Retrofitting

Many existing buildings were designed before current seismic rules or have deteriorated. Retrofitting raises their capacity or reduces their demand so that they can survive a design earthquake. It starts with an evaluation: a rapid visual screening to prioritise buildings, then a detailed assessment of materials, reinforcement, irregularities and expected performance. In India, IS 15988 gives guidelines for seismic evaluation and strengthening of existing reinforced concrete buildings.

Member-level strategies strengthen weak elements. Reinforced concrete jacketing adds a new shell of concrete and steel around a column or beam, increasing strength, stiffness and confinement. Steel jacketing and fibre-reinforced polymer wrapping confine columns and add shear strength with little increase in size. Weak beam-column joints can be strengthened by jacketing through the joint.

Global strategies change how the building as a whole resists earthquakes. New shear walls or steel bracing add strength and stiffness and reduce drift, relieving weak columns. Infilling an open ground storey, or adding walls there, removes a soft storey. Base isolation or dampers reduce the demand instead of raising capacity, which suits heritage buildings where intervention must be minimal. Foundations must be checked for any new loads.

The chosen scheme balances safety, cost, disruption and how long the building can be closed. Retrofitting is most effective when it removes the main deficiency, often a soft storey or weak columns, rather than spreading small improvements everywhere.