Concrete Encased Steel Columns Research Articles

Eccentric-Axial-Load Test for Composite Columns Using Bolt-Connected Steel Angles

AbstractFor better structural performance and constructability, a concrete-encased steel column that uses a steel cage prefabricated with bolt connections was developed. The steel cage consists of ...

Experimental and numerical investigation of high-strength concrete encased steel columns with rectangular-spiral stirrups

In the present study, seismic performance of High-strength Concrete Encased Steel columns with Rectangular - spiral Stirrups (HCESRS) is investigated. The composite specimen was tested under reverse cyclic loading and numerically simulated by the finite element method. Using ABAQUS software, numerical analysis of the specimen was performed, including hysteresis curve, skeleton curve, stress comparison and energy dissipation. Experimental results were compared with the numerical simulation to verify the accuracy of the numerical analysis. The influence of parameter changes such as shear span ratio, axial compression, steel content of I-shape and steel yield strength of HCESRS column were investigated. It was found that as the shear span ratio increases, the ultimate bearing capacity of the member decreases and the deformation capacity increases. Due to the brittle failure characteristics of high strength concrete, the limit value of axial compression ratio of HCESRS column was systematically studied, and effect of axial compression ratio on the ductility for HCESRS column has been investigated. This numerical analysis reliably evaluated and analyzed the seismic response of the HCESRS column.

Axial-moment interaction of high strength concrete encased steel composite columns: Design recommendation

This paper investigates the axial force-moment (N-M) interaction behaviour of high strength concrete encased steel (CES) composite columns. A numerical method is developed to simulate the nonlinear inelastic behaviour of CES composite beam-columns, which includes concrete cover spalling, concrete confinement effect, buckling of longitudinal reinforcing bars, and stain-hardening of the encased steel section. The numerical model is implemented in a self-developed computer program to generate the load-deflection response, moment-curvature relation, as well as the N-M strength interaction diagrams. The accuracy of the proposed analysis method is validated by comparison with the experimental results report in the companion paper and the test data available in literatures. To further understand the resistance of CES columns consisting of high strength concrete and high strength steel, parametric study is performed to evaluate the effect of concrete compressive strength, steel yield strength, load eccentricity, as well as steel area ratio. Finally, a simplified method is proposed to construct the N-M strength interaction curve of CES composite columns. The proposed method can be used to design CES beam columns for concrete grade up to C100 and steel grade up to S960.

Factors Influencing Bond Properties in Concrete Encased Steel Columns at High Temperatures

This paper presents experimental results on factors governing the bond strength between steel and concrete in concrete encased steel (CES) columns at high temperatures. Push-out tests were conducted on 22 CES specimens to investigate the bond strength between concrete and steel under the temperature of 250 °C, while one specimen was tested at room temperatures for comparison. Data obtained from these tests revealed that the bond properties of CES structures between steel and concrete is influenced by a number of factors, including concrete strength, concrete cover thickness, stirrup reinforcement ratio, steel ratio and anchorage length of steel at high temperatures. The formulas for predicting the ultimate bond strength and slip between steel and concrete of CES structures are proposed based on the experimental investigations.

Compressive performance of ECC-concrete encased high strength steel composite columns

The use of high strength steel (HSS) in the construction of concrete encased steel (CES) composite columns is often limited by the strain incompatibility issue between HSS and concrete at peak-load. This study proposes an alternative approach to confine the high strength concrete with Engineered Cementitious Composite (ECC) to improve its compatibility with high strength steel. The main purpose of this study is to experimentally evaluate the axial compressive performance of the proposed composite column cross-section configuration. Behaviours of fifteen short columns including twelve ECC-CES columns are investigated in terms of failure modes, load-deformation curves, ductility and energy absorption capacity. The test parameters included ECC and concrete strengths, ECC cover thickness, steel section shape and column section’s aspect ratio. It was found that ECC generally improved the failure behaviour of high strength steel CES columns and increased the deformation and energy absorption capacity. On average ECC-CES columns showed around 12% and 8% higher ductility and toughness than control concrete column, respectively. A detailed 3D nonlinear finite element model was developed and validated against experimental results. Applicability of current design codes to predict the ultimate strength of ECC-CES columns was also evaluated. Finally, a method to calculate the ECC-CES column’s capacity considering effective material stresses at peak-load was proposed.

