Tubular Members Research Articles

Behaviour of axially-restrained tubular compression members exposed to varying thermal loads

This paper discusses the behaviour of tubular compression members subjected to elevated temperature. The structural integrity of steel structures on exposure to elevated temperature is an important aspect to be considered. In order therefore to understand the behaviour of tubular and joints made of tubular compression members subjected to elevated temperature, studies were carried out. The experimental investigations were carried out on the compressive strength of axially-restrained tubular and joints made of tubular compression members at room as well as elevated temperature (localised). With a view to analyzing the failure modes, microstructural analysis was carried out on the samples extracted from the affected regions. The results reveal that the failure of the axially restrained compression members exposed to elevated temperature is attributed to localised axisymmetric imperfections apart from combined effect of temperature-induced thermal strains and material degradation.

Experimental testing of a self-centring concentrically braced steel frame

Conventional concentrically braced frames (CBFs) undergo many cycles of inelastic deformation during seismic excitation. This inelastic deformation leads to the possibility that a structure will remain in an out-of-plumb position, even if it has performed as required by current design codes. This paper presents an improved steel braced framing system that eliminates such residual deformations in the structure by using a post-tensioning arrangement to ensure the structure self-centres following an earthquake. This is achieved by combining the bilinear elastic rocking response of a post-tensioned frame with the inelastic behaviour of tubular steel bracing members to give a system that both dissipates hysteretic energy and ensures self-centring behaviour, termed the self-centring concentrically braced frame (SC-CBF). This SC-CBF system distinguishes itself from previous self-centring CBFs by employing post-tensioned rocking beam-column connections as opposed to a globally uplifting frame. The mechanics behind the behaviour of the SC-CBF are first described, followed by a discussion of an experimental test setup to validate the concept under quasi-static cyclic testing. Results from a total of nine tests are presented to demonstrate the self-centring behaviour of the SC-CBF. Comparisons with analytical expressions developed for the system demonstrate the SC-CBF performs as anticipated and presents a novel system for the seismic design of steel structures.

Flexural behaviour of stiffened and multi-cell L-shaped CFSTs considering different loading angles

Stiffened and multi-cell L-shaped concrete-filled steel tubular (CFST) members are often subjected to the loads from different loading angles as corner columns. Moreover, the flexural behaviour is the basis for the research of beam-columns and the determination of axial force-bending moment interaction curves. However, little work has been done on the flexural behaviour of these kinds of members at different loading angles. In this study, a total of 11 L-shaped CFST specimens with the variables of sectional type and loading angle were tested under pure bending. The failure mode, bending moment-deflection curves, moments-strain curves, deflection curves, flexural capacity and stiffness were investigated. It was found that the influence of loading angles is significant. The flexural capacity and stiffness of the specimens are highest at 135°, middle at 0° and 180°, and lowest at 45° and 225°. Setting longitudinal stiffeners or using multi-cell cross-section can improve the flexural capacity by about 20%. Then a finite element model was developed incorporating residual stress and cold-formed effect. Considering the effect of loading angles on the basis of the design model at 0°, the simplified formulas for flexural capacity of the stiffened and multi-cell L-shaped CFST members were proposed.

Experimental and numerical studies on square steel-reinforced concrete-filled steel tubular (SRCFST) members subjected to lateral impact

Steel-reinforced concrete-filled steel tubular (SRCFST) structures were proposed in recent years and have been employed in engineering practices. To date, the existing studies mainly focused on the behaviours of SRCFST structural members under static or seismic load. Only very limited studies of the SRCFST structural members subjected to impact load, which might cause severe damage and even structural collapse, can be found in literature. In this study, experimental and numerical studies on square SRCFST specimens under lateral impact load are conducted. A total of eighteen square SRCFST specimens are fabricated and tested via a drop hammer impact device. Various parameters (i.e. impact velocity, boundary condition, impact direction, cross-section of steel profile and axial load level) are considered to evaluate their effects on impact responses of square SRCFST members. The damage modes, impact forces, mid-span displacement responses and energy absorption capacities of the tested specimens are extracted from testing data and analysed. Furthermore, by using the software ABAQUS, numerical model is established and validated with the testing results. With the validated numerical models, the damage evolution, sectional stress and energy distribution of the structure with different parameters and impacting scenarios are evaluated. The impact behaviours of square SRCFST and concrete-filled steel tubular (CFST) members are also compared and discussed.

