Investigation of Bolt Force Reduction Mechanisms in Extended End-Plate Steel Connections Through Numerical Analysis

Abstract

Thousands of existing steel structures rely on standard gravity-designed bolted end-plate connections that fail suddenly under extreme lateral loads, offering no warning and incurring massive remediation costs through full member replacement. This paper presents a systematic parametric study on the strength enhancement of bolted extended end-plate beam-to-column connections using the Component-Based Finite Element Method (CBFEM) implemented in IDEA StatiCa. A baseline connection — comprising an HE 340M column, IPE 500 beam, M36 Grade 10.9 bolts, and a 40 mm end-plate designed to EN 1993-1-8:2005 — is validated against published analytical capacities. Nine distinct joint configurations are then evaluated by independently varying end-plate thickness (40, 45, 50 mm), bolt diameter (M36, M40, M42 Grade 10.9), and stiffener thickness (50, 55, 60 mm) under two loading regimes: elastic design demand (LC1: 834 kNm / 368 kN) and capacity design demand (LC2: 1072 kNm / 448 kN). Results show that increasing end-plate thickness from 40 mm to 50 mm reduces maximum bolt tension by 17.8 %, while increasing stiffener thickness from 50 mm to 60 mm reduces bolt forces by 15.3 %. Increasing bolt diameter beyond M36 is geometrically infeasible owing to bolt-overlap failure. The optimal configuration — 50 mm end-plate combined with 60 mm stiffeners and M36 10.9 bolts — satisfies all Eurocode utilisation checks and shifts the governing failure mode from brittle bolt rupture to ductile beam yielding, demonstrating that localised connection-level upgrades are a cost-effective alternative to full primary-member replacement.

Introduction

This study investigates the structural performance and retrofit optimization of bolted extended end-plate beam-to-column moment connections in steel moment-resisting frames (MRFs). These connections are widely used because they are economical, easy to fabricate, and provide reliable structural performance. However, many existing buildings were designed only for gravity loads and lack seismic detailing, making their connections susceptible to brittle failures such as bolt rupture or end-plate fracture under strong lateral loads. Replacing entire structural members is expensive and disruptive; therefore, this research explores localized, cost-effective strengthening techniques by modifying the end-plate thickness, bolt diameter, and column stiffener thickness using the Component-Based Finite Element Method (CBFEM).

The literature review shows that previous studies have focused mainly on seismic behavior, capacity design, and biaxial loading of end-plate connections. However, research on monotonic loading, multi-directional stress interactions using CBFEM, and practical retrofit strategies for existing gravity-designed structures remains limited. This study addresses these gaps by developing and evaluating code-compliant strengthening solutions that improve connection performance without replacing primary steel members.

A baseline connection consisting of an IPE 500 beam, HE 340M column, 40 mm end plate, 14 M36 Grade 10.9 bolts, and 50 mm continuity stiffeners was modeled according to EN 1993-1-8. The connection was analyzed using CBFEM in IDEA StatiCa, where steel plates were represented by shell finite elements with nonlinear material behavior, bolts were modeled as nonlinear spring elements, and contact, prying action, welds, and panel-zone deformation were accurately simulated.

Three parametric studies were conducted. The first investigated end-plate thicknesses of 40, 45, and 50 mm to evaluate reductions in prying action and bolt tension. The second examined bolt diameters (M36, M40, and M42) to assess whether larger bolts could increase connection capacity while satisfying Eurocode spacing requirements. The third evaluated continuity stiffener thicknesses of 50, 55, and 60 mm to determine their influence on column panel-zone stiffness and force distribution.

The finite element model was first validated against analytical calculations from EN 1993-1-8 under service and ultimate loading conditions. The numerical results closely matched theoretical predictions, with differences below 0.3%, confirming the accuracy of the CBFEM model. Under ultimate loading, the baseline connection reached 98.7% bolt utilization, indicating that bolt failure would govern the connection, which is consistent with previous experimental observations of gravity-designed joints.

The parametric study produced several important findings. Increasing the end-plate thickness to 50 mm significantly reduced prying action and lowered maximum bolt tension by approximately 17.8%, resulting in a more uniform force distribution among the bolts. Increasing the stiffener thickness to 60 mm reduced bolt-force concentration by approximately 15.3% by minimizing column web deformation and flange bending, thereby improving load sharing within the connection. In contrast, increasing the bolt diameter to M40 or M42 was found to be impractical because the larger bolts violated Eurocode minimum edge-distance requirements and introduced the risk of plate tearing. Consequently, the standard M36 Grade 10.9 bolts remain the optimal and code-compliant choice for the connection geometry studied.

A consolidated comparison of all nine models identified the most effective retrofit solutions. The 50 mm end plate provided the greatest reduction in bolt stresses through improved plate stiffness and reduced prying action, while the 60 mm continuity stiffeners enhanced column rigidity and improved force redistribution. Combining these feasible modifications offers the most efficient and economical retrofit strategy, shifting the connection behavior from brittle bolt failure toward a more desirable ductile structural response without requiring replacement of major steel members. The study demonstrates that localized connection strengthening can substantially improve the safety, durability, and seismic performance of existing steel moment-resisting frames while remaining practical and compliant with Eurocode design requirements.

Conclusion

The CBFEM-based parametric study evaluates localized strength enhancements for standard gravity-designed bolted extended end-plate connections. The following conclusions are drawn: 1) Validation Fidelity: The IDEA StatiCa CBFEM model precisely replicates EN 1993-1-8 analytical values for the baseline connection, validating its appropriateness as a tool for systematic parametric assessment, with discrepancies remaining below 0.3%. 2) Prying Action Mitigation: Increasing the end-plate thickness from 40 mm to 50 mm limits plate bending stiffness, directly curbing secondary prying action and resulting in a 17.8% reduction in maximum bolt tension under capacity design demand (LC2). 3) Geometric Constraints: Upsizing the bolt diameter from M36 to M40 or M42 is geometrically infeasible for the IPE 500 configuration. This underscores the necessity of assessing code-mandated edge/end distance overlaps prior to initiating analytical bolt-diameter optimizations. 4) Flange & Web Control: Increasing horizontal continuity stiffeners from 50 mm to 60 mm reduces bolt forces by 15.3%. This is achieved primarily by suppressing local column-flange bending and column-web panel shear deformations. 5) Failure Hierarchy Shift: The combined optimal retrofit (50 mm end-plate, 60 mm stiffener, M36 10.9 bolts) successfully satisfies all Eurocode utilization checks. The connection achieves the intended capacity design hierarchy, shifting the failure mode from abrupt, brittle bolt rupture to desirable, ductile beam yielding. 6) Cost-Effective Retrofitting: The upgrade is executed entirely through local connection modifications (plate replacement and stiffener welding) without altering the primary structural members. This represents a highly cost-effective seismic strengthening strategy for vulnerable gravity-designed steel connections. Future investigations should expand the parametric space to include diverse column and beam sizes and integrate reversed-cyclic loading protocols to analyze the low-cycle fatigue capacity of the upgraded configuration.

References

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Copyright

Copyright © 2026 Basanth Krishnan N, Dr. Krishnachandran. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.