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Publication Number: FHWA-HRT-04-097
Date: August 2007

Measured Variability Of Southern Yellow Pine - Manual for LS-DYNA Wood Material Model 143

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1.12 VARIABILITY BY GRADE

Visual grading includes an assessment of wood defects. When analyzing structures such as guardrail posts, the exact position of the defects will not be known, except in highly controlled laboratory tests. Therefore, a practical approach for addressing defects is to modify the material properties globally as a function of visual grade. This approach requires the implementation of the grade as an input parameter.

Four input options are available for modeling strength and stiffness reductions by grade. The four options are listed in table 17:

Table 17. Input options for modeling strength reductions by grade.
User Input Grade Reduction Factors
QT QC Wood
QT = 0 QC Ignored
1, 1D, 2, or 2D
0.47
0.63
Pine
0.40
0.70
Fir
QT = -1 QC Ignored
DS-65 or SEL STR
0.80
0.93
Pine & Fir
QT = -2 QC Ignored
Clear Wood
1.00
1.00
Pine & Fir
0.0 < QT £ 1.0
0.0 < QC £ 1.0
User-Specified
QT
QC
Pine & Fir

The user may request clear, high-grade (DS-65 or Select Structural), or low-grade (grades 1, 1D, 2, or 2D) wood. This is because significant differences were noted between DS-65 posts and posts of all other grades (1, 1D, 2, and 2D) tested. The DS‑65 posts are significantly stronger than all other posts. In addition, there is no statistically significant difference in response among the other posts, although the dense posts (grades 1D and 2D) tend to absorb more energy than the low-density (grades 1 and 2) posts. Therefore, the posts can effectively be divided into two grades.

Our approach is to implement global strength-reduction factors as a function of grade. Two factors must be applied. One factor, QT, reduces the tensile and shear strengths of clear wood; the other factor, QC, reduces the compressive strengths. The factors are simultaneously applied to the parallel and perpendicular strengths. In addition to applying the predefined quality factors by grade, the user may directly specify the tensile and compressive reduction factors between 0 and 1.

The Standard Grading Rules Handbook for southern pine lumber indicates that the perpendicular-to-the-grain compressive strength of lumber does not always vary with grade (perpendicular tensile strengths are not reported).(26) Therefore, an input flag (IFAIL) is available to allow the user to apply the strength-reduction factors parallel to the grain, but not perpendicular to the grain.

Although strength-reduction factors are a global approach for modeling strength and stiffness reductions caused by defects, the user may want to model local defects in detail. This can be done by the appropriate selection of mesh, local wood properties, and local grain orientation. The modeling of defects in an explicit manner is expected to be time-consuming to set up and may be computationally intensive to run.

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