Tubular T-joints Research Articles

Parametric study on axially loaded square hollow section T-joints at elevated temperature

The paper presents the numerical study of tubular square T-joints under fire conditions. The sequentially coupled thermal-displacement numerical model was proposed to examine the performance of square hollow section (SHS) joints. The developed finite element (FE) model was validated against available experimental test results and applied to study the influence of dimensionless parameters on the strength of SHS tubular T-joints at elevated temperatures. These parameters include brace-to-chord width ratio β, chord-width-to-thickness ratio γ, chord-to-brace thickness ratio τ and loading ratio nF. The obtained results show that the critical temperature of T-joints is mostly influenced by the loading ratio nF and β parameter. Different methods to obtain the joint resistance are presented. The strength of the joint obtained based on EN 1993-1-8 was compared with FE analysis, which showed that the decrease in the joint resistance in brace axial compression follows the curve of Young's modulus reduction factors. Finally, the strength of the joint was calculated based on the component method, with a temperature increase specified for each individual component. This method gave higher results in joint strength, when the non-uniform temperature distribution within the joint was taken into account.

A multi-axial fatigue-oriented strategy for fatigue damage monitoring and assessment of tubular joints

In this paper, a structural health monitoring strategy that performs in-time multi-axial fatigue damage evaluations for welded tubular joints is proposed. First, we developed a stress monitoring program that obtains the reference for the multi-axial fatigue strength assessment, namely, the components of the hot-spot stress and zero point structural stress, from a sensor arrangement strategy representing explicit physical meaning for tubular T-joints. Then, we established a programmable procedure for multi-axial fatigue damage evaluation based on stress monitoring, which is verified through a finite element simulation. Taking practical interests into consideration, we suggest that the proposed strategy is suitable for in-field engineering practice.

Structural behaviour of thin-walled CHS-RHS T-joints: Experimental and numerical assessment

The design of tubular joints with slender cross-sections is currently limited by the validity ranges of established international design standards. Aiming to assess the structural behaviour of tubular joints characterised by thin-walled chord cross-sections, this paper initially reports, an experimental investigation into tubular T-joints consisted of CHS braces welded to thin-walled cold-formed RHS chords, which slenderness values were outside of the current design standard limits. The experimental programme was carried out on 12 cold-formed steel slender cross-section prototypes where the braces were subjected to axial compressive loading. The geometric parameters, including chord and brace thickness and brace diameter were varied. The connected chord face failure was the failure mode observed in all tests. The results of the experimental tests were thoroughly discussed. A second step was used to calibrate a numerical model that was the basis for a parametric analysis involving the most relevant parameters affecting the joint's behaviour. The obtained joint's strengths were compared with those forecasted by ISO 14346 developed for compact cross-sections modifying the chord stress function concerning the elastic cross-section modulus and effective cross-section area when slender cross-section chords are concerned. However, this approach leads to scattered predictions, roughly too conservative. Finally, this paper proposes an alternative design approach for the studied joints that are more accurate in predicting their strength when compared to the presented experimental and numerical results.

Experimental and numerical investigation of ultimate strength of ring-stiffened tubular T-joints under axial compression

The design procedure based on empirical equation for computing the chord strength of tubular joints in offshore structures has been established by various researchers and adopted by American Petroleum Institute and other organizations. Local stiffening of chord may be required to carry the loads from braces from various directions. One such method is to introduce internal circular rings at the brace-chord interface. In the present study experimental and numerical investigation on a ring-stiffened tubular joint have been carried out using 1:8 scale model of commonly adopted configuration used in the industry. Experiments have been conducted to measure the failure loads of stiffened and unstiffened tubular joints of (T-geometry) with plain and flanged ring-stiffeners fitted inside the chord. Models were fabricated using steel material and tests were conducted to measure the chord deformation using displacement transducers. First peak in load-displacement relationship has been used to determine the ultimate capacity. Numerical simulations have been carried out using ABAQUS software based on nonlinear stress-strain relationship of steel. The load-displacement characteristics for stiffened and unstiffened joints have been established. Results obtained from numerical simulation have been compared with that obtained from the experimental studies. Strength enhancement factors for stiffened joints have been calculated and compared with that of the unstiffened joints. The enhancement factors for plain and flanged internal ring-stiffener are noted to be 1.66 and 2.03 respectively.

