Seismic retrofitting is the modification of existing structures to make them more resistant to seismic activity, ground motion, or soil failure due to earthquakes. With better understanding of seismic demand on structures and with our recent experiences with large earthquakes near urban centers, the need of seismic retrofitting is well acknowledged.
State- of – the-art technical guidelines for seismic assessment, retrofit and rehabilitation have been published around the world- such as the ASCE- SEI 41 and the New Zealand Society for Earthquake Engineering (NZSEE)’s guidelines.
The retofit techniques outlined here are also applicable for other natural hazards such as tropical, tornadoes, and several winds from thunderstorms. Whilst current practice of seismic retrofitting is predominantly concerned with structural improvements to reduce the seismic hazard of using the structures, it is similarly essential to reduce the hazard and losses from non- structural elements.
Retrofit strategies are different from retrofit techniques, where the former is the basic approach to achieve an overall retrofit performance objective, such as increasing strength, increasing deformability, reducing deformation demands while the latter is the technical methods to achieve that strategy, for example FRP jacketing.
Increasing the global capacity (strengthening). This is typically done by the addition of cross braces or new structural walls.
Reduction of seismic demand by means of supplementary damping and/ or use of base isolation systems.
Increasing the local capacity of structural elements. This strategy recognises the inherent capacity within the existing structures, and therefore adopt a more cost- effective approach to selectively upgrade local capacity (deformation/ ductility, strength or stiffness) of individual structural components.
Selective weakening retrofit. This is a counter intuitive strategy to change the in elastic mechanism of the structure, while recognising the inherent capacity of the structure.
Allowing sliding connections such as passage way bridges to accommodate additional movement between seismically independent structures.
Addition of seismic friction dampers to brace weak structures and provide damping.