Welded Hollow Spherical Joints Research Articles

Rapid evaluation method for compressive performance degradation of welded hollow spherical joints based on random corrosion distribution

The use of welded hollow spherical joints (WHSJs) is widespread in spatial structures. However, steel is susceptible to corrosion during operation, which can lead to a decrease in the mechanical performance of the joints and ultimately affect the overall safety of the structure. This study focuses on assessing the compressive performance degradation of WHSJs after corrosion. A sophisticated finite element model of corroded joints was developed using the life and death element method. System parameter analysis and sensitivity analysis were conducted, utilizing parameters such as the diameter-to-thickness ratio, axial corrosion ratio, circular corrosion ratio, corrosion depth, envelope ratio, and sphere diameter ratio. Considering the factors mentioned above, a grid-based method was proposed to rapidly evaluate the compressive bearing capacity degradation of corroded joints: When the corrosion area has an irregular and randomly distributed shape, this method can quickly assess the performance degradation of joints by combining "regular corrosion areas that just envelop random corrosion areas" with a "correction factor that considers the degree of random corrosion distribution". This method provides a methodological foundation and technical guidance for evaluating the safety performance of WHSJs and similar joints after corrosion.

Buckling of single-layer reticulated shell structures with corroded welded hollow spherical joints based on multi-scaled model

Buckling of single-layer reticulated shell structures with corroded welded hollow spherical joints based on multi-scaled model

Comprehensive Fatigue Analysis and Practical Design Methodology for Weld Toes in the Sphere of Welded Hollow Spherical Joints in Grid Structures

This paper addresses the issue of fatigue in grid structures, a topic of interest in engineering and academia. The goal is to establish a practical fatigue design calculation method for weld toes in welded hollow spherical joints (WHSJs). The study focuses on commonly used steel tube-WHSJs in grid structures, conducting 25 constant amplitude and four variable amplitude fatigue tests on tube–sphere joints (TSJs) to derive corresponding S-N curves. Using ANSYS, the hot spot stress concentration coefficient Kh at the weld toes in 22 TSJs was calculated, resulting in a numerical solution for Kh ranging from 2.0550 to 4.8600. Based on this, fatigue design methods were established using nominal stress amplitude and hot spot stress amplitude as fundamental parameters. Within a fatigue design reference period of two million cycles, the allowable nominal stress amplitude for TSJs is 22 N/mm2, and the allowable hot spot stress amplitude is 66 N/mm2. The study also conducted macroscopic and microscopic analyses on fatigue fractures of TSJs, revealing that the weld toe in the sphere of TSJs is the primary site for fatigue crack initiation. This research provides practical calculation methods for fatigue design in WHSJ grid structures, contributing to their broader application.

Research on the Tensile Behavior of Stiffened Welded Hollow Spherical Joints Exposed to Fire

Research on the Tensile Behavior of Stiffened Welded Hollow Spherical Joints Exposed to Fire

Experimental investigation of ultimate bearing capacity of corroded welded hollow spherical joints

The basis for analysing and evaluating the safety performance of space structures in service is investigating the effect of corrosion on the residual bearing capacity of welded hollow spherical joints. Thus, accelerated corrosion tests were conducted in this study on five types of welded hollow spherical joints, where the longest corrosion time is 283 d. Subsequently, axial tension, axial compression, and eccentric compression tests were performed after corrosion. The effects of the corrosion time, joint size, steel strength, and eccentric load on the mechanical properties of the corroded joints were then considered. The test results show that the failure mode and strain distribution of the joints are not influenced by corrosion. For the axial force, the ultimate bearing capacity of the corroded joints fluctuates with the corrosion rate (less than 10%); and under the combined action of the bending moment and axial force, the ultimate bearing capacity of the corroded joints decreases with a decrease in the corrosion rate (maximum reduction of approximately 22%). Finally, a simplified calculation method for the ultimate bearing capacity of welded hollow spherical joints after corrosion was proposed. For the atmospheric environment in Qingdao (over a 50 year period), the corrosion coefficient under axial force is 0.87 considering the joint size and comprehensive corrosion influences. Under the combined action of the axial force and bending moment (within 40 mm of the initial eccentricity), the corrosion coefficient is 0.73.