Concrete filled double skin square tubular stub columns subjected to compression load

Concrete filled double skin tubular members (CFDST) consist of double concentric circular or square steel tubes with concrete filled between the two steel tubes. The CFDST members, having a hollow section inside the internal tube, are generally lighter than ordinary concrete filled steel tubular members (CFT) which have a solid cross-section. Therefore, when the CFDST members are applied to bridge piers, reduction of seismic action can be expected. The present study aims to investigate, experimentally, the behavior of CFDST stub columns with double concentric square steel tubes filled with concrete (SS-CFDST) when working under centric compression. Two test parameters, namely, inner-to-outer width ratio and outer square steel tube's width-to-thickness were selected and outer steel tube's width-to-thickness ratio ranging from 70 to 160 were considered. In the results, shear failure of the concrete fill and local buckling of the double skin tubes having largest inner-toouter width ratio were observed. A method to predict axial loading capacity of SS-CFDST is also proposed. In addition, the load capacity in the axial direction of stub column test on SS-CFDST is compared with that of double circular CFDST. Finally, the biaxial stress behavior of both steel tubes under plane stress is discussed.

Investigating local buckling in highly slender elliptical hollow sections through analysis of 3D-printed analogues

Tubular structural members with slender cross-sections are susceptible to failure through local buckling of their tube walls. Previous numerical studies of steel elliptical hollow sections in compression predicted the local buckling modes and the ultimate loads of particularly slender specimens, with the results used to calibrate design methods for slender elliptical sections. Although these numerical parametric studies were conducted across a wide slenderness range, it was only possible to validate the models against experimental results in the low slenderness range since commercially-available steel EHS are intended to satisfy non-slender geometric limits prescribed by structural design codes. Such limitations to the experimental scope are circumvented in the present study through testing of highly-slender specimens produced using additive manufacturing techniques. A total of eight specimens of various cross-sectional aspect ratios and tube wall thicknesses were fabricated at London South Bank University using additive manufacturing techniques, which were then tested in compression; the observed load-deflection behaviour, ultimate loads, longitudinal strains and failure modes are discussed. Through appropriate rescaling of relevant parameters, design predictions for the ultimate load of the 3D-printed analogues are obtained using a design method intended for use with steel elliptical hollow sections. It is shown that the design predictions are safe-sided when compared to the present experimental results, with the accuracy generally increasing with aspect ratio and slenderness.

Cold-formed high strength steel tubular beam-columns

High strength steel is used more often in a variety of civil engineering applications due to its high strength-to-weight ratio and cost effectiveness. This paper presents the experimental investigation on cold-formed high strength carbon steel tubular members subjected to combined compression and bending. The nominal 0.2% proof stresses of the test specimens were 700 and 900 MPa. The test specimens consisted of square hollow sections (SHS), rectangular hollow sections (RHS) and circular hollow sections (CHS). The material properties, global geometric imperfections of the specimens were measured. The behaviour of the beam-column members was investigated through testing 32 specimens which had a nominal member length of 1480 mm. The second order effects were also considered by measuring the mid-height deflections for all specimens. The compression and bending capacities, load-deformation histories and failure modes of the test specimens were also reported. The test results were compared with the values predicted from the American, Australian and European standards. Improved design recommendation is provided for cold-formed high strength steel tubular beam-columns. Based on the experimental results, finite element modelling methodology is also proposed.

Design considerations for joining of tubular members subjected to impact loading

This study explores the effect of impact loading in structural bar and tube welded and threaded joints. Specifically, this work concentrates on stress wave propagation and mechanical impedance to show the effect of geometric parameters on stress distribution and history inside these joined members. In this experimental work, welded and threaded bar-and-tube networks are compared with equivalent length single bars to provide a measure of joint quality. Application to dynamic experimental equipment, known as Hopkinson Bars, is considered herein due to the stringent need for wave uniformity within these systems. Explicit finite element analysis on the experimental setup was performed to provide internal stress history information to provide more insight into the causes of stress risers and spurious load reflections. A key finding in this study shows that joint impact performance reduces when joint size is either too small or too large. This optimized joint size is found to depend on the joint'seffective mechanical impedance. This study serves as a new look at potential joining considerations under impact loading.

