What is the 45-degree arching assumption?

Masonry above an opening tends to bridge the gap by arching, so only the wall inside a 45-degree triangle directly over the opening actually loads the lintel. NCMA TEK 12-3 uses this triangle, with a height equal to half the opening span, as the design masonry load.

When does the full triangle apply?

The full 45-degree triangle applies only when the wall extends at least half the span in height above the opening. If the wall is shorter, the loaded area is a trapezoid capped at the actual wall height, which the tool handles automatically.

How is the design moment found?

The triangular masonry load is converted to an equivalent uniform load and combined with any superimposed uniform load. The maximum simple-span moment is then w*L^2/8 using the effective span (clear span plus bearing). This is the demand the lintel section must resist.

Does this size the lintel itself?

No. It gives you the load and the bending moment demand. You then pick a precast concrete lintel, a steel angle, or a reinforced bond-beam lintel whose published moment capacity exceeds the demand, including a check for shear and deflection.

What about concentrated loads near the opening?

This tool models uniform and triangular masonry loads only. A concentrated load such as a beam reaction landing within the arching region adds directly to the lintel and must be handled separately by a structural engineer.

Masonry Lintel Load Calculator

When you cut an opening in a masonry wall, a lintel carries the masonry above it. Thanks to arching action, the lintel does not carry the whole wall — only a 45-degree triangle of masonry directly over the opening. This calculator applies the NCMA TEK 12-3 method to find that load and the resulting bending moment.

How it works

The loaded masonry forms a triangle whose height equals half the opening span:

triangle height ht = span / 2     (capped at actual wall height h)
If h >= ht:  masonry load W = (1/2) * span * ht * weight        (full triangle)
If h <  ht:  W = (span * h - h^2) * weight                       (trapezoid)
equivalent uniform w_m = W / span
total w = w_m + superimposed uniform load
effective span L = clear span + bearing (default 8 in total)
moment M = w * L^2 / 8

The triangle is the key idea: a wide opening with a tall wall above it loads the lintel far less than the dead weight of the whole wall would suggest.

Example and notes

A 6 ft opening in a wall weighing 55 psf with at least 3 ft of masonry above it gives a triangle 3 ft high, a masonry load of 0.5 × 6 × 3 × 55 ≈ 495 lb, an equivalent uniform of about 82.5 plf, and a moment near 235 lb-ft over a 6.67 ft effective span. Add floor or roof loads separately, then choose a precast or steel lintel rated above the moment demand and confirm bearing and deflection.