OSB Bracing Design for IRC? APA & AWC Say “Not Sure”
The American Wood Council (AWC) and APA know very well that in the International Residential Code (IRC) resistance to lateral loads is hard to quantify exactly. Why? Because in the IRC, the bracing provided is prescriptive, meaning braced wall lines are not “formally designed.” In a presentation created by the American Wood Council, they state that IRC wall bracing is a question mark. (See graphic 1).
Graphic 1: Copy of a table from the AWC presentation that shows the estimated allowable design values for various bracing types found in the IRC and the market. Click to enlarge.
This presentation restates this concept as follows; “This is a relatively formal way to say that we’re not sure exactly what the resistance to lateral loads are being provided by prescriptive bracing. We know from experience that it works under limitation of conventional construction. However, since the wall isn’t formally designed and it lacks elements of a shear wall, such as connections to the foundation or floor, its hard to quantify the resistance to any exact degree.”
Mr. Ed Keith of APA created "Appendix B" for a January 3, 2013 meeting with SBCA, which can be seen in graphic 2. For more detail, please review this January 24, 2013 letter recapping the APA meeting.
What the above graphic is saying is that the IRC has a range of wood structural panel (WSP) or OSB lateral resistance values:
- From a minimum of roughly 150 plf allowable stress design (ASD) (roughly 300 plf ultimate strength) as an unrestrained lateral resisting element (i.e. anchor bolts to the bottom plate);
- To a maximum of about 350 plf allowable stress design (roughly 700 plf ultimate strength) as a fully restrained lateral resisting element (i.e. hold-down connections on both ends of the WSP shear wall).
Testing performed at the SBC Research Institute (SBCRI) proves the point.
Graphic 3: SBCRI testing diagram. Click to enlarge
This graphic at right (click on the image to view a larger version) provides the following insight regarding the allowable stress design of WSPs (ultimate strength values in parentheses):
- WSP (OSB) using only anchor bolts (i.e. anchor bolts to the bottom plate) has an ASD value of roughly 100 plf (200 plf ultimate strength).
- WSP (OSB) with 1100 pounds of restraint has an ASD value of roughly 125 plf (350 plf ultimate strength).
- WSP (OSB) with 2200 pounds of restraint has an ASD value of roughly 250 plf (500 plf ultimate strength).
- WSP (OSB) with 3300 pounds of restraint has an ASD value of roughly 300 plf (600 plf ultimate strength).
- WSP (OSB) with 4400 pounds of restraint has an ASD value of roughly 335 plf (670 plf ultimate strength).
- WSP (OSB) with hold down (w/HD) restraint at each of the OSB panel has an ASD value of roughly 250 plf (500 plf ultimate strength).
The SBCRI testing was also confirmed in this PhD thesis. OSB design values depend on the test method, gravity load applied and all load path connection systems. For more information on wall bracing design value variability, please review the following two articles: APA - IRC Allowable Design Values for OSB Wall Bracing = 175 PLF and OSB Bracing Design for IRC? APA & AWC Say “Not Sure”.
In our January 3, 2013 meeting with SBCA, Dr. BJ Yeh stated the IRC performance of OSB shown in Graphic 4.
SBCA agrees with Dr. Yeh’s assessment and also his point of view as articulated in an article entitled, “APA’s Dr. Yeh on Published Design Values”, and restated here:
“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.”
It follows then that all buyers 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.
For additional information on the performance of wood structural panel, please visit the following webpage on OSB as a Raw Material.