Transverse Impact on Hollow and Concrete Filled Steel Tubular Members

This paper clarifies a review of research progress in the transverse impact process on hollow and Concrete Filled Steel Tubular (CFST) members and their dynamic response under impact loading. This review emphases analytical, numerical and experimental studies recently performed by many researchers. It clarifies the influence of some fundamental parameters on the impact performance such as; axial load level, impact energy, steel ratio, constraining (confining factor), steel tube thickness, diameter-to-thickness ratio of the steel tube. In addition to exploring the effect of concrete and steel material properties and specimen boundary conditions. It also goes through the effect of using Carbon Fiber Reinforced Polymer (CFRP) jackets. The review explained with detailed discussions the stages of impact process, impact energy, dissipated energy, failure modes, life-cycle performance of Concrete Filled Steel Tubular (CFST) members subjected to impact load. The influence of strain rates on the performances of concrete and steel are discussed as well.

Analysis of tubular joint of offshore structure

The Jacket of an offshore structure is made up of tubular members. These tubular members are made up of steel and designed to resist yield and buckling loads. The tubular sections form various types of joints like K joint, KT joint, Y joint, etc. These joints are subjected to various types of cyclic loading conditions, due to which fatigue failure starts and increases as the time passes. As a result, the most critical section of the jacket fails first epically joint. The excessive strains due to axial forces, in-plane, and out plane bending moment accelerate the failure mechanism. In this paper, the comparative study is made between K joint and KT joint for the same loading conditions is made by selecting a finite method approach. By referring to API RP 2A and DNV RP C203. The observations are made on the behavior of joints for the same loading conditions, to the made conclusion of this project. Keywords: K Joint, KT joint, Axial force, in plane bending moment, out plane Bending moment, API RP 2A, DNV RP C203

Confinement of concrete in double skin tubular members under axial compression loads

A hybrid double-skin tubular members (DSTM) consist of three composite materials in one section, include fiber-reinforced polymer (FRP) tube outward, steel tube inward and concrete is in between. These three materials give the member unique properties like protect member from harsh environment, reduce the amount of concrete by the presence of an inner steel tube that allowed the passage of services. Therefore, consider sustainable structure elements, as well as the rising strengthening of members and excellent corrosion resistance. This paper presents an experimental and analytical study of twenty-four circular columns with two types; concrete-filled FRP tube (CFFT) and double skin tubular column (DSTC), with a dimension of 100 × 310 mm which constructed and testing under an axial compressive load till failure, with four variables: numbers of layers of outer FRP tube, the compressive strength of concrete, thickness to diameter ratio and void ratio of inner steel tube. The experimental results showed that the concrete was effected confinement in hybrid DSTC led to very ductile behavior of stress–strain relationship and the number of outer FRP layers was the most effective parameters on DSTCs behavior. The nominal confinement ratio was affected by the thickness of outer FRP tube and compressive strength of concrete inside DSTCs.

Comparison of theoretical and analytical investigation on axial compressive behaviour of concrete filled steel tubular columns

Concrete filled steel tubular (CFST) members has its successive footprints in the construction industry, owing to its high load carrying capacity, improved ductile performance, large energy absorption capacity etc. CFST column has its own advantages like maximizing the useable floor area and minimizing the need of formwork. In this research work, experiments are carried out on 8 circular CFST columns with 60 mm diameter and 2.5 mm thickness. The parameters varied are height (960 mm, 900 mm, 840 mm and 750 mm) and the in-filled materials like normal strength concrete and high strength concrete. The study mainly focused on analysing the codal provisions like Eurocode4, ACI and AS codes with the experimental results. From the study, the influence of slenderness ratio over effect of confinement is observed and evaluated. As the codal provision seems to be too conservative in predicting the compressive behaviour of CFST columns, a modified approach is derived. The analytical expression predicted by this simplified method is applicable for CFST columns with L/D ratio varies from 13 to 16.

CFST Compression members with steel slag as a partial replacement for fine aggregate

Global warming and environmental destruction have become major issues in the last few decades. Use of more and more economical friendly materials in construction is important. Presently in India, steel slag production is about 12 million tonnes per annum, due to limited modes of practices of utilization, steel slags are dumped in yards of each production and engage large area. To overcome these problems steel slag can be used in construction. This paper deals with the study of axial compressive behaviour of Composite column with partial replacement of fine aggregate by steel slag in concrete-filled steel tubular column with different cross-sections. Partial replacement of fine aggregate in Concrete filled steel tubular column will reduce the damage to the ecological balance due to excessive sand lifting from river beds, affecting the ground level. Tests are carried out on the specimens with varying cross-sections and hollow tubular members under axial compression.