Compressive performance of welded hollow spherical joints with external triangular ribs

This study addresses the axial compressive performance of welded hollow spherical (WHS) joints with external triangular ribs. External triangular ribs are able to postpone the local buckling of the joint during axial loading, and thus strengthen the compressive capacity of it. A simplified analytical model is proposed to elucidate the stress distribution on the hollow sphere of axial-loaded WHS joints with external triangular ribs. Then a simplified solution for the yield strength of axial-loaded WHS joints is deduced based on the elastic thin shell theory. The simplified solution indicates that the yield strength of WHS joints with external triangular ribs can be expressed by a coefficient related to the specifications of the ribs and the joints multiply by that of WHS joints without external ribs. Compression tests on 6 full-scale specimens of WHS joints with varied configurations of triangular ribs are conducted. It shows that the failure mode and the strength enhancement of the specimens vary as the specification of the triangular ribs change, and the triangular ribs enhance the strength of the joints up to 77 %. A refined FE model is established and calibrated against the findings from the experiments. Parametric studies on the design parameters which may affect the compressive response of the WHS joints with triangular ribs are fully investigated. It is shown that the compressive yield strength of the joint rises with the increase of the number, height and thickness of the triangular ribs, and decreases with the increase of the angle-of-elevation of the triangular ribs. It is also shown that the effect of the ratio of the diameter of the sphere to that of the steel tube on the compressive yield strength of the joint is much lighter than the others, and deemed as negligible. Regression analysis on the experimental and numerical data is carried out to obtain a practical formula for the compressive capacity of WHS joints with external triangular ribs. This formula is incorporated with the previously developed formula for the compressive capacity of WHS joints with external trapezoidal ribs, to form a general formula applicable for WHS joints with either external triangular ribs or external trapezoidal ribs.

Experimental Study on Fatigue Performance of Welded Hollow Spherical Joints Reinforced by CFRP

The risk of fatigue failure of welded hollow spherical joints (WHSJs) under alternating loads increases due to the inherent defects, the disrepair, and the demand for tonnage upgrades, of suspension cranes. The finite element analysis results revealed that the ranking of the stress concentration factor at the WHSJ was as follows: weld toe in steel tube of tube–ball connection weld > weld toe in steel tube of tube–endplate connection weld > weld toe in sphere of tube–ball connection weld > weld toe in plate of tube–endplate connection weld. Moreover, the peak stress at the weld of the tube–sphere connection was reduced by 32.93% after CFRP bonding reinforcement, which was beneficial for improving the fatigue performance. In this study, 16 full-scale specimens of Q235B WHSJs were tested by an MTS fatigue testing machine to study the strengthening effect of CFRP on the fatigue performance. It was found that the fatigue fracture of WHSJs was transferred from the tube–sphere connection weld to the tube–endplate connection after CFRP reinforcement. According to the fitted S-N curves, the fatigue strength could be increased by 13.26%–18.19% when the cycle number increased from 10,000 to 5,000,000.

Compression behavior of randomly corroded welded hollow spherical joints reinforced by different methods

The welded hollow spherical joint (WHSJ) is one of the most commonly adopted joint types for spatial grid structures. It belongs to thin-walled structures, which are sensitive to corrosion. Actual projects indicate that aged grid structures are often deteriorated by corrosion. An efficient reinforcing scheme for WHSJs is of shortage for actual engineering application. On the basis of this research background, two reinforcing schemes for randomly corroded WHSJs are proposed in this study. The operational way is also suggested. The reinforcing efficiency is summarized. The presented work is mainly conducted based on a numerical analysis. The reliability of the established FE model is verified through experimental results. The influence of geometrical size on the reinforcement efficiency is then systematically analyzed. The work presented in this study can provide a possible way for the efficient reinforcement of grid structures connected by WHSJs.

