High-strength Square Research Articles

Behavior of UHPC filled high-strength square steel tubular stub columns under combined flexural and axial loads

This study experimentally and numerically investigated the behaviors of UHPC-filled high-strength square steel tube stub columns with compact, noncompact and slender sections under combined flexural and axial loads. Eighteen tests were conducted, addressing the gap of limited tests on concrete-filled square steel tubular stub columns with noncompact or slender sections. Additionally, finite element analyses were developed and benchmarked for parametric studies. The results of tests and parametric studies show that (1) steel tube outward buckling and concrete crushing occurred at 1/4–1/2L of the compression zone; (2) the load-axial displacement curves organized into three phases: elastic, elastic-plastic, and recession phase; (3) the compressive strength increased with increasing tube thickness, concrete strength, and steel yield strength. The compressive strength decreased with increasing eccentric ratio and slenderness coefficient. Test and numerical results are utilized to assess the suitability of current design specifications for predicting compressive strength. The evaluation showed that neither GB 50936–2014 nor Eurocode 4 provided reasonable predictions of strength, while AISC 360–16 provided conservative predictions.

Flexural behavior of high-strength square concrete-filled steel tube members subjected to cyclic loadings

This paper investigated the flexural behavior of high-strength concrete-filled steel tube (HSCFST) members subjected to cyclic loadings. Nine cyclic tests of square HSCFST members were conducted. The investigated parameters were the height of steel tube, thickness of steel tube wall, yield stress of steel, and compressive cylinder strength of concrete. A finite element (FE) model was developed and validated. Results from the experimental tests and FE analyses indicated that: (i) the failure modes of HSCFST members were characterized by local buckling and fracture of the steel tube, both occurred earlier with increasing height or yield stress of steel, or with decreasing thickness of steel tube wall, but were delayed when higher strength concrete was used; (ii) using high-strength steel significantly improved the flexural strength, had no obvious influence on the flexural stiffness, but significantly reduced the ductility; (iii) using high-strength concrete improved the flexural strength, stiffness, and ductility; and (iv) the limits by AISC 341-16 on the slenderness coefficient is conservative for HSCFST members subjected to cyclic flexural loadings.

Axial compressive behavior and design of high-strength square concrete-filled steel tube short columns with embedded GFRP tubes

This paper proposed a hybrid column by embedding glass fiber-reinforced polymer (GFRP) tube into high-strength square concrete-filled steel tube (CFT), named as high-strength square steel-concrete-GFRP concrete (SCFC) column. A total of 13 high-strength square SCFC short columns were tested. The test parameters included diameter and thickness of GFRP tube, width and thickness of steel tube, yield stress of steel, and compressive strength of concrete. Results showed that high-strength square SCFC short columns exhibited post-yield strengthening behavior and stable residual strength. The axial strength, confining effect, and ductility were improved because of the embedded GFRP tube. Detailed finite element models were developed and benchmarked to investigate the strength contribution of each component and conduct parametric studies. It was shown that: (i) the concrete infill and the steel tube contributed >90% to the overall compressive strength and (ii) the strength improvement by embedding GFRP tube was closely related to the section size ratio, confinement ratio of the GFRP tube, and confinement ratio of the steel tube. Evaluation of the current equations showed that they were not accurate enough to estimate the axial compressive strength of high-strength square SCFC short columns. New equations were then proposed.

Numerical investigations on flexural behavior of I-shaped CFRP-encased high-strength square concrete filled steel tubes

To enhance the mechanical properties of high-strength concrete-filled high-strength steel tubular (HSCFHST) members, a new type of composite structure is proposed in which an I-shaped CFRP is encased inside the HSCFHST member. The objective of this research is to improve the theoretical foundation for HSCFHST members by analyzing the flexural behavior of inner I-shaped CFRP on HSCFHST members. HSCFHST members' nonlinear finite element models is created using ABAQUS and confirmed by comparing simulation and test data. It is subjected to pure bending stresses. The flexural behavior of the HSCFHST members are examined with respect to failure mode, complete curve, moment sharing ratio, longitudinal and contact stresses in steel tubes and concrete. Additionally, material interaction mechanisms among three components are investigated to explore the range of restraint between the components. To assess a substance's effect and structural parameters on the flexural behavior of the members, including different combinations of the three types of materials, steel's yield strength, concrete's compressive strength, and the ratio of steel. The findings demonstrate that the steel tube's restraint of the concrete is mostly concentrated at its corners on the tensile side. The HSCFHST member's deformation capacity and stiffness were both increased by the usage of the CFRP encased in the member, which also decreased the height of the central axis' upward movement. The flexural load capacity is largely affected by the steel ratio, which also has an impact on steel strength.

