What is arching action in a lintel?

Mortared blockwork above an opening can arch: the load finds a path around the opening into the masonry either side, and only a triangle of wall roughly 45 degrees off each bearing actually rests on the lintel. It applies when there is enough uncracked, continuous masonry above the opening — commonly taken as a height of at least 0.6 times the span — and it reduces the wall load on the lintel dramatically.

How much bearing does a concrete lintel need?

150 mm each end is the usual minimum for a domestic opening, and 200 mm is better on wider spans or where the block below the bearing is hollow. The bearing has to spread the end reaction into the blockwork without crushing it, so on a heavily loaded lintel the bearing stress on the block should be checked against the block's own strength, not just against the concrete.

Can a lintel be cast with the floor slab above it?

Yes, and it is often the better detail — casting the lintel monolithically with a ring beam or floor slab gives continuity, removes a separate operation and avoids a cold joint over the opening. It changes the analysis, though: a lintel built into a continuous beam is no longer a simply supported member and should be designed as part of that beam.

Why does a lintel usually need more steel than the calculation asks for?

Because minimum reinforcement and practical detailing govern, not bending. The moment on a typical domestic lintel calls for well under 100 mm2 of steel, while the smallest arrangement anyone can actually fix — two bars, links and hangers — provides several times that. The calculation is still worth doing: it tells you whether you are in that comfortable region or have left it.

Lintel design — worked example to BS 8110, with arching action

A lintel is a beam over a door or window opening. It is the member every building has dozens of, that almost nobody calculates, and that almost never fails — which makes it a good place to learn something that the bigger members hide: most of what a structural element carries is decided before you reach the bending moment.

Do the loading properly on a lintel and the design falls out in three lines. Do it carelessly and you can be four times out, in either direction.

Arching action — the whole trick

The naive assumption is that a lintel carries the entire column of blockwork above it, all the way to the roof. It usually does not.

Mortared blockwork is quite capable of behaving as an arch. The load above an opening finds a path around it, into the masonry either side, and what is left resting on the lintel is a triangle of wall spreading at roughly 45° from each bearing. Everything outside that triangle arches over.

Triangle base = span L, apex height = L/2
Area = 1/2 x L x L/2 = L2/4
Load per m = (unit weight of wall) x L / 4

The condition is that the arch has somewhere to form. There has to be enough continuous, uncracked masonry above the opening to contain the triangle and the thrust. The ordinary working rule is a wall height above the opening of at least 0.6 times the span. Below that, take the full wall height — the arch cannot develop and it is not safe to pretend it has.

Arching also needs the masonry either side to take the thrust. An opening near the end of a wall, or with another opening close beside it, has nothing to arch into. So does a wall built with the mortar most Nigerian sites actually use for blockwork — weak, and often with unfilled perpends. When any of that is in doubt, carry the full height. It is a cheap conservatism on a member this small.

What else the lintel might be carrying

Arching disposes of the wall. It does nothing about anything else landing on the lintel, and this is where the real loads come from:

A floor slab or beam bearing on the wall above the opening. Point or line loads inside the arching triangle must be carried in full — an arch does not shield a load applied directly to it.
A concentrated load anywhere near the triangle. The usual approach is to disperse it at 45° down to lintel level and add whatever lands within the span.
The roof, on a bungalow where the wall plate runs over the opening.
The lintel's own weight, which on a 225 × 225 section is over a kilonewton per metre and is a third of the total in the example below.

If a slab bears over the opening, the honest answer is often to stop calling it a lintel and design it as a beam — usually as part of the ring beam it is continuous with.

The example

A 2.1 m window opening in a 225 mm rendered blockwork wall of a bungalow. 1.8 m of wall above the opening up to the ring beam, and nothing else bearing on it.

Clear opening 2.10 m
Bearing 150 mm each end
Effective span L 2.10 + 0.30 = 2.40 m
Section 225 wide x 225 deep, grade 25, cover 25
Wall above 1.80 m at 3.9 kN/m2

Loading

Arching? 0.6 L = 0.6 x 2.40 = 1.44 m
wall above = 1.80 m >= 1.44 arching applies

Wall load 3.9 x 2.40 / 4 = 2.34 kN/m
Self weight 24 x 0.225 x 0.225 = 1.22 kN/m
gk = 3.55 kN/m
qk = 0
n = 1.4 x 3.55 = 4.98 kN/m

Without arching, the wall load would have been 3.9 × 1.80 = 7.02 kN/m — three times as much, and the total load more than double. That single test is the largest decision on the page.

