Tubular T-joints Research Articles

Experimental investigation of cold-formed stainless steel tubular T-joints with loaded chord

This study focuses on the cold-formed stainless steel square hollow section (SHS) tubular T-joints prone to failure by chord sidewall yielding, crushing or instability under axial compression in brace member. An experimental investigation was conducted to understand the joint behaviour subjected to chord axial loading during the brace compression. The dimensions of the tubular sections were 80x80x2mm and 40x40x2mm, corresponding to 2γ values of 20 and 40, respectively. Based on the obtained experimental results, it was observed that the T-joint resistances decrease when subjected to chord compressive load, especially in the cases with elevated values for the chord sidewall slenderness (2γ). On the other hand, if a chord tension force is considered, an increase in the T-joint resistance is observed. With the results obtained from the tests, the design standards EN 1993-1-8 in association with EN 1993-1-4, NBR 16239, CIDECT, ISO 14346 and AISC 360-22 were assessed, as well as the predictions proposed by other authors from the literature. Finally, it was found that design codes are very conservative, except for AISC 360-22 [26], which provided unsafe predictions. On the other hand, equations proposed by other authors from the literature presented better results.

Empirical modeling of stress concentration factors using artificial neural networks for fatigue design of tubular T-joint under in-plane and out-of-Plane bending moments

PurposeStress concentration factors (SCFs) are commonly used to assess the fatigue life of tubular T-joints in offshore structures. SCFs are usually estimated from parametric equations derived from experimental data and finite element analysis (FEA). However, these equations provide the SCF at the crown and saddle points of tubular T-joints only, while peak SCF might occur anywhere along the brace. Using the SCF at the crown and saddle can lead to inaccurate hotspot stress and fatigue life estimates. There are no equations available for calculating the SCF along the T-joint's brace axis under in-plane and out-of-plane bending moments.Design/methodology/approachIn this work, parametric equations for estimating SCFs are developed based on the training weights and biases of an artificial neural network (ANN), as ANNs are capable of representing complex correlations. 1,250 finite element simulations for tubular T-joints with varying dimensions subjected to in-plane bending moments and out-of-plane bending moments were conducted to obtain the corresponding SCFs for training the ANN.FindingsThe ANN was subsequently used to obtain equations to calculate the SCFs based on dimensionless parameters (α, β, γ and τ). The equations can predict the SCF around the T-joint's brace axis with an error of less than 8% and a root mean square error (RMSE) of less than 0.05.Originality/valueAccurate SCF estimation for determining the fatigue life of offshore structures reduces the risks associated with fatigue failure while ensuring their durability and dependability. The current study provides a systematic approach for calculating the stress distribution at the weld toe and SCF in T-joints using FEA and ANN, as ANNs are better at approximating complex phenomena than typical data fitting techniques. Having a database of parametric equations enables fast estimation of SCFs, as opposed to costly testing and time-consuming FEA.

An artificial neural network model for determining stress concentration factors for fatigue design of tubular T-joint under compressive loads

PurposeThe stress concentration factor (SCF) is commonly utilized to assess the fatigue life of a tubular T-joint in offshore structures. Parametric equations derived from experimental testing and finite element analysis (FEA) are utilized to estimate the SCF efficiently. The mathematical equations provide the SCF at the crown and saddle of tubular T-joints for various load scenarios. Offshore structures are subjected to a wide range of stresses from all directions, and the hotspot stress might occur anywhere along the brace. It is critical to incorporate stress distribution since using the single-point SCF equation can lead to inaccurate hotspot stress and fatigue life estimates. As far as we know, there are no equations available to determine the SCF around the axis of the brace.Design/methodology/approachA mathematical model based on the training weights and biases of artificial neural networks (ANNs) is presented to predict SCF. 625 FEA simulations were conducted to obtain SCF data to train the ANN.FindingsUsing real data, this ANN was used to create mathematical formulas for determining the SCF. The equations can calculate the SCF with a percentage error of less than 6%.Practical implicationsEngineers in practice can use the equations to compute the hotspot stress precisely and rapidly, thereby minimizing risks linked to fatigue failure of offshore structures and assuring their longevity and reliability. Our research contributes to enhancing the safety and reliability of offshore structures by facilitating more precise assessments of stress distribution.Originality/valuePrecisely determining the SCF for the fatigue life of offshore structures reduces the potential hazards associated with fatigue failure, thereby guaranteeing their longevity and reliability. The present study offers a systematic approach for using FEA and ANN to calculate the stress distribution along the weld toe and the SCF in T-joints since ANNs are better at approximating complex phenomena than standard data fitting techniques. Once a database of parametric equations is available, it can be used to rapidly approximate the SCF, unlike experimentation, which is costly and FEA, which is time consuming.

