Tubular Members Research Articles

Torsional behaviour of tapered CFDST members with large void ratio

This paper investigated the mechanical behaviour of tapered concrete-filled double skin steel tubular (CFDST) members under combined axial compression and torsion. A series of experiments on CFDST specimens were tested including 8 tapered members and 2 normal members with various hollow ratios and axial compression ratios. The torsional performance of tapered members was analyzed on failure modes, torque-rotation angle curves, and torque-shear strain curves. Verified FE models were developed to carry out the full-range analysis, stress and strain distribution, and the contact stresses between the components. The results showed that the failure mode of tapered CFDST columns was the torsional deformation occurred between 0.5H and the top section. The specimens with a high axial compression ratio occurred additionally inclined outward buckling at 0.8H section, giving rise to a decline in torsional capacity. A low level of axial force promotes the torsional bearing capacity. The effects of the taper angle and hollow ratio on the distribution of shear strain and the torsional capacity at different heights are obvious. Furthermore, employing the section with the lowest torsional modulus for tapered CFDST members is conservative as the designed section, because the torsional capacity might be underestimated. Finally, a simplified method defined by a new calculated section for forecasting the torsional performance of tapered CFDST members is proposed.

Analysis on Seismic Performance of Steel-Reinforced Concrete-Filled Circular Steel Tubular (SRCFST) Members Subjected to Post-Fire.

Steel-reinforced concrete-filled steel tubular (SRCFST) columns have a great development prospect in engineering practice due to their high load-bearing capacity, good ductility, and energy consumption capacity. This paper established the post-fire seismic analysis model of SRCFST with a circular-cased H section using the sequential coupled thermal-stress method by ABAQUS. The P-Δ curve, stiffness, ductility, and energy dissipation were calculated. Then, the post-fire seismic performance of CFST members was compared while keeping the total steel ratio constant, and it revealed that the SRCFST had superior ductility to CFST. Finally, the ductility coefficient and skeleton curve were parametrically evaluated. The results of the study showed that the effects of heating time (th), axial compression ratio (n), slenderness ratio (λ), and steel tube ratio (αt) on the skeleton line of SRCFST columns are more significant; the axial compression ratio (n), slenderness ratio (λ), and steel tube ratio (αt) have a negative influence on the ductility subjected to post-fire.

Underwater strengthening and repairing of tubular offshore structural members using Carbon Fibre Reinforced Polymers with different consolidation methods

Offshore structural steel members will need strengthening or repairing due to various reasons. Strengthening is often required due to work over demands or increased environmental loads. Repairing becomes essential as the steel members are usually severely corroded due to the marine conditions. Conventional hot work strengthening and repairing solutions become very expensive and retrofitting using Fibre Reinforced Polymers (FRPs) is an economical alternative. This paper investigates strengthening and repairing of offshore steel tubular members using Carbon Fibre Reinforced Polymers (CFRPs) with the help of experimental testing and an analytical study. Two different specimen sizes which were representative of members in offshore structures namely leg and brace were examined in the study. Partial length strengthening of steel tubular members subject to combined loading of axial compression and bending was carried out using different curing environments (in air and underwater curing). Strengthening and repairing of leg and brace specimens with multiple layers of CFRP retrofitted in underwater conditions were also investigated. Two different consolidation methods (underwater vacuum consolidation and underwater stricture consolidation) were compared with the help of full length strengthening of leg specimens. Although partial length strengthening proved to be less effective, partial length repairing of corroded tubular members were successful in restoring their intact capacity. Full length strengthening for underwater leg specimens were experimentally tested and it was found to improve the structural behaviour in many aspects like ultimate strength, load displacement behaviour, elastic slope, energy absorption, and ductility. A novel technique of underwater vacuum consolidation of retrofit laminates was demonstrated through a set of full length strengthened leg specimens. Underwater vacuum consolidation proved to be advantageous compared to the normal underwater stricture consolidation. An analytical check to identify whether a retrofitted tubular member would reach its target section properties was also introduced. Overall, the paper demonstrates multiple successful applications of FRP composites for underwater strengthening and repairing of offshore steel tubular members. Studies like this expanding the current knowledge on how the FRP retrofitted steel tubular behave within a real offshore structure are essential to increase the uptake of this technology in the oil and gas industry.