Finite element analysis on the residual bearing capacity of axially preloaded tubular T-joints subjected to impacts

Understanding the dynamic behavior of tubular members is necessary for ensuring their safety, even when damaged after an impact. A nonlinear finite element analysis (FEA) was performed to evaluate the residual bearing capacity of axially preloaded square hollow section (SHS) T-joints subjected to impact loading. First, the Bao–Wierzbicki fracture criterion was used to simulate the failure modes and behavior of specimens subjected to static axial compression and impacts, and the numerical results were compared with experimental results in the literature. The bearing capacity error between the numerical and experimental results was within 10%, which verified the accuracy of the finite element (FE) model. Then, the local and global displacements of joints were distinguished with the central line displacement method, and 40 FE models were created. The dynamic responses of joints under impact loading were used to study the resistance mechanism. Joint deformation mainly manifested as outward buckling of the chord webs and indentation of the chord upper flange. The in-plane restraint and support by the brace of the chord increased with the brace–chord width ratio. Finally, the ultimate residual bearing capacity coefficient ψ was defined as the ratio of the ultimate bearing capacities of the impacted and reference specimen, and the influence of different parameters, including axial preloading, impact energy, brace–chord width ratio, and chord diameter, was explored. Axial preloading and impact energy showed a negative correlation with ψ, while the brace–chord width ratio and chord diameter showed a significant positive correlation with ψ.

Damaged Tubular T-joints Retrofitted with Carbon Fiber Reinforced Polymer Subjected to Monotonic and Cyclic Loadings

Owing to seismic, wave, or wind actions, tubular joints used in onshore and offshore civil infrastructures may require retrofitting to withstand both static and cyclic loadings. In the present study, bearing capacities, failure modes, and ductilities of damaged tubular T-joints retrofitted with carbon fiber reinforced polymer (CFRP) were investigated. Two-step loading was applied: first, a pre-determined load was applied to induce chord deformation; this was followed by unloading and CFRP retrofitting. Subsequently, the specimens were reloaded to failure. Results showed that the retrofitted specimens exhibited a maximum capacity similar to and a plastic deformation slightly larger than that of the bare specimen when these subjected to static loading; the retrofitted specimens, which were not severely damaged, exhibited a higher compressive ductility index than that of the bare specimen when these subjected to cyclic loading.

Adhesion failure analyses of laminated FRP composite made bonded tubular T-joints with axially compressed brace

Laminated FRP composite bonded tubular T-joints with constrained chord-ends and compressively loaded brace has been considered in the present research for detailed adhesion failure analysis of the non-planar T-joint region. Computational fracture mechanics based APDL codes have been developed and validated with respect to literature. Three-dimensional stress analyses revealed that Saddle points corresponding to Bottom Toe Line (BTL)/Middle Fillet Line (MFL) in joint regions are predominantly vulnerable towards adhesion/cohesion failures, respectively. Adhesion failure predicted to be the cardinal factor behind joint fracture (over cohesion failure) using Tsai-Wu/Parabolic yield criteria. Strain Energy Release Rate (SERR) estimated through Virtual Crack Closure Technique (VCCT) has been used for quantifying fracture growth. Pre-embedded through-the-circumference adhesion failure (2ϕ = 360°) is primarily susceptible towards mode-I fracture about saddle points with saddle-crown-mid-points and crown points predisposed towards mode-III and mode-II fractures, respectively. Adhesion failures (2ϕ ≤ 180°) are predominantly susceptible towards mode-I radial fracture growth (than circumferential fracture), leading to enhanced chances of adhesion and cohesion failures at Top Toe Line (TTL) and MFL, respectively. Quasi-isotropic ([90/+30/−30/90]2s), and cross-ply ([0/90]4s) are the suggested optimum chord stacking sequences improving fracture resistance of joint against micro (a/fb ≤ 0.3) and macro (a/fb > 0.3) failure ranges.