Parametric study and neural network-based prediction for stress concentration factor of concrete-filled steel tubular T-joint

In this study, an extensive parametric investigation for the stress concentration factor (SCF) of concrete-filled steel tubular (CFST) T-joints was conducted based on a validated finite element (FE) model in which the weld profile, mesh, and contact simulation were specifically and precisely dealt with. Distribution of the SCF on the intersecting line was depicted. Effect of non-dimensional geometric parameters and the elastic modulus of concrete was discussed in detail. Results show that the SCF is monotonically distributed on the intersecting line in the chord side, and the maximum SCF probably located at the crown position. There is the possibility that the maximum SCF of the brace occurs at a position neither the crown nor the saddle. Reducing the length of chord or promoting the elastic modulus of concrete lowers the SCF at most positions, but has no effect on the SCF at chord saddle. A back-propagation neural network (BPNN) model was established, and was trained based on 724 FE results of the SCF from the parametric study. The trained BPNN model predicts the SCF of CFST T-joints with a low error of 14.5%, and it improves the accuracy by 62.0% compared to the current formulae.

Influence of Loading Modes on Stress Concentration Locations in Tubular T-Joints

Influence of Loading Modes on Stress Concentration Locations in Tubular T-Joints

Characterization of fatigue crack growth behavior in welded tubular T-joint

This paper focuses on investigating the fatigue crack growth (FCG) characteristics and residual fatigue life of tubular T-joints which are prone to suffer fatigue damage at the brace and chord intersections under multi-axial stress. Static and fatigue loading tests are performed to investigate the FCG behaviors of tubular T-joints, combining beach marking technique. The evolution of FCG characteristics is studied during the crack growth, convergence and wall penetration through an efficient co-simulation system that established using the multi-scale modeling technique. Fracture morphology analysis is conducted to gain insight into the FCG behaviors combining the scanning electron microscope (SEM) observation. Results indicate that multiple cracks initiate at the crown regions and subsequently evolve circumferentially around the weldments in a doubly-curved shape towards wall-thickness. Interaction effects between adjacent cracks extend the fatigue life along the outer surface, attributed to the premature exposure of converged crack front and the induced variation of stress intensify factor (SIF) distribution along the crack front. The dominant failure mode of tubular T-joints is characterized by an opening mode crack, with the contribution of anti-plane shear mode crack gradually increasing as structural symmetry diminishes. The established co-simulation system shows advantage in capturing the FCG behavior, predicting the fatigue life and characterizing the FCG characteristics with a good balancing of simulation efficiency and calculation accuracy.

Structural Behaviour of FRP-Reinforced Tubular T-Joint Subjected to Combined In-Plane Bending and Axial Load

In this study, 90 finite-element models are used to explore the behaviour of fibre-reinforced polymer (FRP) reinforced joints under combined in-plane bending (IPB) and axial load (AX). The effects of joint geometry, FRP layer count, and AX levels of the chord or brace are considered. Three typical failure modes are observed: chord plastic failure, brace plastic failure, and brace buckling failure. Increasing the number of FRP layers can ensure that failure is chord-related failure in a ductility manner rather than the unexpectedly brace-related brittle failure. Depending on the stress distribution of fibres, FRP reinforcement can restrict the deformation of joints subjected to complex loading patterns. Moreover, added FRP layers efficiently reduce the effect of brace AX on the IPB resistance. Finally, a modified strength equation is established, including the influence of FRP reinforcement, chord AX, and brace AX.

