session 5 ic2011 vestol
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APPLICABILITY OF EUROPEAN STANDARDS FOR STRUCTURAL TIMBER
ON SCOTS PINE ROUND TIMBER Geir I. Vestøl & Olav Høibø
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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
EN 1912: Assignment of visual grades and species 7
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EN 14081-1 Strength graded structural timber with rectangular cross section. Part 1: General requirements
INSTA 142 Nordic visual strength grading rules for sawn timber
EN 14081-4 Strength graded structural timber with rectangular cross section. Part 4: Machine grading – grade machine settings for machine controlled settings
EN 338 Strength classes
Eurocode 5
EN 1912 Assignment of visual grades and species
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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
– Machine grading 9
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RW
EGIAN
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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 10
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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
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Some correlation between dimension and moisture content (r = 0.552)
Strong, even if poor quality
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EN 338 – Strength classification
Strength class is limited by MoE
Should be possible to obtain C40 by rejecting only a small number of pieces
(Vestøl & Høibø, 2010)
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Site effects on density and MoE
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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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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
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# $
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0.2
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MoE and MoR models
Y=μ+f(A,B,C…)+Si+εij
– 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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EGIAN
UN
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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
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!
kh
=150
D
"
# $
%
& '
0.192
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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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