Multi Hazard Seismic Fragility of Very Long Span Cable Stayed Bridges with Tuned Mass Dampers: Parametric Study from 500 M To 700 M Pylons and Programmatic Validation

Cable stayed bridges with ultra-tall pylons are proposed for deep water and strait crossings, yet multi hazard performance and tuned mass damper (TMD) effectiveness across pylon heights remain uncertain. This study presents a reproducible framework for nonlinear time history analysis and seismic fragility of long span cable stayed bridges, explicitly modeling geometric nonlinearity, cable sag, and bearings. A parametric study varies pylon height from 500 to 700 m in 50 m steps for a 1,800 m prototype with fixed base and soil structure interaction models. Tower top TMD, distributed multiple tuned mass dampers (MTMDs), and a combined scheme are evaluated. Fragility functions use maximum likelihood estimation with Sa (T1, 5%) as intensity measure. Wind response uses a Davenport spectrum with aerodynamic admittance and mode shape integrals. A 600 m pylon is near optimal; 500 m raises inertial forces and 700 m increases drifts and buffeting. The combined scheme, with a 1.5% tower top mass ratio and eight MTMDs over the central 40% totaling 2.0%, shifts damage stage 2 (DS2) fragility medians by 45 to 75% and reduces wind root mean square (RMS) deck displacements by 38 to 52%, with local shear rises under 5% near attachments. Optimal MTMD spacing is 0.05L with symmetric twin units per station for torsion control. Validation gives modal frequencies within 3% of a finite element (FE) model and agrees with analytical TMD optima, supporting replication and extension to site specific hazards and geometries.