Uniaxial failure mechanism and design strength of high-strength welded hollow spherical joint

High-strength large-span reticular shells have proliferated in recent years in engineering projects. Thus, appropriate rules should be developed to ensure the safe and efficient design of high-strength welded hollow spherical joints (WHSJ). However, to date, even the compressive failure mechanism of normal-strength WHSJ has not been fully recognized yet. In this paper, the failure mechanism and load bearing capacity of high-strength WHSJ are experimentally studied with four Q460 steel full-scale specimens under uniaxial loads. Also, three-dimensional refined finite element (FE) models are built, and their numerical results are shown to be in good agreement with experimental data. In addition, a micromechanical ductile fracture model, which can account for the effects of both stress triaxiality and Lode angle, is incorporated in FE models to improve the accuracy of joint failure prediction. Furthermore, focusing on the rib stiffener, the effects of two parameters, including the diameter-to-thickness ratio of the spherical joint and the ratio of spherical joint and chord diameters, on the performance of high-strength WHSJ are comprehensively studied. It was found that the failure of WHSJs subjected to uniaxial tension load is the punching shear rupture due to fully developed plastic strain, while the collapse of the WHSJs subjected to uniaxial compression load is caused by elasto-plastic buckling. Finally, new design formulae for the bearing capacity of high-strength WHSJ are proposed and validated using parametric analysis. It will be shown that these formulae help eliminate the potentialrisk in current specification provisions.

Method for considering corroded joints in buckling analysis of single-layer reticulated shell structures

In this paper, an efficient numerical algorithm for considering the corroded joints is originally proposed. This method is utilized to estimate the buckling factor of single-layer reticulated shell structures (SLRSS) connected by welded hollow spherical joints (WHSJs) which is commonly used in reticulated shell structures. WHSJs in actual projects are subjected to corrosion with various severities since they have been utilized 50 years ago. The existing literature mainly focused on the mechanical behavior of WHSJs, and the influence of corroded WHSJs on structural buckling capacity is seldom studied due to the lack of efficient numerical algorithm. In the present study, a numerical method considering the degradation of connection is proposed to investigate the buckling capacity of single-layer reticulated shell. The randomness of bending (axial) stiffness is achieved by double element. The influences of bending and axial stiffness and their capacities are revealed. A probabilistic distribution model of buckling capacity is proposed by performing stochastic numerical analysis. The main contribution of this work is proposing the efficient numerical model for considering degraded connection. The conclusions derived in this study can provide reference for the safety estimation of SLRSS.

Bending capacity of corroded welded hollow spherical joints with considering interaction of tension force and bending moment

Welded hollow spherical joints (WHSJs) are commonly used joints in reticulated shell structures. The WHSJs in actual projects are always subjected to tension force and moment. The appearance of tension force (TF) may affect the probability distribution of moment capacity for WHSJs with random corrosion. The stochastic FE analyses are conducted in this study to investigate the influence of TF on the probabilistic distribution of bending capacity of WHSJs. This study aims to investigate the influence of TF on bending capacity and construct a probabilistic distribution model of bending capacity considering the influence of TF. The proposed method for predicting probability distribution of reduction factor is also validated. Results show that the method has high accuracy in predicting the probabilistic distribution of bending capacity of WHSJs in complex force state. In addition, the proposed method is convenient for actual applications.

Residual behaviour of corroded welded hollow spherical joints subjected to eccentric loads

The evaluation of corroded welded hollow spherical joints (WHSJs) under eccentric loads is necessary to ensure the safety of space structures. In this study, the corrosion characteristics and mechanical properties of standard specimens under different corrosion times were investigated through surface measurements and tensile tests. Next, eccentric compressive tests were performed on four WHSJs subjected to different degrees of corrosion to evaluate the mechanical properties. The test results showed that the logarithmic normal distribution and generalized extremum value distribution could be used to describe the pitting depth distribution precisely. Uniform corrosion was the main index that influenced the corrosion and bearing characteristics under the test conditions; the eccentric compressive ultimate load decreased by 3.8%, 9.9% and 11.8% at 100, 131, and 157 days of acetic acid mist corrosion, respectively, which corresponded to approximately 20, 30, and 40 years of environmental corrosion in Qingdao. In addition, a refined corrosion simulation method, based on corrosion morphology, and two simplified simulation methods were developed. Based on one of the simplified methods, the effects of various corrosion and size parameters on the eccentric performance of the joint were evaluated. The analysis results showed that the corrosion depth and wall thickness were the main factors that influenced the degradation of the joint bearing capacity. A simplified method for designing the eccentric performance of corroded WHSJs was developed and validated.

