Overstated Lumber and OSB Design Values Create a Code-Compliance-Based Market Advantage

Originally published by: SBCAOctober 17, 2013

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Similar to the situation where, when testing for another purpose, SBCRI found that Southern Pine (SP) lumber design values were overstated by a factor of 1.3, SBCRI discovered through testing that the design values of OSB in shear wall applications that meet the code compliance requirements of the International Residential Code (IRC) are overstated by as much as a factor of 1.8. 

This discovery prompted SBCA sent a letter (first pdf link below) on September 23, 2013 to the APA - The Engineered Wood Association (APA) and American Wood Council (AWC) regarding SBCA/SBCRI fundamental unit shear capacity equivalency benchmarking and SBCA/SBCRI findings regarding wood structural panel unit shear capacities from SBCRI testing.

The letter and associated attachments can be summarized as follows:

  • A transparent and easy-to-understand methodology needs to be provided behind why OSB shear wall performance of 500 plf should be allowed to have a performance value equal to 672 plf in the IBC;
  • why OSB shear wall performance of 350 plf should be allowed to have a performance value equal to 600 plf in the IRC;  
  • why OSB shear wall performance of 450 plf with gypsum wallboard attached, as is typical in field applications, should be allowed to have a performance value equal to 840 plf in the IRC; or,
  • why an OSB shear wall that has a tested seismic design coefficient R factor of 2 should be allowed to have an R factor equal to 6.5.

The discrepancy between tested performance in a code-compliant building and the higher design values that are codified into law may be perfectly rational and justifiable, but regardless of the reason, it should be easy for everyone to understand and apply. Otherwise, in each of the cases above, OSB (or more generally wood structural panels) has a legally-adopted competitive advantage in the market that no competing material can easily and cost-effectively achieve.

Suppliers of raw material to the structural building components (SBC) industry, and the engineering community overall, should ensure there is easy access to this information, including any relevant factors that should be considered during the design process because these factors may include good engineering-based reasons to apply them.

On several occasions, the SBC industry has requested clear, concise and straightforward justification for design values, backed up by test data similar to what the Southern Pine industry has done with respect to recrafting its overstated design values. Supporting and transparent test data builds engineering confidence in raw material performance. This is certain to improve construction performance and foster future engineering innovation by the SBC industry.

As the SBC industry has repeatedly stated since August of 2011 in a wide variety of communications, the industry believes that non-transparent “prescriptive code or ICC-ES AC130/AC322 Appendix A-like” approaches to engineering, where design values can easily be overstated or unknown, devalues the work of all professional engineers. Reliable and safe building performance is predicated upon having accurate and fully transparent raw material design properties, including considerations that are needed for successful application or installation.

SBCA/SBCRI's knowledge is backed up by extensive engineering and literature research, as well as over $1.25 million in braced wall testing and engineering analysis. SBCA’s September 23 letter to APA and AWC defines specifically how SBCA/SBCRI is going to use its testing and engineering knowledge for the advancement of engineering in the lateral wall panel design realm. It will use the code-established OSB design values as a foundation for alternative engineered designs for materials and methods of construction that allow for more robust engineering innovation to take place. This engineering will be based on the legally defined OSB performance characteristics and benchmarks that have been promulgated by APA, AWC and their manufacturer members.

We believe there are a wide variety of reasons behind why there has been so little innovation in the wood-based light-frame construction industry. One of the key reasons is the lack of a transparent understanding of lumber and OSB design values. It is certainly a great marketing advantage to codify a design value that is overstated by 1.8 for OSB and 1.3 for Southern Pine lumber. The problem that the SBC industry has is that overstated design values adopted into the legally binding building code completely devalues what the SBC industry does -- re-sell raw material design values through the use of sophisticated software modeling based on proven engineering mechanics-based equations. In other words, it is a serious devaluation of professional engineering.

The SBCA/SBCRI goal is to advance more robust innovation. SBCA/SBCRI data provides the foundation that can move our industry forward, given that innovation cannot wait for fundamental changes in the status quo to take place. Sound engineering and the value that it can provide end-users through the spirit of providing economic liberty through preserving free and unfettered competition as the rule of trade should always prevail.