Tubular X-joint Research Articles

Influence of brace-to-chord angle on the seismic behavior of circular hollow section X-joints under in-plane bending

This paper studied the influence of brace-to-chord angle (BCA) on the in-plane flexural behavior of circular hollow section (CHS) tubular X-joints. The study begins with three CHS tubular X-joint specimens (one orthogonal joint with BCA of 90°, two skew joints with BCA of 70° and 55° respectively) under cyclic in-plane bending moment (IPBM). The test results showed that specimens mainly failed in tearing of the chord wall near the brace/chord intersection after experienced large plastic development, and the crack propagation rate of the chord wall of the skew specimens under negative IPBM is obviously faster than that under positive IPBM, while the propagation rates of the orthogonal specimen under two IPBM is similar. The ductility, strength and energy dissipation of the X-joints are deeply affected by BCA. Decreasing BCA is beneficial to the ductility ratio under positive IPBM, but it is not conducive to the ductility ratio under negative IPBM; decreasing BCA can increasing the flexural strength; both the skew joint with large BCA and the orthogonal joint exhibit higher energy dissipation behavior than that of the skew joint with small BCA. These observations is further confirmed by a simple load transferring model proposed in this study. Finite element (FE) parametric analysis is then carried out to verify test and the load transferring mechanism analysis results. Moreover, FE and test results showed that the negative strength Miu− (under negative IPBM) and the positive strength Miu+ (under positive IPBM) of the orthogonal joints are close, but Miu− of the skew joints are larger than the counterpart of Miu+, and the ratio of Miu− /Miu+ is increased (from slightly more than 1.0 to above 1.2) as BCA decreased (from 70° to 35°).

Experimental and numerical analysis of a novel tubular joint for transmission tower

A novel type of tubular X-joint is evaluated in this study which combines both flange connection and welded connection for practical steel tube transmission tower. Full-scale experiments are carried out to investigate the monotonic behavior of the joints under both axial tensile and compressive load. The failure modes, load-displacement curves, load-strain curves, deformation development and ultimate strength are studied and discussed. Non-linear finite element analysis of the proposed tubular X-joint not only captures the discernible behavior observed during the tests to verify the experimental results, but also help study stress distribution that cannot be directly measured during the tests. Both experimental and numerical results show that the novel type of joints have adequate bearing capacity with good safety margin and the weak parts are mainly concentrated on the vicinity of intersection regions. Finally, several different design codes are used to calculate the ultimate strength of the proposed joints under investigation. The difference between experimental results and current design methods can be attributed to the fact that the connection adopted in the joints is different from the conventional welded tubular X-joint in codes.

New proposal to express notch stress approach results by equivalent SCFs

For the fatigue design of welded structures, such as jacket substructures for offshore wind energy turbines, the structural stress approach is state of the art. Standards and guidelines also allow the application of the notch stress approach for complex welded components, weld roots, or to consider the advantages of the real weld geometry if measured or after grinding within the fatigue design. Since the theoretical background as well as the application of both approaches differs widely, it is not possible to directly integrate the fatigue notch factor Kf according to the notch stress approach into the design formalism of the welded structures based on the structural stress approach. Therefore, this paper derives a methodology to transform the elastic fatigue notch factor Kf into an equivalent stress concentration factor. By applying the introduced concept design engineers are able to selectively combine the advantages of both approaches without adapting the approved software tools to perform the fatigue design of complete welded structures.Furthermore, the presented concept of the equivalent stress concentration factor is an effective tool to perform a direct comparison of the relative fatigue resistance according to both approaches – structural stress approach and notch stress approach – on structural stress concentration level rather than on the level of damages or lifetime. This opportunity of comparison is applied on a tubular X-joint of a jacket substructure. The computed stress concentration factors are in good accordance between both approaches which is similar to the lifetime based results given in literature for application of both approaches on tubular K- and Y-joints.

Simulation of fracture of a tubular X-joint using a shear-modified Gurson–Tvergaard–Needleman model

The full-range behaviour of materials and connections is important in the research of progressive collapse resisting mechanisms and structural seismic behaviour. There have been several recent studies on the fracture simulation of tubular joints. However, the reliability of the fracture models adopted for the tubular members, as well as for the weld materials, has not been examined under various stress states. This paper firstly describes an experimental study on square hollow section X-joint under axial loading. The joint is well tested into the post-ultimate range, and fracture occurs through the thickness of the chord. For the tubular sections and welds used in the joint, three types of coupon tests are carried out for each material, based on which a standard procedure for calibrating the parameters of the Gurson–Tvergaard–Needleman (GTN) model is proposed. Finite-element simulations of the joint test using the calibrated GTN model are then performed. The results show that the original GTN model can predict the initiation of cracks in the X-joint, but fails to predict the fracture propagation, under a shear-dominated stress state, with sufficient accuracy. A shear-modified GTN (referred to as SMGTN) model is developed by combining the GTN model with a maximum shear stress criterion, and shows better performance than the original GTN model, according to the simulated results, in predicting the entire fracture process.

