Thin-walled Concrete-filled Steel Tubular Research Articles

Shear behavior of thin-walled concrete-filled steel tubular columns

Shear failure is a brittle type of failures, which should be avoided in any structure as it could cause property loss and human casualties. The shear behavior of thin-walled concrete-filled steel tubular (CFST) columns was hence investigated in this study, involving the tests of 24 thin-walled CFST columns and considering the following parameters: shear span-to-depth ratios = 0.15, 0.3, 0.5, 1, 1.5, and 2; confinement factor = 0.63, 1.27, 1.44, and 1.70; and axial load ratios = 0, 0.2, 0.4, and 0.6. The test results revealed the insights of the initial shear stiffness, failure modes, ultimate bearing capacity, load-deflection curves, load-strain curves, and ductility of the specimens. Based on the failure modes, a method is proposed to evaluate the load resistance, and explain the relationship between the shear span-to-depth ratio and the peak bearing capacity. A compression strut mechanism considering the confinement provided by the steel tube flange is also proposed. The effect of axial load on the shear strength of the concrete core is negligible when the axial load ratio doesn’t exceed 0.4. The predicted results from using the proposed simple model agree generally well with the experimental results.

Seismic evaluation of stiffened CFST column to RC beam frames

A stiffened concrete-filled steel tubular (CFST) member offers improved composite action over the unstiffened counterpart in terms of delaying local buckling of the square steel tube and enhancing confinement to the infilled concrete. The diagonal ribs, made of steel plates with circular openings and welded to the steel tube, were found particularly effective among various stiffening forms in improving the structural performance of thin-walled square CFST members. To better accommodate this new type of composite columns into engineering practice, the corresponding joints with reinforced concrete (RC) beams were proposed and were found to possess good axial and cyclic behavior. Consequently, composite frames composed of diagonal rib stiffened CFST columns and RC beams are expected to have superior performance and to be widely used in practice. However, the previous research mainly focused on structural members, and no study at the system-level was conducted. In addition, the guidance on seismic design of the stiffened CFST frames in mainstream design codes is lacking. To fill this research gap, this work investigated the seismic behavior of diagonal rib stiffened CFST column to RC beam frames numerically, and fiber-based finite element (FE) models were developed in software OpenSees and were validated extensively by the experimental results. The research parameters included the column-to-beam strength ratio, bond-slip of beam reinforcing bars (rebars) within the joint zone, and shear deformation in ultra-short columns. Besides, unstiffened CFST column to RC beam frames were also designed for comparison. The pushover analysis was then carried out to investigate the effect of different parameters on the strength, deformation mode, and yielding mechanism. In addition, the incremental dynamic analysis (IDA) was also conducted to study the behavior in the elastic stage, elastic-plastic stage and collapse stage. Finally, the design considerations for stiffened CFST frames were proposed. These results will facilitate the use of this efficient form of composite structures.

Seismic behavior of a novel prefabricated thin-walled CFST double-column pier system for simple on-site assembly

To expedite the development of concrete-filled steel tubular (CFST) piers for simple on-site assembly (SOA), a novel prefabricated thin-walled CFST double-column pier system is proposed with the advantages of accelerated on-site construction, facilitated transportation, reasonable constructional cost, and good seismic performance. Two prefabricated thin-walled CFST double-column pier specimens were tested under quasi-static loading, and their failure modes, deformation characteristics, hysterical responses, energy dissipation capacity, stiffness degradation, and residual drift ratio are discussed in detail. The test results indicate that the proposed prefabricated thin-walled CFST double-column pier system shows a fairly good seismic performance with reasonable failure mode, sufficient energy dissipation, and relatively small residual deformation, and the SOA connections can effectively guarantee the overall deformation of the system under a maximum drift ratio of 4.8%. The specimen with double RC tie beams is superior in lateral resistance and initial stiffness to the one with a single RC tie beam, while they show similar performance in ductility and energy dissipation efficiency.

Assessment on seismic performance of circular concrete-filled thin-walled steel tube columns

Concrete-filled steel tubular (CFST) with circular section is a very efficient composite component bearing vertical loads, which takes advantage of the synergy of steel tube and concrete to fully develop the strengths of both materials. To promote the application of circular CFST columns in multi-story buildings or medium-span bridge piers with improved economic efficiency, circular CFSTs using thin-walled steel tubes were studied in this paper. 8 specimens were designed and tested to assess their seismic performance under constant axial compression and cyclic lateral loads. The key test parameters include axial compression ratio, diameter-to-thickness ratio of steel tube, as well as stiffening scheme. The effectiveness of two steel plate stiffeners (longitudinal stiffener, binding stiffener) at the potential plastic hinges of thin-walled circular CFST columns was verified and discussed. Although all specimens failed in a manner characterized by significant local buckling of steel tubes, they still performed a relatively ductile behavior with the ductility coefficients generally exceeding 4.0, and the ultimate drift ratios exceeding 1/40. The steel plate stiffeners did not achieve the expected enhancement in strength and ductility, due to the premature fracture in the steel tube which may be caused by the damage in the welding heat-affected zone. Based on the test results, a numerical model and a simplified hysteretic model were established to predict the seismic responses of the thin-walled circular CFST columns.

