EN338 – what’s happening with grades?

EN338 is a European Standard that specifies strength classes of structural timber (grades with numbers attached for use in design).  It is not the only specification of strength classes, but it is the Standard that contains the definition of the main ones used in construction – commonly known as “C” and “D” grades.

Today the results came through from countries voting on the revision of the Standard – and it passed, with some editorial comments.  So what changes can we expect from this very important document?

Firstly, we see tension grades added to the list.  These have been given the letter T.  Tension strength classes are intended for use in making laminated timber products – mainly glulam – and they have been in use for quite a long time already.  The inclusion in EN338 is intended to simplify matters across all timber related standards – although, as we will see below, this has not really happened yet.

Grading is based on three primary properties: strength, stiffness and density.  For C and D grades the key strength is the major axis bending strength. Since you can only destructively test timber in one way, and because we cannot afford to conduct tests for all properties, the other properties have to be conservatively estimated from the three properties we measure when we establish machine grading settings and visual grading assignments.  So for C and D grades, tension strength, shear strength, and strengths perpendicular to grain are secondary properties, conservatively estimated  from bending strength and density.

The difference between tension grades and bending grades is that tension grades are established based on the results of tension tests, rather than bending tests.  That means that bending strength for the strength class is estimated from tension strength rather than the other way around.  This explains why, if you compare the values for C and T grades you will see that tension strength is better for T grades.

The new version of EN338 also includes some new D classes – extending the range at the high end and filling in some gaps.  This doesn’t mean that all these strength classes are commercially available – it just means they are potentially there to be used.  It will also now be possible to grade hardwoods to the C classes – it makes sense to do this for some hardwoods that have relatively low density (for a hardwood).  This means that the clear distinction between C grades being softwoods and D grades for hardwoods starts to blur – although D grades remain exclusively for hardwood species.

This is not, however, the most important change.  Compared to the previous EN338 there have also been a number of changes to the actual design properties – notably tensile strength of softwoods and compression perpendicular to grain for hardwoods.   This was done partly in an attempt to align the C and T grades, partly to simplify the equations for secondary properties, and partly because new data was presented that showed lower compression strength perpendicular to grain for some hardwood species and possible lower tensile strength for lower grade softwoods.  The density of D70 was also reduced because otherwise it was going to be hard to actually assign any species to this very high strength class.

This means that it will be necessary to check that future designs are made with the updated numbers. 

The characteristic density of C27 has been lowered from 370 kg/m^3 to 360 kg/m^3. This is, in our view, an editorial error that occurred when trying to align the grades in EN14080 with those in EN338.  This will be one new area of conflict with EN14080, which contains a table of equivalence between C and T classes (the other being the reduction in tensile strength for lower C classes).

The CEN committee covering EN338 (TC124 WG2) will discuss a possible reinstatement of the current characteristic density for C27 later this year, but it is not certain this will be approved.

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