Thread by @FlyingQuercus

Sing-a-long version of my presentation tonight at Scottish Timber: Green Drinks @ScotEcoDesign + @AECBnet + ASHS
seda.uk.net/events/green-d…
No spoilers

We need to grade timber because it is variable - indeed there is a lot of variation within a single tree. This is not unique to timber though. We never have perfect knowledge of any engineering materials - even the man-made ones.

But it's not as simple as assessing the density (I wish it was). Density is often not a useful predictor of wood properties like strength and stiffness. We measure it because (we think) it is simple to measure at all stages - but we should be careful not to think it the goal.

You often find images like this in textbooks (eg Dinwoodie) & it clearly shows a correlation between *average* specific gravity (a density) and modulus of elasticity (stiffness) *across many species*

But when we overlay individual board measurements for UK+IE grown spruce we see the relatively large variation within a species (from a relatively small growth area) - and the much poorer correlation between density and stiffness.

Add on data for UK grown larch and we see the same - and what's more some of the larch (which we consider much denser than spruce) is less dense than some of the spruce. When talking about species properties we always need to bear in mind this variation behind the averages.

But, engineers are used to dealing with uncertainty - because we never really know the true material properties or the true loads on the structure. It's all part of the design method behind codes and standards.

What we need to worry about is the possibility that high load coincides with low strength. And so we talk about fifth percentile strengths rather than mean strengths. 5th %ile describes the low end - for which 5% of the pieces are weaker (we don't know which pieces of course)

Right now we have two basic ways of strength grading timber - visual and machine. There are differences in how they are done, but the fundamental basis is the same for both - and both are based on species and growth area. blogs.napier.ac.uk/cwst/quick-sum…

There are a bunch of standards that support this. But one key thing to remember is that for visual grading EN1912 is king. If it changes in a revision there is a reason for that & old documents that mirror it are therefore not reliable. 👏Always check the most recent standards.👏

For construction timber the three primary properties considered in grading are strength, stiffness & density. Key thing here is that this density is defined on the 5th percentile - so this density does not describe actual weight (which can be much higher).

Grading is about making sure the timber has the design properties it is supposed to have. These general strength classes allow designers to do their calculations without knowing the species & origin of the timber that will be used.

Strength classes are therefore a convenience - but they are not essential - all they are is a way of declaring performance.

In modern standards everything has to be underpinned by testing - almost always of full-sized timber with knots and all the other defects it has in reality (ie not clear wood data you often see in old documents)

This figure broadly indicates how UK-grown timber grades. You can get higher grades at the cost of decreased yields.
We are still working on "minor" species, started with these forestresearch.gov.uk/research/timbe…

The following example is based in the handheld grading machine "MTG" from Brookhuis. brookhuis.com/wood-testing-2…
Another portable grading machine is also made by Microtec - the Viscan portable microtec.eu

The most important part of establishing grading is the sampling - it needs to be representative of the timber from the whole growth area. Here we look at larch across the UK.

The grading machine measures "Indicating Property" (in this case dynamic modulus of elasticity). This IP is correlated with the properties we are interested in. By sorting according to thresholds of IP we can see what the properties of the sorted timber are like (collectively)

Before we look at grading possibilities for home-grown timber, a word about species combinations and their names

These are the current visual grading possibilities. But remember, always check against the most up-to-date primary reference. So this is information only.

For British pine we don't currently have many machine grading options - only the old bending-type of machine.

See also forestresearch.gov.uk/research/wood-…

For British spruce we have many options these days (mostly settings calculated by us @EdinburghNapier ).

See also forestresearch.gov.uk/research/wood-…

For larch - almost as many options as for spruce. Unsurprisingly we can get higher grades than for spruce.

(for a full list of all grading machines for machine control in Europe see blogs.napier.ac.uk/cwst/grading-m… )

We also recently did many settings for Douglas-fir, in collaboration with @nuigalway. For Douglas the strength is the limiting property (unlike spruce and larch, for which stiffness is the limiting property). It is also very variable - but high strength classes are possible.

Much more finer detail here blogs.napier.ac.uk/cwst/grade-bri…

Returning back to that larch eg. Notice that the actual achieved strength of the "C16" is much higher than it needs to be. The density is much higher than it needs to be for both grades. Here we are discarding some of the real potential of the timber in exchange for ease of trade

That's we @EdinburghNapier developed some special UK grades that are fitted to the actual properties of our home grown timber - intended for when we don't grade the timber for the general market and don't need to discard potential for ease of trade.

These make much more sense to use if grading timber for a specific project here in the UK.

For more explanation of why see napier.ac.uk/research-and-i…

Density: blogs.napier.ac.uk/cwst/growth-ra…

Not about strength classes: blogs.napier.ac.uk/cwst/why-gradi…
Thinking differently: blogs.napier.ac.uk/cwst/beyond-gr…
Regrading: blogs.napier.ac.uk/cwst/regrading…

Finally, a plug for our new timber architecture and architectural technology degree @EdinburghNapier
napier.ac.uk/engineering

@threadreaderapp unroll