Development and analysis of an innovative high-strength steel adaptable beam to column connection

1 INTRODUCTION In recent years, the construction industry has faced increasing pressure to adopt sustainable practices that minimize environmental impact while maintaining structural integrity and efficiency. This shift is driven by the urgent need to transition from linear construction models, characterized by resource extraction, use, and disposal, to circular approaches that prioritize reuse, recycling, and adaptability. The RFCS Connect4C project emerges as a groundbreaking initiative aimed at advancing the field of steel construction through the development of innovative and adaptable connection systems that align with the principles of the circular economy. The project focuses on the development of demountable and adaptable solutions for beam-column joints and column splices, enabling structures to be easily disassembled and reconfigured for future use. This approach not only extends the lifespan of structural members but also reduces construction waste and lowers greenhouse gas emissions associated with traditional building practices. Among the various joint typologies under investigation, the performance of beam-column joints with long slotted holes is under study, offering significant potential for enhancing circularity in structural design. The concept of circularity in steel construction hinges on designing systems that can adapt to changing needs throughout their lifecycle. Connections that facilitate length adjustments in beams allow for reconfiguration without material loss, promoting flexibility and resource efficiency. By enabling deconstruction rather than demolition, such solutions support a more sustainable built environment, where structural components can be repurposed in new projects, thereby conserving embodied carbon and reducing environmental impact. The study of slotted holes in cleat connections dates to the end of the 20th century. Man et al. (2006) studied the strength and behavior of beam-to-column shear connections with short slotted holes. Their experimental program tested full-scale single and double angle shear connections under shear loads, analyzing failure modes such as angle end tearing, block shear, and bearing failure. Results showed that plate washers were crucial for achieving expected shear capacity in connections with slotted holes. The study proposed new predictive models to improve failure estimation and emphasized the need for further research to refine design recommendations. Among the failure modes in joints under shear, the most common and often times the desired one is plate bearing, since it is a ductile failure mode. The strength model for bearing failure dates back from the 80s with further developments at the end of the 20th century (Kim & Yura 1999, Rex & Easterling 2003, Li et al. 2020). Development and analysis of an innovative high-strength steel adaptable beam to column connection A.S. de Carvalho, T. Bogdan & C. Odenbreit ArcelorMittal Chair of Steel Construction, University of Luxembourg, Luxembourg J. F. Demonceau University of Liege, Liege, Belgium R. Matos & R. Obiala ArcelorMittal, Global R&D, Esch-sur-Alzette, Luxembourg ABSTRACT: In response to the growing demand for sustainable construction practices, the RFCS CONNECT4C (RFCS-02-2022-101112300) aims to enable the circular economy in steel construction through the development of innovative and adaptable connection systems. This paper focuses on the performance of beam-to-column joints featuring long slotted holes, which facilitate horizontal adaptability, allowing for structural adjustments in response to changing requirements over time. By exploring the mechanical behavior of these connections through advanced finite element analyses, the study investigates various geometrical parameters, including plate thickness and slotted hole length, that influence joint classification and shear resistance. The research builds on recent findings concerning failure modes in bolted connections, emphasizing the role of bearing-type failures. The outcomes are expected to contribute to the better understanding of the mechanical behavior of such joints, enabling their practical application, thereby promoting flexibility, reuse, and a reduction in the environmental impacts associated with traditional construction methods. Recently, Može & Beg (2014) investigated the bearing stress behavior of single and two-bolt connections through experimental and numerical analyses. Their study involved 19 mild and highstrength steel connections tested under axial loading to evaluate different failure modes, including bearing, splitting, and net section failures. The authors found discrepancies between experimental results and the EN 1993-1-8:2005 standard, suggesting the latter’s bearing resistance checks were overly conservative. Their proposed modified design check, which primarily considered end distance and bolt pitch along the force direction, provided better alignment with empirical data. Follow-up research confirmed the model’s validity, especially for large end distances, and highlighted friction's role in bearing capacity. A broader statistical analysis of 884 test results (Može 2020) further supported these findings, leading to refined strength models with