Shear behavior of high-strength square concrete filled steel tube members

This paper experimentally and numerically investigates the shear behavior of high-strength square concrete-filled steel tube (HS-SCFST) members. A total of 20 specimens were first tested. The test parameters were the shear span-to-depth ratio, width-to-thickness ratio of the steel tube wall, yield stress of steel, and compressive strength of concrete. Detailed finite element (FE) models were then developed and benchmarked. The benchmarked models were then used to conduct a total of 87 parametric studies. It was shown that (i) the shear strength and stiffness increased with decreasing shear span-to-depth ratio or width-to-thickness ratio, and (ii) the increase in yield stress of steel and compressive strength of concrete improved the shear strength, but had a minor effect on the stiffness. Results from the experimental tests and finite element analyses were also used to evaluate the applicability of several international design provisions (e.g., AASHTO BDS, Eurocode 4, WSDOT BDM and CECS 28) for estimating the shear strength of HS-SCFST members. It was shown that CECS 28 provided the most reasonable estimation of the shear strength, but the contribution of steel tube and concrete infill was not accurately calculated. A new design equation was therefore proposed, considering the effect of shear span-to-depth ratio and the contribution of the steel tube and concrete infill. • Investigates the shear behavior of high-strength CFST members. • Conducts experimental tests and FEM analysis. • Discusses the shear transfer mechanism. • Evaluates the applicability of several international design provisions. • Proposes a design equation considering the effect of shear span-to-depth ratio.

Mechanical behavior of high-strength concrete filled high-strength steel tubular stub columns stiffened with encased I-shaped CFRP profile under axial compression

A new composite column that consists of high-strength square steel tube, high-strength concrete, and carbon fiber reinforced polymer (CFRP) profile with a I-shaped cross section was developed and investigated in this study. This paper experimentally and numerically investigated axially loaded behavior of high-strength concrete filled high-strength steel tubular (HCFHSST) stub column with an encased I-shaped CFRP profile. In the experimental study, axial compression tests were completed on eight specimens to examine the axial load response of the composite column, including the load–displacement curves, failure mode, and strain distribution. Finite element models of the composite column were developed using ABAQUS and validated by comparing numerical results with experimental data. The validated models were used to further investigate the load sharing and transferring mechanism for each component of the composite column. Results showed that the composite column comprehensively used the advantageous merits of concrete, steel tube, and CFRP as a unit to share the applied load at different loading stages: linearly elastic, elastic–plastic, hardening, post-peak descending, and residual plateau stage. Parametric studies were performed that assessed influences of critical design parameters on the axially loaded behavior of the stub column. The examined parameters included the compressive strength of high-strength concrete, yield strength of high-strength steel, and steel ratio. Finally, empirical equations were proposed to determine the ultimate vertical strain and the ultimate axial load capacity of the HCFHSST stub column encased with a I-shaped CFRP profile.

Analysis of stress-strain in the partial heating roll forming process of high strength square hollow steel sections

The demand for roll-formed high strength steel square hollow sections has extensively increased by automotive and construction industries due to their excellent structural performances and attractive appearances. However, due to their low ductility and demand for large forming tool loads, high-strength steels are challenging to be roll-formed at room temperature. Partial heating roll forming is a recently proposed manufacturing technique aiming to overcome these difficulties by applying temperature on bend areas. In this study, the finite element software LS-DYNA was utilized to predict effective plastic strain and stress developments and associated residual stresses induced during the partial heating roll forming method. Furthermore, comparisons were made between cold roll forming and partial heating roll forming methods. The results indicate that the effective stress value during the partial heating roll forming method was lower than the cold forming method. In addition, the product manufactured by the partial heating roll forming method had smaller and uniform residual stress distributions along the section perimeter and across the section thickness than the cold-formed counterpart. Further investigations on corner geometry showed that high strength steel square hollow sections with a minimum bend radius could be successfully manufactured using the partial heating roll forming method.