Experimental and numerical investigations of the compressive behavior of carbon fiber-reinforced polymer-strengthened tubular steel T-joints

A method for strengthening damaged tubular steel T-joints under axial compression by wrapping them with carbon fiber-reinforced polymer (CFRP) sheets was proposed and evaluated. The influence of the CFRP strengthening on the failure mode and load capacity of T-joints with different degrees of damage was investigated using experiments and finite element analyses. Five T-joints were physically tested: one bare joint to obtain the peak load and corresponding displacement (Δ1m), two reinforced joints to provide a reference, and two pre-damaged then retrofitted joints to serve as the primary research objects. The ratio of the pre-loaded specimen chord displacement to the value of Δ1m was considered to be the degree of damage of the two retrofitted joints, and was set to 0.80 and 1.20. The results demonstrate that the maximum capacity of the retrofitted specimen was increased by 0.83%–15.06% over the corresponding unreinforced specimens. However, the capacity of the retrofitted specimen was 2.51%–22.77% lesser compared with that of the directly reinforced specimens. Next, 111 numerical analysis models (0.63 ⩽ β ⩽ 0.76, 9.70 ⩽ γ ⩽ 16.92) were established to parametrically evaluate the effects of different geometric and strengthening parameters on the load capacity of strengthened tubular T-joints under different degrees of damage. The numerical analysis results revealed that the development of equivalent plastic strain at the selected measuring points was moderated by strengthening with CFRP wrapping, and indicated the optimal CFRP strengthening thickness and wrapping orientation according to tubular T-joint parameters. Finally, reasonable equations for calculating the load capacity of CFRP-strengthened joints were proposed and demonstrated to provide accurate results. The findings of this study can be used to inform improved CFRP strengthening of damaged tubular steel structures.

A procedure to generate design load-time histories for fatigue strength assessment of offshore structures

This paper presents general procedures to generate loading histories for fatigue strength assessment of offshore structures based on a storm model and seasonal wave data. Using hindcast wave data of 11 years at one point in the North Atlantic Ocean, the relations among the storm duration, storm crest and the number of levels in a storm are formulated and the statistics of storm duration are obtained. One year, including four seasons is chosen as the reference period of the loading history. Based on the generated loading histories, fatigue crack propagation analyses are performed on a tubular T-joint using a fatigue crack growth model considering the load interaction and R-ratio effect. The crack size uncertainty caused by randomly generated loading histories is investigated. The proposed procedures can lead to a good agreement with the observed significant wave height distribution. The generated loading histories result in median crack growth curves, comparing with those from measured wave data. The crack size is subjected to considerable uncertainty after a period of crack propagation. Ignoring the load interaction effect leads to faster crack growth, which may be the most convenient way to achieve a conservative fatigue crack propagation analysis using only one generated load history.

Experimental and numerical study on static behaviour of uniplanar CFDST-CHS T-joints

Experimental and numerical study on static behaviour of uniplanar T-joints between circular concrete-filled double-skin steel tube (CFDST) chord and circular hollow section (CHS) brace, i.e., CFDST-CHS T-joints for short, are presented in this paper. Twelve uniplanar T-joint specimens, including 10 joints with circular CFDST chords and 2 reference ones with CHS chords, were tested under specified boundary conditions. The experimental parameters included hollow ratio of the chord (χ), brace to chord diameter ratio (β) and axial compression ratio of the chord (n). The failure patterns, load versus displacement (strain) curves and bearing capacities of the specimens were collected and analysed. The composite tubular T-joint specimens showed better structural performance than the reference steel ones. For the CFDST-CHS T-joint specimens, the failure patterns change from the local buckling of CHS brace to the compression-bending failure of CFDST chord with the increase of β. With the local buckling failure of CHS brace, the bearing capacity of the composite tubular T-joint specimens increases with the growing of χ and β. However, with the compression-bending failure of CFDST chord, the bearing capacity of the composite tubular T-joint specimens increases with the rising of n. Advanced numerical simulation on the structural performance of uniplanar CFDST-CHS T-joints was implemented using finite element analysis (FEA) method, and the accuracy of the predicted responses was evaluated against the experimental results. Finally, based on the FEA results, the simplified method for the bearing capacity prediction of the composite tubular T-joints was proposed.