Pseudostatic testing for load-carrying capacity of precast concrete-encased steel composite columns

To combine the advantages of concrete-encased steel columns, high-performance concrete and precast structures, an innovative steel composite column encased in precast high-performance concrete is proposed in this paper. The proposed composite column is composed of a precast outer part and a cast in situ inner part. The precast part can be prefabricated using steel-fiber-reinforced reactive powder concrete (RPC), and the inner part can be cast using conventional concrete. With an aim of exploring seismic behavior of this innovative composite column, three of them were designed and subjected to constant axial compression and cyclic loading, with the main test parameter being the shear span-to-depth aspect ratio. The failure mode, hysteresis loop, skeleton curve, lateral stiffness degradation, energy dissipation and displacement ductility ratio were all analyzed to investigate the effect of aspect ratio on cyclic behavior. Test results indicated that no apparent concrete spalling was found in any of the specimens, due to application of steel fibers in RPC, and the structural steel, precast outer-shell and cast in situ inner part behaved compatibly without any visible slippage. Meanwhile, initial stiffness and load-carrying capacity of the test specimens increased with a decrease in aspect ratio, and energy dissipation and displacement ductility increased with the increasing aspect ratio. Based on the test results, two calculation methods (per AISC 360–10 and JGJ 138–2016) were put forward to predict the load-carrying capacity of the test specimens. The model based on AISC 360–10 overestimated the load-carrying capacities of test specimens, while the model based on JGJ 138–2016 predicted capacities that agreed well with the test results.

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.

A unified approach to evaluate axial force-moment interaction curves of concrete encased steel composite columns

This paper presents a unified approach to evaluate the axial force and moment interaction strength curve of Concrete Encased Steel (CES) composite columns made of different steel and concrete grades. A database was established by collecting the test results of CES composite columns in the literature covering concrete compressive strength ranging from 20 to 104 MPa and steel yield strength from 280 to 913 MPa. A sensitivity study was then carried out to investigate the effect of different design parameters on the accuracy of the current design method EN 1994-1-1 in predicting the ultimate strength of CES composite columns. These design parameters include characteristic strength of materials, steel contribution ratio, concrete cover thickness ratio, longitudinal reinforcement ratio, and volumetric ratio of transverse reinforcement. Analytical study was performed through a self-compiled computer program which was developed based on materials strain compatibility principle. The comparison between the analytical results and test results confirms the validity of the proposed method to provide reasonable prediction of cross-sectional axial-moment interaction curves for CES columns for a wide range of steel and concrete grades. The existing EC4 method, which is based on plastic design principle, was found to be un-conservative in predicting the cross section resistance of CES composite columns with high strength concrete and high strength steel. Further enhancement was made to the proposed method to include both the strain gradient effect and concrete confinement effect to achieve a better agreement with the test results reported in literature. Finally, a simplified method was proposed to construct the axial-moment interaction curves of CES columns, which can be used as a unified approach to design such columns with normal and high strength steel and concrete materials.

Behavior of high strength concrete encased steel composite stub columns with C130 concrete and S690 steel

This paper presents an experimental program that studies the structural behaviour of high strength Concrete Encased Steel (CES) composite columns. The structural performance under compression, including the damage pattern, load-carrying capacity, post-peak ductility, and load-displacement response is experimentally investigated. A total of 14 specimens were tested under concentric compression. The parameters studied in this program include concrete compressive strength, steel yield strength, stirrup spacing, incorporation of steel fiber, as well as the shape of the structural steel section. To evaluate the material compatibility between high strength concrete and high strength steel, two concrete grades (C90, C130) and two steel grades (S500, S690) were used to prepare the test specimens. In addition, 0.5% volume fraction of steel fiber was added in concrete mix to minimize the inherent brittleness of high strength concrete. The comparison between test results and analytical predictions reveals the inability of existing design codes to estimate high strength CES columns, unless steel fiber and dense reinforcement are used in combination. The effect of material strength, steel fibers, volumetric ratios of hoop reinforcement, and shape of steel section on both strength and ductility of CES columns was assessed through a comprehensive parametric study. The analysis of test results demonstrates that steel contribution ratio plays a dominant role in the ductility, whereas increasing hoop reinforcement ratio and adding steel fiber has negligible effect. Finally, a simplified formula is proposed to evaluate ductility of high strength CES columns.

Experimental study on high strength concrete encased steel composite short columns

This paper investigates on the axial compressive behavior of high strength Concrete Encased Steel (CES) composite short columns. Six column specimens made of high strength concrete were tested under monotonic pure compression. The structural performance was investigated including failure pattern, load-carrying capacity, load-deformation response, and post peak ductility. The major parameters affecting the ultimate strength of such composite columns are studied including concrete strength, spacing of transverse reinforcement bars, and the inclusion of steel fibers in the high strength concrete. Based on the experimental work, it is found that the failure mode of CES columns with high concrete strength C120 is characterized by concrete cover spalling, which triggers a sudden drop of compression capacity once the peak resistance is reached. With the decrease of transverse reinforcement spacing, the ultimate load capacity of high strength CES columns can be enhanced due to better concrete confinement but the failure is still brittle with abrupt unloading from the peak load. The test results also indicated that the provision of 0.5% volume of steel fiber is able to prevent the concrete cover spalling, and hence the load-carrying capacity can be enhanced. Finally, the test results together with additional test data collected from the literature are used for comparison with codes’ prediction using the EN 1994-1-1 and JGJ 138-2016. The code methods give excellent prediction of CES columns with normal strength steel and concrete grade up to C90, but they over-estimate the maximum resistance of CES columns with concrete strength higher than 90 MPa. To extend the current design methods for CES members with higher strength concrete, two analytical methods were proposed which shows good correlation with test results, thus enabling the design of CES members with higher strength of concrete.

Seismic damage performance levels for concrete encased steel columns using acoustic emission tests and finite element analysis

Concrete encased steel (CES) columns under low-cyclic lateral loading and high axial compressive ratios were tested using the acoustic emission (AE) monitoring technique to determine their damage performance states. These damage performance states include cover concrete cracking, compressive longitudinal reinforcement yielding, cover concrete crushing or steel flange local buckling, cover concrete spalling, longitudinal reinforcement buckling, and core concrete crushing. Using the test results, a fiber finite element method was calibrated and used to numerically simulate 486 CES columns under lateral loading and determine an empirical expression for the material parameter of the Park-Ang damage model. Use of this modified Park-Ang model in conjunction with the response databank coming from the numerical analysis of the 486 CES columns, finally leads to limit damage values for the damage states of no-damage, minor-damage, medium-damage, serious-damage, and collapse.

Buckling behaviour of high strength concrete encased steel composite columns

This paper provides an insight into the buckling behavior of high strength concrete encased steel (CES) columns through a comprehensive investigation including experimental, numerical and analytical analyses. Three long CES column specimens made of high strength concrete C100 and S355 H-section were tested under axial compression. The maximum test loads obtained from these tests were compared with the buckling resistance predicted by EN 1994-1-1, AISC 360-10 and ACI 318-08. Nonlinear finite element analyses were performed to predict the buckling resistance and trace the load displacement behaviour of these columns. In the finite element model, the column initial imperfections were carefully chosen to predict the maximum resistance and the load-displacement response, and compared with the equivalent imperfection values stipulated in the modern design codes. In order to examine the validity and limitation of the current design approaches in predicting the buckling resistance of CES columns made of high strength concrete, a statistical study was undertaken based on the established database covering a wider range of material strength and geometric configurations. In the statistical study, the buckling resistance and effective flexural stiffness obtained from the tests were compared with the codes' predictions from EN 1994-1-1, AISC 360-10 and ACI 318-08. Finally, the reliability of the current design methods is assessed by correlating their accuracy with respect to the variation of material strength, column slenderness ratio, load eccentricity and steel contribution ratio.

