In-plane, out-of-plane failure and remedial measures

In-plane behaviour (force parallel to the wall)

The wall acts as a shear wall and is stiff and strong. Failure modes:

• Sliding shear: horizontal crack along a bed joint when the vertical load is small.
• Diagonal shear (X-crack): diagonal tension cracking, through the joints (stair-step) or through the units.
• Rocking / toe crushing: a slender wall or pier rotates about its toe, with horizontal cracks at the heel and crushing at the toe.

Out-of-plane behaviour (force perpendicular to the wall)

The wall acts as a thin plate or cantilever in bending; it has little tensile capacity and fails suddenly:

• Overturning / rocking of parapets, gables and free-standing walls about the base.
• Vertical bending: horizontal crack at mid-height between floor and roof.
• Horizontal bending: vertical cracks and collapse at wall junctions or between cross walls.
• Two-way bending: diagonal cracks from the corners when the wall is supported on all edges.

 In-plane              Out-of-plane
 +--------+            ---|||---> force
 |\  X  /|             |wall|  crack at mid-height
 | \  / |  F ->        |~~~~|
 +--------+            ====== base

Remedial and preventive measures

1. RCC or timber bands (plinth, lintel, roof) to tie the walls together and give box action.
2. Vertical reinforcement at corners, junctions and jambs, anchored in the foundation and the roof band.
3. Limit the height-to-thickness and length-to-thickness ratios; provide cross walls, buttresses or pilasters.
4. Small openings (total width limited to about one-third to one-half of the wall length, depending on the number of storeys, as in IS 4326), located away from corners, with a lintel band.
5. A rigid diaphragm (RCC slab) or a properly braced roof anchored to the walls.
6. Good-quality bricks and mortar, proper bond and curing.
7. Repair and retrofit of existing buildings: grouting of cracks, steel mesh with plaster, ferrocement jacketing, and installation of bands.