improved accuracy while maintaining safety. The research contributed to recommendations for the next generation of EN 1993-1-8, aiming for more practical and less conservative design rules. Wald et al. (2002, 2004) investigated bolted connections with slotted holes perpendicular to the applied force, simulating cover plate connections. The studies aimed to apply the component method for predicting resistance, stiffness, and deformation capacity, with preliminary statistical calculations to inform European design rules. Results showed a small reduction in bearing resistance for short slotted holes, increasing with longer slots. Cavène et al. (2017, 2020, 2024) investigated the initial stiffness of bearing connections with slotted holes using experimental and analytical methods. Their study focused on slotted cover plates in bearing-type joints with slots oriented perpendicular to the load direction. They employed Digital Image Correlation (DIC) and Linear Variable Differential Transformers (LVDTs) to measure displacements accurately. By comparing experimental data with theoretical predictions, they developed an analytical model that accounted for the interaction between flexure and bearing effects. Although consistent efforts have been made in the past to characterize the behavior of joints with short and medium slotted holes, there is no definitive information regarding the mechanical behavior of such joints with long slotted holes. Since this type of bolt clearance hole can enhance adaptability in the context of joint reuse, this paper aims to investigate the shear behavior of web cleat beam-to-column connections with long slotted holes through advanced FE analyses. 2 CONNECTION TYPOLOGY The joint typology to be studied is presented in Figure 1. It consists of a double web cleat connection with long slotted holes in the beam leg and short holes in the column leg. The long slotted holes have the function of providing horizontal adaptability, allowing for large tolerances and beam length adjustments. On the other hand, the short slotted holes on the column leg allows the use of different beam sections, with different web thicknesses to be used with the same plate perforation grid. The cleat is made by welding of two rectangular plates, fabricated from high strength steel S690. The dimensions and parameters to be adopted in the joint are the focus of investigation of this paper. Figure 1. Web cleat with slotted holes connection 3 NUMERICAL MODEL The numerical modelling was executed in Abaqus® FEM software, using solid elements. The dynamic implicit solver was used considering a quasistatic simulation. Bolt preload was applied using temperature gradient in the bolt shank. The material of the plates is based on a quadri-linear model. The material for the bolts is based on a elasto-plastic model with linear strain hardening. The mesh is structured, as shown in Figure 2, with 32 elements around the holes. The general element size is 20 mm and 3 elements are considered through thickness of the plates. 3.1 Boundary conditions Two different models for the same beam joint geometry are considered, one cantilever (M1) and one simply supported (M2). Figure 3 Illustrates the options used in the FE software. The cantilever model is used with the intention of classifying the joint according to its stiffness. On the other hand, the simply supported boundary conditions are used with the goal of estimating the shear resistance of the joint and characterize its behavior under shear. Figure 2. Finite element mesh a) b) Figure 3. Numerical model boundary conditions: a) M1: Cantilever model; b) M2: Simply supported model. 4 RESULTS AND DISCUSSION 4.1 Joint classification Given that at the initial stage of development of the joint typology, it is necessary to define the final geometries of the cleat with long slotted holes, several parameters related to the geometry of the plates were evaluated. One example is the plate thickness (Figure 4a) which significantly contributes to its stiffness. It can be observed that, despite the considerable increase in stiffness between the analyzed cases, the connection can still be classified as nominally pinned, except for the case of t=14 mm, where the stiffness is slightly above the threshold for classification as a hinge. It should be noted, however, that the classification also depends on the geometry of the supported member, which in this case has a length of 6.75 m. It is expected that for different lengths of the supported member, the thresholds for classification may be relaxed or become more stringent, thus altering the connection's classification. The effect of the preload level on the bolts was also evaluated (Figure 4b). Similarly, in this case an IPE 360 beam with a length of 6.75 m, connected to an HEB 320 column, was considered. The plate thickness used was 12 mm, and the bolt position in the slotted length was set to 0 mm, meaning it is located at the end of the slotted hole, near the column flange. The preload level was varied between 20% and 70% of the load corresponding to the bolt's ultimate tensile strength. Similar results were observed for preload levels above 40%, where no bolt slip was observed during the analysis. For lower values, it was noted that bolt slip leads to a decrease in the maximum load supported, but does