Steel Connections Research Articles

Component Based Finite Element Method for Steel Joints at Ambient and Elevated Temperatures

This paper introduces Component Based Finite Element Model (CBFEM) which is a numerical design model to analyze and design connections of steel structures with features demonstrated here on a portal frame eaves bolted connection. The connection in CBFEM procedure is analyzed by Finite Element Method FEM. The correct behavior of components is treated by introducing components that well represent its behavior in terms of initial stiffness, ultimate strength and deformation capacity, of bolts, welds etc. As for another FEM design procedures a special care is given to the validation and verification procedures which is demonstrated here in this contribution on example of portal frame eaves welded connection. In this paper, CBFEM is applied to analyze steel beams and columns also at elevated temperature. The main objective of this study is to verify the CBFEM for predicting the resistance of steel members and connections at elevated temperatures.

Investigation of Parameters Affecting Rotational Behavior of Cold-Formed Steel Connection

This study aims to understand the behavior of connection, which holds an important key to efficient designs of the cold-formed steel structure. The focus is on examining parameters that affect the rotational stiffness and behavior of the cold-formed steel connection made of single-lipped channel sections with a bolt and gusset plate system. The numerical study has been conducted using component-based finite element analysis, with the studied parameters including bolt diameters, number of bolts in the group, size of the bolt group, thickness of the cold-formed steel cross-section, and thickness of the connecting plate. The effect of the variables is evaluated and explained by comparison. The result of the study reveals that the rotational behavior of the connection depends on the details of the assembly. For the given base connection, the parameters that give the greatest effect on the rotation stiffness and the moment capacity are the number of bolts and the bolt diameters, whereas the parameter that provides the least effect is the bolt spacing. On the other hand, the bolt group size is the most influential parameter for the member connection. For both types of connections, the thickness of the cold-formed steel section and the thickness of the gusset plate have shown limited effect on stiffness and strength. Using different variables in assembling the connection can also lead to different failure types, either bolt failure or excessive bolt hole deformation, which leads to plate failure. Doi: 10.28991/CEJ-2023-09-11-08 Full Text: PDF

Failure mode and capacity prediction for bolted T-stub connections using ensemble learning

The present study utilizes machine learning (ML) algorithms, namely decision trees, and ensemble learning methods including bagging, random forests, gradient boosting, and extreme gradient boosting to identify the failure mode and predict the corresponding capacity of preloaded bolted T-stub steel connections subjected to tension. A nonlinear finite element model (FEM) was first developed and validated against experimental results from literature. Subsequently, a matrix of 3447 FEMs, covering a wide range of design parameters, were generated to train and test the ML models. In addition, to evaluate the significance of each input parameter in predicting the connections' capacity, different embedded feature selection techniques were employed. Furthermore, different interpretability methods were utilized to uncover the interrelationships between the input features and the modeled outputs to further interpret the developed Blackbox models. The results highlight the effectiveness of ML techniques in predicting the failure modes and corresponding capacity of bolted T-stub connections, supporting their potential as robust and rapid design tools which ensure adequate designs for new connections or effective assessments for existing ones.

Experimental study on behavior of top-and-seat-angle with double web-angle (TSDW) connections under fire conditions

Steel beam-to-column connections are more prone to fracture due to tension in the heating and cooling phase of a fire, imposing an important issue on progressive collapse of steel structures in fire. Experiments are conducted in this study on four top-and-seat-angle with double web-angle (TSDW) connections under room-temperature, heating, heating-cooling and after-fire conditions. The effect of temperature histories and loading states on the failure modes and displacement curves of connections is investigated. A numerical model of full-scale TSDW connections is established and validated, and parametric studies are carried out to study the effect of scale ratios, material properties, angle thickness and stiffeners. The experimental results show that the connection may fail due to tearing of bolt holes or shear failure of bolts in the seat angle. The critical temperature of connections is 710 °C for a load ratio of 0.5, and its residual capacity reduces by 23 % after experiencing a maximum temperature of 980 °C. Loading during fire has a significant effect (60 %) on the residual capacity of connections after fire. It is found from numerical simulation that the scale ratio affects failure modes, critical temperatures, residual capacities and failure displacements of connections, and further work is still needed to quantify structural fire similarity. The yield strength and fracture strain are important factors affecting the behavior of steel connections at both ambient and elevated temperatures. Increasing the angle thickness and applying stiffeners can effectively improve the load-bearing capacity of TSDW connections at room temperature. There is an optimal angle thickness (e.g., 8 mm) for improving residual capacity of connections, and the selection of stiffeners should consider its matching with angles. It is recommended to exclude stiffeners for thick angles (e.g., 12 mm) to avoid adverse effect on the fire resistance and post-fire residual capacity of TSDW connections.

