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Lintel loading and arching action in walls

Short answer

Arching action lets masonry above an opening self-support, transferring load to piers. Only masonry weight within an equilateral triangle of height 0.866L above the lintel is considered, plus any floor loads inside that triangle. This reduced load is used for lintel design; piers carry the transferred loads and must have adequate width.

Explain how loads from the wall above an opening and from floors are considered in the design of lintels and in walls with openings. What is arching action?

When an opening is formed in a wall, the load of the masonry above it is not carried fully by the lintel. The masonry above acts as an arch (arching action) when it is strong enough, and the load tends to flow around the opening to the adjacent piers.

Arching action

Masonry over an opening forms a natural arch if there is sufficient height of masonry and well-bonded abutments (piers) at both sides, so that the horizontal thrust can be resisted. Only a small triangular zone above the opening loads the lintel; the rest of the load arches to the sides.

        load spreads at 45 degrees to piers
          \          |           /
           \         |          /
   pier     \    masonry above /    pier
  ########   \_______|_______/    ########
  ########   |_____lintel_____|   ########
  ########   ||    opening   ||   ########

Load on the lintel (common rules)

  1. Masonry load: only the weight of masonry within an equilateral triangle on the opening span LL (height =0.866L= 0.866L) is taken on the lintel. Masonry above this triangle is ignored.
  2. Floor / roof loads: if a floor slab or beam lies within this triangle (height 0.866L0.866L above the opening), its load is added. If the floor lies above this level, no floor load is taken, because the arching action carries it.
  3. Concentrated loads: a concentrated load within the triangle is taken fully by the lintel. A concentrated load just above the triangle is included only if its 45-degree dispersal falls within the opening span.
  4. The bearing of the lintel on the masonry should be at least 100 mm or L/12L/12, whichever is greater.

Effect on piers

The load of the masonry above the opening is transferred to the piers on both sides. Each pier therefore carries its own wall load, the half-span loads from the adjacent openings, and any concentrated loads. It is checked as a short column using the area factor kak_a, which is low for small sections. Narrow piers between openings are critical, so the code limits their minimum width.

✓ Answer checked by Claude Sonnet 5.5

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