Mechanical behaviour of sawn timber of silver birch under compression loading

Figures & data

Figure 1. Examples of selection locations within the un-edged boards for parallel and perpendicular to grain test specimens, with and without knots (left). Cross-section view with definitions of growth characteristics, distance to pith $d_{\mathrm{p}}$ and growth-ring angle ${\alpha }_{\mathrm{c}}$ (right). Specimen length l, width w and thickness t.

Table 1. Test specimens and specifications of the compression tests.

	Loading direction:	Parallel to grain	Perpendicular to grain
Specimen details	$t\times w\times l$ [mm${}^3$]	$40\times 90\times 240$	$45\times 90\times 70$
	n without knots	26	37
	n with knots	28	36
	$\overline{\rho }$ (without knots) [kg/m${}^3$]	611	620
Test specifications	Guage length [mm]	160	54
	Pre-load [kN]	1.0	0.5
	Loading rate [mm/min]	0.35	0.40

Note: Specimen thickness t, width w and length l, sample size n, and mean density $\overline{\rho }$.

Table 2. Documented growth characteristics and knot parameters.

Parameter	Definition
Distance to pith $d_{\mathrm{p}}$	Distance from centroid of specimen to pith (Figure 1)
Growth-ring width RW	Mean growth-ring width measured (on a representative line) on both cross-sections of the specimen
Growth-ring angle ${\alpha }_{\mathrm{c}}$	Orientation of the growth-ring at centroid of specimen (Figure 1)
Spiral grain ψ	Slope of grain, measured using a scribe (in accordance to EN 1310, CEN, 1997)
Knot area ratio KAR	Ratio of the projected knot area to surface area of the specimen, normal to the load direction (see e.g. Isaksson, 1999)
Knot volume $K_{\mathrm{v}}$	Volume of the representative conical knot section within the specimen
Knot width	Maximum width of all visible knot faces, measured normal to loading direction (in accordance to INSTA 142, 2010)
Knot position	Distance between the centroid of the projected knot area and the centre of the specimen, measured normal to loading direction
Knot orientation	Angle between knot axis and loading direction

Figure 2. Example of digital image correlation contrast pattern applied on the surface of a specimen.

Figure 3. Compression strength parallel to grain $f_{\mathrm{c},0}$ vs. stiffness $E_{\mathrm{c},0}$, density ρ and knot volume $K_{\mathrm{v}}$ (left). Compression strength perpendicular to grain $f_{\mathrm{c},90}$ vs. stiffness $E_{\mathrm{c},90}$, density ρ and knot volume $K_{\mathrm{v}}$ (right).

Table 3. Mean mechanical properties [MPa] and correlation coefficients between mechanical properties.

	All specimens	Without knots	With knots
$f_{\mathrm{c},0}$	48.3 (0.15)	52.7 (0.10)	43.9 (0.14)
$E_{\mathrm{c},0}$	14857 (0.19)	16530 (0.12)	13620 (0.22)
$r(f_{\mathrm{c},0},E_{\mathrm{c},0}$)	0.87	0.91	0.81
$f_{\mathrm{c},90}$	6.8 (0.25)	6.5 (0.28)	7.1 (0.20)
$E_{\mathrm{c},90}$	674 (0.31)	620 (0.37)	729 (0.24)
$r(f_{\mathrm{c},90},E_{\mathrm{c},90}$)	0.83	0.85	0.80

Note: Coefficient of variation (COV) is shown in parentheses.

Table 4. Correlation coefficients between knot free specimens and mechanical properties.

	ρ	$d_{\mathrm{p}}$	RW	${\alpha }_{\mathrm{c}}$
$f_{\mathrm{c},0}$	0.66	0.57	$-0.79$	$-0.26$
$E_{\mathrm{c},0}$	0.67	0.49	$-0.69$	$-0.28$
$f_{\mathrm{c},90}$	0.79	0.43	$-0.04$	0.65
$E_{\mathrm{c},90}$	0.58	0.08	$-0.05$	0.82

Figure 4. Structural timber specimen with knot under compression loads parallel to grain. Equivalent strain images are shown at (a) maximum load, 158 kN ($\sigma =44$ MPa); and (b) after failure at a load of 130 kN ($\sigma =36$ MPa).

Figure 5. Face view of a specimen with knot in compression loads perpendicular to grain. Equivalent strains are shown at maximum load, 22 kN ($\sigma =7.0$ MPa).

Figure 6. Cross-section view of a specimen without knot under compression loads perpendicular to grain. Equivalent strains are shown at maximum load, 18 kN ($\sigma =5.9$ MPa).

CEN, (1997). EN 1310: Round and sawn timber. method of measurement of features. European committee for standardization.

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Isaksson, T. (1999) Modelling the variability of bending strength in structural timber – length and load configuration effects. Thesis (PhD). Lund University.

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INSTA142, (2010), Nordic visual strength grading rules for timber. Suomen Standardisoimisliitto SFS.

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