Performance indices and fragility assessment of steel frame structure with bolted low-yield-point steel connection components

A resilient steel frame structure equipped with replaceable ductile and energy-dissipative connection components was developed in this study. Connective components, such as splice plates and T-stubs are made of low-yield-point steels (LYPs), which have superior ductility and energy dissipation capacity. The plastic hinge can be separated from the column surface by local weakening, thus concentrating local damage within low-yield-point steel connection components (LYP-Cs) to effectively enhance ductility of the structure. Five-story and ten-story bolted steel frame structures with four configuration parameters were designed to investigate the seismic performance and damage development of steel frame structures with LYP-Cs (SF-LYP-Cs). Corresponding numerical models were established. Welded steel frame structures with traditional reduced beam sections (SF-RBS) were also established for comparison. Pushover analyses were conducted to explore the effect of structural fuses and to obtain multi-stage performance indices of the resilient steel frame structure based on energy dissipation proportion (EDP) and damage behavior. Incremental dynamic analysis was performed to verify proposed performance indices, and seismic performance of SF-LYP-Cs was evaluated via fragility analysis. The results showed that LYP-Cs were not only effective structural fuses but also effectively delayed the damage development in the primary members, thus reducing the failure probability of the structures. The multi-stage performance indices were conductive to describe the multiple lines of resistance and effects of structural fuses. A larger weakening degree and smaller beam height mitigated plastic damage development in primary members. When LYP-Cs dominate the system’s energy dissipation, the section sizes of columns and the number of structural stories exert only slight influence on the structural performance indices.

Refined and Simplified Simulations for Steel–Concrete–Steel Structures

Steel–concrete–steel (SCS) sandwich structures have gained increasing interest in new constructions. The external steel plates increase the stiffness, the sustainability, and the strength of the structures under some extreme solicitations. Moreover, the use of these plates as lost prefabricated formwork makes SCS structures modular, enabling higher construction rates. However, for a better understanding of the complex behavior of these structures up to failure, refined numerical simulations are needed to consider various local phenomena, such as concrete crushing in compression and interface interactions. Indeed, the highly non-linear steel–concrete interaction around the dowels is the key point of the composite action. In this contribution, a refined methodology is first proposed and applied on a push-out test. It is especially demonstrated that a regularization technique in compression is needed for the concrete model. Interface elements are also developed and associated with a nonlinear constitutive law between steel connectors and external plates. From this refined methodology, simplified numerical modeling is then deduced and validated. Directly applied to an SCS wall-to-wall junction, this simplified strategy enables the reproduction of the overall behavior, including the elastic phase, the degradation of the system, and the failure mode. The response of each component is particularly analyzed, and the key points of the behavior are highlighted.

Damage Evaluation of T-Stub Connected to Hollow Section Column Using Blind Bolts under Tension

The quantitative calculation and evaluation of seismic damage play a crucial role in ensuring structural safety, conducting performance-based structural analysis, and implementing seismic strengthening measures. However, there is limited research on the damage performance of blind-bolted T-stub steel connections used extensively in prefabricated buildings. In this study, the tensile sub model of a blind-bolted T-stub steel connection in a beam–column joint is investigated. The influence of the flange and web thickness of the T-stub connector, as well as the shear-loaded connecting bolts on the web of the T-stub, on the tensile performance of the sub model are considered. Four tensile destructive tests are conducted on a T-stub connector connected to a hollow section column using blind bolts. The experimental results, including failure modes, force–displacement curves, and strain development in the hollow section column and T-stub, are discussed and analyzed in this study. The test results reveal three main failure modes for this tensile substructure: the plastic deformation of the hollow section column, the bending fracture of the T-stub flange, and the fracture of the T-stub web bolt holes due to compression. Furthermore, a ductile damage finite element analysis method is employed to simulate the fracture damage process of the substructure, and the corresponding damage index is calculated using the typical Park–Ang damage model for evaluation, showing good agreement with the damage classification levels specified in FEMA.