Predictions of compression capacity of randomly corroded WHSJs based on artificial neural network

Welded hollow spherical joints (WHSJs) are commonly used in reticulated shell structures. Corrosion on the surface of WHSJs can remarkably reduce their compression capacity. Pitting corrosion is a typical corrosion type on steel structures. Artificial neural network (ANN) is utilized to predict the compression capacity of WHSJs with random corrosion. Corrosion occurring at different positions can affect the compression capacity in different degrees. The spherical body of WHSJs is divided into several parts, and the mass loss ratio χ is utilized as the representation of corrosion severity. The influences of the number of divided corrosion locations and the probabilistic distribution of T c/T on prediction accuracy is investigated in this study. The applicability of trained ANN for WHSJs with different geometric sizes is also validated. Results indicated that ANNs can be utilized for predicting compression capacity with high accuracy, and the mass loss ratio can be used as the input variable.

Probability distribution model of the bending capacity for WHSJ with multiple randomly pitting corrosion

Welded hollow spherical joints (WHSJs) are commonly used joints in reticulated shell structures. Corrosion that occurs on the surface of WHSJs can considerably reduce the bending capacity of WHSJs. Randomness, including random location and size, is the nature of corrosion. Random size is indicated by random planar size and random thickness. The corrosion of steel structures is always a multiple random problem. A series of nonlinear numerical analysis is conducted in this work to investigate the probabilistic distribution of bending capacity of WHSJs. The objectives of this work are to investigate the influence of corrosion parameters on bending capacity and construct a probabilistic distribution model of the bending capacity of WHSJs with multiple randomly pitting corrosion. The method to determine the probabilistic model of WHSJs in actual condition is also discussed. This work can serve as a basis for the stochastic analysis of integral structures connected by WHSJs with randomly located corrosion.

Influence of compression force on the probability distribution of moment capacity for WHSJs with randomly located corrosion

Welded hollow spherical joints (WHSJs) are commonly used in reticulated shell structures. Corrosion on the surfaces of WHSJs can drastically reduce their loading capacity. WHSJs in reticulated shell structures are constantly subjected to compression force and moment. Compression force may affect the probability distribution of the moment capacity of WHSJs with randomly distributed corrosion. A large number of stochastic FE analyses were conducted in this work to investigate the influence of compression force on the probabilistic distribution of the bending capacity of WHSJs. The objectives of this work were to investigate the influence of compression force on bending capacity and to construct a probabilistic distribution model of bending capacity that considers the influence of compression force. The proposed method was validated and determined to have high accuracy in predicting the probabilistic distribution of the bending capacity of WHSJs in a complex force state.

Residual behaviour of welded hollow spherical joints under corrosion and de-rusting

This paper presents an experimental study of welded hollow spherical joints (WHSJs) under corrosion and de-rusting conditions. First, two scanning tests (3D scanning and scanning electron microscopy (SEM)) and a weight loss test were conducted on the same batch of steel used to manufacture the joints to investigate the corrosion morphology and development of the WHSJs under the same corrosion conditions. Subsequently, axial compression and tensile tests on eight corroded WHSJ specimens without anticorrosive coating (including two reference specimens for comparison) were conducted, and the relevant mechanical properties, such as the failure mode, load-displacement relationship, initial stiffness, bearing capacity, and strain distribution were studied. The results showed that the corrosion morphology of the WHSJs consisted mainly of uniform corrosion with slight pitting corrosion; the corrosion depth exhibited a lognormal distribution, and the initial axial stiffness, yield load, and ultimate load of the WHSJs were influenced by the corrosion and different de-rusting methods. After 100 days of accelerated corrosion tests (equivalent to 20 years in the corrosive environment of Qingdao), the compressive ultimate load decreased by 8.433%, and the tensile ultimate load decreased by 7.235%. Finally, a practical corrosion evaluation method with the statistical distribution and corrosion damage kinetics was established and validated.

