Design Optimization of Railway Steel Bridges Utilizing High Frequency Mechanical Impact Treatment

Sammanfattning

Railway bridges are highly susceptible to fatigue, which typically governs their design. By applying High Frequency Mechanical Impact (HFMI) treatment to enhancefatigue strength, new opportunities arise to effectively utilize Higher Strength Steel(HSS) without increasing dimensions for the cross-section. To evaluate this potential, a Parametric Design Model (PDM) was developed in MATLAB and combinedwith a Genetic Algorithm (GA) to optimize simply supported, single-track, twinI-girder railway bridges (10–40 m spans) across steel grades S355, S460, and S690.The GA optimizes the cross-sectional geometry to minimize total initial production costs taking encompassing material, welding, and painting in to account whilestrictly verifying the designs against Fatigue Limit State (FLS), Ultimate LimitState (ULS), and Serviceability Limit State (SLS) criteria.This thesis then investigates whether it is profitable, and in which combinations ofHFMI and HSS, the potential for material and cost savings is greatest. The results demonstrate that the most significant cost savings are achieved using HFMIin combination with steel grade S355. While steel grades S460 and S690 providesome benefits compared to the As-Welded (AW) case for shorter span lengths of10 m, its economic viability is severely restricted for longer spans. A key findingregarding the failure modes is that while FLS strictly governs the AW that is usedas the reference designs case, the application of HFMI shifts the governing failuremode to ULS for shorter spans, and entirely to SLS deflection for spans over 20 m.To get representative results the optimization was done considering total investmentcost. Conversely, data was analysed comparing both impact on CO2-emissions andtotal Life Cycle Cost (LCC). These comparisons implied some even greater potentialfor design optimization utilizing HFMI if full life cycle were to be considered. Inaddition, an easier comparisons of fatigue design methods was made, which indicatedthat the positive benefits from HFMI is method independent.