Calculation of Creep Strains of Thin-Walled Tubular Members Made of Linear Viscoelastic Materials Under Tension and Torsion

Calculation of Creep Strains of Thin-Walled Tubular Members Made of Linear Viscoelastic Materials Under Tension and Torsion

Numerical investigation into impact responses of an offshore wind turbine jacket foundation subjected to ship collision

Offshore wind turbine structures are increasingly implemented but they could be largely exposed to ship collision in heavy maritime traffic area. This study aims to carry out a systematic parametric study on impact responses of the whole ship-offshore wind turbine jacket foundation system, thereby identifying the most dangerous impacting cases and vulnerable components. The three-point bending tests on DH-36 steel tubes designed by scaling jacket tubular members were performed. The effects of impact locations, collision angles and impact velocities on crashing responses, impact energy distributions and interactive deformation mechanisms were studied. Finally, the most dangerous impact cases and deformation were analyzed. It was showed that the bow impact on a leg tube resulted in the maximum local deformation of the leg and the maximum tower top displacement. The centric impact caused more severe local deformation than eccentric impact. And increasing the impact velocity from 0.5 to 3 m/s could increase the peak bending moment at the impact position of jacket leg. In the most dangerous location, the damage area and local indentation of the leg reached to 5.77 m2 and 916 mm (65.4% of diameter). The study can gain important insights into the development of crashworthy offshore wind turbine structures.

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.

Compressive performance of circular hollow section brace with eccentrically installed rib-stiffened splice plates

Hollow section braces are often adopted in steel frame structures because of their superior performance. To connect tubular members with framing elements, field-bolted connections of the slotted-in single splice plate and gusset plate are majorly used because of their ease of construction. However, the eccentricity between the splice and gusset plate axes reduces the compressive strength of the brace and induces local connection failures under seismic excitations. To improve the compressive strength, this paper proposes a new connection configuration for circular hollow section braces in which cruciform rib-stiffened splice plates are eccentrically installed such that the gusset plate axis coincides with the brace axis. The concept of the proposed connection was first described with theoretical considerations. To demonstrate the effectiveness of the proposed connections, compressive loading tests and subsequent cyclic loading tests were conducted for four test specimens. Furthermore, finite element analyses with long-brace models were performed to demonstrate the good performance of the proposed connection for a wide range of slenderness ratios.

Shear response of circular-in-square CFDST members: Experimental investigation and finite element analysis

This paper firstly presented an experimental work of circular-in-square concrete-filled double-skin steel tubular (CFDST) members under lateral shear load. Sixteen circular-in-square CFDST specimens and four concrete-filled steel tubular (CFST) specimens were fabricated and tested. Crucial design parameters considered in this test including shear-span ratio (m = 0.4, 0.6, 0.8 and 1.0) and hollow ratio (χ = 0.46, 0.59, 0.67 and 0.82) were studied and discussed. The load-displacement curve (P-Δ), shear strain-load curve (γ-P), and failure mode of the tests were analyzed in detail. The testing results show that as the shear-span ratio increases, the shear capacity drops significantly with the change of failure modes from brittle shear failure to ductile bending failure. The hollow ratio is of primary importance in the study of shear strength for CFDST specimens. When m is 0.4–0.8, specimens with a larger the hollow ratio experience a lower shear capacity, and however when m = 1.0, specimens with larger the hollow ratio exhibit a higher shear capacity. Furthermore, using experimental work presented in this study as a basis for model verification, corresponding finite element model was developed to further explore the mechanical properties of CFDST specimens subjected to shear load via using software ABAQUS. Finally, the expression was proposed for predicting the shear capacity of circular-in-square CFDST specimens, in which both the shear-span ratio and hollow ratio were considered. It can be found that the predictions are very compatible to the testing results from this study.

