Crown on dimension lumber 3

[Lion06]

I'm trying to find out if the crown of lumber is dependent on its grade. I know that it's not part of the grading rules, but I was wondering if better graded lumber typically has less of a crown simply because of the nature of the grading rules (better lumber = fewer knots, fewer checks, etc.... - does this translate indirectly to less crown?).

 

[Ron]

SEIT...indirectly, yes. The grading rules address the general quality of the grain, the knots, waning and other properties. When you have excessive crown, it is usually related to the physical characteristics of the board...consistency of the grain, location and frequency of knots.

 

[msquared48]

Also, where the member is cut out of the log.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[SlideRuleEra]

In the USA, lumber is graded on the crown, designated "crook", but in a very round-about way:

1. National Grading Rules have requirements "warp" that vary with the grade.

2. Allowable warp is based on both the grade and the lumber length.

3. Warp is a measurement of crook, bow, cup and twist.

4. All of this is complicated requirement is spelled out in Section 752 of the National Grading Rules.

Here is a paper on Post-Frame Construction Tolerances that, conveniently, has the important part of Section 752 as Appendix "A".

http://bse.wisc.edu/bohnhoff/Publications/ASAE-984002-Construction_Tolerances_Standard.pdf

Note: Section 752 applies to dimensional lumber, too. Not just post-frame.

http://www.SlideRuleEra.net

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[msquared48]

Also, check out the "Wood Handbook", pages 3-7 to 3-10.

Here is an excerpt showing the shrinkage pattern of the member relative to the position where it was cut from the log. The amount of cupping seen does depend on the species too as seen in the relative differences in the radial and tangential shrinkage values.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask