Circular Steel Tube Research Articles

Performance of lightweight coconut shell concrete-filled circular steel tube columns under axial compression

The current study aimed to use coconut shell concrete, a structural lightweight concrete, as an infill material in concrete-filled steel tube (CFST) columns and test it under axial compression. Testing was done on eighteen short, intermediate and long coconut shell CFST columns and six normal-weight CFST short columns for comparison. For both types of columns, the axial load-displacement curves and modes of failure were examined. By varying the length-to-diameter and diameter-to-thickness ratios, the axial capacity of steel tubes filled with coconut shell concrete was assessed. The composite action was verified from the results of the confinement index, strength index and the contribution of the coconut shell concrete as infill concrete. Structural efficiency and energy absorption of the lightweight CFST column was contrasted with its counterpart column. The contribution of coconut shell concrete to the strength of the CFST column was the highest at 61.36% and more significant than that of normal-weight CFST columns. The coconut shell CFST columns were 23.63% lighter than the normal-weight columns, contributing to its higher structural efficiency. These columns also had 8.12% more energy absorption than normal-weight columns. Hence, the results of this investigation revealed that coconut shell concrete has the potential to be utilized in CFST columns. Further, compared to the experimental ultimate loads, the predictions made by the existing codes, EC4 and ANSI/AISC 360 are conservative.

Circular concrete-filled double-skin steel tubular short columns under axial compression: An improved theoretical model and design

Existing theoretical analysis methods for concrete-filled double-skin steel tubular (CFDST) short columns generally ignore the reduction of concrete strength due to brittleness. As a result, these methods cannot accurately capture full range behavior for CFDST columns containing high-strength concrete. By focusing on the reduction effect of concrete strength in CFDST columns due to concrete brittleness, this paper proposes a reduction factor for use in design, for the first time. Based on this reduction factor, a theoretical model for axially compressed circular CFDST short columns considering non-uniform confinement effect of the sandwiched concrete is established. A substantial experimental database of 168 CFDST columns with carbon circular steel tubes is then compiled. By comparing with this database, the proposed theoretical model is shown to achieve greater accuracy than existing approaches. Utilizing this model, the influence of column parameters on the performance of the studied CFDST members are investigated and the results are discussed. It is shown that concrete strength, hollow ratio, yield strength of the outer tube, and the diameter-to-thickness ratio of the outer and inner tubes are influential to the ultimate strength and ductility of CFDST columns. However, the yield strength of the inner tube only affects the ultimate strength of CFDST columns but has a negligible influence on the ductility. Finally, utilizing the experimental and theoretical data presented, the accuracy of existing design codes for concrete-filled steel tubes for the design of CFDST members with or without considering the reduction effect of concrete strength is assessed. The results indicate that Eurocode 4 provides a more reasonable prediction result.

Numerical Modeling of Steel Fiber Reinforced Recycled Concrete Filled Steel Tube Column Under Cyclic Loading

A finite element model (FEM) was created with the aim of analyzing the behavior of steel fiber reinforced recycled concrete (SFRRC)-filled steel tube columns under combined cyclic loading and monotonic axial load. The FEM considered the effect of steel tube confinement on the inner concrete behavior under cyclic loading. The numerical model was described in detail, with a focus on modeling the materials involved (normal concrete, SFRRC, and steel) under cyclic loading. A constitutive concrete model - with and without considering confinement - was based on utilizing a concrete damaged plasticity (CDP) model. The steel tube - concrete core interface was modeled by a surface-to-surface contact. A stress-strain constitutive concrete model, confined by circular steel tubes, was implemented, and validated using experimental results from the literature. The developed FEM considered various parameters: steel tube thickness, volume ratios of steel fibers, besides strengths of both concrete core and the steel tube. The FEM results showed great similarity to the under- cyclic- loading tested columns. The results indicated that the concrete confining pressure must be considered in CDP model. A good correlation between numerical and experimental findings was obvious, including failure modes, and hysteretic curves of load-displacement.

