1. APPLICABILITY OF EUROPEAN
STANDARDS FOR STRUCTURAL TIMBER
ON SCOTS PINE ROUND TIMBER
Geir I. Vestøl & Olav Høibø

2. NORWEGIAN UNIVERSITY OF LIFE SCIENCES
Outline
 Background
– Use of low quality Scots pine in Norway
– Strength grading of structural timer in Europe
 Structural round timber of Scots pine from Norway
– Mechanical properties
– Strength grading
– Applicability of European Standards on round timber
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3. NORWEGIAN UNIVERSITY OF LIFE SCIENCES
Possible use of low quality Scots pine
 Pulp and paper
 Particleboard, fiberboard
 Energy
 Structural round timber
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4. NORWEGIAN UNIVERSITY OF LIFE SCIENCES
Strength of round timber
 Stronger than sawn
timber because of fiber
continuity in the surface
 Juvenile core is
surrounded by mature
wood that is denser,
stiffer and stronger.
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5. NORWEGIAN UNIVERSITY OF LIFE SCIENCES
Strength grading
 Strength grading is allocation of timber pieces into
strength classes with defined properties
 European standardization
– Requirements for density, stiffness properties and
strength properties
– Measurements and calculations
– Visual grading
– Machine grading
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6. NORWEGIAN UNIVERSITY OF LIFE SCIENCES
European standards regarding strength grading
 EN 14081: Strength graded structural timber with
rectangular cross section, part 1-4
 EN 338: Strength classes
 EN 408: Determination of some physical and mechanical
properties
 EN 384: Determination of characteristic values of
mechanical properties and density
 National/regional visual grading rules
– INSTA 142: Nordic visual strength grading rules for
sawn timber
7  EN 1912: Assignment of visual grades and species
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7. NORWEGIAN UNIVERSITY OF LIFE SCIENCES
EN 14081-1
Strength graded structural timber with rectangular
cross section. Part 1: General requirements
 
INSTA 142 EN 14081-4
Nordic visual strength grading Strength graded structural
rules for sawn timber timber with rectangular cross
section.

EN 1912 Part 4: Machine grading – grade
Assignment of visual grades machine settings for machine
and species controlled settings
 
EN 338
Strength classes
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
Eurocode 5
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8. NORWEGIAN UNIVERSITY OF LIFE SCIENCES
European standards applicable on round timber?
 EN 338: Strength classes
 EN 14251: Structural round timber - Test methods
– (Corresponds to EN 408)
 EN 384: Determination of characteristic values of
mechanical properties and density
– Corrections for moisture content, dimensions and sample
size
 National/regional visual grading rules
 Missing:
– EN 1912: Assignment of visual grades and species
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– Machine grading
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9. NORWEGIAN UNIVERSITY OF LIFE SCIENCES
Visual grading of Scots pine round timber
 Ranta-Maunus (1999)
– A: C30 (Scots pine and Norway spruce)
– B: C18 (Scots pine)
– Strength reducing characteristics, geometrical
characteristics, biological characteristics
 Boren & Barnard (2000)
– C30 (Scots pine)
– C24 (Scots pine)
– Knot size, knot sum, annual ring width, tapering
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10. NORWEGIAN UNIVERSITY OF LIFE SCIENCES
Tested materials
 Density, modulus of elasticity and
bending strength
 Visual characteristics
 533 logs
– 75 mm – 250 mm
– Sampled from 150 trees from 10
locations
– All logs with diameter >75 mm
tested
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11. NORWEGIAN UNIVERSITY OF LIFE SCIENCES
Uncorrected values of density, MoE and MoR
 Some correlation between dimension and moisture content
(r = 0.552)
 Strong, even if poor quality
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12. NORWEGIAN UNIVERSITY OF LIFE SCIENCES
EN 338 – Strength classification
(Vestøl & Høibø, 2010)
 Strength class is limited by MoE
 Should be possible to obtain C40 by rejecting only a small
number of pieces
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13. NORWEGIAN UNIVERSITY OF LIFE SCIENCES
Site effects on density and MoE
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14. NORWEGIAN UNIVERSITY OF LIFE SCIENCES
Site effect on bending strength
 It is not possible to obtain
C40 from all sites without
rejecting a large number
 Models showed that visual
grading left significant
effects of site
 Predicting MoR from MoE
is more accurate, and not
as dependent on site
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(Vestøl & Høibø, 2010)
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15. NORWEGIAN UNIVERSITY OF LIFE SCIENCES
Corrections according to EN 384
 MoE is corrected for moisture content
– Correction coefficient = 0.02
 MoR is not corrected for moisture content
 MoR is corrected for dimension by dividing on
" 150 % 0.2
kh = $ '
# D&
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!
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16. NORWEGIAN UNIVERSITY OF LIFE SCIENCES
MoE and MoR models
 Y= +f(A,B,C…)+Si+ ij
– f(A,B,C…) = covariate function
– Si = random effect of site (i = 1–10)
 Tested covariates
– Density, annual ring width, grain angle
– Maximum knot size, relative maximum knot size
– Maximum knot group, relative maximum knot sum
– Tapering
– Moisture content
– Dimension
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17. NORWEGIAN UNIVERSITY OF LIFE SCIENCES
MoE, Correction for moisture content
 MoE = f(MC, density, tapering, knot sum)
– Fixed effects model:
• RMSE = 1953 N/mm2, R2 = 0.61
– 16.0% of random variance due to site
 The model predicts a correction for moisture content
– Average correction coefficient: 0.02 (as in EN 384)
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18. NORWEGIAN UNIVERSITY OF LIFE SCIENCES
MoR, Correction for moisture content
 MoR = f(MC, ln(D), ln(density), tapering, knot size, knot sum, grain angle)
– Fixed effects model
• RMSE = 6.1 N/mm2 , R2 = 0.75
– 11.3% of random variance due to site
 The model predicts corrections for both moisture content and dimension
– Average correction coeffisient for moisture content: 0.04
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19. NORWEGIAN UNIVERSITY OF LIFE SCIENCES
MoR, Correction for dimension
 MoR = f(MC, ln(D), ln(density), tapering, knot size, knot sum, grain angle)
– Fixed effects model
• RMSE = 6.1 N/mm2 , R2 = 0.75
– 11.3% of random variance due to site
 The model predicts corrections for dimension as described in EN 384
" 150 % 0.192
kh = $ '
# D&
!
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20. NORWEGIAN UNIVERSITY OF LIFE SCIENCES
Conclusions
 Visual grading leaves significant residuals due to site that
cannot be accounted for.
– Visual grading rules have to be either conservative or limited to
timber from a specific region.
 The model indicates that MoE of round timber can be corrected
for moisture content as described in EN 384.
 The model indicates that bending strength can be corrected for
dimension as described in EN 384, but it should also be
corrected for moisture content.
 The correction coefficients should be estimated from a
controlled experiment without correlation between dimension
and moisture content
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