not significantly affect the initial stiffness of the element. All cases were classified as nominally pinned connections. When evaluating the effect of the distance of the bolt group relative to the column flange (Figure 4c), by varying the bolt positions along the slotted length, it was observed that as the distance between the first row of bolts and the column flange increases, both the stiffness and the strength of the connection decrease. The results were assessed considering a plate with a thickness of 12 mm and a preload level of 70%. When evaluating the effect of using different bolt diameters (Figure 4d), no significant impact on the connection's stiffness was observed. When using M20 bolts, bolt slip was observed, as expected, due to the smaller cross-sectional area, which corresponds to a lower load required to achieve the 70% preload applied. Considering that the joint can be classified as a pinned joint, the main parameter for its design it’s shear resistance. a) b) c) d) Figure 4. M1Cantilever model: a) Effect of plate thickness; b) Effect of bolt preload; c) Effect of bolt position along the slotted hole; d) Effect of bolt diameter 4.2 Shear resistance Following the connection classification, its shear resistance is evaluated. As presented earlier, if the appropriate geometric dimensions are adopted, cleat connections with long slotted holes behave as pins, which should primarily resist shear forces. The concentrated load was applied near the connection region (to generate high shear forces in the connection) and the connection was considered at only one end of the beam. The evaluations presented in this section refers to a connection with 70% of preload on the bolts, which are M24 10.9 bolts. The first evaluation was related to the assessment of the effect of cleat thickness, as shown in Figure 5a. For this, it was needed to consider a cleat with weaker yield strength if compared to the beam. This was done since the beam web of an IPE 360 is less thick than the cleat in all the cases analyzed. However, even considering a beam with S960 steel and a cleat with S690 steel, the failure mode was predominantly by bearing in the beam web, with a contribution of the cleat in bearing. That is, elongation was observed in both plates. The effect of the slotted hole length on the bearing failure of the cleat hole was also evaluated (Fig. 5b). The lengths were varied between 75 mm and 150 mm. It can be observed that the length of the slotted hole has a significant impact on the shear resistance of the connection, considering this failure mode. The effect of different beam sections, with predominantly the same height was assessed (Fig. 5c). This was done in order to assess the effect of beam web thickness in the failure by beam web in bearing. To secure this failure mode, a beam with S355 steel and a cleat with S690 steel was considered. Additionally, it is also presented the shear strength prediction of prEN1993-1-8, based on Može & Beg (2014) model. It could be noted that after the characteristic elongation of the bearing failure in the slotted cleat, bolt pullout occurs, where the bolt starts to penetrate the elongated opening of the slotted hole. To mitigate this effect, the use of a washer plate was evaluated. As shown in Figure 5d, the washer plate prevents the bolt from penetrating the slotted hole opening in the event of bearing failure. a) b) c) d) Figure 5. M2-Simply supported FEM model: a) Effect of plate thickness. b) Effect of slotted hole length. c) Evaluation of beam section (focus on web thickness). d) Effect of using a plate washer. 5 CONCLUSION This research has explored the innovative design and analysis of beam-to-column connections featuring long slotted holes within the framework of the Connect4C project. The aim is to enhance adaptability in steel structures, promoting the principles of the circular economy through designs that facilitate disassembly and reuse. The findings demonstrate that the incorporation of long slotted holes significantly influences the mechanical behavior of joints, particularly in terms of shear resistance and failure modes. The advanced finite element analyses revealed that factors such as plate thickness and bolt preload levels play critical roles in defining the performance and classification of these connections. Notably, the study identified the mechanism of bearing-type failures as a predominant mode, highlighting the need for design models that accurately predict these behaviors and ensure reliable performance under various loading conditions. Further research needs to be conducted, by refining strength models and investigating the interaction between flexure and bearing effects, which will contribute to more practical design recommendations aimed at enhancing the safety and efficiency of bolted connections with long slotted holes. 6 ACKNOWLEDGEMENTS This Research is funded by RFCS, through the grant RFCS-02-2022-101112300, in the frame of the Connect4C Research Project. 7 REFERENCES Cavène, E. et al. 2017. Experimental study of cover-plate bolted joints with large or slotted holes. Cavène, E. et al. 2020. Experimental study of slotted hole bolted cover-plate connection using full field measurement. Structures 23: 573–587. Cavène, E. et al. 2024. 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