Design Value Concepts by APA’s Dr. Yeh

Originally published by the following source: SBCAAugust 13, 2018
by SBCA Staff


An SBCA member recently sent the following quote from Dr. BJ Yeh, P.E., APA Director of Technical Services, which provides an interesting perspective (click on the image to view a larger version):

While we do not have a direct reference for this quote, SBCA completely agrees with Dr. Yeh. Although a slight modification to this statement to add clarity to the concept Dr. Yeh is advocating follows -- “Design values published by each product manufacturer (i.e. by each OSB manufacturer, each lumber manufacturer, each alternative product manufacturer, etc.) are in place to provide safety and reliability to building designs. Therefore, the designers, building officials and consumers are expecting product performance to comply with the published design values.”

Assuming that this quote from Dr. Yeh is accurate, and there is no reason to believe that it is not, SBCA members collectively believe that accurate design values are important for truss design. For example, SBCA members take responsibility for their scope of work when they provide professionally engineered and sealed truss design drawings.

Dr. Yeh is advocating that anyone using OSB or lumber in a building application can rely upon the accuracy of the design values being provided by the OSB or lumber manufacturer. When SBCA members undertake structural product designs, as Dr. Yeh states, they are buying reliable design values for OSB or lumber, based on the grade purchased. SBCA members rely upon accurate lumber and OSB design values every day as they design trusses and truss repairs. The same holds true for any product that has design values used to undertake structural design.

It follows then that all buyers of these products should consider requesting a mill specific engineer’s seal for the design values sold by that mill for the grade of OSB or lumber being purchased. The following two examples, of an engineer’s seal next to a great stamp, are obviously made up to make Dr. Yeh’s point. However, it would be easy enough for a lumber or OSB mill to provide a seal for the design value tables for the grades and sizes that they produce.

This is really what Dr. Yeh is asking for when he says “Design values published by each product manufacturer are in place to provide safety and reliability to building designs.” The implication of Dr. Yeh’s statement is that OSB and lumber design values are the responsibility of the OSB and lumber manufacturer.

As mentioned above, each OSB and lumber manufacturer should then be willing to take responsibility for their product’s published grade, and associated design values by providing an engineer’s seal validating them.

Or in the alternative, since Dr. Yeh has proposed this concept, groups like APA could provide an engineer’s seal for global OSB design values for OSB product grades. The same would hold true for lumber sizes, grades and design values by “APA like” grading agencies, including but not limited to, SPIB, WWPA, WCLIB, CLMA, Timber Products Inspection, etc.

SBCA believes that the “each product manufacturer published design value concepts” advocated by Dr. Yeh would provide the reliability he acknowledges are expected by all buyers. It is clear that OSB and lumber mill specific sealed design values would provide confidence in OSB and lumber engineering so that safe and reliable building designs are the outcome.

SBCA’s hope is that all CMs begin to ask their OSB and lumber suppliers for mill specific sealed design values for each grade they purchase. Dr. Yeh suggests that this is a reasonable expectation. SBCA agrees and as the saying goes, if not, why not?

Thank you Dr. Yeh for your very valuable insight.