified with the current test results.

Simplified impact force models for novel FRP-steel–concrete composite columns under axial impact loading

This study aims to investigate the axial impact resistance of FRP tube-confined concrete-encased cross-shaped steel column (FCCSC). The numerical simulations were conducted utilizing the ABAQUS software. The finite element (FE) models developed in this study were validated and upgraded by fitting the relevant test data of FRP-confined concrete-filled steel tube (FCFST). Subsequently, a significant number of models were established and executed with four variable parameters, namely impact mass, impact velocity, concrete strength and FRP tube thickness. Based on the analysis results, typical axial impact states were summarized and defined. Then, the simplified impact force time history models were developed for predicting the dynamic responses of FCCSCs subjected to axial impact loads. The dynamic damage index for axial impact resistance and the corresponding design flow of FCCSCs also have been proposed.

Experimental investigation and predictive modeling of shear performance for concrete-encased steel beams using artificial neural networks

Experimental investigation and predictive modeling of shear performance for concrete-encased steel beams using artificial neural networks

Seismic performance of concrete-encased CFST column to RC beam joints: A practical model

This study proposed a practical model of the concrete-encased concrete-filled steel tubular (CFST) column to reinforced concrete (RC) beam joints based on the experimental and analytical investigations in companion publications. The analytical work was conducted to figure out the load transfer mechanism in the panel zone with considerations of the geometric features, material strengths and the axial load level. The strength calculation method was proposed according to the mechanical principle and parametric study for the beam, column and panel zone, respectively. The strength calculation method was proved to be consistent with the measured results and could reflect the actual failure mode. Then, a hysteretic model for the panel zone was developed that considered the stiffness degradation, which was verified by test results and FEA simulations. Finally, the practical model was proposed, which could simulate the failure mode, the strength and the cyclic behaviour of the composite joint. The predicted results of the proposed practical model coincided nicely with the test results of both composite members and joints, which indicated that the practical model could be used in the seismic analysis of the specified frame structures.

Axial stress-strain characteristics and confinement mechanism of concrete-encased steel composite columns: An analytical model

Axial stress-strain characteristics and confinement mechanism of concrete-encased steel composite columns: An analytical model

Shear strength model for steel shape-reinforced concrete girders considering compatible steel–concrete interaction

Steel shape-reinforced concrete (SRC), also known as concrete-encased steel, offers numerous benefits compared with conventional reinforced concrete (RC) or bare steel due to its combination of RC and structural steel sections; therefore, SRC members find extensive use as transfer elements or girders in engineering applications. When evaluating SRC girders’ shear resistance, the direct strength superposition of RC and structural steel is basically used in most design codes, ignoring the steel–concrete interaction under shear, which has proven unreasonable because of the brittle shear behavior of RC. This paper suggests a novel method for assessing SRC girders’ shear resistance mechanism. The proposed method involves modeling the RC portion by a hybrid truss-arch analogy; in addition, based on the strain compatibility condition, the steel profile’s contribution is determined by applying the yielding criterion. Finally, the rationality of the proposed model is validated through a comprehensive comparison with previously published models and a set of 45 experimental test data samples available in the existing literature, and the results show that the proposed model can reasonably predict the maximum load-carrying capacity of SRC girders and is superior to existing shear strength models and code-based equations.

Seismic performance of concrete-encased hexagonal CFST column base: Design method

This paper presents the parametric analysis and design method for the concrete-encased hexagonal concrete-filled steel tubular (CFST) column base under combined compression and biaxial bending. The corresponding experimental and numerical investigations on the seismic performance of the concrete-encased hexagonal CFST column base under combined compression and biaxial bending have been presented in the companion paper (Yang et al., 2023). The finite element model established in the companion paper is used to study the influence of key parameters on the load bearing capacity and the distribution of internal force. These parameters include those of the CFST column, the outer RC, the plate-anchor component, the axial load ratio and shear connectors. A total of six failure modes are identified, including three types of flexural failure and three types of shear failure. The ultimate capacity and ductility of the column base with flexural failure at the bottom or top of the plate are higher than that of other four failure modes, which are determined as the reasonable failure modes for practical application in the design and analysis of concrete-encased column bases. A novel design approach for the concrete-encased CFST column base is introduced. A simplified method is proposed to predict the flexural resistance of the concrete-encased CFST column base, following up with the shear strength calculation of outer RC to determine the stirrup arrangement. Some constructional recommendations are also proposed.