3D numerical study of axial compression behavior of concrete-filled steel tubular columns from mesoscopic perspective

This study investigates the axial compression behavior of Concrete-Filled Steel Tubular (CFST) short columns from a mesoscopic perspective. Concrete is regarded as a three-phase composite material consisting of aggregates, mortar, and interfacial transition zones (ITZs). The Concrete Damaged Plasticity (CDP) model was employed to simulate the nonlinear mechanical response and fracture characteristics of concrete. Verified against experimental results, the numerical model excels in realistically simulating the uniaxial compression behavior of CFST short columns. Subsequently, factors influencing the macroscopic behavior of CFST short columns were investigated. Results indicate that the random aggregate model adopted for concrete enables a more realistic prediction of the behavior of CFST short columns. Additionally, circular section steel tubes demonstrate a stronger confinement effect compared to square ones. With increasing lateral confinement, the properties of CFST short columns, such as bearing capacity and ductility, are significantly enhanced.

Seismic Response and Collapse Analysis of a Transmission Tower Structure: Assessing the Impact of the Damage Accumulation Effect

This paper delves into the impact of the damage accumulation effect, which leads to the degradation of material strength and stiffness, on the seismic resistance of transmission towers. Building upon the elastic–plastic finite element theory, a mixed hardening constitutive model is derived for circular steel tubes, standard elements in transmission towers, incorporating the damage accumulation effect. A user material subroutine, UMAT, is created within the LS–DYNA framework. The program’s validity and reliability are established through axial constant–amplitude loading tests on single steel tubes. The subroutine is employed to conduct the incremental dynamic analysis (IDA) of an individual transmission tower and to contrast it with the structure utilizing the Plastic Kinematic material model, assessing the discrepancies in tower top displacements and segment damage indices (SDIs) at both macroscopic and microscopic scales. The results shows that the Plastic Kinematic model inflates the seismic performance of the transmission tower. When considering the damage accumulation effect in structural failure, the damage index of the members increases, leading to a reduction in both the structural strength and stiffness. The dynamic response in the plastic phase becomes more pronounced, and the onset of structural failure is accelerated. Consequently, structural analysis under seismic conditions should account for the damage accumulation process. Through the delineation of member and segment damage, the extent of damage to transmission tower segments can be quantitatively assessed. Subsequently, the ultimate load–bearing capacity and the most vulnerable location of the transmission tower can be ascertained. Finally, this paper provides a detailed analysis of the transmission tower collapse process under seismic action and summarizes the mechanism of collapse for the structure.

Research on axial compressive behavior with circular steel tubes reinforced by sleeved tube

To study the behavior of the axial compression circular steel tube reinforced by a sleeved tube, the finite element model of casing tube reinforcement was established based on the existing research results. The mechanical properties and failure mechanism of the reinforced casing tube specimen were systematically studied by finite element parameter analysis. The results show that in the elastic stage, the outer-sleeved tube is less involved in the force. After entering the elastic-plastic stage, the outer-sleeved tube starts to work and is stressed together with the inner steel tube. Slenderness ratio, diameter-thickness ratio, and ferrule spacing are the main factors affecting the ultimate axial compression capacity of casing reinforcement components. When other variables are consistent, the ultimate axial compression capacity of specimens after reinforcement with different initial stress ratios is basically the same. The increase of the ferrule width and thickness has little effect on the improvement of the ultimate axial compression capacity of the reinforcement components. The load value after casing reinforcement calculated by the current standard is much higher than the test load value of the component. According to the results of parameter analysis and the current standard, the design method for the ultimate axial compression capacity of the circular steel tube reinforced by a sleeved tube was put forward.

Load-carrying capacity of hole threads in TOBs bolted curved T-stubs to circular steel tube connections

This study investigates the load-carrying mechanism and proposes precise strength prediction methods for hole threads in Thread-fixed One-side Bolts bolted curved T-stubs to circular steel tube connections. A series of parametric analysis was performed with an advanced 3D finite element model considering critical parameters including tube wall thickness, bolt angle and bolt size. The axial load distribution along the hole threads and the shear stress distribution at the thread root were analyzed in detail. It was found that there were three typical load distribution patterns along hole threads, which were (1) linear pattern; (2) concave parabola pattern; and (3) convex parabola pattern, and the ratio of bolt stiffness to tube wall stiffness was identified as the pivotal factor affecting the axial load distribution. Meanwhile, the analysis results indicated that the bending moment on the TOBs did not affect the load distribution along hole threads, but led to a non-uniform stress distribution along the thread in the same turn. In the end, the prediction formulas for the carrying capacity of hole threads are presented by modifying Yamamoto’s equation based on the analysis findings. The formulas are validated against the finite element analysis results and previous test results. The prediction can reach the highest accuracy when adopting the proposed failure criterion 2, with average errors of − 0.001 and − 0.002, respectively, demonstrating the accuracy and reliability.

