Circular Hollow Section Joints Research Articles

Experimental investigation on seismic performance of CHS T-joints in salt spray environment

This paper aims to investigate the seismic performances of circular hollow section (CHS) T-joints with different regions of corrosion damage. A total of seven CHS T-joint specimens with various corrosion degrees were applied to conduct the quasi-static tests of brace axial cyclic loading, and the effects of corrosion damage on the hysteresis behavior and failure mode were discussed. Experimental results indicated the brace corrosion only had a slight influence on the seismic performance of CHS joints, while the corrosion difference on two sides of the brace might cause an additional bending moment and damage accumulation to accelerate the plastic collapse of the connection zone. Compared with the corrosion on the brace, the resistance of CHS joints was more susceptible to the corrosion located in the chord wall, resulting in the axial cyclic carrying capacity and stiffness of CHS joints significantly decreasing with the increase of chord corrosion level. However, the increasing corrosion on the chord would affect the final fracture behavior and fracture position, so the total energy dissipation and deformation of CHS joints presented a complicated and nonlinear degradation trend. In addition, corrosion damage simultaneously had an impact on reducing the bending capacity of the chord, and this phenomenon not only caused the differences between the degradation rates of axial tensile and compressive strength of CHS joints but also affected the failure mechanism and local deformation of CHS joints under the brace axial cyclic loading.

Optimization‐improved thermal–mechanical simulation of welding residual stresses in welded connections

AbstractThis paper presents a novel computer‐aided computational framework to determine the optimum shape parameters in a welding heat source model using a coupled supervised Gaussian process regression (GPR) and genetic algorithm (GA) approach in estimating the welding residual stresses. The experimental X‐ray diffraction (XRD) approach validates the optimization‐improved thermal–mechanical simulation method. The validation of the proposed approach includes the welded plates with simple geometry to welded pipe‐to‐plate and the circular hollow section joints with complex topologies. The optimization‐derived shape parameters in a welding heat source model lead to close estimates of the temperature history and the welding residual stresses for these types of welded joints considered in this study.

A local digital twin approach for identifying, locating and sizing cracks in CHS X-joints subjected to brace axial loading

This paper aims to introduce a strain-interfaced local digital twin solution for a welded circular hollow section (CHS) X-joint subjected to brace axial loading. The solution comprises a series of machine learning algorithms to (1) identify the presence of cracks, (2) locate the cracks and (3) quantify the extent of cracking. These algorithms make use of strain readings in the vicinity of the crack to perform the diagnosis, representing a remote sensing methodology, thereby eliminating physical inspections. The validation of the proposed methodology includes two experiments – one each in the high and low cycle fatigue regime – demonstrating its wide scale applicability. The success of these experiments highlights the strong potential of affordable strain sensors in crack diagnosis assessments for CHS joints.

Wrapped Composite Joints for Circular Hollow Section Structures

AbstractAn innovative bonding joining technology for truss/lattice circular hollow section (CHS) supporting structures is being developed as replacement for welded joints. Wrapped Composite Joint uses bonding instead of welding to transfer the joint forces and bending moments between the chord and brace CHS members through a dedicated fibre reinforced polymer composite wrap. It is aimed to replace all welds in different types of joints such as X, K, KK and splice joints in fatigue and/or corrosion dominated designs of truss/lattice structures such as offshore wind jackets and floaters, fish farms, truss bridges, etc. Composite material is known for good fatigue performance from aerospace industry and wind turbine blades. Number of joint experiments showed superior fatigue performance of the wrapped composite joints compared to welded CHS joints. This new concept allows for huge reduction of thickness of CHS members resulting in significant truss/lattice structure weight reduction and increase of fatigue lifetime. This paper presents state of development and outlook of the wrapped composite joint technology in terms of design, production and joint performance for ultimate limit state, fatigue and influence of the environment. Framework of prediction methods and design principles based on ongoing experimental and numerical research will be presented. The scope includes material, interface, and joint level experiments from small to full scale (100‐600 mm member diameter) and finite element modeling of complex steel‐composite bonded joints. Advantage of using state‐of‐the‐art monitoring techniques such as Digital Image Correlation, Fibre Optics Strain Measurement and 3D scanning is highlighted

