Revaluation of Structural Properties of Machine Stress Graded Plywoods
For designers working with structural plywood a number of different plywood stress grade standards may apply. The plywood standard AS/NZS2269.0:2008 and the timber design standard AS1720.1:2010 list different grade stresses for the same plywood grades. In addition plywood standard 2269.0:2008 lists three techniques for determining plywood stress grades, two of which were not in existence when the original plywood strength properties were developed.
This project related the strength and stiffness properties of current softwood/hardwood plywood resources from both Australian and New Zealand against the existing plywood stress grades.
The research showed plywood stress grades better matched AS1720.1:2010 rather than AS/NZS2269.0:2008. As a result Australian designers of plywood structures should use the characteristic values from AS1720.1:2010. Researchers believe plywood stresses in AS/NZS2269.0:2008 should either be amended to reflect those in AS1720.1:2010 or deleted, leaving AS1720.1:2010 as the only source.
The planned revision of NZS3603:1993 should also adopt the plywood stress grades from AS1720.1:2010.
The project confirmed that plywood grade stresses in AS1720.1:2010 align well with the F8, F11 & F14 plywood resource from both Australia and New Zealand.
The plywood used in this study was selected using a plywood machine stress grader in an effort to understand the effect of machine stress grading plywood on plywood properties. In the report the F rating that has been assigned is that from the machine stress grader, it is not the company branded F ratings. In fact the machine grader sample was made up from a mix of non-structural, F5, F8 & F11 (company branded) plywood, this was done to maximise the property range within the machine grader sample.
Tables 5 and 6 primarily show the ineffectiveness of machine stress grading plywood. The samples of plywood in Tables 5 & 6 are not representative of company branded product and accordingly the results cannot be interpreted as such.
All the New Zealand plywood used in report was kindly provided from one mill. It is also worth acknowledging that no machine graded plywood is sold in NZ.
The companies producing plywood currently operate under a third party verification scheme and accordingly end users can have confidence that structural plywood will have the properties claimed for it.