Concrete-filled and bare 6082-T6 aluminium alloy tubes under in-plane bending: Experiments, finite element analysis and design recommendations

The application of aluminium alloys in structural engineering is growing owing to their high strength-to-weight ratio, aesthetic appearance and excellent resistance to corrosion. However, the low modulus of elasticity of aluminium poses adverse effects on the flexural response of structural members made of aluminium alloys. In case of tubular members, the performance can be improved with the addition of concrete infill. Research on the flexural response of concrete-filled aluminium alloy tubes is still minimal. This study presents experimental and numerical investigations on the behaviour of concrete-filled and bare 6082-T6 aluminium alloy tubular members under in-plane bending. In total 20 beams, including 10 concrete-filled aluminium alloy tubular (CFAT) and 10 bare aluminium alloy tubular (BAT) specimens, were tested. The specimens comprised of square and rectangular hollow sections and were filled with 25 MPa nominal cylinder compressive strength concrete. The experimental results are reported in terms of failure mode, flexural strength, flexural stiffness, ductility and bending moment versus mid-span deflection curve. Compared to the BAT specimens, the counterpart CFAT specimens have shown remarkably improved flexural strength, stiffness and ductility due to the concrete infill and the improvement is more pronounced for the specimens with thinner sections. Finite element models of BAT and CFAT beams were developed by taking into account the nonlinearities in geometry and material and validated against the experimental data. A parametric study considering a broad range of cross-sections and different concrete grades was conducted based on the validated models. The FE results have shown that the flexural strength of the BAT and CFAT members increases with the increase of cross-sectional aspect ratio, wall thickness and concrete grade. The results obtained from experiments and numerical analysis for BAT members were used to assess the flexural capacity predictions and the applicability of the slenderness limits provided in the European standards. It was demonstrated that the slenderness limits provided by Eurocode 9 are conservative for Class A aluminium sections. Hence, revised Class 1, Class 2 and Class 3 limits are proposed which appear to be better applicable to Class A aluminium alloys. In the absence of design specifications for CFAT flexural members, the design rules for concrete-filled steel tubular flexural members provided by Eurocode 4 were adopted and the material properties of steel were replaced with those of aluminium alloy. It was shown that the proposed design methodology is suitable for the design of CFAT flexural members. Moreover, a slenderness limit for compact sections of CFAT flexural members is proposed based on Eurocode 4 framework.

Flexural Strength Evaluation of Multi-Cell Composite L-Shaped Concrete-Filled Steel Tubular Beams

Concrete-filled steel tubular (CFST) members have been widely used in industrial structures and high-rise residential buildings. The multi-cell composite L-shaped concrete-filled steel tubular (ML-CFST) cross-section, as an innovative, special-shaped structural arrangement, may solve the issue of normal CFST members protruding from walls and result in more usable interior space. Currently, no design rules are available for the application of ML-CFST members. One of the primary objectives of the present study is to develop recommendations in line with the unified theory to evaluate the bending moment resistance of ML-CFST beams. According to the unified theory, the bending moment resistance of an ML-CFST beam is related to the compressive strength (fsc) and the flexural strength index (γm) of a composite section, in which the accuracy of γm and fsc are affected by a confinement effect factor (ξ). Nevertheless, the original expression of ξ is not suitable for ML-CFST sections, since the appreciable effect of the irregular shape on confinement is neglected. Considering the cross-sectional geometry and boundary conditions of the cells, an equivalent shape factor to modify the confinement effect was proposed in this study through dividing the infill concrete into highly confined areas and less confined areas. An adequate formula to calculate the fsc and an approximate expression of γm for the ML-CFST sections was then developed. Furthermore, four-point bending tests on eight specimens were carried out to investigate the flexural performance of the ML-CFST beams. Lastly, the proposed formulas were assessed against experimental and numerical results. The comparisons show that the proposed unified theory-based approach produced accurate and generally conservative results for the ML-CFST beams studied.