Ultimate capacity and stability of topologically optimised shear plates in compliance with structural Eurocodes

Topology optimisation (TO) is increasingly used to assist metallic parts manufacturing in automotive, aerospace, and biomedical sectors. A similar trend is observed in civil engineering structures, particularly in steel construction joints representing crucial steelwork components. Yet, the widespread adoption of TO to steel members in structural engineering has been hampered by difficulties complying with code requirements and challenges in utilising cutting-edge software in real, heterogeneous, and complex cases. To fill this research gap, a methodology is developed in the current study to implement the requirements of Eurocode 3 in the TO process. Nonlinear Finite Element Analyses (NLFEA) are performed on the web cover-plates of a well-known bolted splice connection between segments of a steel beam to validate the code-compliant methodology. Results show that it is possible to reduce up to 87.5% of the plate initial volume while keeping the connection original capacity. Also, it has been found that the initial volume can be reduced up 20% without affecting the original plate capacity, and up to 60% while ensuring that the ultimate plastic displacement is larger than that of the original plate. Based on extensive buckling analyses, critical loads exceed ultimate loads from 9.3 to 12.4 folds, thus ensuring excellent stability performance. The results of this study demonstrate that validation with NLFEA is a crucial step for achieving actual code-compliant optimised steel joints, and that the linear elastic TO could not satisfy the safety requirements. Moreover, encouraging findings regarding stability and ductility of (extremely) optimised parts suggest the adequacy of TO for designing steel connections. The volume of topologically optimised solutions significantly decreased, contributing to the decarbonisation targets for steel construction.

DRVENA KONSTRUKCIJA CENTRALNOG DELA KROVA CRKVE U NIKŠIĆU

The paper presents the solution of the timber roof structure of the central nave in the town of Nikšić in Montenegro. The central part of the roof is shaped like a regular pyramid over an octagonal base. The basic structural system consists of eight curved timber girders, which are supported in the corners of the octagon and are connected at one point at the top of the roof. Above these girders, in their plane, the secondary supporting structure was added, in order to form the pyramidal shape of the roof. Considering the curved shape of the girders of the basic structural system, the design of the structure foresees glued laminated timber for the basic elements of the roof structure. The solution includes a presentation of all elements of the roof construction, according to the hierarchy of load transfer, as well as a solution of steel connectors for the mutual connection of timber elements.

Corrosion Analysis and Prevention of Tubing Nipples in Four Wells

A high-temperature and high-pressure gas field in an oil field requires the use of super 13 chromium material due to corrosive environments. In the recent production process, there have been four consecutive cases of corrosion, fracture, and perforation of tubing nipples, which have brought enormous economic losses to the oilfield. This article analyzes the failure causes of these four tubing nipples and proposes preventive measures. In this paper, the macroscopic morphology, chemical composition, mechanical properties, metallography, and microscopic morphology of four failed tubing nipples have been analyzed. Combined with the field service conditions and string structure of the tubing nipples, it is believed that the failure of the tubing nipples is mainly due to the connection of carbon steel and stainless steel materials, forming a typical galvanic corrosion galvanic cell. The geometric and environmental factors of the materials exacerbate this corrosion. Eventually, severe galvanic corrosion occurred on the inner surface of the tubing nipple, resulting in severe wall thinning or partial perforation, and four tubing nipples failed. The failed tubing nipples in Wells A, B, and C have erosion marks, which are caused by the action of high pressure and high flow rate fluids in the well; The inner surface of the tubing nipple in Well D is rusty yellow, mainly due to the corrosion of its inner wall caused by the acid fracturing fluid injected earlier. At the same time, the string structure of four wells shows that the tubing nipple is located at the variable diameter node of the string. When the fluid in the well flows from the large hole into the smaller hole, the fluid in the well will change there, and its flow rate and pressure will also increase, further exacerbating the erosion in this area. With the development of deep wells containing carbon dioxide in oil fields and the application of carbon dioxide injection secondary oil recovery technology, the use of 13Cr martensitic stainless steel is also increasing. In order to avoid the occurrence of similar accidents that will bring huge economic losses to the oil field, it is recommended that the material of the pipe string (including accessories) of such wells be confirmed using a portable direct reading spectrometer before entering the well.