Failure Mechanism Analysis of Tubular X-Joint under Lateral Impact Force

To study the failure mechanism of tubular X-joint under lateral impact force, the nonlinear finite element method is adopted to simulate theoretical analysis model of tubular X-joint. Before analysis, the precision of finite element analysis (FEA) model is verified based on experimental data firstly. On the base of analyzing mechanism for tubular X-joint under lateral impact loading, the development law of deformation and stress is clarified, and energy transform pattern and the varying rules of impact force-deformation curve is analyzed.

Brittle failure caused by lamellar splitting in a large-scale tubular joint with fatigue cracks

This paper reports a new incidence of brittle failure by lamellar splitting in a large-scale tubular X-joint and examines the possible causes of this failure. The X-joint, with multiple pre-existing fatigue cracks at the weld toe along the brace-to-chord intersection, experiences brittle failure during a monotonic in-plane bending test. Post-test sectioning of the material around the brace-to-chord intersection reveals lamellar splitting in the mid-thickness of the chord wall instead of rapid extensions of the fatigue cracks in the through-thickness direction. The lamellar splitting observed in this test differs from the conventional lamellar tearing both in its appearance and in its causes. The elongated nonmetallic inclusions concentrated at the mid-thickness, as revealed by the microscopic scanning, leads to delamination cracking near the mid-thickness and subsequently to the brittle failure of the joint. The material requirements in prevailing engineering codes do not suffice to prohibit the lamellar splitting failure observed in this study, which may cause catastrophic failures in engineering structures designed with insufficient redundancy.

Welder-optimized CJP-equivalency welds for tubular connections

This paper will present the experimental and numerical investigation on the fatigue crack failure at the root of the enhanced partial joint penetration welds in the circular hollow section X-joint specimens subjected to constant-amplitude brace in-plane bending actions. The enhanced partial joint penetration welds, which include a portion of the brace wall as the inherent “back-plate” for the welds, reduce the workmanship requirement on the welding procedure and improve the quality control of the welded joint. This study compares the fatigue crack developments observed at both the weld toe and the weld root in three large-scale X-joint specimens with different post-weld surface treatments. Root cracking does not take place in as-welded tubular X-joint specimens, of which the fatigue life depends on the weld toe failure. In specimens with post-weld grinding treatments to enhance fatigue resistance of the material near the weld toe, however, the finite-length root face triggers the initiation and propagation of fatigue cracks with an applied number of cycles more than that required to initiate a toe crack in the same tubular X-joint. Ultimate strength testing of specimens with large fatigue cracks, and post-test sectioning, reveals that fatigue failure weld-toe fatigue cracks remain more critical than the root cracks.

Tests and behaviour of cold-formed stainless steel tubular X-joints

The paper presents a series of tests on cold-formed stainless steel tubular X-joints. The tubular X-joint specimens were tested without chord preload as well as with three different levels of preload applied to the chord members. The test specimens were fabricated from square and rectangular hollow sections as brace and chord members. A total of 32 tests was performed. High strength stainless steel (duplex and high strength austenitic) and normal strength stainless steel (AISI 304) specimens were tested. The test results were compared with the design strengths obtained using the CIDECT Guide and Eurocode for carbon steel structures. It is shown that the design strengths predicted by the current design specifications are very conservative for the test specimens calculated using the 0.1%, 0.2%, 0.5% and 1.0% proof stresses as the yield stresses. The 0.2% proof stress is comparatively more reasonable to predict the design strengths of stainless steel tubular X-joints for both ultimate limit state and serviceability limit state.

Analysis and calculation of axial stiffness of tubular X-joints under compression on braces

This paper introduces the influence factors of axial stiffness of tubular X-joints. The analysis model of tubular joints using plate and shell finite element method is also made. Systematic single-parameter analysis of tubular X-joints is performed using Ansys program. The influences of those factors, including ratio of brace diameter to chord diameter (β), ratio of chord diameter to twice chord thickness (γ), ratio of brace wall thickness to that of chord (τ), brace-to-chord intersection angle (θ), and chord stress ratio, ratio of another brace diameter to chord diameter, in-plane and out-of-plane moment of braces, etc., on stiffness of tubular X-joints are analyzed. Two non-dimensional parameters—joint axial stiffness factor η N and axial force capacity factor ω N are proposed, and the relationship curve of the two factors is determined. Computational formulas of tubular X-joint axial stiffness are obtained by multi-element regression technology. The formulas can be used in design and analysis of steel tubular structures.