Seismic performance of concrete-encased hexagonal CFST column base: Experimental and numerical analysis

This paper presents the seismic behaviour of concrete-encased hexagonal concrete-filled steel tubular (CFST) column base under combined compression and biaxial bending. A total of 12 column base specimens are tested in terms of failure modes, load-displacement relationship, strain distribution, strength and stiffness degradation and energy dissipation capacity. The main parameters include the height of the outer reinforced concrete component, the loading angle of lateral load, the axial load ratio, the cross section of the CFST column and the base type. A finite element model is established, validated, and used to study the distribution of internal force. The effects of some key parameters on the internal force distribution of column base are studied, including the second-order torsion, the loading angle, the loading path of the lateral load and the sustained load. Three types of flexural failure modes are identified in both experimental and numerical investigations. The outer RC component as well as the axial load ratio have a significant influence on the failure mode of the column base. The concrete-encased column base tends to fail at the top section of the base plate with a higher axial load level. The outer reinforced concrete not only resists external load with the CFST column and plate-anchor component together, but also improves the strength and ductility of the CFST column.

Experimental and numerical investigation on precast CECFST column-to-column connections subjected to axial and eccentric compression

Concrete-encased concrete-filled steel tube (CECFST) is a new type of composite member featuring superior structural performance and high construction productivity. This form of structural members has been increasingly used as columns in high-rise buildings and as piers in long-span bridges. Recently, to promote the application of CECFST columns in countries short of labour, e.g., Singapore, North America and Europe, an advanced precast CECFST column-to-column connection with rapid construction and excellent tensile performance was developed by the authors. However, behaviour of the developed precast connection subjected to axial and eccentric compression has not been studied so far. In this paper, an experimental programme including five connection tests was conducted to investigate compression performance of the developed precast connection. The test results showed that compression resistance of the precast connection could achieve that of CECFST columns, indicating that it is feasible to connect CECFST columns by the newly proposed precast connection to enhance construction productivity. Furthermore, an extensive finite element (FE) modelling programme was carried out to generate more numerical data by conducting parametric studies. The experimental and numerical results showed that connection configuration plays a pivotal role in load-carrying capacity of the precast connection. Based on EN 1994-1-1, one simplified strength prediction model was proposed for the proposed precast CECFST column-to-column connection. Compared with experimental and numerical results, the predicted strength was conservative and consistent, indicating that the proposed approach can be used in the design of the precast connection.

Experimental and numerical study on seismic behavior of partially steel-reinforced concrete beam-to-steel tube column joint

Steel-reinforced concrete (SRC) beams are commonly used in the transfer structures of high- and super-high-rise buildings since concrete-encased steel elements have good resistance to buckling and seismic performance. However, the full-length steel shapes utilized in SRC beams are ineffective and pose many construction challenges. This study proposes a new steel-reinforced concrete beam-square steel tube (SST) column joint in which the SRC beam is embedded with a partial-length steel shape. Experimental tests are conducted on three joint specimens with varying cross-sectional dimensions of the SRC beam and lengths of the steel shape. The experimental results show that all the specimens exhibit good deformation, load-carrying capacity, and stable hysteresis responses. The three specimens demonstrate a considerable strength drop at drifts exceeding 5%. Besides, significant plastic deformation occurred at the column base region and in a limited region of the steel beam flange near the beam-column joint. Finite element models are developed to study the effects of single-side steel length, shear stud spacing, and thickness of beam concrete cover on the seismic performance of the proposed joint. The elastic deformations that caused the total column drift were theoretically analyzed. The predicted joint initial rotational stiffness using the proposed equation agrees well with the test and numerical results. The study’s limitations are also discussed.

Maximum shear strength of concrete-encased steel composite beams: A novel strength model and FE verification

Due to the high load-carrying capacity and rigidity, concrete-encased steel (CES) beams are widely applied as transfer elements in heavily-loaded structures. However, the accurate shear strength prediction method is still under investigation because of the difficulty in the shear contribution decomposition of steel shape and reinforced concrete. Current design codes apply the strength superposition for CES beams, namely that their shear strength can be obtained by directly superimposing the shear strengths of RC and steel shape, which has been proven questionable by existing research. Therefore, it needs to carefully decouple the multiple shear transfer mechanisms in CES beams and explore their roles in a composite structure. In this paper, a novel shear strength model is proposed, in which a CES beam can be divided into a shear web and a composite truss consisting of RC with two steel flanges, and strain compatibility is applied to evaluate the shear contribution of these two parts. After that, an elaborate FE model is established to further quantitatively investigate the shear contribution of the composite truss and the shear web. Finally, the proposed model is verified with 61 existing specimens in the literature, and the comparison between the calculated and test results indicates that the proposed model could predict the shear strength of CES beams accurately.

Axial compression behavior of FRP-confined steel-reinforced columns with a flange-broadened cross-shaped steel

Due to the hybrid confinement effect and reforming effective confinement area, the composite action of square columns is enhanced by utilizing cross-shaped steel on fiber-reinforced polymer (FRP) tube-confined steel-reinforced columns. These novel hybrid columns are FRP-confined concrete-encased cross-shaped steel columns (FCCSCs). This study adopted flange-broadened cross-shaped steel to develop the potential advantages of FCCSCs. Twenty-two specimens were tested under monotonic and cyclic axial compression, and the test parameters were column form, FRP thickness, cross-shaped steel dimension, and slenderness ratio. FCCSCs with broad steel flanges exhibited high axial load capacity and axial deformability. In addition, increasing the thickness of the flange, web, or FRP tube resulted in stronger hybrid confinement and outperformed strength behavior in confined concrete. Slender FCCSCs presented lower axial capacity and higher lateral deflection. Finally, the calculation method for compressive load capacities of slender FCCSCs is proposed.