Shear strength of stiffened square thin-walled concrete-filled steel tubular columns

In comparison to extensive researches on compressive or flexural behavior of stiffened square concrete-filled steel tubular (CFST) columns, few were conducted on shear behavior. The cyclic-shear behavior between stiffened and unstiffened square thin-walled CFST columns was different, and the cyclic-shear behavior of CFST columns stiffened by the diagonal ribs, which were welded on the adjacent sides of a square tube, was significantly improved, as demonstrated by recent experimental results. This work uses detailed finite element (FE) analysis and parametric analysis to explore the shear mechanism and shear contribution of each component in the stiffened CFST column. The refined FE model was developed using ABAQUS and verified by the test results of both stiffened and unstiffened CFST columns subjected to cyclic and monotonic shear loading. Based on the analysis of test results, a method based on the degree of the cross-sectional plasticity was proposed to distinguish shear failure from bending failure, as typical bending failure phenomena (concrete crushing and tube buckling) were also observed at the ends of column specimens with shear failure. Analytical results showed that the steel tube and diagonal ribs resisted the shear through shear stress, and the concrete resisted the shear through the developed compression strut confined by the horizontal stress of steel. A shear model considering the interaction between concrete and steel was then established. The mechanics-equations and regression-equations were proposed and compared with the shear test results reported in the existing literature and FE analysis. Both types of equations were found to well predict the shear strength of both stiffened and unstiffened square CFST columns.

Cyclic-shear behavior of square thin-walled concrete-filled steel tubular columns with diagonal ribs

There is no doubt that stiffeners for square concrete-filled steel tubular (CFST) columns can improve the composite action in terms of delaying local buckling of steel tubes and enhancing the confinement on infilled concrete. Among various proposed stiffening forms, diagonal binding ribs, made of perforated thin-walled steel plates and welded at all the four pairs of adjacent sides of a square tube, are found particularly effective. Recent research has experimentally evaluated the axial compressive and cyclic-bending behavior of square thin-walled CFST columns stiffened by diagonal ribs. However, further investigation is necessary to better understand their cyclic-shear behavior. In this work, four diagonal ribs stiffened square thin-walled CFST columns and one unstiffened counterpart were designed and tested under combined cyclic-shear load and constant axial compression. Main parameters included the shear span to depth ratio, axial load ratio, width-to-thickness ratio of the steel tube, and thickness of diagonal ribs. Test results showed that the diagonal ribs improved the confinement from the steel tube to the infilled concrete. The stiffened specimens exhibited satisfactory cyclic-shear behavior with much higher load capacity, better ductility and larger energy dissipation than the unstiffened counterpart. Numerical modeling of the shear specimens was then conducted, indicating that the concrete and steel part contributed to the shear strength by 41.3%-57.4% and 42.6%-58.7%, respectively. Finally, the test results were compared with current design provisions, the average ratio of the predicted to tested shear strength were 0.60 and 0.66 for the calculation methods in the AISC specification and Eurocode, respectively.

Eccentric compression behaviour of diagonal rib-stiffened thin-walled square concrete filled steel tubes

Thin-walled square concrete filled steel tubular (CFST) members have attracted more attention in practice because of less steel consumption, high construction efficiency, and convenient arrangement for furniture or curtain wall. Compared with circular members, the local buckling of thin-walled square CFST members is a big concern since tube buckling often results rapid strength and ductility degradation. So in this investigation, diagonal ribs were welded on the internal tube corners to improve the performance of square CFST members. Nine diagonal rib-stiffened square CFST specimens, with large width-to-thickness ratio B/t=100, were experimentally investigated under eccentric compression. The main test parameters included column slenderness ratios (λ=13.9, 34.6, 55.4) and loading eccentricities (2e/B=0, 0.25, 0.50, 1.00). Detailed finite element models were established and verified. The composite effect of the stiffened column was elaborated through combining test results and the finite element analysis, followed by feasibility of column–column connection details and parametric studies. Finally, design methods for calculating the member resistances subjected to axial and eccentric compression were proposed.