Study on seismic behavior of CHS T-joints under quasi-static cyclic in-plane bending

This paper aim to study the in-plane flexural behaviors of circular hollow section (CHS) T-joints by quasi-static cyclic loading test and finite element (FE) analysis. Two specimens under cyclic in-plane bending (IPB) were carried out to investigate the effect of the brace-to-chord diameter ratios (a key parameter for the CHS joints and denoted as β) on the seismic behaviors of the T-joints. Moreover, finite element (FE) analysis were performed to study the effect of β, γ (another key parameter for the joints with name of the chord radius-to-thickness ratio) and the chord axial pressure on the in-plane flexural behavior of the T-joints. Test results showed that both two specimens failed in tearing of the chord wall at the heat-affected zone (WHAZ), and both two specimens exhibit good ductility and energy dissipation behavior. Both specimens mainly rely on the plastic development of the chord wall near brace/chord intersection and the crack propagation (after cracking) to dissipate energy input by the earthquake. Test results also found that the current specification conservatively predict the in-plane flexural strength of the T-joints (about 20 %–30 %), especially for the joints with larger β. Test and FE results also show that the increase of β (or the decrease of γ) will effectively improve the flexural strength, energy dissipation capacity and ductility ratio. The results also shows that chord pressure stress will adversely affect the in-plane flexural behavior of the T-joints, especially when the stress is larger than 0.6 times of the chord yield strength. A FE parametric study was performed to investigate the effect of β on the force transmission characteristics of the T-joints under IPB, and the force transmission characteristics in elastic state was further interpreted by a simple semi-circular arch analytical model.

Fire resistance and design assessment of Q355 circular hollow section joints

This paper presents an experimental investigation on fire resistance of circular hollow section (CHS) T-, Y- and K-joints. A total of 15 specimens subjected to brace axial load were heated inside an electric furnace, including 5 T-joints, 5 Y-joints and 5 K-joints. The temperature inside furnace follows ISO 834 standard fire curve. The principal parameters considered in this study include the dimensionless geometric parameters of joints β and γ, as well as the load ratio n. It is found that the failure mode of the axially loaded CHS joints under fire is chord plastification. Referring to the temperature-deformation curves of these CHS joints, it is suggested to adopt 1.5% ovalisation of the chord diameter as the failure criteria of the CHS joints under fire. The fire resistance of the CHS joints is seriously reduced with an increase in the load ratio n. By comparing the temperature development and the critical temperatures of the CHS joints obtained in the tests with the ones predicted by design codes, it is found that the measured temperature development curves of the CHS joints are significantly different from the calculated ones before the joint temperature reaches 550 °C. Further investigation is essential for temperature development analysis of CHS joints, especially in the aspects of the section factor Am/V of CHS joint. The fire resistance performance of CHS T- and Y-joints is shown to be reasonably estimated with the resistance method provided in modern design codes, even though the estimation on CHS K-joints is relatively conservative.

Deformation and strain limits for IPB-loaded high strength steel CHS joints

In this study, existing deformation limits are first re-examined to investigate if they can be rationally extended to high strength steel circular hollow section (CHS) joints that are subjected to in-plane bending (IPB) moment. It is pointed out that existing deformation limits, which have been developed and validated primarily for mild steel joints, need to be modified when high strength steels are involved. By noting that the ductility of IPB-loaded joints can be significantly reduced with the use of high strength steel, a new deformation limit is proposed which allows less deformation to less ductile high strength steel joints. The deformation limit, proposed in terms of joint rotation angle, is validated both numerically and experimentally. In addition, a recently proposed strain limit criterion is also discussed. To provide a guide for obtaining converged strain from finite element (FE) analysis, mesh sensitivity study is comprehensively conducted. It is shown that the element size required for the convergence of strain is substantially smaller than that required for obtaining satisfactory global response such as joint load-deformation relationship. By applying a systematic FE modeling strategy, numerical investigation is made to check the feasibility of the 5% strain limit criterion which has recently been advocated by the revised draft of ISO 14346. While the limiting principal strain of 5% is shown to be reasonable for CHS-to-CHS joints loaded by IPB, for longitudinal branch plate-to-CHS joints, a lower limiting strain appears more appropriate.