Structural performance of rigid shear connectors in concrete encased steel composite columns

Concrete encased steel (CES) composite columns possess a wide variety of advantages in constructing concrete-steel composite frames. In order to achieve the composite behaviour of these columns, it is necessary to transfer the force between steel and concrete, effectively. Therefore, shear connectors should be used on the contact surface between concrete and steel. In this research, behaviour of the rigid shear connectors in CES composite columns is numerically studied. The results – obtained from the finite element analysis – were validated against the experimental results. Afterwards, parametric studies were conducted to investigate the effective parameters on the behaviour of the rigid shear connectors. The results show that the length, the thickness and the penetration depth of the shear connector, the compressive strength of concrete and the thickness of concrete cover have a noticeable effect on the strength and stiffness of the shear connector. Moreover, increasing the length and the penetration depth of the shear connector, and the thickness of the concrete would increase the residual strength level. In general, the rigid shear connector is mobilized under a much smaller slippage (between concrete and steel), as compared to the stud shear connector, and provides greater stiffness and strength. Moreover, due to being encased in reinforced concrete, it can also provide sufficient ductility. Finally, based on the parametric studies, a formula is suggested to predict the ultimate shear capacity of the rigid shear connector in the CES composite columns.

Axial compression behavior of prefabricated L-section thin concrete encased steel short columns

A prefabricated L-section thin concrete encased steel (L-TCES) column, which could be prefabricated in shop, connected on site, and not convex out of wall, was proposed. Changing the proportion of steel sectional area, concrete strength, stirrup spacing and the concrete encasement thickness outside the flange, experimental and numerical investigations were conducted on the axial compression performances of the proposed composite column, and the failure modes, load-displacement curves, load-strain curves and characteristic resistances were analyzed. According to the results, the concrete encasements and reinforcements can restrain the steel buckling, and increasing the proportion of steel sectional area or the concrete strength can improve the ultimate resistance and stiffness of the column. Increasing the thickness of concrete encasement outside the flange could prevent the concrete from cracking, and compared with steel column, the resistance of steel in the composite column can be more fully utilized under the ultimate load. The formulas to calculate the design and ultimate resistance for the composite column with closed stirrup were proposed, providing results with satisfactory accuracy. The simplified model of load-displacement curve was proposed and validated against the test results.

Temperature gradient impacts on concrete-encased steel I-girder: an ANN optimization approach

Temperature gradient impacts on concrete-encased steel I-girder: an ANN optimization approach

Negative stiffness-enhanced seismic damping technology for an over-track complex with concrete-encased steel columns

The resilient transit-oriented model stimulates the necessity of seismic performance enhancement or retrofitting of over-track complexes with concrete-encased steel slender supporting columns. This study proposes a novel low-damping-ratio-based vibration control approach and easy-to-use design for over-track complexes by flexibly installing the negative stiffness amplification system (NSAS) at the multistory of the over-track building and lower podium. Moreover, a multilocation-oriented design strategy and fitted formulae are developed for the NSAS-damped complex with enhanced energy-dissipation efficiency and an adjustable structural modal shape. By utilizing the negative stiffness device, dashpot, and tuning spring, the NSAS is theoretically constructed, and a simplified model of the NSAS-damped over-track complex is established. Stochastic response analysis and parametric investigation are performed to quantify the benefit of the NSAS-damped story over the conventional control method. Then, easy-to-use design formulae are provided in the initial design and parameter modification stages to realize the simultaneous control of the upper and lower structures. Based on a typical over-track complex with concrete-encased steel slender supporting columns, design cases are analyzed for the NSAS and conventional viscous dampers to verify the applicability of the proposed NSAS and design. The results indicate that the target-story located NSASs contribute a higher-efficiency approach to realize the significant performance improvement or retrofit of the over-track complexes without perturbing the daily function. Particularly, NSASs tailored for distinct stories in the over-track complex effectively satisfy the modification demand for structural modal shape and enhanced energy dissipation efficiency, which cannot be realized by conventional viscous dampers or the existing design of NSASs in normal buildings. In addition, the NSAS-damped story demonstrates a released stiffness abruptness and robust seismic performances against various earthquakes.

Cyclic Loading Test for Beam–Column Joints of Precast Concrete Beam and Concrete-Encased Steel Angle Column

Cyclic Loading Test for Beam–Column Joints of Precast Concrete Beam and Concrete-Encased Steel Angle Column