Tests on Mechanical Properties of Concrete-Filled Double-Skin Circular Steel Tubes Connected by Thread Under Axial Compression

Tests on Mechanical Properties of Concrete-Filled Double-Skin Circular Steel Tubes Connected by Thread Under Axial Compression

Machine learning and nonlinear finite element analysis of fiber‐reinforced polymer‐confined concrete‐steel double‐skin tubular columns under axial compression

AbstractFiber‐reinforced polymer (FRP)‐confined double‐skin tubular columns (DSTCs) are an innovative type of hybrid columns that consist of an outer tube made of FRP, an inner circular steel tube, and a concrete core sandwiched between them. Available literature focuses on hollow DSTCs with limited research on DSTCs made with inner steel tubes filled with concrete. Overall, DSTCs have many applications, highlighting the importance of studying the effects of concrete filling and strength on the composite system. To address this gap, finite element models (FEMs) and both traditional and innovative machine learning (ML) techniques were used to develop accurate models for predicting load‐bearing capacity and confined ultimate strain under axial loads. A comprehensive database of 60 experimental tests and 45 FEMs simulations of columns was analyzed, with five parameters selected as input variables for ML‐based models. New techniques like gradient boosting (GB), random forest (RF), convolutional neural networks, and long short‐term memory are compared with established algorithms like multiple linear regression, support vector regression (SVR), and empirical mode decomposition (EMD)‐SVR. Regression error characteristics curve, Shapley Additive Explanation analysis, and statistical metrics are used to assess the performance of these models using a database containing 105 FEMs test results that cover a range of input variables. While EMD‐SVR and GB perform well for confined ultimate strain, the suggested EMD‐SVR, GB, and RF models show superior predictive accuracy for confined ultimate load. To be more precise, for confined ultimate load prediction, EMD‐SVR, GB, and RF obtain values of 0.99, 0.989, and 0.960, respectively. The values for GB and EMD‐SVR at confined ultimate strain are 0.690 and 0.99, respectively. However, design engineers are limited by the “black‐box” nature of ML. In order to solve this, the study presents an open‐source GUI based on GB, which gives engineers the ability to precisely estimate confined ultimate load and strain under various test conditions, enabling them to make well‐informed decisions about mix proportion.

Study on the Bending-Shear Properties of Concrete-Filled Circular Carbon Fibre Reinforced Plastic Steel Tubes.

In order to study the bending-shear performance of CFRP concrete-filled steel tubes, static tests were conducted on 15 circular concrete-filled CFRP steel tube bending-shear specimens. For all specimens, Ds was 120 mm, ts was 2 mm, and ml was 1. The shear displacement (V-Δ) curve of the specimen and the collaborative work between the steel tube and CFRP are discussed. ABAQUS was applied to simulate the V-Δ curve and failure mode of the specimen. We explored the effects of CFRP layers, material strength, the steel ratio, and the shear span ratio on the bending-shear performance of components. The experimental results show that a steel tube and CFRP can work together. As the shear span ratio increased, the bearing capacity and stiffness of the specimen decreased. An increase in the number of transverse CFRP layers could improve the bearing capacity of the specimen, but it had no significant effect on the stiffness. Calculating the elastic stage stiffness and bearing capacity of 15 short columns of test and FE curves revealed an average error of 6.71% and a mean square error of 0.83 for the elastic stage stiffness. The simulation results of the established finite element model are in good agreement with the experimental results. The average error of the bearing capacity was 3.88%, with a mean square error of 0.94. Based on experimental and finite element results, the moment shear correlation equation for concrete-filled CFRP steel tube bending-shear members is presented.