Numerical Analysis of Axial Cyclic Behavior of FRP Retrofitted CHS Joints

This paper aims to numerically investigate the cyclic behavior of retrofitted and non-retrofitted circular hollow section (CHS) T-joints under axial loading. Different joints with varying ratios of brace to chord radius are studied. The effects of welding process on buckling instability of the joints in compression and the plastic failure in tension are considered. The finite element method is employed for numerical analysis, and the SAC protocol is considered as cyclic loading scheme. The CHS joints are retrofitted with different numbers of Fiber Reinforced Polymer (FRP) layers with varying orientation. The results show that the welding process significantly increases the plastic failure potential. The chord ovalization is the dominant common buckling mode under the compression load. However, it is possible to increase the energy dissipation of the joints by utilizing FRP composite through changing the buckling mode to the brace overall buckling.

Strength prediction of steel CHS X-joints via leveraging finite element method and machine learning solutions

This study employs high-fidelity finite element method (FEM) and machine learning (ML) solutions to determine the accurate strengths of steel circular hollow section (CHS) X-joints with minimal computational effort. FE analysis methods have helped to improve the understanding of the complex structural behavior of CHS joints. However, the routine use of FE analysis imposes high computational demands and requires a thorough verification of numerical models. Alternatively, this study explores the feasibility of using ML methods for estimating the strengths of CHS X-joints. High-fidelity FE models were first built to generate about 4000 datasets. Each dataset included seven joint parameters, namely, steel grade, chord diameter, brace-to-chord diameter ratio, chord diameter-to-thickness ratio, brace-to-chord thickness ratio, chord length-to-radius ratio, and chord stress level. Two ML models; a support vector regression (SVR) and a deep neural network regression (DNNR) were formulated, extensively trained, tested, and validated via comparison with experimental data. The validated DNNR model provided a more accurate prediction of the joint strength with an average error of 0.8%. On the basis of extensive joint strength data readily available from the proposed ML solution, the joint strength margin implied in current design equations was evaluated from various perspectives via variation of key joint parameters.

Enhanced crack sizing and life estimation for welded tubular joints under low cycle actions

This study examines the low-cycle fatigue of circular hollow section (CHS) X-joints made of S355 steels, through a combined experimental and numerical investigation, assisted by non-destructive crack sizing methods, namely the Alternating Current Potential Drop and Ultrasonic Phased Array. The comparison of the two methods reveals the discrepancy in the crack sizing, and illustrates the fatigue crack initiation and propagation in CHS joints. This study extends the Improved Modified Neuber’s Rule to estimate the local energy indicator for CHS X joints. The numerical analysis implements a continuum damage mechanics method, which estimates fatigue lives of test specimens with reasonable accuracy.

A new method for calculating hot-spot stress in gap DKK CHS joints under arbitrary loading

This paper presents a new method to calculate stress concentration factors (SCF) and hot-spot stresses in gap double K (DKK) circular hollow section (CHS) joints used in construction of offshore wind turbine jacket support structures. The focus of this paper is to overcome important limitations of existing analytical methods due to their unsuitability for application under arbitrary loading and their inability to account for carryover stresses. The proposed new analytical method consists of two steps: in the first step, an arbitrary brace loading condition in any DKK CHS joint is decomposed into symmetric and anti-symmetric axial, in-plane bending and out-of-plane bending unit load cases. In the second step, 11 key SCFs are identified and used to calculate SCF distributions around the four weld perimeters of the brace-chord connections under the aforementioned unit load cases, including carryover SCFs. These SCF distributions can be combined to calculate stress distributions and hot-spot stresses under any arbitrary brace loading condition in the DKK joint. Based on the results of an extensive numerical parametric study, regression equations have been found for calculating the 11 key SCFs. The proposed method has been demonstrated to calculate hot-spot stresses much more accurately than the most widely current used method.