Failure of single hat-shaped thin-walled tubular composite T-joints under impact loading

Abstract This paper presents an experimental and numerical investigation into the failure characteristics of single hat-shaped thin-walled tubular T-joints made of twill-woven carbon fiber reinforced thermoplastic (CFRTP) under impact loading. One flat panel and one hat-shaped shell were prepared via a high temperature and pressure molding process first, and then were joined together via (a) adhesive bonding and (b) hybrid bonding and riveting, to form the T-shaped thin-walled tubular joints. Drop weight impact tests were conducted for the T-joints with three impact velocities. Load-time/displacement curves were measured and four distinct failure modes were identified through analyzing damage images and micrographs. It is noted that the specific energy absorption (SEA) capability increases with impact velocity and can be affected significantly by the deformation of the hat-shaped shell, but appears to be less sensitive to the joining forms. Nonlinear finite element analyses based on selected progressive damage model were conducted to predict the failure and energy absorption characteristics of the T-joints under an impact loading. There exists a good correlation between the simulation and experimental results, which validates the present numerical modeling method.

Static Strength of Square T-Joints Reinforced with Collar-Plates under Axial Compression or In-Plane Bending

In typical square tubular joints, the most common failure mode is local yielding or buckling of the chord at the brace/chord intersection. Thus, in the current paper, a collar-plate, as external reinforcement, is welded around the weld toe to increase the stiffness of the chord surface. Firstly, theoretical formulas, based on the yield line principle which is often used to examine thin-walled members suffering from local collapse mechanisms, for calculating the capacities of the collar-plate-reinforced square tubular T-joints under axial compression and in-plane bending are deduced. Then, experimental tests and finite element (FE) simulations are carried out on square tubular T-joints reinforced with collar-plates. The experimental results showed an improvement in the bearing capacity of SHS joints reinforced with collar-plate by 78 and 89% compared with the corresponding un-reinforcement joints. The FE results, as well, proved that using the collar-plate reinforcement is an efficient method to improve the static strength of a square tubular T-joint under either axial compression or in-plane bending. Finally, through the comparison between the FE results and the theoretical formulas, the validation range of the formulas for predicting the strengths of the collar-plate-reinforced square tubular T-joints is specified. For the joints under axial pressure, the validation range of the proposed formula is 0.45 ≤ βc ≤ 0.8, 2γ ≥ 20 and τc ≥ 1.5, while for the joints under in-plane bending is βc ≤ 0.8, 15 ≤ 2γ ≤ 30 and τc ≥ 1.50.

Detailed investigation of adhesive fillet tubular T-joint of laminated FRP composite tube under axial compressive load

Composite structures have wide applications in mechanical fields. Such structures consist of several joints, which may run a risk of eventually weakening a structure in case of excessive weight. It is, therefore, necessary to strengthen a structure so as to reduce the chance of failure. The present study was undertaken with a model of tubular T-joint having adhesive fillet. It also focussed on three-dimensional stress analysis on adhesive fillet. The model was made of laminated fibre-reinforced polymer (FRP) composite tubes with adhesive triangular fillet, subjected to axial compressive loading. The modelling and analysis were carried out by using finite element method (FEM) for three different joints with different geometrical parameters. The model was validated by using the available finite element model results. Based on the present finite element model results, the axial stress (σz) at brace-fillet intersection, circumferential stress (σφ) and circumferential-axial stress (τφr) at chord-fillet intersection were found to have greater effect as compared with other stress components. The maximum value of stress component could be noticed in between the brace and the chord joint region, which could eventually cause failures. The effect of adhesive fillet on deflection was studied in details by using ANSYS 18.1, and results showed that deflection of adhesive fillet model could be reduced by 12.36% as compared with non-fillet condition. Based on the present research findings, the presence of adhesive fillet was found to significantly reduce deflection effect as well as improve joint strength and stability of the structure.

