Guidelines for Seismic Design of Road Bridges
2018 Edition

IRC SP 114 suggests selecting seismic analysis techniques based on bridge type, pier height, span length, ground conditions, and seismic zoning. For simpler cases like simply supported spans with lower pier heights in zones II and III, the Elastic Seismic Acceleration Method (ESAM) is advised. For medium to complex bridges, especially with higher piers or in zones IV and V, the Elastic Response Spectrum Method (ERSM) is preferred. Time History Analysis is reserved for critical structures or those on complex sites. The standard provides detailed tables correlating bridge characteristics with suitable methods to ensure accurate seismic response evaluation.

The code mandates that reinforced concrete bridge components use under-reinforced sections to promote ductile flexural failure, with closely spaced transverse reinforcement in plastic hinge zones to confine concrete and prevent buckling of longitudinal bars. Plastic hinges are designed through capacity-based approaches to ensure predictable inelastic behavior. For steel structures, detailing focuses on preventing premature buckling in compression zones and ensuring ductile joint behavior, allowing only plastic or compact sections in critical regions. These provisions aim to achieve overall ductile performance, essential for seismic resilience in zones III, IV, and V.

Seismic isolation devices per IRC SP 114 are suitable for multi-span continuous bridges with fundamental periods up to 1 second, excluding Type III soft soils. These devices function by elongating the structure's period and enhancing damping, thereby reducing seismic forces and displacements. Types covered include low and high damping elastomeric bearings, lead-rubber bearings, viscous fluid dampers, and friction sliding dampers. Design verification requires thorough testing of strength and displacement capacities, except for simple elastomeric low damping and flat sliding bearings. Detailed parameters such as damping ratios, force-displacement characteristics, and maximum allowable displacements must be obtained from manufacturers.

Hydrodynamic effects on submerged bridge components are considered by evaluating horizontal water pressures induced during seismic events. Two principal approaches are used: the cylinder analogy method, applied with static seismic coefficient methods, calculates total hydrodynamic force and pressure distribution; and the added mass method, employed in dynamic analyses like Response Spectrum or Time History, incorporates the mass of water acting with the structure. These forces act up to the scour depth on piers, wells, piles, and connecting elements and must be combined with inertial seismic forces to ensure comprehensive structural safety.

Assessment involves evaluating soil layers beneath the water table for loose sands susceptible to liquefaction. Standard Penetration Tests (SPT) and Cone Penetration Tests (CPT), along with laboratory grain size analyses, are performed. The Factor of Safety (FOS) against liquefaction is computed as the ratio of Cyclic Resistance Ratio (CRR) to Cyclic Stress Ratio (CSR). If FOS is less than one, the soil is considered liquefiable, necessitating mitigation measures such as ground improvement or foundation design modifications. The evaluation process follows the detailed criteria and formulas outlined in IRC SP 114, particularly in Appendix A-5.