Behavior of Circular Hollow Steel-Reinforced Concrete Columns under Axial Compression

The circular hollow steel-reinforced concrete (HSRC) column consists of an inner circular hollow steel tube and outer circular hollow reinforced concrete (RC). This design provides several advantages, including being lightweight, having a wide sectional profile, and having a high flexural stiffness. This paper aims to investigate the behavior of the circular HSRC columns under axial compression through testing and finite element (FE) modeling. An FE model was established to simulate the circular HSRC columns under axial compression, which was validated against the test data. Additionally, the load distribution and the interface stress between the outer hollow RC and inner steel tube were analyzed. Subsequently, a systematic parametric analysis was conducted on the diameter (d) and thickness (t) of the steel tube; slenderness ratio (λ); strength of concrete (fcu); yield strength of steel tube (fsy), longitudinal rebar (fly), and stirrup (fgy); as well as the stirrup spacing (s). The critical influencing factors of the circular HSRC columns under axial compression were identified. fcu, λ, d, fly, and fsy dramatically influence the bearing capacity, and the stiffness is notably affected by λ and fcu. Finally, three simplified design methods were summarized and evaluated for calculating the bearing capacity of the circular HSRC columns under axial compression.

Feasibility of using superelastic shape memory alloy in plastic hinge regions of steel bridge columns for seismic applications

AbstractThis paper contributes to the further development of seismic resilient infrastructure by introducing a novel prototype bridge which integrates tubular steel piers with superelastic shape memory alloy (SMA). The proposed bridge pier is composed of a circular steel tube which is bonded to a superelastic SMA tube in regions of high stress. The pier is distinguished by its ability to minimize residual drifts following inelastic deformations induced by cyclic loading. Three‐dimensional continuum finite element (FE) models are utilized to examine its lateral behavior. Experimental data is used to demonstrate the effectiveness of the continuum FE procedure in replicating the cyclic response and capturing both global and localized behaviors. A novel composite material model is proposed to represent the degradation of strength and accumulation of irreversible strains in the cyclic response of superelastic SMAs. An iterative procedure for the calibration of this material is presented. Investigations, employing the calibrated FE procedure, focus on the quasi‐static cyclic response of steel piers with superelastic SMA in the plastic hinge zone, aiming to identify the optimal length of the SMA tube for achieving a self‐centering response with reduced residual deformation. The study is then further expanded to examine the seismic response of a bridge structure incorporating such piers. Development of the FE model for the prototype bridge includes the modelling of the piers using continuum elements, while the superstructure, bearing units, abutment walls, and backfill material are modelled using discrete elements. Nonlinear time history analyses are undertaken to investigate the effects of column wall thickness and materials used in the plastic hinge zones of the piers. Dynamic FE study results indicate that bridges employing such piers are capable of returning to their original position, provided the SMA tube is of adequate length.

Investigation on axial compression performance of self-compacting fly ash concrete filled square steel tube column: Tests and numerical simulation

Self-compacting concrete has the advantages of high fluidity, high uniform density, and low porosity. To promote the engineering application of self-compacting concrete, this article studies the axial compression test of ten square steel tube self-compacting concrete columns. The effects of design parameters such as different grades of concrete strength and internal structures of circular steel tubes, square steel tubes, and H-shaped steel on the mechanical properties of self-compacting fly ash concrete filled square steel tube columns are studied. The failure mode, characteristic load and characteristic displacement, ductility, and energy dissipation capacity of each specimen are analyzed. On the basis of the test, the finite element analysis model of steel tube self-compacting concrete column is established, and the test results and simulation results are compared and analyzed. The results show that: the failure mode of the square steel tube specimen is the multilayer middle and upper ring buckling failure. Internal structural measures of square steel tube and H-shaped steel can improve the bearing capacity, ductility, and energy dissipation capacity of the specimen. It is not recommended to use the specimen with an outer square and an inner circle design. The results of the finite element simulation have a high degree of matching with the test results, which intuitively reflects the changes in force of each component.

Seismic behavior of double-cell CFST columns with table tennis racket-shaped cross section

Multicell concrete-filled steel tube (CFST) columns with excellent mechanical performance have been used in super high-rise buildings in recent years. According to the requirements of a 265-m tall building, the double-cell CFST columns with an innovative cross section of table tennis racket were applied. Hence, six columns were designed according to the prototype columns and subjected to low-frequency cyclic loading. The impacts of different shear-span ratios (SSRs), axial load ratios (ALRs), and steel ratios on the seismic behavior were elaborated and clarified. The test results indicated that the seismic performance varied in different loading directions owing to the asymmetric cross section, and the columns performed better in the positive loading direction resulted from earlier and more sufficient confinement of the circular steel tube, which was also reflected by the measured strain development. With SSR deceasing from 2.5 to 1.7, earlier and more severe damage was recorded. The resisting capacity and stiffness were significantly improved at a cost of ductility loss. Increasing ALR from 0.2 to 0.4 brought about more damage and worse deformability. However, a proper increase in axial load was beneficial to the resisting capacity and stiffness, and its influence on the restoration capacity was associated with SSR. Increasing the steel ratio (by 33.36%) could effectively postpone and alleviate the local buckling of steel tube, thereby comprehensively improving the seismic performance. A finite element (FE) analysis of such columns was performed to carry out the damage evolution and parametric study, and provide references for their application in practical engineering.

