Bolted steel joints are critical components in steel structures, influencing force transfer, deformation and energy dissipation mechanisms. In traditional analysis of steel structures, beam-column joints are often simplified as either pinned or rigid. While this simplification eases analysis and reduces computational demands, it fails to accurately capture the semi-rigid joint behaviour, underestimating designs, especially in multi-story steel structures where the cumulative effects of joint flexibility and frictional slip can be relevant. The complexity of accurately modelling semi-rigid joints in finite element analysis (FEA) arises from challenges such as geometric and material nonlinearities, frictional contact interactions, and detailed meshing, all of which increase computational demands, necessitating the use of simplified joint behaviour models. This study identifies and attempts to address a persistent gap in efficiently modelling these semi-rigid joints during structural analysis.

The primary goal of this MSc thesis is to investigate the behaviour of semi-rigid joints and propose a relatively cost-efficient approach to incorporating more realistic joint behaviour into structural analysis. To achieve this, the research explores the frictional behaviour and potential relaxation of bolted joints, focussing on the relative sliding of plates in frictional contact. The study specifically examines how these factors influence load transfer, energy dissipation, and overall structural performance. In multi-story constructions, the cumulative effect of small joint relaxations at each story level can lead to increased lateral sway and amplified second-order effects, raising significant concerns about structural integrity.

The research begins with a detailed examination of an individual beam-column joint assembly consisting of top, bottom and web plates welded to the column flange and bolted to the beam flanges and webs. Through numerical simulations in ANSYS, the study analyzed the joint under both monotonic and cyclic loading conditions, focussing on the force transfer mechanisms and relative deformations between various components of the connection. A parametric study, varying friction, bolt pretension, and loading magnitude, was performed to further explore how these parameters influence the stiffness, energy dissipation, and overall behaviour of the joint assembly. Based on the force-displacement response of different components of a beam-column connection, a Component-Based Spring Method (CBSM) is proposed to simplify the modelling of semi-rigid joints in 2-D multi-story frame structures. This method involves creating equivalent spring elements that simulate the load transfer and deformation behaviours observed in various components of a connection. The study then extended the analysis to the global behaviour of a three-story steel frame structure, comparing the performance of rigid and semi-rigid frames. The simplified CBSM approach was validated by applying it to a three-story steel frame structure and comparing the results with those obtained from detailed solid connection modelling.

The investigation into the local behaviour of the beam-column connection identified key components, such as the Top Plate & Top Beam Flange, Web Plate & Beam Web, and Bottom Plate & Bottom Beam Flange, that govern the force transfer and deformation within the joint assembly. The force-displacement curves of each component provided insights into the pre-yield and post-yield behaviour of the connection. The results revealed that the friction coefficient and bolt preload significantly influence the stiffness and energy dissipation capacity of the joints. Higher friction coefficients and bolt pretension enhanced joint stiffness but reduced energy dissipation, indicating a trade-off that must be balanced in design if dynamic vibration is a concern. The Component-Based Spring Method was validated against detailed solid models, achieving accuracy in frame deflection results of 93% to 96% while reducing computational time by 99.5%. The analysis of the global behaviour of joints within the frame structure revealed that semi-rigid frames exhibit significantly higher lateral deflections, up to 40.63% more when compared to rigid frames, highlighting the risks of oversimplified rigid joint models. These findings highlight the importance of accurately modelling the behaviour of semi-rigid joints to ensure realistic structural performance and the safety of steel frame structures.