Analysis and Design of Short FRP-Confined Concrete-Encased Arbitrarily Shaped Steel Columns under Biaxial Loading

The FRP-confined concrete-encased steel column is a new form of hybrid column, which integrates advantages of all the constituent materials. Its structural performance, including load carrying capacity, ductility, and corrosion resistance, has been demonstrated to be excellent by limited experimental investigation. Currently, no systematic procedure, particularly for that with reinforced structural steel of arbitrary shapes, has been proposed for the sectional analysis and design for such novel hybrid columns under biaxial loading. The present paper aims at filling this research gap by proposing an approach for the rapid section analysis and providing rationale basis for FRP-confined concrete-encased arbitrarily shaped steel columns. A robust iterative scheme has been used with a traditional so-called fiber element method. The presented numerical examples demonstrated the validity and accuracy of the proposed approach.

Compressive behaviour of large rupture strain FRP-confined concrete-encased steel columns

Fibre-reinforced polymer (FRP)-confined concrete-encased steel columns (FCSCs) have emerged as a new form of hybrid columns. An FCSC consists of an outer FRP tube, an inner steel section, and concrete filled in between. The concept of FCSCs not only provides a durable and ductile structural form for new construction, but also can be practiced as an effective method for the retrofitting/strengthening of existing steel columns. This paper presents the first ever study on FCSCs with a large rupture strain (LRS) FRP tube [i.e., polyethylene terephthalate (PET) FRP tube]. A total of 12 circular specimens (six FCSCs and six FRP-confined concrete columns) were tested, with the main test variable being the thickness of the PET FRP tube. The test results showed that the buckling of the encased steel section can be well prevented by the constraint from the surrounding FRP-confined concrete even when the axial deformation is large. On the other hand, the encased steel section can provide additional confinement to the concrete in FCSCs, leading to excellent structural performance of the columns in terms of both axial strength and deformation capacity.

08.31: Analysis of the load transfer between materials in composite concrete encased steel columns loaded axially

ABSTRACTThe analysis of the load distribution between steel and concrete parts of the composite concrete encased I‐shaped steel columns subjected to short‐term axial load is presented. The variable parameters were: two loading conditions − load scheme I (specimens loaded through the entire cross‐section simultaneously), load scheme II (specimens loaded only through the steel profiles protruded from the top end) and concrete class (C25/30 and C40/50). Obtained in the experiment longitudinal strains of both steel and concrete were used to compute the force transferred through the steel and concrete parts of the columns. To provide a reference level for the discussion on the results the analytical solution assuming the perfect bond between steel and concrete, Hooke's law for steel and the Rüsch formula for unconfined concrete were used. The force for which the stress in steel and concrete computed using the strain readings obtained from the experiment starts to deviate from the full bond analytical model predictions can be interpreted as the force for which the loss of adhesive bond occurs. Furthermore on the basis of the equilibrium of the infinitesimal cross section of a column, method for estimating the shear stress at which the debonding occurs is shown. The analysis showed that for columns under the load scheme I composite action of steel and concrete is in effect for the entire load range regardless of the concrete class. For elements under the load scheme II the composite action of both materials is in effect only up to a specific value of the force that changes along the height of the column. Presented method can be applied to composite steel‐concrete columns under any configuration of boundary conditions subjected to axial compression. Moreover it can be used to specify the area for which the shear connectors should be provided, to validate constitutive models of shear stress – slip behaviour that can be used is FEM analysis and to analyse adhesive bond properties also in columns made of another materials, for example reinforced concrete members strengthened with FRP sheets.

Finite element analysis on seismic performance of EWECS composite column

A new hybrid Engineering Wood Encased Concrete-Steel (EWECS) structural system is developed in Japan to solve the limitation of story number for wooden structure. In this structural system, the EWECS column consist of concrete encased steel (CES) core and wood panel as a column cover. An experimental study has been done to investigate the seismic performance of the EWECS column, which is compared with CES column without concrete cover. The main parameter is the presence of wood panel. In order to validate the experimental results, a 3D finite element (FE) model is built by using ANSYS APDL v.14. The FE results indicate that FE EWECS column model has excellent performance, which has a stable spindle-shape hysteresis loops. The presence of wood panel contributes to improve the maximum flexural capacity of the EWECS column about 13%. In addition, the stress distribution on the FE models has good correlation with failure mode of the EWECS column specimens. Generally, the numerical results are able to accurately simulate the seismic behaviour of the EWECS column on test results.