Investigation on bearing resistance of thin-walled circular steel tube subjected to eccentric loading

In the engineering structures using circular steel tube (CST), the eccentric loading of CST members often occurs due to installation error and structural form. Existing research mainly focuses on the bearing resistance of circular steel pipes under the condition of eccentricity at one end. In practical application, both ends of CST members also may be eccentric. And with the increase in eccentricity, material yielding may occur prior to flexural buckling. So far, it’s lack of reports about the mechanical behavior of CST members subjected to eccentric loading at both ends and the boundary between material yielding and flexural buckling. This paper presents experimental, numerical and theoretical studies on bearing resistance of thin-walled circular steel tube with slenderness ratio of 30, 40 and 50, subjected to eccentric loading at one end and both ends, respectively. The study reveals a significant discrepancy in the prediction of bearing resistance for circular tubes subjected to eccentric loading at both ends according to existing design codes. Considering the synthesis of bending moment and deflection caused by eccentric loading at both ends, the calculation method of the bearing resistance based on flexural buckling of CST is established, which enhances the prediction accuracy of test verification. In addition, a theoretical boundary between the two failure modes appearing in CST members under eccentric loading at one and both end(s) - flexural buckling and reaching the material yielding strength - was established as dominated by the slenderness ratio and loading eccentricity.

Strength design of built-up radially battened columns subject to axial compression and arbitrary directional bending moment

Built-up radially battened columns (RBCs), comprising several identical circular steel tubes interconnected by multiple radial-shaped battens, represent an innovative column design that combines aesthetic appeal with exceptional load-bearing capacity. This makes them well-suited for architectural applications in large public buildings such as stations and airports. The axial compressive strength design methods for the built-up RBC have been extensively explored in the authors' prior studies. However, in real-world engineering applications, built-up RBCs may experience simultaneous axial compression and varying directional bending moments, particularly when considering potential eccentric compression or wind and earthquake loads in public buildings. Currently, strength design methods for this loading condition are still lacking. To address this deficiency, this paper primarily delves into the failure mechanism and strength design methods for built-up RBCs experiencing combined axial compression and arbitrary directional bending moments. Initially, the sectional strength of the built-up RBC is investigated by using a shell finite element model (FEM) for the built-up RBC. It is revealed that the compression-bending sectional strength of the built-up RBC exhibits significant differences with varying bending directions and tube numbers. Moreover, in some bending directions, the M-N sectional strength curve of the built-up RBC exhibits convexity. By integrating theoretically derived equations grounded in the principles of materials mechanics with FEM numerical calculations, concise and practical design equations are proposed for accurately predicting the sectional strength of the built-up RBC under combined axial compression and arbitrary directional bending moments. Subsequently, The buckling strength of the built-up RBC is studied using the FEM, considering initial geometrical imperfections consistent with first-order flexural buckling and accounting for geometric nonlinearity. It is noted that the bending direction of the built-up RBC may deviate during loading. Besides, the M-N buckling strength curve of the built-up RBC is not as convex as its M-N sectional strength curve. Based on calculations from numerous examples, practical design equations for predicting the buckling strength for the built-up RBC under axial compression and any directional bending moments are introduced. The key findings of this paper contribute to the compression-bending strength design of RBCs, enriching RBC design theory and promoting the widespread application of built-up RBCs in actual engineering projects.