Compressive strength of corroded special-shaped welded hollow spherical joints based on numerical simulation

Special-shaped welded hollow spherical joints (WHSJ) are increasingly used in several important spatial grid structures. The corrosion of WHSJ is inevitable and can eventually threaten structural safety. There is no research or comparison on the compressive strength of corroded special-shaped WHSJ. Therefore, through the simulation and analysis of the corrosion depth, corrosion mode, corrosion size, joint size, and steel material, the compressive strength of three types of special-shaped WHSJ after corrosion was studied, and the corresponding evaluation method for compressive strength was established. Results indicated that the loss of wall thickness (Tc/T) and the ratio of grooving corrosion (2Wc/D) were the key factors affecting the compressive strength of the special-shaped WHSJ after corrosion. The effect of corrosion on the bearing capacity was related to the size ratio (d/D). WHSJ connected by the H-beam were most affected by corrosion; the failure mode was basically buckling failure, but punching failure might occur in the H-beam WHSJ. The results obtained could form the basis for the selection of the WHSJ shapes and the evaluation of the bearing capacity of the space grid structure with special-shaped WHSJ after corrosion.

Ductile Fracture Analysis of Welded Hollow Spherical Joints Subjecting Axial Forces with Micromechanical Fracture Models

Two micromechanical fracture models, void growth model (VGM) and stress modified critical strain (SMCS) model, were adopted to distinguish the failure mechanism of welded hollow spherical (WHS) joints under axial load based on FE analysis. Ductile fracture was successfully predicted for WHS joints under axial tension. The predicted fracture location is at the weld toe between WHS joints and circular hollow section members, which is consistent with corresponding test results. The predicted fracture load is lower than the peak load on the load–displacement curve, which indicates that the failure mechanism of WHS joints under axial tension is fracture due to inadequate strength and the fracture load should be taken as the ultimate load bearing capacity of the joints. A simplified SMCS model was proposed and verified for ductile fracture prediction of WHS joints under axial tension. Micromechanical fracture analysis was also conducted on WHS joints under axial compression. It was obtained by both VGM and SMCS model that no fracture would occur before the load reached its peak value, the reason of which was discussed by tracing the variation of the equivalent plastic stain and stress triaxiality at the potential location of fracture. Therefore, the failure mechanism of WHS joints under axial compression is losing stability with the depression of the sphere cap and the peak load on the load–displacement curve should be taken as the ultimate load bearing capacity of WHS joints.

Load capacity of corroded welded hollow spherical joints subjected to bending moment and axial force

Welded hollow spherical joints (WHSJs) have been extensively used in space lattice structures. Corrosion is inevitable in the service life of WHSJs, and it can significantly reduce their load capacity and seriously threaten their structural safety. A series of nonlinear numerical analyses was conducted to investigate the moment and axial force interaction for corroded WHSJs. The moment-rotation surface of corroded WHSJs subjected to axial force was derived. The influence of the location, size, and thickness of corrosion of WHSJs on their compressive capacity were revealed in this study. Two types of corroded shapes were investigated. Results indicate that the geometrical parameters exert a significant influence on the moment and axial force interaction. An analytical equation has been proposed to quantify the influence of axial force on the bending stiffness and moment capacity of corroded WHSJs.

Mechanical behavior of welded hollow spherical joints with diameter exceeding 1.0 m

The welded hollow spherical joint (WHSJ) with diameter greater than 1.0 m is commonly employed in long-span tubular transmission towers. It has a relatively higher ratio of diameter to thickness compared to traditional spherical joints, since the larger thickness leads to difficulties in fabricating a spherical joint with a diameter of more than 1.0 meter, and will also lead to deterioration of mechanical properties of materials. Therefore, longitudinal and transverse stiffeners are usually set inside to avoid local buckling and improve bearing capacity. In this study, experimental tests and finite element (FE) analyses of full-scale WHSJs with a diameter of 1400 mm under the multi-axial load are conducted. The results indicate that all specimens have similar failure modes, namely the indentation of the hollow sphere near the main tube, and the failure of the longitudinal stiffeners at the same position. In addition, the setup of longitudinal stiffeners would obviously improve the ultimate bearing capacity (UBC) of WHSJ. After the validity of the FE model is verified, the parametric study is carried out. The results indicate that the UBC of stiffened WHSJ can be approximated by the sum of the resistances of the corresponding unstiffened WHSJ and the stiffeners. Accordingly, design equations are proposed for stiffened WHSJs with diameter greater than 1.0 m. The results obtained by proposed equations are compared with that obtained by tests and FE analyses. They are basically in good agreement, which shows the validity of the equations.