Experimental and numerical investigation of underwater composite repair with fibre reinforced polymers in corroded tubular offshore structural members under concentric and eccentric axial loads

Repair of offshore steel structural members become essential due to the corrosive marine environment in which they exist. Structural repair using composite material has significant advantages in these cases than the conventional repair by welding or bolting steel sleeves. However, the composite repair technology with fibre reinforced polymers (FRP) is not very well established for their usage on load bearing offshore steel structural members either in underwater or above water conditions. The study had attempted to do a pilot experimental study on steel tubular members under the combination of axial compression and bending loads to address this gap in knowledge. The specimens were grouped into four categories: intact, corroded, repaired in air and repaired underwater. The repair scheme remained the same for in air and underwater repairs with one layer of glass and two layers of carbon fibre reinforced polymers, but with resin and repair methodology being different for both. The repaired specimens showed notable improvement in the ultimate strength from the corroded members to match the intact strength. Underwater repairs proved to be adequate to replicate the ultimate strength achieved by the conventional in air repairs. Investigations into parameters like load displacement behaviour, energy absorption, ductility and strain values also revealed that the underwater repair performed very similar to the conventional in air repair. A simplified Finite Element model simulating the repaired specimens was also presented in the paper which had predicted the experimental behaviour with great accuracy. In conclusion, the underwater and above water repair of structural steel tubular members using FRP composite materials seem a very viable solution for the corroded load bearing members in the offshore industry, however more detailed study on long term performance and cyclic behaviour of the composite repaired members is yet to be carried out.

Composite patch repair of damaged tubular members from flare boom structures subjected to compressive loads

Conventional repair method for steel structures requires cutting and welding processes, which in offshore platforms may impose several disadvantages. Composite material repair has become an alternative for the rehabilitation of damaged structures and has been widely applied in other industries. In this work a numerical-experimental investigation evidences the effectiveness of the composite patch repair for flare boom corroded tubular elements subjected to axial compressive loads. A finite element model developed to determine the minimum composite laminate thickness that recovers the structural element strength is validated through numerical simulations. Twelve API 5L grade B reduced scale steel tubes with central perforations and similar longitudinal and transverse slenderness, respectively defined by length to radius of gyration of the cross-section area and diameter to thickness ratios, are tested covering the range of interest for full scale members. The composite material consists of a hybrid fabric made of carbon and glass fibers impregnated with epoxy resin. Severe localized corrosion is considered conservatively, idealized as a circular perforation in half the element length. In many cases the maximum load of the repaired tube even exceeded the intact tube capacity. Moreover, it is shown that sufficient load capacity recovery is achieved for the conditions assumed.

Thermal field of large-diameter concrete filled steel tubular members under solar radiation

Concrete-filled steel tubular (CFST) members have been widely used in engineering, and their tube diameters have become larger and larger. But there is no research on the thermal field of large-diameter CFST structure. These studies focused on the thermal field of the large-diameter CFST structure under solar radiation. The environmental factors and the actual placement position were considered, and the finite element model (FEM) of the thermal field of CFST members under solar radiation (SR) was established. Then the FEM was verified by practical experiments. The most unfavorable temperature gradient model in the cross-section was proposed. The testing results showed that the temperature field of the large-diameter CFST member section was non-linearly distributed due to the influence of SR. The temperature field results of CFST members with different pipe diameters indicated that the larger the core concrete diameter was, the slower the central temperature changed, and there was a significant temperature difference between the center and the boundary. Based on the numerical model, the most unfavorable temperature gradient model in the section was proposed. The model showed that the temperature difference around the center of the circle is small, and the boundary temperature difference is significant. The maximum temperature difference is 15.22oC, which appeared in the southern boundary area of the specimen. Therefore, it is necessary to consider the influence of SR on the thermal field of the member for large-diameter CFST members in actual engineering, which causes a large temperature gradient in the member.

Behaviour and design of ultra-high-strength CFST members subjected to compression and bending

A comprehensive experimental and numerical study on the axial and flexural behaviour of square concrete-filled steel tubular (CFST) members incorporating S960 ultra-high-strength steels and high-strength concrete (C70 and C100) is reported in the present paper. An experimental programme that consisted of eighteen stub and slender CFST members under various loading conditions was conducted. The test results showed that the nominal compressive and flexural strength of ultra-high-strength CFST members were achieved. Steel coupon tests, concrete compressive strength tests and geometric imperfection measurements were also carried out. In addition, numerical models which accounted for geometric imperfections and residual stresses were developed. A confined concrete model was employed in the numerical analysis to simulate concrete confinement effects. Validation against the experimental results demonstrated the accuracy and reliability of the developed model. A series of parametric studies were performed to evaluate the effects on the ultimate strength by changing the steel yield strength, concrete compressive strength, steel tube width-to-thickness ratio and member slenderness ratios for different loading conditions. Comparisons of the results were made with the current codes of practice including AS/NZS 2327, Eurocode 4 and ANSI/AISC 360, which indicated that all the provisions were conservative in predicting the axial and flexural resistance of CFST members with recommended curves.

