Seismic Evaluation, Repair and Strengthening of Masonry Buildings - Guidelines
2009 Edition

IS 13935 outlines specific methods for repairing cracks in masonry walls based on crack size: For minor to medium cracks (0.5 mm to 5 mm), pressure injection of non-shrink cement-polymer grout through ports spaced roughly at wall thickness is recommended, starting injection from the bottom or one end after cleaning the cracks. For larger cracks exceeding 5 mm or areas with crushed concrete, removal of loose material followed by filling with expansive or quick-setting cement mortar is advised, along with additional shear or flexural reinforcement if necessary, and covering with mortar. Severely damaged members may require replacement. Additionally, wire mesh strengthening involves applying 20-40 mm thick cement or micro-concrete layers on both sides of the wall reinforced with galvanized steel wire fabric connected by rods spaced 300-400 mm apart, ensuring durable structural restoration.

The standard classifies buildings into Important and Ordinary categories based on occupancy and function: Important buildings (with an importance factor of 1.5) include hospitals, schools, monuments, emergency service buildings, large community halls, power stations, important industrial facilities, VIP residences, and any building housing more than 100 occupants. Ordinary buildings have fewer than 100 occupants and do not fall into the important category. This classification influences seismic design forces, with important buildings subjected to 50% higher design forces, impacting retrofitting and design requirements as per IS 4326 and IS 13828.

For strengthening masonry openings, IS 13935 recommends use of galvanized wire mesh reinforcement (typically gauge 10 or 13) with wire spacing at 25 mm. Vertical seismic belts are installed on jambs and piers around door and window openings, ensuring coverage on both faces of the wall. Typical belt widths vary from 200 mm to 580 mm depending on building category and wall length. Wire mesh is combined with single or paired high-strength steel bars (HSD or TOR) of diameters ranging from 8 mm to 16 mm, depending on the structural requirements. Additional strengthening can include steel beam lintels or flat iron ties to reduce arch thrust and prevent cracking.

IS 13935 in conjunction with IS 1893 Part 1 identifies various plan irregularities such as torsion effects, re-entrant corners, diaphragm discontinuities, out-of-plane offsets, and non-parallel lateral systems, as well as vertical irregularities including mass and geometric changes and discontinuities in vertical lateral force-resisting elements. Buildings exhibiting these irregularities, especially in seismic zones III, IV, and V, require detailed seismic evaluation, often involving advanced nonlinear or dynamic analyses. Plan irregularities can increase expected damage by one grade, while vertical irregularities can cause severe damage, necessitating specialized design or strengthening measures. Special attention is also needed for structures on liquefiable or landslide-prone sites.

The standard specifies the use of cement and steel as primary repair materials, including steel bolts, rods, angles, and expanded metal. Admixtures enhancing mortar or concrete properties, such as non-shrink and bonding agents, are used. Reinforcing meshes and micro-concrete (fine aggregate concrete with fibers or ferrocement) are applied to cracked surfaces. Epoxy or polymer-cement mixtures are utilized for crack injection and bonding. Repair techniques involve component-wise masonry removal and reconstruction with richer mortar, application of reinforcing mesh fixed to both sides covered with micro-concrete, and epoxy grouting for RC elements. Seismic strengthening includes increasing lateral strength by enlarging structural members, improving connections for load transfer, removing weak features, and avoiding brittle failure modes through proper reinforcement.