Earthquake protection of load bearing masonry

Principles for masonry

• Use a simple, symmetrical plan and avoid long walls (keep the length between cross-walls to about 4-6 m).
• Keep the building low (limit the number of storeys as per NBC 202 / IS 4326).
• Use good mortar (cement-sand 1:4 to 1:6, not mud) and fill all joints; use bricks with a crushing strength of at least 3.5 N/mm^2 (class as per code).
• Keep openings small and well placed (see below).

Measures to make masonry earthquake resistant

1. Horizontal bands: RC plinth, lintel and roof bands tying all the walls together.
2. Vertical reinforcement: steel bars at corners, T-junctions and jambs of doors/windows, anchored in the foundation and the roof band. Typically one bar of 10 mm (single storey) to 12 mm (more storeys) at each corner.
3. Openings: total width of openings not more than 50% of the wall length in a single storey; piers between openings at least 340 mm wide; openings at least 600 mm from an inside corner (or as per the NBC 202 table).
4. Cross walls and buttresses: provide cross-walls within the spacing limits, and buttresses on long walls, to prevent out-of-plane failure.
5. Good bonding at corners: use toothing or stitching (and dowel bars) so that the walls do not separate.
6. Light, well-anchored roof: use light roofing and anchor trusses or rafters to the roof band; avoid heavy mud roofs and unanchored parapets, chimneys and gable ends.
7. Strong foundation: firm, uniform soil with a plinth band; avoid mixed foundations.
8. Seismic gaps and separation: for L or T plans.
9. Quality control: good workmanship and curing.

 Wall elevation with bands and vertical bars
  |==================| roof band
  | |  []     []  |  |  vertical bar at jamb
  |==================| lintel band
  |                  |
  |==================| plinth band
  ######## footing

Factors to be considered for seismic safety (configuration)

• Simple rectangular plan with symmetrical walls; length : breadth not more than 3.
• Regular elevation with continuous walls; low height and a light roof.
• Limited, well-placed openings.
• Cross-walls at regular spacing; good corner bonding.
• Uniform foundation on firm soil, with a plinth band.
• Seismic gap between different blocks.
• Good-quality bricks, mortar and workmanship.