Structural Performance and Compression Resistances of Thin-Walled Square CFST Columns with Steel Lining Tubes

AbstractThis paper proposes an innovative thin-walled square concrete-filled steel tubular (CFST) column with a circular/octagonal steel lining tube, in which the outer steel tube and the lining tu...

Seismic behavior of thin-walled square CFST columns with diagonal rib/circular liner stiffeners

This paper focuses on two novel enhancement schemes for thin-walled square concrete filled steel tubular (CFST) columns, i.e., diagonal rib stiffener and circular liner stiffener, with the advantages of sufficient confinement, easy fabrication, and flat outer surface. Seven thin-walled square CFST columns were tested under pseudo-static loading, considering the parameters of stiffener type, the width-to-thickness ratio of steel tube, and axial load ratio. The test results indicated that all the stiffened specimens showed excellent seismic performance with higher lateral resistance, better ductility, and greater energy dissipation than the unstiffened counterpart. The specimens with diagonal rib stiffeners effectively restricted the local buckling of the square steel tube but suffered a weld fracture between the tube plate and the diagonal rib at a large drift ratio. The circular liner stiffener could provide sufficient confinement to the core concrete. Even in the case of a high axial load ratio of 0.6, the circular liner stiffened specimen still showed a ductile behavior with the ultimate drift ratio (DR0.85) of 1/35 and the ductility coefficient (μ) of 4.3. The hysteretic behavior of the thin-walled square CFST columns with diagonal rib/circular liner stiffeners was predicted by both the FE model and the simplified theoretical model, in which the post-buckling strength of the thin-walled square steel tube and the confinement effect of the stiffeners are considered.

Degradation of Axial Ultimate Load-Bearing Capacity of Circular Thin-Walled Concrete-Filled Steel Tubular Stub Columns after Corrosion.

This work aimed to investigate the effects of steel tube corrosion on the axial ultimate load-bearing capacity (AULC) of circular thin-walled concrete-filled steel tubular (CFST) members. Circular thin-walled CFST stub column specimens were made of steel tubes with various wall-thicknesses. These CFST column specimens were subjected to an accelerated corrosion test, where the steel tubes were corroded to different degrees of corrosion. Then, these CFST specimens with corroded steel tubes experienced an axial static loading test. Results show that the failure patterns of circular thin-walled CFST stub columns with corroded steel tubes are different from those of the counterpart CFST columns with ordinary wall-thickness steel tubes, which is a typical failure mode of shear bulging with slight local outward buckling. The ultimate strength and plastic deformation capacity of the CFST specimens decreased with the increasing degree of steel corrosion. The failure modes of the specimens still belonged to ductile failure because of the confinement of outer steel tube. The degree of steel tube corrosion, diameter-to-thickness ratio, and confinement coefficient had substantial influences on the AULC and the ultimate compressive strength of circular thin-walled CFST stub columns. A simple AULC prediction model for corroded circular thin-walled CFST stub columns was presented through the regression of the experimental data and parameter analysis.

Nonlinear post-fire simulation of concentrically loaded rectangular thin-walled concrete-filled steel tubular short columns accounting for progressive local buckling

Abstract The repair of fire-damaged thin-walled rectangular concrete-filled steel tubular (CFST) columns in engineering structures after fire exposure requires the assessment of their residual strength and stiffness. Existing numerical models have not accounted for the effects of local buckling on the post-fire behavior of CFST columns with rectangular thin-walled sections. This paper describes a nonlinear post-fire simulation technique underlying the theory of fiber analysis for determining the residual strengths and post-fire responses of concentrically loaded short thin-walled rectangular CFST columns accounting for progressive local buckling. The post-fire stress-strain laws for concrete in rectangular CFST columns are proposed based on available test data and implemented in the theoretical model. An innovative numerical scheme for modeling the progressive local and post-local buckling of CFST thin-walled columns is discussed. The nonlinear post-fire simulation model is verified by experimental data and then used to investigate the significance of local buckling, material strengths and width-to-thickness ratio on the post-fire responses of CFST stub columns. The proposed post-fire computer model is shown to be capable of predicting well the residual stiffness and strength of concentrically loaded thin-walled CFST columns after fire exposure. A design formula is proposed that estimates well the post-fire residual strengths of CFST columns. Computational results presented provide a better understanding of the post-fire behavior of CFST columns fabricated by thin-walled sections incorporating local and post-local buckling.