Optimized design of fillet welds for CHS joints according to EN 1993‐1‐8

AbstractDesigning fillet welds using the directional method of EN 1993‐1‐8 requires the consideration of force components, converted to stresses, located in the plane of the weld throat. For circular hollow section (CHS) joints, considering these ”stress components“ can be exceedingly difficult. An approach to applying the directional method to CHS joints is developed in this paper. The directional and simplified methods of EN 1993‐1‐8 are then evaluated against available finite‐element data, validated from recent tests on weld‐critical CHS X‐joints according to the standard evaluation procedure of EN 1990. Despite inherent non‐uniform loading of the weld around the perimeter of the brace, it is shown that taking the total weld length to be effective is safe for both methods. Procedures are given so that these methods can be used to design fillet welds in CHS joints as ”fit‐for‐purpose“, as permitted by EN 1993‐1‐8 and ISO 14346, and new design charts are produced for minimum fillet welds that develop the axial capacity of the connected brace.

Experimental study on joint resistance and failure modes of concrete filled steel tubular (CFST) truss girders

Concrete Filled Steel Tubular (CFST) structures are increasingly used not only for columns in tall buildings but also in the arch trusses of many arch bridges and in the truss girders of buildings and bridge decks. Therefore, the chords of Circular Hollow Section (CHS) truss arches and girders are increasingly filled with concrete, effectively making them CFST structures. In addition to the strength and stiffness of the CFST members, the failure mode of the CFST joint connecting them to the tubular member is also affected by the concrete filling. In this study, truss girders with different web arrangements were tested, and their behaviour investigated. The girders were not slender because they were designed to attain the peak limiting state for joint failure rather than chord failure due to bending moments. Moreover, two other types of girders were tested: one without concrete-filled chords (CHS girder) and another with only the upper chord filled with concrete; thus allowing an investigation of how a concrete-filled chord affects joint failure mode. The geometry of the CHS girder joints was such that only chord face failure and punching shear failure could occur. The former required an inward deformation that was prevented by the concrete filling in a CFST girder with similar geometry. Finally, the study considers extending the Eurocode 3 and the AWS D1.1 code formulae, originally proposed for CHS joints, to calculate the resistance of CFST joints.

INTERACTION STRENGTH FOR CHS-TO-LONGITUDINAL PLATE JOINTS UNDER AXIAL LOAD AND IN-PLANE BENDING MOMENT

The circular hollow section (CHS) has lots of advantages in excellent structural properties, architecturally attractive features. Thus, CHSs are used extensively in many applications in buildings, bridges, towers, and offshore. In CHS, it is necessary to understand the various complex shape of CHS joint because loads act simultaneously according to joint type. The use of high-strength steel has been continuously increased, but the current design equation in AISC 2011 or KBC 2016 limits maximum yield strength 360 MPa. This paper studies an interaction strength of high-strength CHS-to-longitudinal plate X-joint subjected to combination of plate axial load and in-plane bending moment. For the study, numerical analysis based on experimental tests was carried out. The analysis was performed according to variables for determining shape of joint, i.e., chord slenderness, plate width-to-CHS diameter ratio, and utilization ratio. Investigations have shown that a linear summation of the ratios for axial load and in-plane bending moment may be used as conservative approximation.

Simplified Effective Notch Stress calculation for non-overlapping circular hollow section K-Joints

This paper presents two simplified procedures for the calculation of the Effective Notch Stress (ENS) on non-overlapping circular hollow section (CHS) K-joints. The proposed procedures aim to alleviate the modeling challenges associated with the traditional ENS calculation on joints with complex geometry, such as the CHS joints. The first procedure is an extension of the recently proposed extrapolation method, similar to the extrapolation in the widely-used Structural Hot-Spot Stress (SHSS) approach. The second procedure provides ENS estimations based on the parametric relationship between the ENS and SHSS. Through an extensive numerical study, this investigation develops parametric equations supporting each procedure, covering practical geometric range for CHS K-joints under balanced brace axial load. Both simplified procedures demonstrate close agreement with the traditional ENS calculation and provide reasonable fatigue life assessments with a substantial number of experimental data, based on the S-N framework.