Experimental and theoretical investigation on residual bearing capacity of CFDST-T joints after lateral impact

This study investigates the residual performance of concrete-filled double-skin steel tubular (CFDST) T-joints after lateral impact. One hollow T-joint and five CFDST-T joints with various steel tube thickness and hollow rate were first tested by a pendulum machine. The axial compression test was subsequently performed to explore the residual performance in terms of failure mode, residual bearing capacity, ductility, energy dissipation, and strain evolution. Results indicated that the failure modes of the CFDST-T joint and hollow T-joint were characterized by global bending and severe local indentation, respectively. Besides, the CFDST-T joints exhibited the strongest residual bearing capacity with a hollow rate of 0.52 and the greatest ductility of 0.63. Moreover, increasing the outer tube thickness improved the residual bearing capacity and simultaneously reduced the ductility coefficient while the inner tube exerted little effect on residual performance. Simplified calculation formulas were finally derived to predict the impact residual deflection and residual bearing capacity of the CFDST-T joint based on parametric analysis. This study will provide a reference for future research to contribute to the design of more reasonable CFDST-T joints and promote their application.

Numerical analysis of SCF of Tubular T-joints strengthened with prestressed CFRP

Fatigue failure is one of the main concerns of welded tubular joints owing to inevitable welding defects and severe stress concentration along the intersection. In addition, the complex geometry of the joint makes it inconvenient to reinforce by traditional repair methods. Carbon fiber-reinforced polymer (CFRP) composite has been widely used for structural strengthening due to superior strength-to-weight ratio, good fatigue and corrosion resistance, as well as ease of installation. Although tubular joints strengthened by using non-prestressed CFRP sheets have been investigated in references, application of prestressed CFRP has not been reported. This study proposed a form of prestressed CFRP sheets for the fatigue strengthening of tubular T-joints. Three-dimensional finite element models were developed for both CFRP-strengthened and unstrengthened tubular T-joints to calculate the hot spot stress distribution, which was verified using experimental test data and Lloyd’s Register (LR) equations. A parametric study was then carried out to explore effects of CFRP strengthening parameters on the SCF reduction coefficient ψ, including the prestressing level (p), CFRP width (w), and CFRP layers (n). It was found that prestressing CFRP changed the stress distribution and values along the weld toe. With the increase of prestress, the position of the maximum stress gradually moved from the saddle point to the crown point. Besides, the strengthening parameters had significant effects on reduction of the hot spot stress along the weld toe.

Finite Element Analysis of Corrosion Tubular T-Joint Repaired with Grouted Clamp

A finite element (FE) analysis of corroded circular hollow section T-joints repaired using grouted clamps is presented in this study. To ensure an accurate simulation, experiments on a T-joint with uniform corrosion and an intact T-joint with grouted clamp are reproduced separately using an FE model. The experimental and FE modeling results correlate closely, displaying similar failure behaviors and load–displacement responses. Subsequently, a total of 56 FE T-joint models with varying degrees of corrosion on the chord, repaired by grouted clamp, were then analyzed numerically. The corrosion was artificially applied to the chord’s outer surface at depths of 10%, 20%, and 30% of the chord’s thickness. The models also account for variations in joint geometry, dictated by parameters β and γ, which range from 0.565 to 0.678 and from 21 to 28, respectively. Furthermore, grouted clamp’s properties were examined, including sleeve length, thickness, strength, as well as the thickness and strength of the grout, in relation to the complexity of the repair. The grouted clamp demonstrated significant repair capability, increasing the ultimate strength of the corroded joint by up to 2.23 times. Reinforcements that are both thicker and longer substantially enhance the joint’s ultimate strength. However, inappropriate repair construction results in an abrupt termination of the load–displacement curve and a brittle failure phenomenon outside reinforced chord region. Additional weight from the grouted clamp requires reasonable control, and sleeve overlapping ratio should be guaranteed to be greater than 0.7. The joint bearing capacity can be efficiently increased by thicker grouts and sleeves only in ductile failure cases. The confinement effect and mechanism of the grouted clamp in joint deformation were visually analyzed as stress distribution from FE analysis results. Finally, a prediction equation is proposed to estimate the static strength of the repaired joint through regression analysis.

Numerical investigation of ultimate bearing capacity of circular and square concrete-filled double steel tubular T-joints