Cyclic tests on hybrid buckling-restrained braces with Fe-based SMA core elements

The present study is focused on the development and testing of a novel buckling-restrained brace (BRB) system. The Fe-based shape memory alloy (FSMA) plate is used as the yielding core segment, whereas the steel plates are adopted in the fabrication of the elastic end segments as well as the buckling-restraining systems. As a proof-of-concept, a miniature BRB with extruded FSMA core was tested under quasi-static loading to explore its force-resisting capacity, displacement ductility, and the energy dissipation potential. Five full-scale brace sub-assemblages were fabricated using the short-core FSMA BRBs (named as, FSBRB) and the elastic hollow tubular members. The short-length FSMA yielding core plates were welded to the steel elastic segments of BRBs. Four hot-rolled steel angle sections were used to fabricate the buckling-restrained mechanism for the short-core BRBs. All brace sub-assemblages were subjected to reversed-cyclic gradually increasing displacements as per ANSI/AISC 341–16 loading protocol. The main parameters varied in this study were the length of FSMA core plates, the size of the air gap between the core plates and restrainers, the position of stoppers, and the detailing of welding connections between the FSMA core plates and the elastic steel segments. All brace sub-assemblages exhibited stable and nearly symmetric hysteretic response and excellent cumulative displacement ductility. Though the heat-affected FSMA plates exhibited a considerable reduction in the strain capacity, the proposed detailing scheme resulted in the better strength-adjustment factors and the displacement ductility of the FSBRB specimens.

Investigation on the behaviour of CFRP strengthened CHS members under monotonic loading through finite element modelling

During the past several years, civil infrastructure has made extensive use of tubular hollow steel members as structural elements. However, the rehabilitation and strengthening of tubular hollow steel members is a major concern due to design errors, environmental effects, reduction in material properties over time caused by corrosion and the need to withstand increased loads. A comprehensive investigation on the strengthening of tubular hollow steel members is therefore required. Although carbon fibre reinforced polymer (CFRP) strengthening technique has shown its effectiveness to improve the structural response of different steel members, studies on the structural response of circular hollow section (CHS) steel members strengthened with CFRP is limited. In the present study, an effective finite element (FE) model is first developed by comparing the FE model simulated results with the authors' experimental results. This is followed by a parametric study on the effect of the bond length of CFRP, number of CFRP layers, ratio of CFRP thickness to CHS wall thickness, diameter to thickness ratio of CHS and steel grade of CHS on the structural behaviour of the strengthened member subjected to monotonic loading. These parameters had noteworthy effects on the behaviour of the CFRP strengthened CHS steel members under monotonic loading. The CFRP strengthened CHS steel members showed an improvement in the moment capacity, secant stiffness, energy dissipation capacity and ductility compared to their bare CHS steel members. It can be concluded that the effectiveness of the CFRP strengthening technique is enhanced with the increase in the ratio of CFRP thickness to CHS wall thickness and/or increasing the diameter to thickness ratio of CHS. In contrast, the effectiveness of the CFRP strengthening technique is decreasing with the increasing of the steel grade.

Experimental investigation and FE modelling of the flexural performance of square and rectangular SRCFST members

In this study, experimental investigation and finite element (FE) modelling of the flexural performance of square and rectangular steel-reinforced concrete-filled steel tubular (SRCFST) members are carried out. Six square SRCFST specimens and four rectangular ones are fabricated and tested to obtain their failure modes, the strain data, the deformation and the moment capacity under four-point bending. The effects of profiled steel configuration, loading planes, shear-span ratios (a/D) and depth-to-width ratios (D/B) on the flexural performance of SRCFST members are evaluated. Moreover, the experimental data reported in this study are employed to verify the numerical model comprehensively. Thereafter, the verified FEA model is utilized to evaluate the stress distribution, the interaction between steel and concrete, the moment resisted by each component and the role of embedded profiled steel. Finally, the measured bending moment capacities are adopted to calibrate the accuracy of existing design method on SRCFST members stipulated in Eurocode 4 (EC4) (2004), while the results demonstrate that high accuracy for the prediction of bending moment capacity can be obtained.