Material Compatability & Optimisatioin for Homegrown Mass Timber Systems

 

The current homegrown timber supply is focussed on production of sawn timber for general construction, fencing, and wood packaging materials. Therefore, there is a need to adapt the existing supply chain to make it suitable for scaled-up volume production of engineered wood products. 

Furthermore, in order to complete with laminated products and graded timber, the value return from the UK timber resource will have to be maximised via appropriate selection, sawing, drying, grading and handling.

The properties of timber lamellas used for the manufacture of mass timber products, such as Cross Laminated Timber (CLT) and Glue Laminated Timber (GLT), determine the performance of the end-product. For this reason, a number of key properties of this raw material must be carefully considered to ensure that the in- service performance of the end-product meets expectations, while also being commercially viable.

Wood properties and production parameters interact, so both must be considered together. Each CLT manufacturer has their own approach to selection of boards for panel manufacture (standard thicknesses and strength classes). This is usually dictated by the type, and availability, of raw material in their supply chain, as well as the end-product requirements of the markets they serve. Similarly, the cross-section and the make-up of the layers differ from manufacturer to manufacturer. 

Edinburgh Napier University’ (ENU) Centre for Wood Science and Technology (CWST) provided guidance and outlined best practice for utilising UK-grown softwood for mass timber manufacture. Opportunities for a manufacturer to optimise are outlined, but since this depends on their individual priorities, and target markets, no single optimised solution is given here. Instead, the potential for manufacturers to optimise to their own situation is presented.

Principle Outputs

  • Recommended production parameters for UK-based CLT production facility

  • Recommendations for strength grading method

  • Recommendations for appearance grading

  • Recommendations for kiln drying

Conclusion

C16+ is intended to be used as replacement for C16 when grading to a single grade and has the same (almost 100%) machine grading pass rate as C16. It allows to take advantage of the extra strength and density. One other advantage of C16+ is that (via the equations in EN 384) it has mean density sufficient to meet the glulam mean density requirement for reaction to fire classification without need for further testing.

The general perception of Irish and UK-grown spruce is that it “grows too quickly” and is “therefore of low density” and “therefore of low strength”. This is an incorrect understanding of the wood quality and the drivers of wood properties. Research has shown this for several years, but the perception is hard to shift. In particular, there remains a perceived link between wide growth rings and low wood quality that is largely unsupported in reality. The misconception is partly fed by the “slow-grown timber” marketing message used by importers for many years.

This has implications for the market perception for CLT made form Irish and UK-grown spruce, especially compared to imported CLT. This is not an accurate reflection of actual wood quality and the Transforming Timber project’s research, evidence base, and learning platform is the first step to correcting the longstanding misconception.

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