Lateral load effect on wall with and without opening

Wall without opening

A solid wall acts as a monolithic shear wall (in-plane) or plate (out-of-plane). The lateral force is carried by one wide section, so the stiffness and strength are high. Typical failure: a single large diagonal crack, or sliding or rocking at the base. The capacity is calculated on the full length $L$: $V = f_s\,L\,t$.

Wall with openings

Doors and windows break the wall into piers (vertical strips between openings) and spandrels (horizontal strips above and below the openings).

• Stiffness and strength fall sharply; the lateral force is shared among the piers in proportion to their stiffness (stiffness $\propto$ $t$ and falls with $(h/L)^3$ for slender piers).
• Stress concentrations at the corners of openings cause diagonal cracks radiating from the corners.
• Short, stocky piers fail in shear; slender piers fail by rocking (flexure); spandrels crack in shear and flexure.
• Out-of-plane: the section is weaker at the opening, cracks run from the corners of the opening to the edge, and the wall can fail at lintel or sill level.

 No opening          With opening
 +---------+         +----+-----+----+
 |  \   /  |         | pier|open| pier|
 |   \ /   |   ->    | /   |    |  \  |
 +---------+         +----+-----+----+
  single X-crack      cracks from the corners

Hence IS 4326 limits the total length of openings (not more than 50 percent of the wall length for one storey, 42 percent for two storeys and 33 percent for three storeys), sets a minimum distance of an opening from an inside corner and a minimum width of the pier between openings (the values depend on the building category), and requires lintel bands and sill bands, or reinforcement around openings.