Experimental and numerical behavior of eccentrically loaded square concrete-filled steel tubular long columns made of high-strength steel and concrete

The high-strength materials are increasingly used in the concrete-filled steel tubular (CFST) column to improve its mechanical behaviors. This study aims to evaluate the fundamental behavior of high-strength concrete-filled high-strength square steel tubular (HCFHSST) long columns under eccentric compression. A series of tests were conducted on eccentrically loaded HCFHSST long columns to investigate the mechanical characteristics and failure mechanisms. Numerical investigations were systematically performed to explore the load-carrying performance of the column. Meanwhile, parametric studies were implemented to evaluate influences of several key parameters on the strength increase factor (SIF), P–M, and P/P0–M/M0 curves of the HCFHSST members, respectively. Furthermore, the equations were proposed to predict the peak-loads of HCFHSST long columns under eccentric loading. The experimental and simulated peak-loads were compared with the results of various design codes and recommended equations to assess the feasibilities of those calculation systems. The results demonstrated that the bending moment–curvature curves of the experimental specimens could be categorized into three types considering the second-order effect. Additionally, it was numerically observed that using high-strength steel can enhance the yield load proportion of the column, and the concrete withstands most of the applied load when utilizing high-strength concrete as infill. Based on the peak-load calculations of codes EC4, AISC 360, and GB 50936, it was found that code GB 50936 presented a better result on average while was unsafe for some cases with high e/B ratios. The equations proposed in this study can provide an improved prediction and hence are suggested.

Transverse impact behavior of high-strength concrete filled normal-/high-strength square steel tube columns

High strength materials and dynamic loadings are two important research topics for building structures. In this study, high-strength concrete filled square steel tube columns (HSCFST) using S690 along with Q355 structural steel subjected to transverse impact loadings were experimentally tested by a drop hammer tester, and the impact force, deformation, and energy absorption of such specimens were obtained. Results showed that HSCFST has great impact resistance, showing high impact force plateau value and small deflection. The influence of steel strength, concrete strength, steel ratio, and impact energy on impact resistance was deeply analyzed. Compared with normal-strength materials, using high strength steel can improve its impact resistance but the contribution is not as great as the increment of yield stress, whilst using the high-strength concrete has limited effect. Besides, a refined finite element (FE) method by ABAQUS/Explicit employing the rate-dependent constitutive model for S690 was adopted to simulate the dynamic responses of HSCFST under impact loadings, performing a good agreement with tests. This study provides the basic experimental data of HSCFST subjected to transverse impact and develops a reasonable FE model to predict its impact resistant performance, contributing to the related studies of HSCFST components.

Key technology and equipment for local heating roll-forming of high strength square tube

The application of advanced high strength steel and the development innovation manufacturing technology is an effective way to realize the automobile lightweight and improve the safety performance design of high strength steel products. In this paper, a local heating “square to square” roll-forming process was proposed to solve the difficult problems of the introducing high strength steel into the traditional roll-forming equipment. Two kinds of inductors structure were compared and analyzed. Based on the “round to square” process, the size design of the rollers in forming process was deduced by the mutual relationship of the angle. And the relationship between the frame layout was given. The calculation of example and the key technical equipment were completed. Based on this new technology, the performance test of high strength tubes was carried out. It was found that the mechanical properties of the corner and the dimensional accuracy of the section are significantly improved. The research shows that this key technology and equipment of hot roll-forming can effectively overcome the problems of high strength steel.

Local-global interaction buckling of square high strength concrete-filled double steel tubular slender beam-columns