Experimental and numerical investigation on behavior of CFRP-strengthened circular hollow section gap K-joints

An efficient technique of Carbon Fiber Reinforced Polymer (CFRP) application was proposed to promote joint capacity of general tubular K-joints fabricated from Circular Hollow Section (CHS) members. Using this technique, in order to understand the static performance of CFRP-strengthened CHS joints, a systematic investigation was carried out by means of both experiments and finite element method. Three CHS gap K-joints strengthened with CFRP sheets were tested under static axial force in braces, whilst one additional joint was served as reference joint without CFRP. The results of failure modes, deformation, Mises stress, strain intensity and load-bearing capacity of the joints were presented and compared. A series of finite element models were developed and validated for the joints with and without CFRP reinforcement. A parametric study was conducted to evaluate the effect of variables (length, layers and mechanical properties of CFRP) on load-bearing capacity. The research results revealed that the proposed technique of CFRP installation was efficient to promote performance of in-service CHS gap K-joints. Moreover, the layer of CFRP sheets has significant effect on load-bearing capacity, but the effect was negligible for either length or mechanical properties of CFRP sheets. Finally, formulas were proposed for calculating ultimate load-bearing capacity of CHS gap K-joints with CFRP composites, and their calculation results matched well with the experimental and numerical results respectively.

Translating the material fracture resistance into representations in welded tubular structures

This paper presents an approach to integrate a lower-bound material J–R curve into the load–deformation relationship for circular hollow section joints. The lower-bound J–R curve derives from a regression analysis of the experimental data for a wide range of structural steels reported in the literature and follows a linear relationship up to a maximum crack extension of 2.0mm. The deterioration in the joint strength caused by ductile tearing adheres to a reduction factor based on the crack area. The J–R curve modified load–deformation responses compare closely with reported experimental results for both welded tubular joints and large-scale tubular frames.

Influence of tensile chord stresses on the strength of circular hollow section joints

AbstractThe current version of the Eurocode for the design of joints does not include any brace load capacity reduction for welded hollow section joints with tensile chord preloading as opposed to those without. However, the more recent 2nd edition of the CIDECT design guides or the ISO standard for welded hollow section joints includes a strength reduction due to tensile chord stresses. This paper presents the results of experimental and numerical investigations into how tensile chord stresses influence the capacity of circular hollow section X‐joints. The conclusion of this study is a new chord load function proposal for circular hollow section joints subjected to tensile chord stresses.

Test on the Suspended Dome Structure and Joints of Dalian Gymnasium

The roof of Dalian Gymnasium, of span 145.4 m ∞ 116 m, rise span ratio 1/10, truss height 2.4 m and strut height 10.0 m, is the largest suspended dome structure in Asia. Different tension schemes have an important effect on the performance of a structure. It is necessary to investigate the characteristics and feasibility of different tension schemes. In addition, to check the accuracy of the numerical model and its calculated results and to investigate the mechanical performance when subjected to construction and design loads, as well as safety considerations during construction and when in service, a scale model of the suspended dome with scaling factor 1:10 was constructed and models of some key joints at full scale. The results show that the theoretical strut internal forces and displacements of joints, are in good agreement with test results. Therefore, tension steps in the cable can be controlled by internal forces in the struts. The lifting of struts is a continuous process of adjustment. The size of the cable needed to be accurate during the test, which indicates that the controllability of this method is relatively difficult. However, when tensioning diagonal cables, the diagonal cable force can be an effective control because these is a sufficient number of controllable tension points and a relatively small cable force. More importantly, because the geometrical dome forming accuracy of this method is higher, the latter method is suggested for use during construction. The displacements of the joints in the upper reticulated shell of the suspended dome, the axial forces in the struts and the stresses in the members present, vary linearly with test load which indicates that the vertical stiffness of the overall structure is well maintained when conducting static tests on the model. The test CHS (circular hollow section) joints which were not internally strengthened, failed with a large amount of plastic deformation. However, the ultimate bearing capacity of CHS joints strengthened by inside gusset plates was obviously improved. Some key CHS joints are suggested for strengthening with inside gusset plates, during the construction process. The work of this paper provides a reference for similar structures.

Re-evaluation of the moment capacity of CHS joints

Current design formulae for circular hollow-section (CHS) T and X joints loaded by brace in-plane or out-of-plane bending moments are based on a limited number of tests available from the early 1980s. In the last three decades, many experimental studies and numerical research programmes have been conducted in this area. This study re-evaluates the brace moment capacity strength of T and X joints. The available data are checked against possible member failures, while the experimental data are carefully screened with respect to outliers. Detailed comparisons are made between finite-element results and ‘reliable' test data obtained from different research programmes. Based on the selected data, the existing strength equations are updated.