Experimental and parametric studies of SCFs in FRP strengthened tubular T-joints under axially loaded brace

An experimental program was carried out on an axially loaded steel welded tubular T-joint made of circular hollow sections and strengthened with Glass/vinyl ester to study the effect of Glass Fiber Reinforced Polymer (GFRP) material on the Stress Concentration Factors (SCF) at both crown and saddle points on the chord member. Four tests were executed. The first test was performed on the plain joint which served as the reference for comparison, and the remaining three tests were performed on the strengthened joints with various FRP strengthening schemes. The arrangement of tests as well as specimen details together with most pertinent results are discussed. A significant drop in SCF values was observed for the FRP-strengthened T-joint as compared to the SCFs of the corresponding un-strengthened joint. The finite element model of the T-joint specimens were made and verified against the experimental data. Furthermore, extensive numerical analyses comprising of 1188 finite element models were performed to achieve a better understanding of the effect of the FRP parameters as well as the joint’s geometric parameters on the chord's SCFs. Additionally, through nonlinear regression analysis, SCF design equations were proposed for the fatigue design of axially loaded FRP strengthened tubular T-joints.

Accurate evaluation of fracture parameters for a surface-cracked tubular T-joint taking welding residual stress into account

An efficient numerical analysis procedure based on FEM was implemented for evaluating the effect of welding residual stress (WRS) on mixed-mode stress intensity factors (MM-SIFs) for a non-planar surface-cracked tubular T-joint with a rounded weld toe. A multi-pass welding simulation was carried out using real welding conditions by means of an effective in-house welding simulation code. A fully compatible 3D finite element mesh generation system was implemented to model non-planar perpendicular surface-cracked tubular T-joints with rounded weld toes. The adequacy of the mesh generation system was verified with the reference solutions. A fully automated mapping code was developed to assign the six components of the simulated WRS to each element face over the crack surface. A developed fracture approach was employed to accurately compute MM-SIFs resulting from WRS in which normal and shear components of WRS were considered. MM-SIFs were calculated carefully for several surface cracks with different shapes under different loading conditions. The behavior of MM-SIF solutions resulting from the combination of external load and WRS was influenced significantly by the behavior of those produced due to WRS only. In addition, the value of the computed SIFs resulting from the combination of external load and WRS was almost doubled for mode-I SIF compared to those obtained by external load only and WRS only especially near the crack end locations. The behavior of the calculated fracture parameters was also discussed.

Effect of geometric parameters and combined loading on stress distribution of tubular T-joints

Steel tubular structures are widely used in the construction of offshore platforms and T-type junctions are extensively used in this domain. The tubular members are welded, which generates significant stress concentrations at the edges. The stress levels reached in these critical places are used to assess lifetimes based on fatigue curves from tests conducted on standard samples. This study is devoted to the modeling and analysis of T-type welded tubular structures for the determination of hot spots stresses (HSS) at the chord/brace intersection, A numerical analysis was carried out to study the effect of a combined loading composed of an axial loading and a continuation of rational bending, that best assimilate real conditions, as well as the effect of normalized geometric parameters α, β, g on the distribution of stress concentration (area and values) of T-joints. The mechanical behaviour has been modeled in 2D using quadrangular and triangular thin-shell elements by the finite element method (FEM). It is the most appropriate approach because it considers all geometric complexities and singularities of the structure, while the efforts as well as the computation time are considerably reduced compared to an experimental study or to complex FE models implementing solid elements. In this study, we use the COMSOL-MULTIPHYSICS® software...

Extended Formulation of Stress Concentration Factors for CFST T-Joints

In previous research, the authors numerically investigated 212 finite-element (FE) models of concrete-filled steel tubular (CFST) T-joints under axial force in the brace to derive formulae for stress concentration factors (SCFs). The formulations involve four nondimensional parameters: diameter ratio β, diameter-to-thickness ratio of chord 2γ, thickness ratio τ, and relative chord length α. In the current study, the earlier formulation was extended to include four additional loading conditions: in-plane bending (IPB) in the brace, out-of-plane bending (OPB) in the brace, axial compression in the chord, and IPB in the chord. The validity of the new SCF formulae was demonstrated by comparing the SCFs obtained using the formulae with the results of numerical analysis.

