TR26 is a structural timber grade in common, long standing, use in the UK for trussed rafters. It doesn’t appear in EN338 because it is not part of the C and D class system. Properties can be found in the, now withdrawn, British Standard BS5268 but these design values relate to the permissible stress design system – the strength values are not compatible with limits states design as used in the Eurocodes (EN1995). So where can you find design values for TR26?
Well, one place values can be found is the Standard for machine grading settings, EN14081-4. This standard has not been updated for a while and now contains only a tiny fraction of the settings tables for grading machines now approved. It was decided that it was impossible to keep this Standard up to date and it will soon be withdrawn (settings tables are, instead, made available via grading machine manufacturers, and SG18 for Notified Bodies).
Strength classes like TR26 are defined by three properties:
- Bending strength (major axis) – specifically the lower 5th percentile
- Bending stiffness (major axis) – the mean
- Density – specifically the lower 5th percentile
In a few of the tables in EN14081-4 we can see that
- Characteristic bending strength = 28.3 N/mm^2
- Characteristic bending stiffness = 11.0 kN/mm^2
- Characteristic density = 370 kg/m^3
The rest of the important design properties are governed by the equations in EN338 – equations that will shortly be moved to a revised version of EN384, and changed in the process. One of the changes is the formula for tensile strength parallel to grain.
This means that when the new versions of EN384 and EN338 are published in 2016 – this will make TR26 slightly better – raising the tensile strength parallel to grain from 17 N/mm^2 to 18 N/mm^2.
There is one oddity here in that the equation for mean density (which is not changing) actually gives a value of 440 kg/m^3 when rounded to the nearest 10 kg/m^3. However, in the table of the current (2009) EN338 the value quoted for C27 (which has the same 5% density as TR26 in that version) is listed as 450 kg/m^3.
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