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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Possible use of low quality Scots pine
Pulp and paper
Particleboard, fiberboard
Energy
Structural round timber
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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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