High-strength square concrete-filled double steel tubular (CFDST) slender beam-columns with a circular internal steel tube subjected to eccentric loads may undergo interaction local-global buckling. No computational studies on the interaction local-global buckling of slender square CFDST beam-columns have been reported and their behavior has not been fully understood. This paper describes a mathematical model for the simulation of the interaction local-global buckling behavior of square high-strength CFDST slender beam-columns under axial compression in combination with uniaxial bending. The mathematical model is formulated by the fiber approach, accounting for confinement provided by the internal circular steel tube, and geometric and material nonlinearities. An incremental-iterative numerical procedure is designed to quantify the local-global interaction buckling responses of slender CFDST columns. Efficient numerical solution algorithms implementing the inverse quadratic method are developed for solving the nonlinear equilibrium dynamic functions of CFDST columns. The formulation proposed is verified by existing experimental data on CFDST columns as well as double-skin concrete-filled steel tubular (DCFST) slender columns. The developed computational model is employed to study the local-global interaction buckling behavior of CFDST columns made of high-strength materials with various important parameters. Simplified design models are proposed for determining the ultimate axial strengths of slender square CFDST columns under axial compression and the interaction curves of CFDST slender beam-columns loaded eccentrically. It is demonstrated that the computational and design models predict well the interaction local-global buckling behavior and strength of slender square CFDST beam-columns.

Optimization of electroporation-based multiple pulses and further improvement of transformation efficiency using bacterial conditioned medium for Nannochloropsis salina

Development of transgenic strains of microalgae is necessary to obtain microalgal strains that are capable of producing high levels of oils and a variety of bio-products. However, microalgae have a much lower transformation efficiency in comparison to other micro-organisms due to the presence of thick cell walls. Here, a two-pronged approach for optimizing the electroporation parameters combined with the application of secondary metabolites from myxobacteria to weaken the cell wall was used to maximize the transformation efficiency of Nannochloropsis salina (Eustigmatophyceae). When the electroporation parameters were set at 50 pulses of square waves at a field strength of 12,000 V cm−1, a transformation efficiency of up to 53 per 108 cells was achieved for N. salina in the early exponential phase of growth. Next, cultivation of the microalgae using bacterial conditioned F2N medium containing 10% supernatant of the myxobacteria strain DZ2 resulted in a moderate inhibition of the growth rate and distinctive morphological changes. Flow cytometry confirmed an increase in the overall cell size as well as granularity. When the bacterial medium conditioned cells were employed for transformation under optimized electroporation conditions, the transformation efficiency improved 2.71-fold up to 144 per 108 cells. Taken together, a combinatory approach for electroporation using high field strength square wave pulses and conditioning cells with myxobacterial supernatant is a promising method for the efficient genetic transformation of microalgae.

Experimental and numerical behaviour of eccentrically loaded high strength concrete filled high strength square steel tube stub columns

Using high strength materials in concrete filled steel tube (CFST) columns is expected to achieve better structural performance and fulfil the requirements of sustainable construction. To study the mechanical behaviour of eccentrically loaded high strength concrete filled high strength square steel tube (HCFHSST) stub columns, this paper describes 12 tests with different eccentricity ratios and steel ratios. The cubic strength of high strength concrete under investigation was 110.5 MPa, and the yield strength of the high strength steel was about 434 MPa. Curves of load-lateral deformation were presented, along with values of ductility index, and the minimum ductility index based on the steel ratio of columns was suggested. Finite element analysis (FEA) software ABAQUS was applied to simulate HCFHSSTs. The analytical results were in good agreement with the experimental ones. The load-lateral deformation curve was divided into four stages: elastic, elastic-plastic, plastic hardening and descending. The confinement effect of steel tube at various stages was analysed. The parametric studies were carried out to evaluate the influences of the eccentricity ratio, concrete compressive strength, steel yield strength and steel ratio on the strength reduction factor (SRF), concrete contribution ratio (CCR), P-M and P/Pu-M/Mu interaction curves of the HCFHSST members. The bending moments at balanced points of P/Pu-M/Mu curves calculated by the plastic stress distribution models (PSDM) and FEA models were compared. The ultimate bearing capacities obtained from the tests and the values calculated from the AISC 360, GB 50936 and CECS 28: 90 design codes were compared. Finally, the formulas were proposed to predict the P/Pu-M/Mu curves for the HCFHSST stub columns subjected to eccentric load. The proposed formulas’ predictions agreed well with the test results.

Fatigue testing of butt-welded high strength square hollow sections strengthened with CFRP

This paper presents a study on the residual stress, hardness and fatigue behaviour of butt-welded high strength square hollow section (SHS) steel tubes strengthened with carbon fibre reinforced polymer (CFRP). SHS tubes, with a nominal yield stress of 780MPa and an ultimate tensile strength of 860MPa, were butt-welded using the Gas Tungsten Arc Welding (GTAW) welding method. The hardness in the heat affected zone (HAZ) dropped to around 80% of that of the parent metal of the SHS steel. The strength in HAZ of the butt-welded SHS tubes is about 93% of that of the parent metal of the SHS tube. The residual stress of the butt-welded SHS tubes is about 20–50% 25-45%of the yield stress along the longitudinal direction and 15% of the yield stress along the transverse direction. The fatigue life of CFRP strengthened butt-welded SHS beams was extended for about 5 times comparing to the butt-welded beams without CFRP strengthening.

