Causes and crack patterns of out-of-plane failure

Causes of out-of-plane failure

Out-of-plane failure occurs when the lateral (inertia) force acts perpendicular to the wall plane and the wall bends. The main causes are:

1. Low tensile and flexural strength of masonry and weak mortar bond.
2. High slenderness: large height-to-thickness or length-to-thickness ratio.
3. No lateral support: missing or weak connection to cross walls, floors and roof; flexible roof not acting as diaphragm.
4. No bands or ties at floor and roof; weak connection at wall corners.
5. Large openings and long unsupported spans, parapets and gables unbraced.
6. Heavy roof or floor that increases inertia force; poor construction (poor bond, thin walls).
7. Cantilever or free-standing parts (parapets, boundary walls).

Two typical crack patterns

1. Horizontal crack at mid-height (or base) of the wall: the wall bends vertically between floor and roof supports and breaks like a beam; parapets overturn with a horizontal crack at the base.
2. Vertical crack at wall corners or junctions (corner separation): the wall bends horizontally between cross walls, and the two walls separate and fall outwards. In walls supported on four sides diagonal cracks from corners to the centre appear.

 Horizontal crack           Corner crack
  |    |                     |   |
  |~~~~|  <- mid-height     -+   +-
  |    |                    crack