Local buckling of steel plates in concrete-filled steel tubular columns at elevated temperatures

Local buckling remarkably reduces the strength of steel plates in rectangular thin-walled concrete-filled steel tubular (CFST) columns at ambient temperature. This effect is more remarkable at elevated temperature. However, there have been very limited experimental and numerical investigations on the local and post-local buckling behavior of steel plates in CFST columns at elevated temperatures. This paper presents numerical studies on the local and post-local buckling behavior of thin steel plates under stress gradients in rectangular CFST columns at elevated temperatures. For this purpose, finite element models are developed, accounting for geometric and material nonlinearities at elevated temperatures. The initial geometric imperfections and residual stresses presented in steel plates are considered. Based on the finite element results, new formulas are proposed for determining the initial local buckling stress and post-local buckling strength of clamped steel plates under in-plane stress gradients at elevated temperatures. Moreover, new effective width formulas are developed for clamped steel plates at elevated temperatures. The proposed formulas are compared with existing ones with a good agreement. The effective width formulas developed are used in the calculations of the ultimate axial loads of rectangular CFST short columns exposed to fire and the results obtained are compared well with the finite element solutions provided by other researchers. The initial local buckling and effective width formulas can be implemented in numerical techniques to account for local buckling effects on the responses of rectangular thin-walled CFST columns at elevated temperatures.

Experimental and analytical studies on CFRP strengthened circular thin-walled CFST stub columns under eccentric compression

Carbon fiber reinforced polymer (CFRP) strengthened concrete-filled thin-walled steel tubular (CFST) column can improve its load carrying capacity and resolve local bugles or corrosion of thin-walled steel tube. However, little attention to CFRP strengthened CFST columns has been paid. This paper reported an experimental and numerical analysis on eccentric compressive behavior of circular CFST stub columns partially-wrapped by CFRP strips. A series of circular composite columns, including nine CFRP strengthened CFST stub columns and one bare CFST stub column, were tested subjected to eccentric compression. Moreover, a nonlinear finite element (FE) modeling in considering contact interactions of the composite columns was developed and verified by the test results in terms of eccentric load (N) - longitudinal shortening (δ) curves and failure patterns. Then, the influence of extensive parameters on the eccentric compressive behavior of CFRP strengthened circular CFST columns was also evaluated. Meanwhile, a simplified empirical method on the eccentrically-loaded stub composite columns was proposed on the basis of unified theory. The experimental and analytical data indicated that the eccentric compressive strength of CFRP strengthened circular CFST stub column was obviously influenced by load eccentricity, CFRP confinement factor, steel strength, core concrete strength and CFRP strength. The proposed simplified empirical formulas may provide a considerable approach for designing this type of composite structures in engineering practice.

Performance of concrete-filled steel tubular column-wall structure subjected to ISO-834 standard fire: Experimental study and FEA modelling

This paper reports a full-scale fire testing of concrete filled thin-walled steel tubular (CFST) column-wall structure. All together eight specimens were designed and tested under axial compression load and subjected to ISO-834 standard fire. Load ratio, fire-exposure time and insulation scenarios were varied in the testing program. Cement mortar, aerated concrete block, and mineral wool were used as the insulation layer materials. Test results including temperature to time relations, axial deformation, and fire resistance as well as failure modes were described. A finite element analysis (FEA) model was established in order to simulate the thermal and structural behavior of CFST column-wall under fire. The FEA model was verified against test data and turned out to be reasonably accurate in predicting the temperature distribution, axial displacement, fire resistance and failure modes of CFST column-walls. Sensitivity studies were carried out to determine some major parameters in the FEA model. Discussion was made on the influence of thermal resistance, spalling of the insulation layer and confinement of inner concrete on the modelling of CFST column-wall structures.

Seismic performance assessment of blind bolted steel-concrete composite joints based on pseudo-dynamic testing

This paper aims to investigate seismic behavior and dynamic responses of semi-rigid composite joints composed of steel-concrete composite beams and concrete-filled thin-walled steel tubular (CFTST) columns. A range of pseudo-dynamic tests (PDTs) was performed on two full-scale blind bolted end-plate joint specimens, including one flush and one extended end-plate typed joint. The input ground motion adopted for the PDTs was Ispara seismic acceleration record and scaled to represent various seismic hazard levels. Using Ispara earthquake record with different peak ground accelerations (PGAs), a lot of useful data about hysteresis behavior, stiffness degradation and energy dissipation capacity was obtained. The data was also estimated in detail to characterize the seismic performance of this kind of composite joints. Dynamic responses of beam-to-column joint including displacement and acceleration responses, and dynamic magnification factor were discussed. The experimental results indicated that the structural responses of the test joints under seismic actions coincided with the desired performances analyzed for varying damage levels. The study affirms that the blind bolted end-plate composite joints to CFTST columns possess good energy dissipation seismic performance, so they can provide sufficient ductility and adequate strength to satisfy the seismic design requirement.

