Seismic safety of Masonry buildings

General guidance regarding the seismic safety of masonry buildings

  • A single storey building using one brick walls will be relatively safer than three storey one. The fourth storey, if added, will be unsafe, and make the tower stories more vulnerable.
  • Use of half-brick thick (10 to 11.5 cm or 4 to 4.5” thickness) load bearing walls will make the storey very unsafe during seismic intensity VIII on MSK intensity scale and if used in 3rd or 4th storey, it may have catastrophic failure.
  • Too many window openings make a wall weaker, and use of smaller size piers less than 18 inches (45cm) in width between them will increase the damageability even higher.
  • Richer cement-sand mortar of 1:4 mixtures (1part cement by 4 parts of sand) makes the masonry stronger against earthquake shaking as compared with 1:6 mortars by factor of 2.5 to 3. Also 1:6 mortars are stronger than lime-cinder or lime-surkhi mortar.
  • Use of clay mud mortar produces the weakest masonry. Its strength in dry condition reduces to less than 50% when the walls get wet during rains. Hence, use of good plastering is essential to protect such masonry during rainy months.
  • Longer walls between consecutive cross walls are found weaker than shorter walls. The length is controlled by limiting its height to thickness ratio.
  • Taller walls between any two floors are found to be weaker than shorter walls. The storey height is controlled by limiting its height to thickness ratio.
  • All four walls enclosing a room should be properly connected at each corner. Walls not so connected will easily separate at corners and overturn under the earthquake motion.
  • The most important seismic safety requirement is provision of seismic bands in all stories in all external as well as internal walls. These bands maintain the integrity of the whole building as one unit under earthquake shaking. Besides the earthquake safety, they also increase the stability of the walls under the vertical loads.
  • The roof structure of the sloping roofs needs its integrity through bracing and proper connectivity with the walls. Such integrity is automatically provided by reinforced concrete slabs wherever used for floors and the roof.