Analysis

M = nL2/8 = 4.98 x 2.40^2 / 8 = 3.58 kNm
V = nL/2 = 4.98 x 2.40 / 2 = 5.97 kN

Simply supported, because a lintel sitting on blockwork has no reliable restraint at its ends. Even where it is cast into a ring beam, designing it simply supported and detailing the top steel for continuity is the safe combination.

Flexure

d = 225 - 25 cover - 8 link - 6 = 186 mm
K = 3.59e6 / (225 x 186^2 x 25) = 0.018 <= 0.156
z = 0.95d = 177 mm
As = 3.59e6 / (0.95 x 460 x 177) = 46 mm2
As,min = 0.13% x 225 x 225 = 66 mm2 governs

Provide 2T12 bottom = 226 mm2 PASS
Top 2T10 hangers to carry the links

The bending needs 46 mm². The smallest thing anyone can build needs 226. You cannot fix links without two bars in the top and two in the bottom to tie them to, and nobody is putting 8 mm main bars in a lintel. The section is five times stronger than it needs to be, and there is no cheaper version of it.

Shear

v = 5.98e3 / (225 x 186) = 0.14 N/mm2
vmax = 0.8 sqrt(25) = 4.0 N/mm2 PASS
vc with 226 mm2 at d = 186 = 0.62 N/mm2
v < vc + 0.4, so nominal links only

Spacing 0.75d = 140 mm governs -> T8 @ 125

The links are not resisting shear here — 0.14 against a concrete capacity of 0.62 is not close. They are there to hold the cage together, restrain the compression bars and control cracking, and their spacing is set by the 0.75d rule rather than by any force.

Deflection

L/d actual 2400 / 186 = 12.9
Basic simply supported = 20
MF large, capped at 2.0 (steel is barely stressed)
Allowable 20 x 2.0 = 40
Check 12.9 <= 40 PASS by a factor of three

Compare this with the slab, where deflection is what governs everything. A lintel is short and deep, so stiffness is never the problem. See span/depth ratios for where the modification factor comes from.

Bearing, which is the check people skip

The lintel passes everything. The block underneath it might not. The end reaction of 6 kN spread over 150 × 225 mm is trivial, but on a wide opening carrying a slab it is not:

Bearing stress = end reaction / (bearing length x wall thickness)
Compare with the blockwork's compressive strength, not the concrete's

Nigerian hollow sandcrete blocks vary enormously in strength, and the shell of a hollow block under the end of a lintel is a small area of quite weak material. Where the reaction is significant, the usual answers are to lengthen the bearing to 200 or 225 mm, to fill the blocks below the bearing with concrete, or to run the lintel through as part of a ring beam so there is no discrete bearing at all.

Rules of thumb, and where they run out

For ordinary domestic openings in a bungalow, with arching and no slab bearing over:

Clear opening	Usual section	Usual steel	Bearing
up to 1.2 m	225 x 150	2T12 bottom, T8 links	150 mm
1.2 – 2.4 m	225 x 225	2T12 bottom, T8 links	150 mm
2.4 – 3.0 m	225 x 300	2T16 bottom, T8 links	200 mm
over 3.0 m	Design it as a beam, and check the arching assumption very carefully

These are a sanity check on a calculation, not a substitute for one. Every one of them assumes arching applies and nothing else bears on the lintel. Put a floor slab over a 2.4 m opening and the table is wrong by a factor of three. That is exactly the case where a lintel does fail, and it is why the loading is worth more attention than the design.

What this leaves out

Openings close together or near a wall end, where the arch has nothing to thrust against.
Combined lintel and ring beam acting continuously — a better detail, but a different analysis.
Point loads from trusses or beams landing near the opening, which need dispersing to lintel level.
Boot lintels and composite action with the masonry above.
Deflection limits for the window itself. Structural deflection limits are generous; a large glazed unit under a lintel may need a tighter one so the frame is not loaded.

Run it yourself

Structura's lintel module does the arching test explicitly — it tells you on the sheet whether the 45° triangle applied or whether the full wall height was carried, which is the number worth checking — then flexure, shear links and deflection with a PASS or FAIL on each. Single members are free to run, as many as you like.

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