Simplified Computational Procedure for the Design of Special Moment Resisting Steel Connections

Simplified Computational Procedure for the Design of Special Moment Resisting Steel Connections

Testing, analysis and design of wire and arc additively manufactured steel bolted connections

Capitalizing on Wire and Arc Additive Manufacturing (WAAM), 3D printed steel has shown significant potential in manufacturing large-scale steel structural elements in the construction industry. Due to the intrinsic difference in material properties, the mechanical performance of WAAM steel connections requires further investigation. In this paper, a total number of 24 WAAM steel coupon specimens with three different print layer orientations and 36 WAAM single-shear bolted connection specimens with one bolt of different design configurations were fabricated, dimensionally measured with 3D scanning technique, and tested under monotonic tension. The failure modes and ultimate capacities of the bolted connections were analysed, focusing on the print layer orientations of the WAAM steel plates. The current codified design provisions and design approaches proposed in the literature for steel structures were further evaluated by comparing the failure modes and ultimate capacities of the bolted connection specimens. This research shows that specimens with different print layer orientations present anisotropy phenomena in the coupon tests and bolted connection tests with differences of up to 10% and 20%, respectively. The relatively accurate predictions of the ultimate capacity of the WAAM steel bolted connection specimens following the current steel design standards are significantly compromised by predicting the incorrect failure modes, which could be attributed to the influence of anisotropic material properties of the WAAM steel plates and the failure modes of tilt-bearing and the end-splitting not being considered in current design provisions. This research conducted systematic experimental investigations focusing on the material properties of WAAM steel and the structural behaviours of WAAM single-shear bolted connections, which could potentially promote the application of WAAM technology in construction industry.

Experimental study on self-centering steel connections based on phased energy dissipation mechanism

Traditional steel beam-column connections and column bases often experience yield or buckling, which makes repair difficult. The recent development of resilient seismic structures has addressed this issue. Resilient seismic structures can achieve self-centering functionality through prestressed steel strands and dissipate energy through energy dissipation elements, while protecting the main components such as beams and columns from damage. This paper presents an experimental study on connections that combine friction and plastic deformation of T-stub to achieve energy dissipation. Four specimens were designed by varying the cross-sectional form of the T-stub flanges and the material of the friction plates. Quasi-static tests were conducted under low-cycle reversed loading to observe the test phenomena. The test results including moment-rotation response, deformation, stiffness, energy dissipation capacity, force of high strength strands and strain were analyzed to investigate the seismic performance of the connections. It was found that weakening the cross-section of the T-stub enabled uniform yielding and energy dissipation. Reducing the friction force between the friction plates and the T-stub web enhanced the self-centering capability while decreased the energy dissipation capacity. Overall, the connections exhibited good seismic performance.

Influence of steel connector in laser-welded sandwich panels subjected to low-frequency plane shock wave

Experimental study was performed to investigate the responses of laser-welded sandwich panels subjected to a low-frequency plane shock wave inside an explosion container. The square tube and H-beam with different thicknesses were selected as connectors for the sandwich panel. The damage mechanisms of the three specimens were investigated, including the cores and face sheets. Numerical simulation was also used to discuss the influence of different peak pressure. The results show that one of the two web plates of the core cell was obviously in tension and the other one was in compression impacted by the low-frequency plane shock wave. The sandwich panel with a connector has excellent anti-blast performance in effectively reducing deformation even if the mass of the connector and the cell core is approximately equal, and the square tube was a better choice as a connector than the H-beam. The damaged area of the sandwich panel with square tube is usually in the boundary along the transverse, and the structure can withstand the critical low frequency plane shock wave much more than the initial sandwich panel.

