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
Teglmursten gruppe 1 med f_b = 10 MPa og M10-mørtel (f_m = 10 MPa): f_k = 0,55 · 10^0,70 · 10^0,30. Da 10^0,70 · 10^0,30 = 10^1 = 10, bliver 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.
Eksempel (metoden): en sten med prøvet middeltrykstyrke f_st = 30 MPa. Aflæses δ i tabel A.1 til fx 0,8 for stenens højde/bredde, bliver f_b = 0,8 · 30 = 24 MPa. δ-værdien SKAL aflæses i tabellen for den konkrete sten — 0,8 er her kun et illustrativt tal, ikke en fast værdi.
Mortar class and specialist mortars
Formlen ovenfor gælder almindelig mørtel (general purpose mortar) med α = 0,70 og β = 0,30. Bruges tyndfugemørtel (limfuge til fx porebeton) eller letmørtel, ændrer både K-værdien og eksponenterne sig — EN 1996-1-1 pkt. 3.6.1.2 har egne udtryk for de mørteltyper (bl.a. en anden potens på f_b og bortfald af mørtelleddet for tyndfuge). Brug altid det udtryk, der passer til den faktiske mørteltype.
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