In-plane and out-of-plane behaviour of masonry

Masonry walls resist lateral load (earthquake, wind) in two ways, depending on the direction of the force relative to the plane of the wall.

In-plane behaviour

The force acts parallel to the plane of the wall; the wall acts as a shear wall and is strong and stiff in this direction. Failure modes:

1. Sliding shear (horizontal cracks): sliding along a bed joint, usually near the base or at a damp-proof course, when the vertical load is small.
2. Diagonal shear (X-cracks): the principal tension exceeds the tensile strength of the masonry, giving stair-stepped cracks through the joints or cracks through the bricks; the most common failure of squat walls.
3. Rocking (flexural failure, toe crushing): a tall, slender wall or pier rotates about its toe, with horizontal cracks at the heel and crushing at the toe.
4. Pier and spandrel failure in walls with openings: short piers between openings fail in shear (diagonal cracking or bed-joint sliding), and spandrels crack in shear or flexure.

 Diagonal shear     Sliding shear      Rocking / toe crush
 +--------+         +--------+         +--------+
 |\      /|         |        |         |        |
 | \    / |         |        |         |  ----  |  horiz. crack
 |  \  /  |   F ->  |========| F ->    |        |
 |  /  \  |         |  slide |         |        |
 | /    \ |         +--------+         +--/---\-+
 +--------+                              toe crush

Out-of-plane behaviour

The force acts perpendicular to the plane of the wall; the wall bends like a slab spanning vertically and/or horizontally. Masonry has very low tensile strength and no ductility, so this is the weakest direction.

1. Overturning / rocking as a cantilever: gables, parapets and free-standing walls tilt about their base (horizontal crack at the base).
2. Vertical bending between floor and roof: horizontal crack at mid-height; the wall spans one way like a beam.
3. Horizontal bending between cross walls: vertical cracks at the corners where walls meet; the wall then bulges and collapses outward.
4. Two-way bending (arching action): when the wall is supported on all sides, cracks run diagonally from the corners of the wall towards the centre (like a yield-line pattern).
5. Delamination of the leaves in thick or multi-leaf walls, and separation of walls at corners because of poor connection.

 Corner separation        Vertical bending (mid-height crack)
   |   |                   +------------+
 --+   +--               ->|            |   <- force
   | crack |               |~~~~~~~~~~~~|  mid-height crack

Remedies

Provide RCC bands and tie beams at plinth, lintel and roof levels; vertical bars at corners and jambs; adequate cross walls and rigid diaphragm roofs; limits on wall height and length (H/t and L/t); small openings kept away from corners; and good mortar and bond.