Creep Prediction Model of Concrete-Filled Steel Tube under Different Core Concrete Conditions

To reasonably predict the creep law of concrete-filled steel tubular (CFST) members under different working conditions, four groups of creep tests were carried out, and the creep curves of CFST specimens with defects were obtained. Based on the creep prediction formula given in the code for the design of CFST highway arch bridges issued in 2015, the creep prediction models of CFST under different core concrete conditions are studied. The results show that the concrete microcracks around the cavity defects are continuously generated and expanded during the compression process, resulting in the dislocation and sliding of the concrete. It increases the additional deformation of CFST; debonding defects hinder the redistribution of stress between the steel tube and the core concrete, weaken the interaction effect between the steel tube and the concrete, and increase the creep deformation of the core concrete. According to Different core concrete states, appropriate reference factors are selected to modify the creep prediction formula of CFST, which greatly improves the creep prediction accuracy of CFST members and defective members.

Simplified bar-system model for tubular structures by considering local joint flexibility

Local joint flexibility (LJF) may produce significant effect on performance of tubular structures. According to the local deformation characteristics at tubular joints, a simplified bar-system model is presented by using tie-strut members to simulate the local flexibility at joints. Based on proposed equations for calculating the LJFs of different types of tubular joints in literature, analytical equations for determining the dimensions and material properties of the presented tie-strut members are deduced. Additionally, nonlinear static pushover test was conducted for a small-scale tubular structure specimen and linear vibration test was carried out for another specimen. Load-displacement curves at some specified positions in the static test and displacement-time curves at some specified positions in the vibration test are obtained. Three different finite element models are then presented, which include using 3-D shell elements to simulate the entire tubular structures (3-D model), using beam elements to simulate the entire structures with rigid connection assumed at tubular joints (rigid frame model), and using beam elements to simulate the tubular members and using the presented tie-strut model to simulate the tubular joints (presented bar-system model). Finite element results obtained from the three different methods are compared with the experimental results, and the comparison indicates that both 3-D model and the presented bar-system model can produce accurate predictions compared to the experimental results while the rigid frame model overestimates the stiffness and load-carrying capacity and underestimates the vibrating amplitude of the specimens. As the presented bar-system model has much higher efficiency compared to 3-D model, it is more challenging for analyzing both linear and nonlinear behavior of tubular structures in practical design.

Performance of reinforced concrete-filled steel tubular (RCFST) members subjected to transverse impact loading

Reinforced concrete-filled steel tubular (RCFST) members possess excellent load-bearing properties and fire resistance, and have been applied in a range of practical engineering projects. In addition to static and seismic loads, RCFST members may also be subjected to transverse impact loads, such as those from vehicle/vessel collisions and terrorist attacks. An experimental and numerically study into the dynamic behaviour of RCFST members subjected to transverse impact is therefore the focus of the present paper. Nine specimens were tested using a drop-weight experimental setup, and the instantaneous impact force and deformation states were recorded. A finite element model was established, considering strain rate effects, and validated against the test results. Parametric studies were then conducted in which the effects of the material selection, cross-section characteristics, specimen scale and impact conditions were analysed. Finally, design proposals for RCFST members under transverse impact loading are presented.

Experimental and numerical evaluation of the axial capacity of cracked tubular members

This paper presents the experimental and numerical results for the axial capacity of cracked tubular steel members. Experimental tests of 11 columns in compression with simulated cracks of different sizes, defined as the percentage of the circumference (12%, 23.5% and 38.5%). The crack-tips were further treated by drilling a crack arresting hole. These specimens were then modelled by finite element analysis which were verified to match the experimental test. The DNVGL-RP-C208 standard was used as basis for performing the numerical finite element analysis. In addition, the capacity of the columns was calculated according to the 2004 revision of the NORSOK N-004 standard. The experimental tests indicated that the capacity in compression did not change significantly with the presence of cracks and crack arresting holes. The results from the numerical finite element analysis show a good agreement with the experimental work. However, the compressive capacity according to NORSOK N-004 shows a significant deviation to the safe side.