Fire resistance of thin-walled tubular T-joints with internal ring stiffeners under post-earthquake fire

Abstract The objective of this paper is to computationally investigate the fire resistance of earthquake-damaged thin-walled tubular T-joints stiffened with internal rings at elevated temperatures. An earthquake can cause severe damage to the T-joints, thus may lead to the reduction of fire resistance after that. Additionally, given the needs in improving the performance of T-joints using stiffeners, the behaviors and fire performance of these stiffened T-joints under post-earthquake fire (PEF) is of great interests. A computational model, validated with the existing experimental results, is established to conduct the analysis of the fire resistance behavior of the stiffened tubular T-joints under PEF. Then, by using this model, the stiffening strategies for tubular T-joints using 1 internal ring, 2, and 3 internal rings, are evaluated along with the unstiffened case to investigate their effectiveness in improving the fire resistance capability under PEF. Later, a parametric study is performed for the stiffened T-joints with 3 internal rings under PEF using the validated computational model. 24 finite element models are analyzed under different damage levels, by considering the following key geometric parameters: the diameter ratio (β), the wall thickness ratio (γ) of the T-joint's chord and brace members, the diameter-thickness ratio (α) of the chord member, and the thickness ratio η of ring-stiffener and chord along with the damage variable ( D ˜ ) to account for the level of the T-joint being damaged. The results on different stiffening strategies and the parametric study reveal that the significant influence of the damage variable and the sensitivities of other influencing parameters. The findings from this study provide needed evidence for potential design recommendations for post-earthquake fire resistance of stiffened tubular T-joints.

Compressive Behavior of Damaged Tubular T-joints Retrofitted with Collar Plate

The static bearing capacity and failure modes of damaged tubular T-joints retrofitted with collar plates under axial compression were investigated. Two-step loading was applied: First, the chord deformation was loaded to the pre-designed degree, followed by unloading and collar plate installation. Subsequently, the specimens were reloaded. The maximum capacity and corresponding displacement (Δ1,m) were determined, and the ratio of the chord deformation of the other joints to Δ1,m was utilized to define the damage degree. The maximum capacity of retrofitted tubular T-joints could be up to 13–77% in second step loading, compared with that of unreinforced T-joints; however, the capacity decreased by 2–10% compared to that of directly reinforced T-joints. Sixty-nine finite element models were generated. The effects of chord wall thickness, chord diameters and collar plate lengths on the bearing capacity under different damage degrees were analyzed. A satisfactory effect could be obtained by using a suitable size of expanded collar plates under a constant damage degree. Retrofitting with collar plates could mitigate the development of equivalent plastic strain in the joint intersection, even for considerably damaged tubular T-joints. A modified formula considering the damage degree was proposed for bearing capacity prediction of retrofitted tubular T-joints.

Compression capacities of cold-formed high strength steel tubular T-joints

The paper presents a test programme on cold-formed high strength steel tubular T-joints with fully supported chord members. The T-joints consist of brace members made up of square, rectangular and circular hollow sections, while chord members were made up of square and rectangular hollow sections. The T-joints were fabricated by both brace and chord members made up of square and rectangular hollow sections as well as circular hollow section braces were welded to square and rectangular hollow section chord members. The brace and chord members were welded by semi-automatic gas metal arc welding process. The steel grade of 900 MPa was used for the square, rectangular and circular tubular members, while the steel grade of 960 MPa was used for the square and rectangular tubular members only. A total of 22 tests was conducted where an axial compression was applied through the brace members, while the chord members were resting on a solid flat base. The ratio of brace-to-chord width (β) ranged from 0.33 to 1, brace-to-chord thickness (τ) from 0.53 to 1.27, chord width-to-thickness (2γ) from 20.6 to 38.7, and chord side wall slenderness (h0/t0) from 12.7 to 38.6. The test results were compared with the nominal strengths obtained from the Eurocode 3 (EC3) and CIDECT for the assessment of the existing design rules for tubular T-joints with fully supported chord members. It is found that the existing design rules are not applicable for the design of cold-formed high strength steel tubular T-joints with fully supported chord members.