Characteristic compressive strength for masonry f_k is calculated as K · f_b^0,70 · f_m^0,30. Here are the K coefficients, the limits and a worked example.
Introduction
Before you can check a brick wall's load-bearing capacity, you must know the wall's characteristic compressive strength f_k. It combines the stone's normalized strength (f_b) and the mortar's strength (f_m) in a power formula from EN 1996-1-1.
Power formula
K-coefficients (table 3.1)
| Stone type | Group | K (general purpose mortar) |
|---|---|---|
| Brick / fired clay and sandlime | 1 | 0,55 |
| Brick / fired clay and sandlime | 2 | 0,45 |
| Lightweight concrete | 1 | 0,55 |
| Lightweight concrete | 2 | 0,52 |
| Aerated concrete | — | 0,55 |
Example
Clay bricks group 1 with f_b = 10 MPa and M10 mortar (f_m = 10 MPa): f_k = 0.55 · 10^0.70 · 10^0.30. Since 10^0.70 · 10^0.30 = 10^1 = 10, f_k = 0.55 · 10 = 5.50 MPa.
Where does f_b come from? The form factor δ
f_b in the formula is not the stone's declared raw strength. It is the stone's normalised mean compressive strength: the tested mean compressive strength converted to a theoretical stone of 100 × 100 mm. The conversion uses a shape factor δ per DS/EN 772-1, because a low, wide stone measures different compressive strength than a high, narrow one — geometry affects the test.
Example (the method): a unit with a tested mean compressive strength f_st = 30 MPa. If δ is read from table A.1 as e.g. 0.8 for the unit's height/width, f_b = 0.8 · 30 = 24 MPa. The δ value MUST be read from the table for the specific unit — 0.8 is only an illustrative figure here, not a fixed value.
Mortar class and specialist mortars
The formula above applies to general purpose mortar with α = 0.70 and β = 0.30. If thin-layer mortar (glued joints for e.g. aerated concrete) or lightweight mortar is used, both the K value and the exponents change — EN 1996-1-1 cl. 3.6.1.2 has its own expressions for those mortar types (including a different power on f_b and the mortar term dropping out for thin-layer). Always use the expression that matches the actual mortar type.
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