Experimental and numerical analysis of blind bolted moment joints to CFTST columns

Experiments and numerical analysis were conducted to investigate the mechanical behavior of the blind bolted end plate joints to concrete-filled thin-walled steel tubular (CFTST) columns. Four blind bolted end plate joints to CFTST columns subjected to the monotonic loading were tested. Then, finite element analysis (FEA) modeling of the tested specimens was developed, and the results obtained from FEA modeling were verified against those from the test results. Parametric studies were conducted to investigate the influence of end plate thickness, axial load level, steel ratio and slenderness ratio etc. on moment-rotation curves of composite joints. Some methods of effective provisions were proposed and discussed in order to enhance the connection behavior. The test and numerical analysis results indicated that the typed joints to CFTST columns behaved in a semi-rigid and partial strength manner. The proposed innovative blind bolted joint using reliable and effective solutions could be applied in the mid- and low-rise buildings.

Seismic behavior of blind bolted end plate composite joints to CFTST columns

The seismic behavior and failure mechanism of a novel type of composite joints are presented in this paper. These composite joints are composed of composite steel-concrete beams and square concrete-filled thin-walled steel tubular (CFTST) columns. The steel beams are connected to the CFTST columns using end plates with blind bolts, while the cast-in-situ RC slabs are attached to steel beams with shear connectors. Some quasi-static tests were performed on flush and extended end plate joints and horizontal cyclic displacements at the column ends were experienced during the test. A detailed account of the results and observations from the cyclic tests are investigated, and the main behavioral aspects are discussed. The seismic performance of the blind bolted composite joint to CFTST column is estimated in terms of the hysteretic behavior, strength and stiffness degradation, ductility, and energy dissipation capacity, etc. The results of this study show that the novel composite joints exhibit adequate ductility and energy dissipation, whilst they behave in a semi-rigid manner according to the EC3 specification.

Biaxially loaded high-strength concrete-filled steel tubular slender beam-columns, part II: Parametric study

Biaxially loaded high strength rectangular concrete-filled steel tubular (CFST) slender beam-columns with large depth-to-thickness ratios, which may undergo local and global interaction buckling, have received very little attention. This paper presents the verification of a multiscale numerical model described in a companion paper and an extensive parametric study on the performance of high strength thin-walled rectangular CFST slender beam-columns under biaxial loads. Comparisons of computer solutions with existing experimental results are made to examine the accuracy of the multiscale numerical model developed. The effects of the concrete compressive strength, loading eccentricity, depth-to-thickness ratio and columns slenderness on the ultimate axial strength, steel contribution ratio, concrete contribution ratio and strength reduction factor of CFST slender beam-columns under biaxial bending are investigated by using the numerical model. Comparative results demonstrate that the multiscale numerical model is capable of accurately predicting the ultimate strength and deflection behavior of CFST slender beam-columns under biaxial loads. Benchmark numerical results presented in this paper provide a better understanding of the local and global interaction buckling behavior of high strength thin-walled CFST slender beam-columns and are useful for the development of composite design codes.

Stability Bearing Capacity of Concrete Filled Thin-Walled Circular Steel Tubular Column under Axial Compression

The bearing capacity formula of concrete filled thin-walled steel tubular (CFTST) short column was established based on limit equilibrium method, and the reasonable value fork1in Richart Strength Model was regression introduced adapted with concrete stress-strain relations. According to the elasticity modulus theory, the calculation formulas of stability bearing capacity of CFTST slender column were deduced, reasonable considering the interactions between steel tube and concrete. And the calculation process is relatively simple which avoiding complicated iterations used in the conventional calculation method. Finally, the comparative evaluation on calculation formula was done based on 79 groups of test results of concrete filled steel tubular column at home and abroad. Results of calculation formula agree well with the test results in safe range.

Unified Solution of the Ultimate Bearing Capacity for Stiffened and Thin-Walled Square CFST Short Columns under Axial Compression

Directed against the stiffened and thin-walled square concrete-filled steel tubular (CFST) short columns, the paper simplifies the square CFST columns into the circle ones by introducing the strength reduction coefficient of concrete and the equivalent constraint reduction coefficient. On the basis of the unified strength theory, the united solution of the ultimate bearing capacity for square CFST columns is deduced after considering the influence of strength-differential in tension and compression and intermediate principal stress. Some influencing factors are also analyzed in the paper. Through comparing the results of proposed formula with that of experiments of relevant literatures，the rationality of proposed formula is proved. The solution may provide theoretical references for the study of the ultimate bearing capacity for stiffened and thin-walled square CFST short columns.