From Beam to Foundation: Lintels, Padstones and Strip Footings

Follow one 41 kN beam reaction from floor to padstone, down the wall and into a strip footing, with every number shown, plus the lintel triangle.

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Take the beam reaction (kN), check it on the padstone, spread it down the wall below, add the wall's own line load, then divide the total by the allowable bearing pressure to get the footing width. Use factored loads (1.35G + 1.5Q) for the padstone, and unfactored loads for the bearing pressure.

Most guides cover one link of that chain. This post follows one load the whole way down, with the numbers.

Values are typical and illustrative. A competent engineer must check them for any real design.

What is the example?

A knock-through in a ground floor wall of a two-storey house. The opening is 3.6 m clear, so a steel beam spans 3.8 m effective between padstones. Above it sit a first floor, a one-storey cavity wall and a trussed rafter roof. The ground is firm clay. Loaded widths are explained in loaded width, and the general method in manual load takedown.

How do you get the load onto the beam?

Turn each area load into a line load by multiplying by the loaded width, then add the masonry and the beam's own weight.

Source for the area loads: typical UK domestic values from BS EN 1991-1-1 and its UK National Annex, as used in the manual takedown. The floor is a timber floor, with its permanent load rounded up to 1.0 kN/m² to cover finishes. Snow at 0.6 kN/m² is a lowland figure; check it for the site.

The full height of masonry above has been put on the beam. That is conservative. Some of the wall will arch over the opening, which the lintel section below covers.

What is the beam reaction?

For a simply supported beam, the reaction at each end is wL / 2, with L the effective span of 3.8 m.

Permanent reaction = 15.1 × 3.8 / 2 = 28.7 kN
Variable reaction  =  6.3 × 3.8 / 2 = 12.0 kN

Characteristic     = 28.7 + 12.0 = 40.7 kN
ULS (1.35G + 1.5Q) = 1.35 × 28.7 + 1.5 × 12.0 = 56.7 kN

Keep the two numbers apart. The ULS figure (56.7 kN) is for the beam design and the padstone check. The characteristic figure (40.7 kN) is the one that travels down to the ground for the bearing pressure. This is the same split as in the manual takedown: never mix factored and unfactored loads in one column.

Using the straight 1.35G + 1.5Q combination is slightly conservative. The 6.10a and 6.10b pair in the UK National Annex to BS EN 1990 can give a lower figure.

How do you check the padstone?

The beam end must bear on a padstone that spreads the load into the masonry without crushing it. The check in principle is:

1. Find the bearing area. A 440 mm long × 215 mm wide padstone gives 94,600 mm².
2. Find the bearing stress at ULS: 56,700 N / 94,600 mm² = 0.60 N/mm².
3. Compare it with the design compressive strength of the masonry, fd, increased by the enhancement factor for a concentrated load.

Clause 6.1.3 of BS EN 1996-1-1 allows an enhancement factor, β, on the strength under a local load. It can be up to 1.5, but it reduces as the loaded area becomes a bigger share of the wall area, and it depends on how far the load is from the end of the wall. Work it out from the clause and the UK National Annex rather than assuming 1.5.

The strength fd comes from the unit strength, the mortar mix, the wall type and the partial factor on masonry. All of those vary with the job. For this example, assume fd = 2.0 N/mm² for illustration only. A bearing stress of 0.60 N/mm² against 2.0 N/mm² passes before any enhancement, which is a utilisation of about 0.30.

A designer must check the masonry strength and the enhancement factor for the real wall. They must also check:

• The padstone's own strength and thickness.
• The bearing of the steel flange on the padstone.
• Eccentricity of the reaction.
• The stress lower down the wall, once the load has spread but before other loads are added.

A padstone check that passes does not mean the wall below passes. That is the next step.

How does the point load spread down the wall?

A point load does not stay concentrated. It spreads as it travels down the masonry, so its effect on the wall and foundation is smaller than the full reaction at one spot.

The commonly used hand assumption is a spread of 45 degrees from each edge of the padstone, on the side where the wall continues. The padstone is 0.44 m long and the wall is 2.8 m from beam bearing to the underside of the footing (2.1 m of ground floor wall plus 0.7 m of substructure).

Length at foundation level = 0.44 + 2.8 = 3.24 m
Spread of beam reaction    = 40.7 / 3.24 = 12.6 kN/m

The beam ends at the jamb of the opening, so the load can only spread one way, along the wall. If the wall continues past the opening, this works. The limits are:

• The spread cannot go past the end of the wall, a corner, an opening or a movement joint. It stops there.
• It only works in solid, well bonded masonry. Through a cavity it stays in the leaf that carries it, unless the leaves are tied across.
• Spread zones from neighbouring point loads overlap, and the overlapping loads must be added, not counted twice.
• 45 degrees is a convention. Some designers use a steeper angle for a more conservative result, and BS EN 1996-1-1 has its own spread rules for checking the wall at mid-height. Know which method your calculation uses and say so.

If the pier below the beam is short, say only 0.9 m long, the load cannot spread to 3.24 m. It acts over 0.9 m, and the line load jumps:

Spread of beam reaction over a 0.9 m pier = 40.7 / 0.9 = 45 kN/m

Same beam, same reaction, almost four times the line load at the foundation. This is the number that catches people out, and it is why a knock-through often needs a new or widened foundation under the pier.