Development of 3D models with augmented reality for steel and reinforced concrete structure subjects

In the academic sphere of undergraduate Civil Engineering courses, students find it difficult to abstract three-dimensional visualisations from two-dimensional images presented in project documentation. The aim of this article is therefore to present an alternative to assist in the teaching and learning process of steel and reinforced concrete structural design content. Three-dimensional models of steel connections and reinforced concrete structural elements were developed with functionality for the application of Augmented Reality (AR). The models were developed in Revit software and exported to the Sketchfab platform, where it is possible to use the AR feature via an application. Additional information, such as texts, can be added to the models, which expands the possibilities for developing didactic activities. Both tools used proved to be very suitable for the purpose of developing digital objects for teaching engineering.

Seismic design and experimental testing of precast tolerance concrete shear wall subject to cyclic load

A precast tolerance concrete shear wall (PTCW) characterized by proper-length steel connectors is proposed to adjust the tolerance and improve the joint failure model of the precast wall. The design method for this wall is also presented in this study and verified by test results. Four full-scale walls, including two cast-in-situ walls (RCWs) and two precast walls (PCWs) with axial compression ratios of 0.2 and 0.3, respectively, were tested under lateral loading. The seismic behavior of the panels, including the failure modes, hysteretic behavior, strength envelope curves, stiffness degradation, energy dissipation, and steel strains, was analyzed and discussed herein. The test results showed that all four walls were in a flexural-shear failure mode, and the PCWs showed better performance in terms of hysteretic behavior, strength envelope, stiffness, and energy dissipation, particularly under an axial compression ratio of 0.3. Furthermore, the peak load of PCW1 (axial compression ratio is 0.2) and PCW2 (axial compression ratio is 0.3) increased by 5.93% and 15.51%, and the yield load increased by 12.1% and 11.39% compared with those of RCW1 (axial compression ratio of 0.2) and RCW2 (axial compression ratio of 0.3), respectively. The equivalent viscous damping coefficients of all four panels were higher than 0.05. The lateral bearing capacity of the walls increased with an increase in the axial compression ratio. Moreover, no visible damage or slippage was observed when the PCWs were demolished after testing. The main nonlinear strain was concentrated on the vertical bar and not on the connectors, indicating that the plastic hinge position was transferred from the joint interface to the wall panel. The results indicated that the use of steel connectors to strengthen the joints can achieve the seismic design concept of strong joints and weak members.

A cyclic GTN model for ultra-low cycle fatigue analysis of structural steels

Ultra-low cycle fatigue (ULCF) is a critical concern in the seismic design of steel structures due to its adverse effects on the ductility and energy dissipation capacity of steel connections. The Gurson-Tvergaard-Needleman (GTN) model, a well-accepted micromechanical fracture model, cannot be applied directly to ULCF analysis, as it requires proper estimation of the effects of cyclic loadings on the ductile fracture of materials. In this paper, a cyclic GTN (C-GTN) model was proposed for the ULCF prediction of materials, in which the evolution of microvoids during ULCF loadings was addressed for tensile and compressive half load cycles, respectively, and the effect of the cumulative plastic strains on the material’s resistance to ductile fracture was considered by the reduction of the critical void volume fraction for void coalescence. Then, a VUMAT subroutine was programmed for the C-GTN model, in which the cyclic hardening behavior of materials was simulated with the Voce-Chaboche model. Finally, the C-GTN model was calibrated for the G20Mn5QT cast steel based on previous tests on notched round bar specimens of the material. The applicability of the C-GTN model and the VUMAT subroutine were validated in the ULCF analysis of double-hole plate specimens of the G20Mn5QT cast steel.

