Background. In the building sector, it is widely considered that construction timber has become poorer in quality during recent decades. Builders frequently find it difficult to define exactly what is meant by poorer timber, but they report noticing an increased amount of deformed timber, leading to increased amounts of rejections.
Material. In a study, 47 fast-grown and 47 slow-grown butt logs of Norway spruce, exceeding 38 cm at the top, were sawn to studs. The logs were obtained from two stands in southern Sweden. One of these stands was 65-year-old at felling whereas the other was 105-year-old. The logs were sawn into three planks sized 75 x 300 x 6000 mm, where the centre plank was reserved for future studies.
The planks were divided in the middle, after which one half was ripped into six studs (47 x 75 x 3000 mm) before being dried, together with the unripped planks, to a moisture content of 12%. After drying, the unripped planks were sawn into six studs. All the 1128 studs were measured after drying with respect to bow, crook and twist.
Results. The largest proportion of studs that were acceptable with regard to deformation requirements in "Purchasing rules for wall studs", was obtained by sawing the slow-grown logs, drying the pieces and then ripping them into studs. Of these, 71% of the juvenile, 79% of the intermediate and 86% of the mature studs were acceptable as wall studs. When ripping the undried pieces from slow-grown logs and drying the studs only 30% of the juvenile, 66% of the intermediate and 75% of the mature studs were acceptable as wall studs. When studs were obtained from fast-grown logs that had been ripped after drying, 28% of the juvenile, 58% of the intermediate and 58% of the mature studs were acceptable as wall studs. The poorest result was found for studs originating from the fast-grown logs that had been ripped before drying, where only 28% of the juvenile, 55% of the intermediate and 51% of the mature studs were acceptable as wall studs.
Conclusions. Norway spruce may be considered as a very suitable material for wall studs. When sawing, drying and ripping large-sized logs of slow-grown spruce, with a mean width of annual rings less than 34 mm for the twenty rings closest to the pith, we may expect that 8090% will be accepted as wall studs, provided that good sawing and drying techniques are used. With fast-grown spruce, the yield decreases to about 5060%, mainly since studs taken near the pith are not acceptable as wall studs.
Key words: deformation, wall studs.
Correspondence: T. Thörnqvist, Södra Timber AB, S-351 89 Växjö, Sweden
Telefax: +46-470-711721
E-mail: thomas.thornqvist@timber.sodra.se