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SEISMIC DESIGN OF RAILWAY BRIDGES
Serge MONTENSSYSTRA
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SPECIFICITIES OF RAILWAY BRIDGES
• HIGH LONGITUDINAL PIER STIFFNESS REQUIRED:Rail‐structure interaction requirements are severe: check differential longitudinal displacement underbraking‐acceleration loads < 5 mm
HIGH PIER STIFFNESS → SHORT VIBRATION PERIOD → HIGH SEISMIC ACCELERATION
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• RAIL‐STRUCTURE INTERACTION ANALYSIS ALLOWS CONSIDERING DISPATCHING OF BRAKING‐ACCELERATION LOADS THROUGH THE RAILS
→ LESS CONSERVATIVE DESIGN OF PIER STIFFNESS → REDUCTION OF LONGITUDINAL SEISMIC FORCES
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• IN HIGH SEISMIC AREAS, RAIL‐STRUCTURE INTERACTION ANALYSIS IS REQUIRED IN ORDER TO DESIGN LESS STIFF PIERS, AND OPTIMIZE THE SEISMIC BEHAVIOUR
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USE OF LOCK‐UP DEVICES (OR SHOCK TRANSMISSION UNITS)
• Lock‐up devices could be used to dispatch the loads under earthquake to several piers, but check RSI displacement at SLS (problem of gap when loading lock‐up device)
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‐ Transverse bearings necessary on abutments→ R = 1 (response modification factor) for smallbridges
‐ Long viaducts in valleys: piers of variousheights→ no simultaneous plastic hinging at all piers→ R small, often R = 1
TRANSVERSE EARTHQUAKE
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« SERVICE » EARTHQUAKE
• FOR MODERATE EARTHQUAKES, THE TRAIN OPERATION SHOULD RESTART AFTER THE EARTHQUAKE:
→ REDUCED EARTHQUAKE HAS TO BE DEFINED (WITH REDUCED RETURN PERIOD), not defined in Eurocodes
→ THE STRUCTURE HAS TO BE CHECKED FOR THIS REDUCED EARTHQUAKE: NO REINFORCEMENT YIELDING, LIMITED HORIZONTAL DISPLACEMENTS
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EXAMPLE OF SHEAR KEYS FOR HSR VIADUCTS
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DAMPERS
• DAMPERS CAN BE USED IN ORDER TO DECREASE THE DECK ACCELERATION AND SUBSTRUCTURES SEISMIC FORCES
• THE BRIDGE MUST BE USABLE AFTER « SERVICE » EARTHQUAKE → FUSE ELEMENTS CANNOT BE USED
• THE LONGITUDINAL DISPLACEMENT UNDER « SERVICE » EARTHQUAKE MUST BE LIMITED TO 20mm IN ORDER TO LIMIT RAIL STRESSES
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EXAMPLES OF DAMPERS
• A9 VIADUCT ON PERPIGNAN – FIGUERAS HSR LINE: 10 DAMPERS OF 1500 kN
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EXAMPLES OF DAMPERS
• VENTABREN VIADUCT ON MEDITERRANEAN HSR LINE
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CONCLUSION
• IN HIGH SEISMIC AREAS, RAIL‐STRUCTURE INTERACTION ANALYSIS IS REQUIRED IN ORDER TO DESIGN LESS STIFF PIERS, AND THEN OPTIMIZE THE SEISMIC BEHAVIOUR
• FREQUENTLY WE HAVE R=1 TRANSVERSELY• « SERVICE » EARTHQUAKE HAS TO BE DEFINED, AND SPECIFIC CHECKING PERFORMED
• DAMPERS CAN BE USED, BUT CAREFULLY