Experimental and Finite Element Method Investigation of Axial Load Carrying Capacity of Concrete Filled Circular Steel Tube Columns According to Different Slenderness Ratios

Experimental and Finite Element Method Investigation of Axial Load Carrying Capacity of Concrete Filled Circular Steel Tube Columns According to Different Slenderness Ratios

Fire test and simulation of circular steel tube confined steel-reinforced concrete columns

Steel tube confined steel-reinforced concrete (STCSRC) columns are novel steel-concrete composite columns where the steel tubes are disconnected at beam-column connections. Few researches have been conducted on the fire behaviour of STCSRC columns by now, though there are some relevant studies on the fire performance of steel-reinforced concrete-filled steel tubular (SR-CFST) columns. However, the mechanical behaviour of STCSRC columns under fire exposure differs from that of SR-CFST columns due to the discontinuous external steel tube. It is essential to carry out a study on the fire performance of STCSRC columns. The fire tests of four axially loaded STCSRC columns were carried out in this paper, and the failure mode, distribution of cross-sectional temperature, fire resistance, and lateral deflection were discussed. A finite element model was developed and validated with test results, and then a parametric analysis of the STCSRC stub and slender columns under fire exposure was performed. The load ratio, steel tube strength, the area ratio of steel tube to concrete, and slenderness ratio have an adverse effect on the fire resistance of STCSRC columns, whereas cross-sectional dimension, area ratio of steel section to concrete, and strength of steel section have a favourable impact. Concrete strength has a different influence on the fire resistance of STCSRC stub and slender columns. Finally, based on the specifications JGJ/T 471-2019 and EN 1994-1-2, simplified calculation methods for assessing the fire resistance of STCSRC columns were proposed.

Structural performance of circular CFST columns with circumferential gap imperfections

The present study investigated the influence of circumferential uniform gap imperfections (≤ 1.0%) on the structural behavior of circular concrete-filled steel tube (CFST) columns under axial compression. The study involved testing 20 CFST columns with circumferential uniform gaps with varying gap ratios (0.2%–1.0%), diameter-to-thickness ratios (18–43), and concrete strength (68.5 MPa and 114 MPa). The presence of uniform gaps (0.2%–1.0%) reduced the confinement effect of steel tube on core concrete, and peak load capacity decreased by 9.0%–24.0%. The normalized concrete strength in high-strength concrete specimens with gap ratios ranged between 0.89 and 0.99, showing no interaction between steel tube and core concrete due to the less deformable nature of high-strength concrete. Core concrete exhibited a combination of vertical splitting and diagonal cracks distributed across the height of specimens with an increase in gap ratio, and local steel tube bulging was dispersed around the edge regions. A theoretical procedure was developed to assess the impact of uniform gaps on the stress state of steel tube and core concrete by determining their respective load contributions. The suggested approach reasonably estimated the gap ratio's nominal influence on the behavior of steel tubes and its considerable influence on the behavior of core concrete. Considering the experimental limitations, gap ratios were broadened through finite element modelling to investigate the gap ratio effect further. After examining the confinement mechanism of CFST columns and finite element results, an analytical formulation to predict the peak load capacity of CFST columns with circumferential uniform gaps was proposed and verified.

Experimental investigations into stocky composite columns of concrete-filled circular S690 steel tubes under compression

Composite columns of concrete-filled steel tubes (CFSTs) are very efficient in providing large structural resistances and rigidities. However, high strength S690 steel tubes are not covered in current design codes. Therefore, a series of experimental investigations into stocky composite columns with circular CFSTs under compression were conducted. A total of 21 stocky composite columns with different tube diameters and thicknesses made of Grades S355, S460 and S690 steel and Grade C60/75 concrete were tested under different loading conditions. Moreover, extensive measurements on the surface strains of the steel tubes were made during tests. Data analyses on the test results reveal that all composite columns exhibit enhanced section resistances and ductility at large deformations, indicating that the use of S690 steel and C60/75 concrete in these composite columns are highly efficient. A simple method for determining confining pressures provided by the steel tubes to the concrete cores in these composite columns is proposed, which involves a systematic analysis on the measured data and the use of von Mises yielding criterion without any complicated plasticity formulation. Based on the proposed method, the resistance contributions of both the steel tubes and the concrete cores are successfully quantified. Comparison between the measured and the predicted resistances using EN 1994–1-1 of these composite columns reveals that while the section resistances of these composite columns are safely predicted, the resistances of the steel tubes are underpredicted while those of the concrete cores are overpredicted correspondingly. The findings highlight the need to improve the predictability of the current design rules, and to give increased resistances of the steel tubes when high strength S690 steel tubes are used.