Assessment of Concrete Filled Steel Tubular Members: An Experimental Review

In this world with rapid development, durable and fast construction techniques lead to the invention of composite materials that are robust and advantageous over conventional materials. Recently invented Concrete-filled steel tubular members are the composite members used in civil engineering works to replace conventional steel and concrete members. This paper deals with an overview of the experimental performance of various types of composite members such as columns, short columns, stub columns, beam-columns, and slender columns deals with various loads such as axial compression, flexural load, cyclic bending, long-term sustained load, torsional load, and impact load. Effects due to the variation in the parameters like steel and concrete strength, diameter to thickness ratio, axial load level, shapes of tubes of these composite members on load-carrying capacity, flexural stiffness, ductility, torsional capacity, and cyclic performance of these members are discussed in this paper. The future scope is mentioned for study related to these composite members in the paper.

Behaviour and design of eccentrically loaded CFST columns with high strength materials and slender sections

High strength concrete-filled steel tube (CFST) columns have been adopted in many constructions requiring great load-carrying capacities, large floorplans, cost-effective and time-saving solutions such as modern skyscrapers or high-rise buildings, but their performance is still insufficiently understood due to the lack of experimental investigations and design specifications. The test program in this study consists of eight stub square CFST columns fabricated from high strength materials and slender sections which are beyond the permitted limits specified by many current design provisions. The failure patterns, load and moment resistances, load-mid-height deflection and load-axial strain responses were used to evaluate the overall performance of the test specimens. Furthermore, a numerical model for CFST columns, taking into account the actual factors such as the confinement on concrete cores, initial imperfections and residual stresses on steel tubular members, was developed and demonstrated a good agreement with the experimental results. A parametric study then adopted this verified model to explore the confinement effect on concrete cores and the influence of various material strengths, eccentricity ratios, section slendernesses and member slendernesses on the performance of composite columns. Finally, the load-moment interaction curves of specimens under different loading conditions obtained from experiment and numerical analysis were compared to the predictions of AISC 360-16, Eurocode 4 and AS/NZS 2327 for validating these specifications. In short, this study is expected to advance the understanding in the area of eccentrically loaded CFST columns with high strength materials and slender sections.

Maximum bending moment capacity of circular perforated offshore steel tubular members

Offshore steel tubular structures may experience perforations during their lifespan due to effects of the corrosive marine environment, which leads to deterioration of their strength capacity and lifetime shortening. There is a lack of procedures to quantify the structural capacity loss in view of the damage dimensions and geometrical characteristics of the tubular members. In this work, the maximum bending moment capacity of circular perforated steel tubular structures is evaluated experimentally and numerically. The effects of the magnitude of the perforation and the diameter-to-thickness ratio are discussed. In a first phase, reduced scale experimental tests and numerical simulations are developed considering tubular structures under pure bending moment load through a four-point configuration. Subsequently, a parametric analysis is conducted employing a simplified numerical model, then a design expression is adjusted and compared with numerical predictions and experimental measures to determine its reliability. The numerical model and the proposed expression showed absolute errors below 3% in relation to the experimental measures. Thus, the proposed expression can be adopted as an alternative criterion to assist in decision-making for supervision, repair or replacement of perforated tubular structures subjected to bending moment.