How wide does the strip footing need to be?

Divide the characteristic line load by the allowable bearing pressure. Add the weight of the footing.

Assume the wall below already carries 30 kN/m of characteristic load (other floors, roof and its own weight), as in the foundation load worked example. The allowable bearing pressure in the example is 100 kN/m², a typical presumed value for firm clay.

That number is an example only. The real allowable bearing pressure must come from a site investigation, or from a ground assessment by someone qualified to give it. Do not take it from a blog post.

With the load spread over 3.24 m:

Line load = 30 + 12.6 = 42.6 kN/m
Footing weight, 300 mm thick concrete = 24 × 0.3 = 7.2 kN/m²
Width = 42.6 / (100 - 7.2) = 0.46 m
Build 600 mm
Check: 42.6 / 0.6 + 7.2 = 78 kN/m², under 100 kN/m²

With the load on a 0.9 m pier:

Line load = 30 + 45 = 75 kN/m
Width = 75 / (100 - 7.2) = 0.81 m
Build 900 mm

Characteristic loads are used throughout, because the allowable pressure is a service-level value. ULS loads are then used to design the footing's thickness and reinforcement. The wall-based version of this step is in foundation load on a wall.

How does the 45 degree and 60 degree lintel triangle work?

This is a separate question from the beam. A lintel over a window or door does not usually carry the whole wall above it. Masonry arches over the opening, so the lintel only carries the masonry in a triangle above it, plus any loads that land inside that triangle.

BS 5977-1 (Lintels: method for assessment of load) is the UK standard for this. Its scope is limited to modest spans and to openings with nothing else in the triangle, so check it for the job in front of you.

The angle is where sources disagree:

• Many UK references and manufacturer guides draw the triangle as equilateral, with 60 degree base angles. The height is then about 0.87 × the span.
• Other guidance, including most North American material, uses 45 degree base angles. The height is half the span.

The exact geometry and the conditions in BS 5977-1 should be checked against the standard or the lintel manufacturer's own guidance before you rely on either. The geometry matters because it decides whether a floor is inside the triangle.

What the rule means in practice:

• Masonry inside the triangle goes on the lintel as a load, along with the lintel's own weight.
• Floor and roof loads above the apex are generally treated as arching over the opening, and drop out of the lintel load.
• Floor loads that land within the triangle, or concentrated loads such as a beam bearing, must go on the lintel.

Arching only works if the masonry can resist the sideways thrust. Typical conditions are:

• Running bond, not stack bond.
• Enough wall above the opening for the triangle to form.
• Solid masonry on both sides of the opening, with good bearing at the ends.
• No control joint, corner or other opening close to the lintel.

Where these are not met, assume no arching and take the full wall and floor loads. The same applies to a steel beam in a knock-through: the beam example above used the full wall for exactly that reason. Manufacturers publish safe working loads for their lintels (tested to BS EN 845-2), so the usual route for a standard lintel is to find the load and pick from the table.

What goes wrong most often?

• Smearing a beam reaction along the wall as a line load, then not checking the pier or the padstone.
• Spreading a point load past a corner, an opening or the edge of the wall.
• Using ULS loads for the bearing pressure, or characteristic loads for the padstone.
• Assuming arching in stack bond, next to a control joint, or without enough masonry on either side.
• Taking a presumed bearing pressure that was never confirmed by a ground investigation.

For the wider method of working loads down a house, see the load takedown worked example. A takedown that treats every opening as a line load will understate the pier and the foundation beneath it.

FAQ

How do you calculate the load on a lintel?

Work out the weight of masonry in the load triangle above the opening, add the lintel's own weight, then add any floor or roof load that lands inside the triangle. Loads above the apex are generally taken as arching to each side. Check the triangle geometry and the arching conditions against BS 5977-1 or the manufacturer's guidance.

How do you check a padstone?

Divide the factored beam reaction by the padstone's bearing area to get a stress. Compare it with the design compressive strength of the masonry, increased by the enhancement factor from BS EN 1996-1-1 clause 6.1.3. Then check the padstone itself and the stress lower in the wall. A designer must confirm the masonry strength.

How does a point load from a beam spread through a wall?

A common hand assumption is a spread of 45 degrees from the edges of the bearing, on the side where the wall continues. It stops at corners, openings and movement joints, and overlapping zones must be added together. If the wall below is a short pier, there is little room to spread and the line load at foundation level is much higher.

How do you calculate strip foundation width?

Divide the characteristic line load, including the footing's own weight, by the allowable bearing pressure. In the example, 42.6 kN/m at 100 kN/m² needs about 460 mm, so 600 mm is built. The allowable pressure must come from a site investigation, not from a typical value.

What is the difference between characteristic and ULS loads?

Characteristic loads are unfactored and are used for bearing pressure and settlement. ULS loads are multiplied by partial factors (1.35 for permanent, 1.5 for variable) and are used to design the beam, padstone, wall and footing reinforcement. Keep them in separate columns throughout.