Flexural behavior of high strength concrete filled high strength square steel tube

To study the mechanical behavior of high-strength concrete filled high-strength square steel tube (HCFHST) under pure bending load, six specimens with different steel ratio were tested. The corresponding nonlinear finite-element models were established to analyze the mechanical properties. The load-displacement curves obtained from the numerical analyses are consistent with the experimental results. In addition, the influences of different materials are analyzed in this paper. The moment-curvature relationship can be divided into three stages: elastic stage, yield stage and hardening stage. And the ultimate bearing capacity increased with the steel ratio, steel yield strength and concrete compressive strength. Moreover, the ultimate bearing capacity of the experiment and finite-element analysis (FEA) model is compared with the requirements of codes: AISC-LRFD (1999), AIJ (1997), EC4 (1994) and GB50936-2014 (2014). The test and FEA result were the most compatible to the calculated result of EC4 (1994). And find the results in this paper is the most compatible to the code EC4 (1994).

Finite Element Simulation on Behavior of the High-Strength Concrete Filled High-Strength Square Steel Tube Middle-Long Columns under Axial Compressive Load

In this paper, using ABAQUS, 16 high-strength concrete filled high-strength square steel tube middle-long columns’ axial compression process were simulated. The load-deflection relationships were obtained and the new combination in improving the bearing capacity and plastic deformation has a great advantage. Realization of length variation slenderness ratio by changing the length of column, this paper also study the influence of slenderness ratio, the main parameters of the high-strength concrete filled high-strength square steel tube middle-long column. It is found that both bearing capacity and the plastic capacity are associated with slenderness ratio.

Experimental and analytical investigation of high-strength concrete-filled steel tube square columns subjected to flexural loading

The concrete-filled steel tube (CFT) columns have several benefits of high load-bearing capacity, inherent ductility and toughness because of the confinement effect of the steel tube on concrete and the restraining effect of the concrete on local buckling of steel tube. However, the experimental research into the behavior of square CFT columns consisting of high-strength steel and high-strength concrete is limited. Six full scale CFT specimens were tested under flexural moment. The CFT columns consisted of high-strength steel tubes (<TEX>$f_y$</TEX> = 325 MPa, 555 MPa, 900 MPa) and high-strength concrete (<TEX>$f_{ck}$</TEX> = 80 MPa and 120 MPa). The ultimate capacity of high strength square CFT columns was compared with AISC-LRFD design code. Also, this study was focused on investigating the effect of high-strength materials on the structural behavior and the mathematical models of the steel tube and concrete. Nonlinear fiber element analyses were conducted based on the material model considering the cyclic bending behavior of high-strength CFT members. The results obtained from the numerical analyses were compared with the experimental results. It was found that the numerical analysis results agree well with the experimental results.

The Behavior of High-Strength Square Steel Tube Columns Filled with Concrete

The Behavior of High-Strength Square Steel Tube Columns Filled with Concrete

Prediction of hysteretic behavior of high-strength square concrete-filled steel tubular columns subjected to eccentric loading

This study aimed at predicting the structural behavior of high-strength square CFT (concrete-filled steel tube) columns. First, the material models of the existing steel tube and concrete were compared, and a nonlinear fiber element analysis method was proposed. To verify the proposed fiber element analysis method, the behavior of CFT columns made from high-strength materials was investigated experimentally. CFT members consisted of high-strength steel tubes (yield strength; fy=913MPa) and high-strength concrete (fck=91.3 MPa). The moment-rotation relationships for hollow and concrete-filled steel tubes were compared. In addition, the P-M interaction diagrams for the experiment result and AISC-LRFD code provisions were compared. Finally, the result of the fiber element analyses was compared with the test results.

Development and Validation of Fiber Model for High-Strength Square Concrete-Filled Steel Tube Beam-Columns

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