Effect of member orientation on static strengths of cold-formed high strength steel tubular X-joints

Rotation of brace and/or chord members has a significant effect on the static strengths of tubular joints. Brace-rotated and bird-beak tubular joints are obtained by rotating the brace and/or chord members along their respective longitudinal axes by angles corresponding to their cross-sectional diagonal planes. This paper presents an experimental investigation of 29 cold-formed high strength steel (CFHSS) brace-rotated (BR), square bird-beak (SBB) and diamond bird-beak (DBB) tubular X-joints. The tubular members were made of S960 steel, having a nominal yield strength of 960 MPa. An axial compression load was applied through the braces of brace-rotated and bird-beak tubular X-joints. The welds connecting braces and chords of brace-rotated and bird-beak tubular X-joints were laid using the automatic gas metal arc welding process. The measured values of brace-to-chord width ratio (β) ranged from 0.33 to 0.83, effective brace-to-chord width ratio (β’) ranged from 0.25 to 0.87, brace-to-chord thickness ratio (τ) ranged from 0.67 to 1.28 and chord width-to-chord thickness ratio (2γ) ranged from 25.3 to 39.1. Three failure modes were observed in this study, namely chord face failure by plastification, a combination of this with chord sidewall failure (i.e., combined failure), and chord crown failure by plastification. Nominal strength prediction methods, from prior studies on member-rotated tubular joints as well as from the prEN 1993-1-8 (2021) and CIDECT (2008, 2009), were evaluated against the static strengths obtained from the tests. Reliability analysis was also performed to check the reliability levels of the existing design rules. For the range of tests undertaken, it was found that the current design rules of RHS-to-RHS, CHS-to-RHS and CHS-to-CHS tubular X-joints given in the CIDECT (2008, 2009), when used in conjunction with a material factor of 0.80, were found suitable and reliable for the design of CFHSS BR, SBB and DBB X-joints, respectively. In addition, the effect of change of member-rotation on the joint strengths and load–deformation curves was also assessed by comparing the structural performance of member-rotated tubular X-joints with the those of corresponding identical traditional RHS-to-RHS X-joints (Pandey and Young [1]).

Axial Capacity Prediction of Concrete-Filled Steel Tubular Short Members Using Multiple Linear Regression and Artificial Neural Network

The estimation of the ultimate capacity of rectangular or circular shaped steel tubular members filled with concrete, such as columns, beams, and beam-column connections, requires a detailed structural study to be carried out. Therefore, identify the concrete strength the member subjected to axial-load only. Using the Levenberg-Marquardt artificial neural network, this paper investigates the concrete-filled steel tubular (CFT) members axial strength. 201 experimental specimens were collected from the literature to obtain the best results, and a wide range of geometric and material properties of CFT members were included. The proposed design and specimens illustrate the practicality and effectiveness of the chosen CFT column approach to classify real structural results.

Numerical investigation on the structural performance of octagonal hollow section columns

This study is initiated by the increasing use of steel tubular members with octagonal hollow sections (OctHSs) in civil structures. Previous studies primarily focused on the local buckling behaviour of cold-formed steel OctHSs. However, investigations on the global buckling behaviour of OctHS long columns remain scarce. Thus, the main objective of this paper is to numerically investigate the structural performance of the OctHS columns. A finite element method (FEM) was developed for tubular columns and validated against relevant experimental results reported in the literature. Subsequently, a parametric study on 239 OctHS columns was carried out. The applicability of design methods outlined in Eurocode 3, ANSI/AISC 360-16 and ASCE/SEI 48-19 was evaluated. The evaluation shows that the design method in Eurocode 3 provides conservative predictions for the load-bearing capacity of OctHS columns, while the ANSI/AISC 360-16 design method was slightly unconservative for the strength predictions. The design method in ASCE/SEI 48-19 gave inaccurate predictions for the load-bearing capacity of the OctHS columns. Modifications to the design methods in ANSI/AISC 360 and ASCE/SEI 48-19 were proposed to improve the accuracy of strength predictions and to obtain safe structural designs. The reliability of the design method in Eurocode 3 and the modified American design methods was assessed through the reliability analysis and found to satisfy the reliability requirements.