Effect of Sustained Load on Ultimate Strength of High-Strength Composite Columns Using 800-MPa Steel and 100-MPa Concrete

AbstractTo investigate the effect of sustained loads on the ultimate strength of high-strength composite columns using grade 800-MPa steel and 100-MPa concrete, ultimate load tests were performed after long-term loading. Two concrete-encased steel (CES) columns were tested under concentric loading, and a CES column and a concrete-filled steel tube (CFT) column were tested under eccentric loading. The test results showed that stress-relaxation in the concrete and gradual stress-transfer to the steel and longitudinal bars occurred, and the ultimate shrinkage strain and ultimate creep coefficient of the high-strength concrete were estimated as 553 μe and 1.44, which are lower than those of normal-strength concrete but belong to the ranges specified in concrete standards. To confirm the tests, a numerical parametric study was performed, considering the age-adjusted effective modulus and relaxation. The results of the experimental and numerical studies showed that, in the case of the high-strength composite co...

Eccentric Axial Load Capacity of High-Strength Steel-Concrete Composite Columns of Various Sectional Shapes

AbstractTwo concrete-filled steel tube columns and four concrete-encased steel columns using high-strength steel (yield strength fys=913, 806, and 812 MPa) and high-strength concrete (compressive strength fc′=94, 113, 104, and 184 MPa) were tested to investigate the effect of various sectional shapes and configurations on the eccentric axial load carrying capacity. This study focused on maximizing the contribution of the high-strength steel, preventing early crushing of the concrete (1) by using steel tubes or closely spaced ties for lateral confinement, (2) by using ultra high-strength (200 MPa) concrete with a high-crushing strain, and (3) by placing L-shaped steel sections at the corners of the cross section. The test results showed that the steel tube successfully restrained early concrete crushing and developed its full plastic stress; unlike expectation, early crushing occurred in the ultra high-strength concrete column; and the concrete-encased L-section column had higher peak strength and flexural...

Analytical behavior of eccentrically loaded concrete encased steel columns subjected to standard fire including cooling phase

A nonlinear 3-D finite element analysis (FEA) model was developed to predict the behavior of eccentrically loaded concrete encased steel (CES) columns subjected to ISO-834 standard fire including heating and cooling phases. The finite element model has been validated against published tests conducted at elevated temperatures. Comparisons between the predicted results and the test results show that this model can accurately predict the behavior of CES columns under fire. The FEA model was then used to investigate the typical temperature-time curve and mid-height lateral deformation-time curve of eccentric compression CES columns in a complete loading history including initial loading, heating and cooling. It is shown that the temperature delay is obvious at the inner layers of concrete. The fire resistance of a CES column should be checked for the full process of fire exposure until temperatures everywhere in the column start to decrease. The lateral deformation of the column still gradually increases during the cooling phase and the column may fail during that phase. There is a large residual deformation after the fire exposure. Furthermore, the variables that influence the behavior of the CES columns under fire were investigated in parametric studies. It is found that the main parameters which influence the lateral deformation-time curve of the column during the full process of fire exposure are load ratio, slenderness ratio, duration time, depth to width ratio and steel ratio, and the main parameters which influence the residual deformation ratio of the column after fire are load ratio, duration time, cross-sectional depth and steel ratio.

H型鉄骨内蔵CES柱の復元力特性のモデル化に関する研究

Concrete Encased Steel (CES) system consisting of fiber reinforced concrete (FRC) and steel is a new composite structural system proposed by the authors, and is being conducted continuous and comprehensive studies to make it practical. In existing studies, the influence of the axial force ratio, the H-shaped steel ratio, the shear span ratio, the concrete fiber contents ratio (the ratio of fiber volume to concrete volume) and the flange width of H-shaped steel on the deformation capacity were investigated. Also the deformation capacity was examined by using the above five parameters. Then equations to evaluate the deformation capacity were proposed. It is clarified that the proposed equations show good agreement with the test results. In this paper, hysteresis models for CES columns to use the earthquake response analysis are proposed with considering the deformation capacity. The accuracy of the model is verified by comparing with the test results.