The punching shear problem in composite T-joints with doubler plates has been evaluated in various design codes of practice. However, code provisions do not address the behavior of concrete-filled double steel tubular T-joints filled with concrete, in which concrete contributes to a portion of the shear force to resist punching. The present study presents a finite element modeling analysis to investigate the ultimate load-bearing capacity of circular and square concrete-filled double steel tubular T-joints. A set of 24 double-skin concentric tubular joint specimens were modeled in ABAQUS. A subsequent parametric study was carried out to examine the influence of infill concrete compressive strength, steel tube cross section shape, and section width-to-thickness ratio. Accordingly, stress evolution pattern, buckling mode, stress-strain response, and load–displacement curve of the entire T-joints were investigated. Based on the results obtained, it was revealed that recession in the flange of the primary component, as well as the heaving of the primary component web, are the main reasons for the higher stress levels. It was also seen that when the compressive strength of the infill concrete is 45 MPa, the maximum stress level in circular concrete-filled double steel tubular joint is 2.6 × 102 MPa, while that in the square joint is 2.566 × 102 MPa. The corresponding values for concrete with a compressive strength of 30 MPa are 2.58 × 102 MPa and 0.455 × 102 MPa, respectively. It was also observed that when the width-to-thickness ratio is increased from 20 to 33.33 in circular T-joints, the ultimate load-bearing capacity is reduced by 15%.

Experimental and Numerical Studies on the Stiffening of Tubular T-joint of Offshore Jacket Structures

Experimental and Numerical Studies on the Stiffening of Tubular T-joint of Offshore Jacket Structures

Corrosion effect on stress concentration factor in tubular T-joints under axial loading

A stress concentration factor (SCF) is used to determine concentrated stress at hotspots for fatigue evaluation of steel fixed offshore platforms. The varying SCF significantly affects the fatigue damage of the hotspot according to the exponential function. Current standards have proposed empirical formulas to determine the SCF at the hotspots of the tubular joints, but it depends on the geometry and some specific limitations. When a tubular joint is corroded, its geometry will be changed, making these limitations unsuitable, especially for the case of non-uniform corrosion. Therefore the formulas are no longer suitable to apply in these cases. This paper uses ABAQUS-based numerical simulation to analyze concentrated stress at saddle and crown points on the chord of non-uniform corroded tubular T-joints under axial loading in some different scenarios. The research indicates that thickness and dimension of corroded zone around the joint intersection substantially influence stress concentration factor and fatigue damage at the mentioned hotspots.

Numerical Study on Ultimate Strength of Non-uniform Corroded Tubular T-joints under Compression

Numerical Study on Ultimate Strength of Non-uniform Corroded Tubular T-joints under Compression

Structural reliability assessment of offshore tubular t-joints under static axial brace loading

This paper presents a comprehensive study on the structural reliability assessment of tubular T-joints under axial brace loading in offshore structures. The investigation combines numerical modeling and experimental analysis to understand the behavior of these joints and assess their structural integrity. Based on Hot sport stresses in the joints. The numerical investigation reveals that the brace region experiences higher stress concentrations compared to the cord region, with maximum stress concentrations observed at the 90-degree position. Experimental results confirm these trends, although slight discrepancies exist between numerical and experimental stress concentration factors (SCFs), with percentage difference of about 12 percent on the average for both Chord and Brace. The likely cause of this discrepancies is due to Fabrication imperfections and limitation in terms of Mesh size on the part of the software. The findings contribute to the knowledge of tubular T-joint behavior and have implications for designing mitigation measures. Further research is needed to analyze additional parameters and refine design guidelines to enhance the performance of tubular T-joints in offshore structures.

Design of Cold-Formed High-Strength Steel Diamond Bird-Beak Tubular T-Joints and X-Joints

Numerical investigation and design of cold-formed high-strength steel (CFHSS) diamond bird-beak (DBB) T- and X-joints are presented in this paper. The 0.2% proof stress of braces and chords was 960 MPa. Tests of CFHSS DBB T- and X-joints undergoing compression loads were carried out by the authors. The test results were used to develop accurate finite element (FE) models in this study. A comprehensive FE parametric study was then performed using the verified FE models. The nominal strengths predicted from the literature and European code were compared to the joint failure strengths and ultimate capacities of 244 DBB T- and X-joints specimens, including 224 FE specimens investigated in this work. The failure of DBB T- and X-joints specimens at chord crown locations was identified as the dominant failure mode. It has been shown that the design provisions given in the literature and European code are unsuitable and uneconomical for the cold-formed S960 steel grade DBB T- and X-joints investigated in this study. Hence, accurate, less dispersed, and reliable design equations are proposed in this work, using two design approaches, to predict the joint failure strengths and ultimate capacities of the investigated DBB T- and X-joints.

