Seismic Evaluation and Strengthening of Existing Reinforced Concrete Buildings - Guidelines
2013 Edition

Key factors for seismic vulnerability assessment include using adjusted seismic forces that reflect reduced service life and acceptable risk, evaluating structural strength, ductility, stiffness, and continuity, checking for material degradation such as cracks and corrosion, and applying performance objectives that prioritize life safety. The assessment also incorporates allowances for data uncertainties and references international guidelines like FEMA and Eurocode for comprehensive evaluation and retrofit planning.

For buildings with inadequate lateral force resisting elements, the standard recommends member-level strengthening by retrofitting individual deficient columns and beams through techniques such as concrete or steel jacketing to enhance their strength, stiffness, and ductility. When multiple critical members are deficient, structural-level reinforcement by adding braced frames or shear walls is advised to improve overall lateral stiffness and reduce displacement. Additionally, the introduction of new structural elements like shear walls and steel bracing can significantly augment the building's lateral load capacity.

The evaluation of shear stress involves calculating average shear stresses in columns and shear walls by dividing applied shear forces by their respective cross-sectional areas. In compression columns, shear capacity must be adequate to develop the required moment capacity, following IS 13920 guidelines for shear reinforcement detailing. The standard emphasizes adequate anchorage and the use of shear transfer reinforcement (such as dowel bars) in walls added between columns. Compliance with IS 13920 ensures proper shear design and reinforcement spacing.

Supplemental damping and base isolation serve to enhance seismic performance by reducing structural demands. Base isolation minimizes seismic forces transmitted to the building by decoupling it from ground motion, making it especially effective for stiff, low-rise, heavy-mass structures. Supplemental damping involves installing energy dissipation devices that absorb seismic energy, reducing displacements and vibrations, which is beneficial for flexible buildings capable of inelastic deformation. Both approaches are selected based on the building’s dynamic properties to improve resilience.

Irregularities such as weak or soft storeys and mass discontinuities must be managed by identifying them first, then mitigating their effects through structural interventions. Adding shear walls or braced frames at weak storeys, with at least two walls per orthogonal direction positioned opposite each other and away from the stiffness center, helps increase strength and reduce torsional irregularities. Balancing the stiffness and mass centers minimizes eccentricities. For plan irregularities, seismic gaps can be introduced to separate the building into regular blocks, preventing pounding. Ensuring these new elements are integral to the lateral load path is critical for effective performance.