Probabilistic Calibration of Target Safety Dependent Safety Factors for the Design of Steel Concrete Composite Beams

Lawal D. Aminu, Jibrin M. Kaura, Uwemedimo N. Wilson

Abstract


This study presents a reliability-based calibration of partial safety factors for steel–concrete composite beams designed in accordance with Eurocode 4. The research evaluates the reliability performance of composite beams under ultimate limit states and calibrates partial safety factors to achieve specified target reliability levels. Limit state functions were developed for flexural and shear failure modes. Uncertainties in concrete compressive strength, steel yield strength, dead loads, live loads, and model uncertainty were represented using probabilistic models obtained from established literature. Structural reliability analyses were conducted using the First-Order Reliability Method (FORM) implemented in CodeCal software. Reliability indices ranging from 2.0 to 4.3 were investigated, with emphasis on the Eurocode target reliability index of β = 3.8. The influence of importance factors on structural reliability was also examined. The results showed that reliability is highly sensitive to load variability and importance factors, with optimum performance occurring at importance factors between 0.5 and 0.8. Reliability-based calibration produced optimized partial safety factors for both flexural and shear limit states. At β = 3.8, the calibrated flexural safety factors were γM = 0.98, γG = 1.59, and γQ = 1.35, while the calibrated shear safety factors were γM = 1.05, γG = 1.42, and γQ = 1.50. Comparison with Eurocode values indicated that permanent-action factors require upward adjustment, whereas material and variable-action factors can be reduced while maintaining the target reliability level. The study concludes that reliability-based calibration provides a rational basis for optimizing partial safety factors for steel–concrete composite beams. The calibrated factors improve the consistency of safety margins and achieve target reliability levels while promoting structural economy and design efficiency. 


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