Seismic retrofit is the modification of an existing structure to improve its resistance to earthquake loading. It is a critical practice for aging buildings, bridges, and infrastructure that were designed to outdated codes or no seismic provisions at all.
Seismic retrofit is the set of interventions applied to an existing structure to improve its seismic performance. Retrofitting is necessary when a structure was designed to outdated codes, when its use has changed to a more critical occupancy category, when damage or degradation has reduced its capacity, or when new information about seismic hazard at its site has emerged. Unlike new design, retrofit must work within the constraints of an existing structure β its geometry, materials, foundations, and operational requirements β which makes retrofit inherently more challenging and often more expensive per unit of performance improvement.
Retrofit strategies fall into three broad categories. Local strengthening targets specific deficient elements β adding steel jackets or FRP wraps to columns, adding shear reinforcement to beams, or strengthening beam-column joints. Global strengthening modifies the entire structural system β adding new shear walls, braced frames, or moment frames; installing base isolation or supplemental damping devices; or introducing new lateral force paths. Demand reduction reduces the seismic demand on the structure β typically through base isolation (for stiff structures) or damping devices (for all structures), which reduce both force and displacement demands. The choice of strategy depends on the structure type, the performance objectives, the available budget, and the constraints on construction (occupancy, access, downtime).
Seismic retrofit is increasingly important worldwide because much of the built environment predates modern seismic codes. In the United States, thousands of older buildings in high-seismic regions have been retrofitted β often in response to specific ordinances (such as Los Angeles' URM and soft-story ordinances, or San Francisco's soft-story ordinance). In Japan, retrofit became a national priority after the 1995 Kobe earthquake. In Iran, seismic retrofit of existing buildings, schools, hospitals, and infrastructure is an ongoing national program, particularly in Tehran and other high-hazard regions. The choice between retrofit and replacement is rarely simple: retrofit typically has a lower environmental footprint, preserves historic or cultural value, and avoids the social disruption of demolition and new construction, but it can also be more expensive than replacement if the existing structure is severely deficient. Modern performance-based retrofit frameworks β such as ASCE 41 in the United States β define specific performance objectives (immediate occupancy, life safety, collapse prevention) for different seismic hazard levels and provide analytical procedures for demonstrating compliance. The most common retrofit technique in Iran involves adding steel or concrete shear walls, adding steel bracing, or using FRP wraps to strengthen masonry and concrete structures.