Resilient performance of steel connections with low yield point fuses considering the effect of RC floor slab

The beam-column connection of a steel frame equipped with replaceable low yield point steel cover plates (SF-LYCP) was proposed to address the significant repair challenge and financial losses caused by beam-column connection brittle failure after an earthquake. According to an experimental study, the SF-LYCP exhibited outstanding bearing capacity, ductility, energy dissipation, and post-earthquake rapid recovery performance. To explore the effect of the reinforced concrete (RC) floor slab on the mechanical behavior of SF-LYCP in a natural working environment, the finite element method of steel beam-column connection with low yield point (LYP) steel cover plates considering the effect of the RC floor slab (SF-LYCP-RCslab) was developed and analyzed by ABAQUS. The results demonstrated that the proposed finite method was able to accurately predict the behavior of SF-LYCP-RCslab. The load capacity, initial stiffness, and energy dissipation capacity of the SF-LYCP-RCslab were increased in comparison to the SF-LYCP. The LYP cover plates in the SF-LYCP-RCslab still played the desirable role of structural fuses. However, the concrete of the RC floor slab had severe damage and prevented the quick replacement of repairable LYP cover plates. Based on these findings, four improved strategies for the SF-LYCP-RCslab were proposed to reduce concrete damage and increase the replacement convenience of damaged elements. Comprehensively considering the bearing capacity, ductility, energy dissipation, structural fuse function, concrete damage, and post-earthquake rapid recovery performance, the improved strategy, which adopted a profiled steel–concrete composite floor slab with a gap, was recommended in steel frame beam-column connection design.

Benchmarking by high heat flux testing of W-steel joining technologies

For a future commercial fusion reactor, the joining of tungsten and steel will be of vital importance, covering the main part of the plasma facing area. However, the large difference, of more than a factor of 2, in the coefficient of thermal expansion (CTE) of W and steel results in high thermal stresses at their interface. The cyclic nature of the operation can cause fatigue effects and could result in a premature failure of the joint.One possible solution is the insertion of a functionally graded material (FGM), with varying the CTE, as an interlayer between tungsten and steel, which could reduce these stresses. In this study, two processes, atmospheric plasma spraying (APS) and spark plasma sintering (SPS), are utilized to manufacture such FGMs. The gradation was accomplished by using two or three layers with a thickness of 0.5 mm each.Another principle is the insertion of a ductile metal interlayer, which reduces the stress by plastic deformation. Vanadium and titanium foils of varying thickness were chosen, as both have a CTE in between W and steel and V forms a solid solution with W and Fe. These and a direct W-steel joint as baseline reference were made by current-assisted diffusion bonding. All samples consist of 3 mm thick W and steel tiles allowing a direct comparison of the different technologies.An efficient high heat flux benchmark test procedure was developed and performed to investigate and compare the potential of the different joining technologies. For this, the complete stacks were brazed on actively cooled copper cooling modules and tested with high stationary heat loads of up to 5 MW/m2 with 200 cycles at each level in the JUDITH 2 facility. Detailed thermal analysis including comparison with prediction based on FEM simulation are presented to understand the cause of the failure and track the degradation. This study allows to help focusing the further development of W-steel joining technologies.

Effects of material property and thickness on block shear strength in lean duplex stainless steel welded connections

Recently, studies have been carried out to apply lean duplex stainless steels, with less than 1.5% nickel, which provides higher material strength and lower cost compared with austenitic stainless steel for building structures. This study compared with existing experimental and finite element analysis results on the structural behaviours of cold-formed lean duplex stainless steel welded connections with different tensile strength (tensile strength-to-yield stress ratio) and plate thickness despite identical steel grade. The main variables are plate thickness and weld lengths. The material tensile strength of a 3.0 mm thick plate was 22% lower than that of a 1.5 mm thick plate. All specimens showed block shear facture in the base metal. Block shear strengths predicted by current design codes and the proposed equations from previous studies were compared with test strengths. KDS 41 31 00-19/AISC2016 and EC3 design specifications underestimated block shear capacity by on average 24%, 43%, respectively for 1.5 mm and 3.0 mm thick specimens, and equations suggested in previous studies of stainless steel welded connections did not sufficiently account for the influence of stress triaxiality by material property and plate thickness difference on the block shear strength of lean duplex stainless steel specimens. Therefore, thickness-dependent modified strength equations were recommended based on experimental and numerical studies of lean duplex stainless steel welded connections and comparison of carbon steel and other stainless steel connections.