Finite element analysis of ultimate load for tubular X-joints subjected to axial compression loads

The ultimate load of tubular X-joints subjected to axial loads is analyzed using finite element method.The effect of the weld size on the ultimate load is considered in the nu-merical modelling for tubular X-joints.In finite element analy-sis,sub-zone mesh generation method is used to produce the mesh of a tubular X-joint.The entire tubular X-joint is divided into several different zones according to the requirement of the computation accuracy.The mesh of each zone is generated separately,and the finite element mesh of the entire structure is obtained by merging the meshes of all the sub-zones.This sub-zone mesh generation method can produce meshes of dif-ferent qualities according to different stress gradients.Based on this method,the relationship between the applied loads and the displacement of tubular X-joints subjected to axial compression loads is analyzed using ABAQUS(2000) software,and the ultimate load of X-joints is then obtained.Thereafter,through the analysis of 50 models of tubular X-joints,the effects of geometrical parameters and material parameters on the ultimate load of tubular X-joints are investigated.

Elastic–plastic crack driving force for tubular X-joints with mismatched welds

This study describes the elastic–plastic driving force in surface cracks located at weld toes near the saddle point of circular hollow section X-joints, with strength mismatch between the chord material and welds. The remote loading at the brace end imposes displacements acting along the brace axis. The 3-D finite element models couple a global, topologically continuous mesh and a separate, local crack-front model through mesh-tieing. The numerical solver computes the elastic–plastic crack driving force ( J -value) through a domain-integral approach. Comparisons of the elastic–plastic J -values evaluated from a continuous model and from a mesh-tied model for a simple plate configuration, which represents key features of the brace–chord intersection near the saddle point, verify the accuracy of J -values computed from mesh-tied models containing both homogeneous and mismatched material properties. The numerical analyses employ stress–strain curves for representative high-strength steels now used in offshore construction. The yield strength of the welds follows σ y w = m σ y c , where m denotes the mismatch ratio and σ y c is the chord yield stress. The strain hardening property of the welds remains the same as that of the chord material. Unlike historical research on weld mismatch effects for simple fracture specimens, the surface crack in the tubular X-joint resides in the base metal (chord) adjacent to the weld toe rather than in the welds. The computed J -values demonstrate that the crack driving force increases with increased weld strength — and thus a higher potential for initiation of ductile tearing. The numerical results show that a large elastic–plastic crack driving force exists for joints with a high chord radius to wall thickness ratio ( γ ) or with a small brace to chord diameter ratio ( β ) . For joints with β > 0.8 , the model with uniform material properties ( σ y w = σ y b = σ y c ) exhibits the largest crack driving force among the different mismatch ratios.

Optimization of Stiffening Rings' Position in Welded Tubular X-Joint

Summary Experimental results are presented to show that the position of the stiffening rings in welded tubular X-joints has a great impact on the value of the stress concentration factor (SCF) at the hot spot as well as its location. Preliminary guidelines for an optimal ring position are proposed. proposed. Introduction In offshore structures, such as semisubmersible drilling vessels and fixed platforms, tubular members comprise the main load-carrying components. The platforms, tubular members comprise the main load-carrying components. The design of joints between these tubular members is complicated because the radial flexibility of the tube walls leads to significant stress concentrations. Since many of these structures are subjected to alternating loads, the presence of stress concentrations can lead to an early fatigue failure. Ring stiffeners often are used as a means of reducing the SCF and thus increasing the fatigue life. This has been confirmed by the results of Maeda et al. and Matoba et al. The effect of the position of ring stiffeners on the maximum SCF value and, hence, the fatigue life is, to my knowledge, not available in the open literature. This paper represents a preliminary study of the effect of positioning of both internal and external ring stiffeners on the value and positioning of both internal and external ring stiffeners on the value and the location of the maximum SCF. It should be recognized that this paper is not intended to give a generalized recommendation for the optimal ring position but rather to alert designers and fabricators to the danger of position but rather to alert designers and fabricators to the danger of unoptimized position. Poor positioning of the rings can result in very little improvement in the state of stress as well as the expense and drawbacks associated with welding. Experimental Details The experiments were conducted on welded tubular X-joints with an angle of intersection equal to 900 (this joint also is classified as double-T joint). JPT p. 328