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.

What is the Masonry Lintel Load Calculator?

Computes the load on a masonry lintel over an opening using the 45-degree triangular arching assumption from NCMA TEK 12-3. Enter opening span, masonry unit weight, and wall height above the opening to get the triangular masonry load plus any superimposed uniform load, with the resulting design moment. It runs free in your browser on Gera Tools, with nothing uploaded.

Masonry Lintel Load Calculator

Name: Masonry Lintel Load Calculator
Creator: Gera Tools
License: https://creativecommons.org/licenses/by/4.0/

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.

Why the 45-degree arching assumption makes physical sense

Masonry is strong in compression but weak in tension. When a wall spans an opening, the masonry above the opening naturally redistributes load by forming a compression arch — units above and to the sides of the opening carry load diagonally around the gap. The 45-degree triangular load region represents the masonry that sits inside the potential arch but hasn’t fully transferred its load to the surroundings; the lintel must carry this triangle.

Outside that triangle — in the triangular regions to either side of the opening and extending up at 45 degrees — the masonry load arches into the adjacent wall, bypassing the lintel entirely. This is why the formula does not simply multiply wall height by span by weight, which would significantly overestimate the demand.

The NCMA TEK 12-3 document formalizes this approach and is the standard reference used by masonry designers in North America.

When the wall height limits the triangle

The full triangle (height = span/2) applies only when there is enough masonry above the opening to form the complete arch. If the wall terminates before the triangle can fully develop — for example, the opening extends close to the top of a low parapet — then the loaded area is a trapezoid rather than a triangle, and the calculation changes:

If h < span/2:   W = (span * h - h^2) * weight_per_sqft

This formula integrates the area of a partial triangle (or trapezoid). The calculator handles this automatically based on the wall height you enter.

Worked example

Scenario: A 6 ft clear opening in an 8-inch CMU wall weighing 55 psf, with 4 ft of masonry above the opening, and a 200 lb/ft roof load bearing on the wall above.

Step 1 — Check whether full triangle applies:

Triangle height = 6 ÷ 2 = 3 ft
Wall height above = 4 ft, which exceeds 3 ft → full triangle applies

Step 2 — Masonry load on lintel:

W = 0.5 × 6 × 3 × 55 = 495 lb total triangular load
Equivalent uniform = 495 ÷ 6 = 82.5 plf

Step 3 — Add superimposed load:

Total w = 82.5 + 200 = 282.5 plf

Step 4 — Effective span and moment:

Effective span L = 6 ft + 8 in bearing = 6.67 ft
Moment M = 282.5 × (6.67)^2 ÷ 8 = 1,573 lb-ft

Select a lintel whose published moment capacity exceeds 1,573 lb-ft — with appropriate safety factors per the applicable masonry code (TMS 402 or similar). Confirm bearing stress, shear, and deflection limits as well.

Common lintel types and when to use this calculation

Precast concrete lintels: Widely available in standard widths for 4”, 6”, 8”, 10”, and 12” CMU. Published moment capacities in manufacturer tables; confirm the span you’re designing against.
Steel angle lintels: Common in brick veneer above window/door openings. The same load triangle applies; size per AISC for bending plus check for lateral torsional buckling on the outstanding leg.
Reinforced bond-beam lintels: A continuous reinforced course built into the CMU wall itself, spanning the opening. Designed per TMS 402 reinforced masonry provisions.

Add floor or roof loads separately, then choose a precast or steel lintel rated above the moment demand and confirm bearing and deflection. For concentrated loads (beam reactions) within the arch region, consult a structural engineer — the simple triangular method does not apply.