Behaviour and design of cold-formed high strength steel RHS T-joints undergoing compression loads at elevated temperatures

This study presents a comprehensive numerical program aiming to investigate the static resistances of cold-formed high strength steel (CFHSS) T-joints with square and rectangular hollow section (SHS and RHS) braces and chords at elevated temperatures. The included angle between brace and chord members was 90°. The static behaviour of simply supported SHS and RHS T-joints undergoing compression loads through brace members was investigated at four elevated temperatures, including 400°C, 500°C, 600°C and 1000°C. The mechanical properties at elevated temperatures given in the literature for cold-formed S900 steel grade tubular members were used in this study. The numerical investigation was performed using the finite element (FE) models developed and validated by Pandey et al. [1] and Pandey and Young [2] for cold-formed S960 steel grade T-joints at ambient temperature and post-fire conditions. In total, 756 FE T-joint specimens were analysed in this numerical study, including 189 FE T-joint specimens for each elevated temperature. The tubular T-joint specimens were failed by chord face failure, chord side wall failure and a combination of these two failure modes. The resistances of investigated T-joints at elevated temperatures were compared with the nominal resistances predicted from design rules given in European code and CIDECT using the mechanical properties at elevated temperatures. Overall, it is shown that the current design rules given in European code and CIDECT are uneconomical and unreliable. As a result, economical and reliable design rules are proposed in this study through two design approaches for predicting the resistances of cold-formed steel SHS and RHS 90° T-joints of S900 grade at elevated temperatures.

New proposal for the bending moment capacity of tubular T-joints in carbon steel and stainless steel structures

The expected failure mode of T-joints between rectangular hollow sections subjected to bending moment with a ratio of the brace width (b1) over the chord width (b0) below 0.85 (i.e., β=b1/b0≤0.85) is the chord face failure, produced by yielding. This paper presents the development of a numerical model to reproduce this phenomenon, calibrated against experimental laboratory tests, which is then used to carry out a large parametric study, where the influence of the main geometric parameters involved in the joint resistance –the chord width (b0), the chord thickness (t0), the chord radius (R0), the brace width (b1), the brace height (h1) and the weld thickness (a)– is assessed for two different materials: S275 carbon steel and EN 1.4301 austenitic stainless steel. The results derived from the parametric study suggested that a different yield line mechanism may develop for higher β-values (β≥0.45) than that considered in current European and American codes. Hence, a new formulation for estimating the bending resistance of T-joints exhibiting chord face failure modes is presented based on a different yield line mechanism developed in this paper. This formulation includes additional geometric parameters that are dismissed in the current formulation, such as the chord radius and the weld thickness, which are found to have a non-negligible influence on the joint resistance. Moreover, the proposed formulation is adjusted in order to be used for joints with lower β-values without losing accuracy. Additionally, the new proposed formulation is adapted for stainless steel joints by combining it with the Continuous Strength Method (CSM) formulation with promising results. Finally, the soundness and safety of the proposed formulation is statistically demonstrated.

Experimental investigation on cold-formed steel elliptical hollow section T-joints

A test campaign on cold-formed steel elliptical hollow section T-joints with the chord simply supported and the brace axially compressed is described in this paper. Four different sizes of elliptical hollow sections with the section aspect ratio ranged from 1.65 to 3 were employed to fabricate the T-joint specimens. Thirty-one T-joint specimens with three different types of member orientations were considered. Robotic gas metal arc welding was adopted to fabricate the elliptical hollow section T-joint specimens. The width ratio between brace and chord (β) ranged from 0.29 to 1. The brace-to-chord thickness ratio (τ) ranged from 0.60 to 1.77 and the chord width-to-thickness ratio (2γ) ranged from 10.6 to 52.3. All the T-joint specimens were tested with the chord simply supported and the brace axially compressed. The load-carrying capacities, load-deformation responses and failure modes of the elliptical hollow section T-joints are presented and discussed. Since no codified design provision is available for elliptical hollow section T-joints investigated herein, the feasibility of the design provisions originally developed for tubular T-joints with conventional cross-sections (i.e. circular hollow section and rectangular hollow section) as specified in the CIDECT and Eurocode 3 as well as the design recommendations proposed for hot-rolled steel elliptical joints was examined. Comparison results indicate that the joint strength predictions via these design methods are quite conservative and scattered for cold-formed steel elliptical hollow section T-joints.

Experimental and numerical study on the dynamic behavior of concrete-filled double steel tubular T-joint under impact loading

Experimental and numerical study on the dynamic behavior of concrete-filled double steel tubular T-joint under impact loading